Technical Briefs 01-92

8/3/2019 Technical Briefs 01-92

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Powell Technical Brief #1Fast Bus Tr ansf er

Powell Electrical Group Powell Electrical Systems, Inc.PO Box 12818 Houston, TX 77217

2005 Powell Industries, Inc. All rights reserved .

Tel: 713.944.6900 Fax: [email protected]

April 23, 1990

Fast bus transfer is normally used for transferring a bus supplying motors to an emergency power sourceon failure of the normal source of power. It is essential that this transfer be accomplished with a minimumof "dead time" to prevent loss of critical motors or damage to the motors on re-energization.

Two schemes of operation are used for fast transfer. In the first, the trip signal to the opening breakerand the close signal to the closing breaker are given simultaneously. With this method, there is apossibility of overlap between the two sources, which may lead to the incoming breaker closing into afault. This can be prevented by adding a few milliseconds of time delay to the closing signal. In thesecond scheme, the closing signal of the second breaker is initiated by a "b" contact of the openingbreaker. This may be either standard "b" contact or a fast "b" contact.

We have recently run timing tests on the "Dash 3" PowlVac circuit breaker to determine fast transferdead times. The result, which apply to 5PV0250-3 and 15PV0500-3 breakers, both 1200A and 2000A,are given in the following table.

Dead Time, ms Source of Closing Signal No Arcing With Arcing

Simultaneous Close and TripSignals 7.0 - 17.0

(1.0)* - 9.0

Trip Then Close, Using Fast "b"Contact 53.0 - 63.0 45.0 - 55.0

Trip Then Close, Using Standard"b" Contact 57.5 - 67.5

49.5 - 59.5

Possible overlap

Baldwin Bridger, P.E.Technical Director


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Powell Technical Brief #2Closing and Lat ching Capabi l i t y of Medium Volt age Power

Circuit Breakers




May 18, 1990

ANSI Standard C37.06-1987, American National Standard for Switchgear - AC High Voltage CircuitBreakers Rated on a Symmetrical Current Basis - Preferred Ratings and Related Required Capabilities,includes a column in Table 1 headed Closing and Latching Capability. In older editions of this standard,the current value in this column was given in rms kiloamperes, and was determined by multiplying themaximum symmetrical interrupting capability by 1.6. In the 1987 edition, this current is expressed in crestkiloamperes, and the value is determined by multiplying the maximum symmetrical interrupting capabilityby 2.7.

Other standards had previously required the closing and latching current to have a crest value of 2.7times the maximum symmetrical interrupting current, so the performance required of the circuit breakerhas not really changed. Only the method of stating the requirement has changed. This change was madeto bring the ANSI standard in line with the IEC standard, which also expresses closing and latchingcapability in crest amperes.

Since many specification writers will be using older standards, or copying older specifications, we willprobably see both methods of specifying closing and latching current used in specifications for manyyears. The following table gives both sets of values.

Closing and Latching Capability per ANSIC37.06

RatedMaximum

Voltage

kV, rms

Rated ShortCircuit Current

kA, rms

Nominal

MVA 1979 Edition kA, rms 1987 Edition kA, Crest

4.76 29 250 58 97

4.76 41 350 78 132

8.25 33 500 66 111

15.0 18 500 37 62

15.0 28 750 58 97

15.0 37 1000 77 130


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Powell Technical Brief #2Closing and Lat ching Capabi l i t y of Medium Volt age Power

Circuit Breakers


page 2

If the specified value of closing and latching current matches a value from either edition of thestandard, we can assume that a standard breaker is desired. If there is any possibility ofconfusion, the specifier should be contacted to determine which basis is being used to specifythe close and latch rating.





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Powell Technical Brief #3Capaci t ance Curr ent Swi t ching Capabi l i t y of PowlVac

Ci rcui t Breakers




June 7, 1990

We have recently had capacitance current switching tests performed on our "Dash 3" PowlVac circuitbreakers, using GE interrupters. The results of these tests showed that these breakers are qualified asdefinite purpose circuit breakers, in accordance with ANSI Standard C37.06-1987, Table 1A, for bothisolated and back-to-back switching of capacitors.

Table 1 lists the maximum rating of capacitor bank that can be switched by each rating of circuit breakerwhen applied in accordance with ANSI/IEEE Standard C37.012-1979. The values in the table werecalculated using a total current multiplier of 1.25 for ungrounded capacitor banks and 1.35 for groundedbanks. These multipliers include allowances for higher than normal voltage, capacitor tolerance, andharmonic components in the current. See ANSI/IEEE C37.012-4.7.1. When PowlVac circuit breakersare used in a back-to-back switching situation, inrush currents and frequencies must be limited to thevalues given in Table 1A of ANSI C37.06-1987. This may require the addition of reactance between thetwo capacitor banks.

Table 1: Capacitor Bank Switching Capability of "Dash 3" PowlVac Circuit Breaker s

Maximum Nameplate Rating of Capacitor Bank,MVAR

Ungrounded Bank Grounded BankCircuit Breaker Type and

RatingSystem Voltage

kV1200A

Breaker2000A

Breaker1200A

Breaker2000A

Breaker2.4 2.09 3.33 1.94 3.08

4.16 3.63 5.76 3.36 5.34 05PV0250 4.76kV 250MVA

4.76 4.15 6.60 3.85 6.11

11.5 10.04 15.93 9.30 14.75

12.47 10.88 17.28 10.08 16.00

13.2 11.52 18.29 10.67 16.94

13.8

12.05

19.12

11.15

17.71

15PV0500 15.0kV 500MVA

14.4 12.57 19.95 11.64 18.48


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Powell Technical Brief #3Capaci t ance Curr ent Swi t ching Capabi l i t y of PowlVac

Ci rcui t Breakers


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Note: This table does not apply to PowlVac circuit breakers using Mitsubishi interrupters. We have nottested those breakers for capacitance current switching capability, but we do have some data fromMitsubishi that allows us to apply them. Such applications should be referred to me for checking.





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Powell Technical Brief #4Umbi l i cal Cor d Used on Powl Vac Ci r cui t Br eaker s




July 28, 1990

Occasionally, customers or prospective customers question our use of a manually-operated controldisconnect ("umbilical cord") on our PowlVac circuit breakers. Some of the questions asked, and ouranswers to them, are:

Q. Why does Powell use an umbilical cord for its control disconnect?

A. The use of the umbilical cord is part of our user-friendly design, which locates all circuit breakercontrol accessories in the front of the cell. In addition to the control disconnect, these devices include themechanism-operated cell switch (MOC) and the truck-operated cell switch (TOC). In our PowlVac design, these devices are located where they may be observed by an operator inserting or removing thecircuit breaker, allowing the operator to check alignment and operation when the circuit breaker isinstalled. These devices are also available for servicing without removing the circuit breaker from the cell.

Q. Is this design safe?

A. Yes. The umbilical cord's plug mechanism is mechanically interlocked with the circuit breaker toinsure safe operation. Interlocks provided include:

The circuit breaker cannot be inserted into the cell without plugging in the umbilical cord. Once the circuit breaker racking mechanism has been operated to start the circuit breaker

insertion process, the plug cannot be removed. It is therefore not possible to disconnect thecontrol circuits of a circuit breaker that is in service.

Unplugging the umbilical cord trips the circuit breaker if it is closed and discharges the closingspring if it is charged. Since the plug must be removed in order to remove the circuit breaker fromits cell, these interlocks insure that the circuit breaker is open and all energy storage springs aredischarged when the circuit breaker is taken out of the cell.

Q. Why does Powell differ from all other manufacturers in the method of disconnecting the control connections to the circuit breaker?

A. Powell does not differ from "all other manufacturers". While the umbilical cord design has not beenused frequently in the United States, other American manufacturers have used it. It is also commonly

used in Europe. We chose to use this design because we think it offers superior performance in total.


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Powell Technical Brief #4Umbi l i cal Cor d Used on Powl Vac Ci r cui t Br eaker s


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Q. Does the umbilical cord design meet ANSI standards?

A. Yes. This design, including required interlocking, is covered in detail in ANSI/IEEE StandardC37.20.2-6.2.7. The PowlVac circuit breaker meets these requirements.





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Powell Technical Brief #5Compar ison of Porcelai n & Cycloaliphat ic Epoxy Insulat ion




July 29, 1990

PowlVac vacuum circuit breakers and metal-clad switchgear use a primary insulation system ofcycloaliphatic epoxy. This insulation has given excellent results in the eight years since we firstintroduced PowlVac , but we still have customers who request porcelain.

Powell is far from alone in using cycloaliphatic epoxy insulation. The material has been in common use inEurope for a generation, and other U. S. users include Westinghouse, S&C and Square D. It is especiallyinteresting to see the first two of these companies using cycloaliphatic epoxy. A few years ago, both werestrong proponents of porcelain insulation.

Although there are many formulations of cycloaliphatic epoxy and a number of varieties of porcelain,each of which has its own specific qualities and parameters, there are a number of general comparisonswhich can be made.

First, in the physical area, the following relationships are typical:

Cycloaliphatic epoxy ("cyclo") weighs less than 70% of porcelain's weight. The thermal coefficient of expansion of cyclo is 1/20th that of porcelain. The tensile strength of cyclo is about 11 times that of glazed porcelain. The compression strength of cyclo is 4 to 6 times that of glazed porcelain. The flexural strength of cyclo is 16 to 18 times that of glazed porcelain. The Izod impact strength, unnotched, is about the same as glazed porcelain. Dimensional and shape control is much easier in cyclos than in porcelain. While the repairability of cyclos is limited, porcelain is unrepairable.

In the electrical area, you will find:

The dielectric constant of cyclo is only about two-thirds that of porcelain. The temperature class of porcelain is much higher than that of cyclo, but cyclo mixtures with

temperature classes of 105 C or 130 C are readily available. The track resistance of cyclo is slightly less than that of porcelain. The water absorption of cyclo is slightly greater than that of porcelain, but is still in the range of

2/10's of 1%.


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Powell Technical Brief #5Compar ison of Porcelai n & Cycloaliphat ic Epoxy Insulat ion


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Finally, cyclo exhibits excellent resistance to common industrial chemicals, is readily washable,and has excellent erosion resistance and weathering properties.

In summary, we believe that the excellent physical properties of cyclo make it the insulatingmaterial of choice in spite of some small sacrifice in electrical properties. This is especially truefor applications requiring great strength under severe dynamic loading, such as supportinsulators in circuit breakers and switchgear.





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Powell Technical Brief #6Ef f ect of Solar Radiat ion on Out door Met al-Enclosed

Swi t chgear


July 30, 1990

From time to time we get questions about the rating of outdoor metal-enclosed switchgear which isexposed to solar radiation. It is fairly obvious to anyone who thinks about it that switchgear sitting out inthe sun gets hotter than switchgear sitting in the same ambient air temperature inside a building where ithas no solar exposure. How should we handle this extra heat?

Metal-enclosed switchgear built to ANSI standards, as is all Powell switchgear, is rated in accordancewith the usual service conditions set forth in those standards. All four of the ANSI product standards wecommonly use (C37.20.1 for low voltage switchgear, C37.20.2 for metal-clad switchgear, C37.20.3 forinterrupter switchgear, and C37.23 for bus duct) include as one of the usual service conditions that theeffect of solar radiation is not significant. Thus, all testing and rating of switchgear ignores the effect ofsolar radiation.

When switchgear is installed in a location where solar radiation is significant, there is another ANSIstandard to give guidance in properly applying the switchgear. ANSI/IEEE C37.24-1986, IEEE Guide for Evaluating the Effect of Solar Radiation on Outdoor Metal-Enclosed Switchgear , gives the informationnecessary to allow calculating the derating of the continuous current capability of switchgear exposed tothe sun. This standard is site-specific; the derating depends on the location of the switchgear installation.

As a switchgear manufacturer, we assume that our customers specify switchgear ratings in accordancewith the usual service conditions given in the product standards. We further assume that the specifier will

do the necessary evaluation and either limit his loads or upgrade his ratings to take care of any solarradiation derating that is needed. If requested, we will be glad to discuss this derating with ourcustomers, and to assist them with the calculations if necessary, but we should not be expected toautomatically quote a 2000A circuit breaker where a 1200A circuit breaker is specified, just because theinstallation is outdoors in Yuma, Arizona.





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Powell Technical Brief #7Seismic Test ing of PowlVac Swi t chgear


September 29, 1990

We often see specifications that call for switchgear "to be suitable for use in seismic zone X", where Xmay be any number from 0 to 4, depending on the location of the final installation of the switchgear.Unfortunately there is no ANSI standard that defines "suitable for use in seismic zone X". Seismicrequirements for nuclear generating station equipment, which do exist in standards, are not stated interms of seismic zones, but are site specific.

ANSI Standard A58.1-1982, Minimum Design Loads for Buildings and Other Structures , gives someguidance for the seismic loading that various items must withstand, using the basic formula:

where is the lateral force to be designed for,

is the seismic zone coefficient, which varies from 0.125 for Zone 0 to 1 for Zone 4,

is the occupancy factor, which varies from 1 for Category I to 1.5 for Category III,

is the horizontal force factor, which is 0.3 for all machinery in a building,

and is the weight of the equipment.

From basic mechanics, Force = Mass x Acceleration. In the above formula, F p is a force. W p is a weight,which is the product of a mass and the acceleration of gravity, or g. It follows that the product of Z, I andCp is a dimensionless coefficient for g. For a worst case situation, where the switchgear is installed in acritical occupancy in Zone 4, the value of this coefficient is 1 x 1.5 x 0.3, or 0.45. Since seismic testing isperformed in terms of acceleration rather than force applied, the test level for a worst case installation

should be 0.45 g.

The other aspect of suitability is the performance of the equipment under the specified conditions. Here,we have absolutely no guidance from ANSI standards. Based on past experience and input from varioususers, Powell has decided that the following are reasonable criteria for suitability:




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1) There shall be no structural damage that prevents normal operation of the equipment after the event.

2) No doors or covers shall open during the event.

3) The circuit breakers shall not open or close during the event except on command.

4) The circuit breakers shall not move from the fully connected position during the event.

5) After the event, it shall be possible to open and close the circuit breakers and rack them into and out

of the connected position.6) Primary and control fuses shall remain in their fuse clips.

7) Transformer rollout drawers shall not come open during the event.

8) After the event, primary circuits shall withstand a 27 kV power frequency withstand test (hipot). Thevalue of 27 kV is chosen because it is the power frequency withstand voltage specified for field testing of15 kV metal-clad switchgear.

About four years ago, Powell had samples of PowlVac metal-clad switchgear tested for the ability towithstand Zone 4 seismic forces. These samples were single-unit equipments, to give the narrowest

structure possible, and had the heaviest circuit breakers installed in the highest positions in which theyare ever used. They were therefore worst-case seismic samples.

Based on the requirements of ANSI A58.1-1982, we chose to use 0.45 g as the zero period acceleration(ZPA) value for these tests. The seismic experts at Southwest Research Institute in San Antonio took thisvalue and developed a required response spectrum (RRS) that peaked at about 1.8 g at 3.5 Hz forvertical acceleration and about 1.9 g at 2.5 Hz for horizontal acceleration, with a minimum value of 0.45 g(the ZPA) at frequencies above 32-33 Hz. Full seismic tests were done by Southwest Research Instituteat these values of acceleration.

The eight criteria listed on the previous page were used to judge the performance of the equipmentunder seismic test. In addition, the circuit breakers were successfully closed and tripped on commandduring the seismic test. Except for a minor problem with the transformer rollout drawer, the equipmentperformed as required. The rollout drawer fastening system was reinforced, and the equipmentperformed successfully on retest.


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page 3

Based on these tests, standard PowlVac metal-clad switchgear is suitable for use in seismic zones 0, 1and 2. With the addition of holding clips at the transformer rollout drawers, PowlVac is suitable for use inzones 3 and 4.





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Powell Technical Brief #8Prevent ing Volt age Feedback in Synchronizing Ci rcui t s


October 22, 1990

Many synchronizing schemes use two lamps in series, connected from the incoming voltage source tothe running voltage source. This "dark lamp" synchronizing indication can be used by an operator tosupplement the meter and synchroscope readings to insure synchronism before closing the incomingcircuit breaker.

This scheme, however, can allow energizing of a supposedly dead bus if the synchronizing switch isaccidentally left in the "ON" position. The two lamps will be in series with the secondary of the busvoltage transformer, and this circuit will be connected across the energized incoming voltage transformersecondary. The portion of this voltage which appears across the bus voltage transformer will be steppedup by the ratio of the bus voltage transformer, and this higher voltage will be applied to the switchgearbus.

To prevent this voltage feedback, a dead bus relay ( 27B ) should be connected in the circuit as shown inthe figure below. For simple synchronizing schemes, where one or more generators are manuallysynchronized to a common bus, this circuit with its one 27B relay is satisfactory. For more complexschemes, involving automatic synchronizing, machine-to-machine synchronizing, or synchronizing to autility source, a more complex circuit may be necessary to insure that no voltage feedback circuits exist.All synchronizing circuits should be reviewed carefully to prevent voltage feedback through thesynchronizing lamps.





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Powell Technical Brief #9Fuses f or Use in DC Cont r ol Ci r cui t s


January 9, 1991

The majority of control circuits in metal-enclosed switchgear, particularly in metal-clad switchgear, aresupplied from a dc power source. For nearly half a century Powell and other switchgear manufacturershave used 250-volt cartridge fuses (so-called "Code fuses") to protect these control circuits. Typical fusetypes are Bussmann Type NON and Shawmut Type OT. The application of these fuses to this type ofcircuit has been generally successful and has been generally accepted by our customers.

From time to time, however, someone raises the question of the dc rating of these fuses. Bussmannadvises me that the Type NON has been tested successfully for 10 kA interrupting capability at 250 V dc,which is the rating commonly ascribed to these fuses. Based on this test data, we can safely apply thesefuses to dc control circuits where the short circuit level of the control circuit is 10 kA or less. The typicalcontrol battery used for switchgear can deliver a short circuit current of about 10 times its one-minutedischarge rating, so it would be a very unusual dc control circuit that had a short circuit capability inexcess of 10 kA.

Another question sometimes raised is whether or not these fuses are UL listed for dc applications. Theanswer is no. If a fuse with a UL listing for dc use is required, we should use either Fusetron Type FRN-Ror Low-Peak Type LPN-RK. These fuses are dual-element time delay types which may be used in thesame fuse blocks used for Type NON fuses.





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Powell Technical Brief #10Tr ansient Recovery Vol t age (TVR) Values f or Test ing

PowlVac

Circui t Breaker s




January 10, 1991

The interrupting performance of any circuit interrupter is affected by the transient recovery voltageappearing across the first pole to interrupt. Both the absolute value of this voltage and its rate of rise areimportant in determining the interrupter's ability to meet its interrupting rating. The required values oftransient recovery voltage are included in ANSI/IEEE C37.06-1987, along with the other ratings of circuitbreakers.

The conventional way of specifying the rate of rise of the transient recovery voltage is to specify the peakvalue (E2) and the time required to reach that peak (T2). The rate of rise is then determined by dividingE2 by T2. The nominal values are those for a full rated short circuit interruption. For lower currents, bothhigher peaks and faster times are specified. Table 6 of ANSI/IEEE C37.06-1987 lists the multiplyingfactors to be applied to E2 and T2 for interrupting currents below the full rating of a circuit breaker.

Table 1 of ANSI/IEEE C37.06-1987, which gives the preferred ratings of indoor oilless circuit breakers,such as PowlVac breakers, calls for E2 to be 1.88 times the breaker's rated maximum voltage for testsat 100% of the circuit breaker's interrupting rating. Unfortunately, values of T2 are not standardized,leaving the manufacturer with no guidance on this subject. In order to assign some reasonable value toT2, Powell decided to use the rate-of-rise values given in Table IIA of IEC Standard 56, interpolatingbetween the listed values to match the ANSI voltage ratings, and multiplying the rate-of-rise values by E2to obtain T2. The values obtained by this method were used in the testing of PowlVac circuit breakers,and are given in the table below.


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Powell Technical Brief #10Tr ansient Recovery Vol t age (TVR) Values f or Test ing

PowlVac

Circui t Breaker s


page 2

PowlVac Transient Recovery Voltage Test Values

Transient Recovery Voltage

Rated Maximum Voltage = 15kV

Rated Maximum Voltage = 4.76kV

Current % of InterrupterRating

7 to 13 33.00 29 1137 10.47 19.8 529

20 to 30 31.86 29 1098 10.11 19.8 510

40 to 60 30.17 49 615 9.58 33.1 289

100 28.20 73.6 383 8.95 49.4 181





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Powell Technical Brief #11Consequences of Vacuum Int err upt er Fai lure




March 1, 1991

Users and prospective users of vacuum circuit breakers frequently ask us what happens if a vacuuminterrupter fails to interrupt. The short answer to this question is that the interrupter is usually destroyedand must be replaced. However, this short answer needs some additional comment to be reallyinformative.

First, failure of a properly applied vacuum interrupter to interrupt a fault current within its rating is a veryrare event. In the 8 years that we have been building PowlVac vacuum circuit breakers, we havemanufactured over 3200 breakers. Assuming an average of two years in service for these breakers, wehave a history of nearly 20,000 interrupter-years of service. We have never heard of a failure to interruptby any of these circuit breakers. We are proud of this history, but, based on industry statistics, we are notsurprised by it.

Second, even if an interrupter does fail, the consequences are not the disastrous burn down that somepeople imagine. During some recent design tests of a prototype of a new version of the PowlVac breaker, we drove an interrupter far past its rated contact life span and had a failure. Photo 1 shows thefailed interrupter. When failure occurred, the internal shield was burned through and the ceramicenvelope, exposed directly to the arc, broke apart. The arc continued for several cycles, until the circuitwas opened by a backup circuit breaker. Aside from the failed interrupter, the only damage to the circuitbreaker was a small area of smoke and burn discoloration on the nearby insulating material. Photo 2shows this area, which was about 6 inches square. Five minutes with an industrial cleaner and a coupleof paper towels removed all but about one square inch of this discoloration. The remaining area seemedto be singed, but there was no detectable erosion of the surface of the insulating material. Had thisbreaker been in service, it could have been returned to service immediately after replacing theinterrupter.


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Powell Technical Brief #11Consequences of Vacuum Int err upt er Fai lure


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Summing up, interrupter failures are rare, and when they do happen, most are not a major disaster.

Photo 1Failed Vacuum Interrupter

Photo 2Discolored Insulation at

Failure Location





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Powell Technical Brief #12Cont inuous Curr ent Car rying Capabi l i t y of Low Volt age

Circuit Breakers




March 4, 1991

Various types of low voltage circuit breakers have differing continuous duty capabilities. Some are ratedto carry 100 percent of their trip rating continuously, while others are rated to carry only 80 percent oftheir trip rating continuously. It is important that we understand the difference and apply these breakersproperly.

The general run of molded case circuit breakers in frame sizes of 400 A and below are rated to carry only80 percent of their rated trip current on a continuous basis. Particularly when these breakers aremounted close to each other in a panelboard, the extra heat generated by carrying 100 percent of the triprating will both lead to false tripping and cause long-term degradation of the insulating material of whichthese breakers are made.

On the other hand, all low voltage power circuit breakers and the general run of insulated case circuitbreakers are capable of carrying 100 percent of their trip rating on a continuous basis.

Some confusion can arise when using large molded case circuit breakers, in frame sizes of 600 A andabove. These breakers may be rated either 80 percent or 100 percent, depending on the model and themanufacturer. As you would expect, the 100% breaker costs considerably more than the 80% breaker.Some models have both 80% and 100% ratings available. The 100% rated breaker may require a largerenclosure and/or more ventilation than the 80% rated breaker of the same model.

Please observe the following application rules:

1) Apply MCCB's in 400 A frame size and smaller based on continuous loads of not more than 80% ofthe circuit breaker's trip rating. If trip ratings are selected by our customer, assume that they are basedon the 80% load requirement.

2) Apply insulated case breakers and low voltage power circuit breakers based on continuous loads ofnot more than 100% of the breaker's trip rating. If trip ratings are selected by our customer, assume thatthey are based on the 100% load requirement., Be sure that the insulated case breakers selected are100% rated.


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Powell Technical Brief #12Cont inuous Curr ent Car rying Capabi l i t y of Low Volt age

Circuit Breakers


page 2

3) Apply large molded case circuit breakers based on either the 80% or the 100% rating, making surethat the breaker selected fits the application, and that adequate space and ventilation is provided for thebreaker chosen. If trip ratings are selected by our customer, be sure that you understand which basiswas used for selection.





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Powell Technical Brief #13Fut ure Use of Space in Powel l Equipment




March 27, 1991

Powell's switchgear and motor control equipments frequently include space which is not used by activeswitching devices, but is available for future use. This space varies in the amount of equipment present,and is called by many different names. Some of the terms used include space, future, future space,equipped space, space only, spare, and blank. Unfortunately, there are no industry standards definingthese terms and their use varies widely throughout the industry, so there is often confusion betweenspecifier and manufacturer or between engineering and shop personnel about what is desired on aparticular job.

In order to minimize the confusion, we have adopted the following terms and descriptions in Powell forinternal use:

Spare - A complete, ready-to-operate unit, including the drawout switching device (circuit breaker ormotor starter) and all required secondary devices, fully wired. A spare differs from an active unit only inthat the spare has no assigned function in the power system.

Fully Equipped Space - A spare without the drawout switching device. Includes all required secondarydevices and wiring, a finished unit door, primary buswork and disconnecting devices, and all cell partsrequired for inserting the drawout switching device.

Equipped Space - Includes a door with cutouts for primary switching devices but not for secondary andcontrol devices, primary disconnecting devices and riser bus connecting them to the main bus, and allcell parts required for inserting the drawout switching device. No primary or secondary devices areincluded, and wiring is minimal.

Blank Space - A blank door, no primary or secondary devices, buswork, wiring, or cell parts required forinserting the drawout switching device. Steelwork should be done so that the blank space can beequipped in the field with little or no cutting or welding.

Blank - An area that can never be used for a primary switching device. This area is made unusable bythermal limitations of the equipment, inability to bus to the area or to maintain proper isolation of bus oroutgoing leads, or some similar problem.

Related to these definitions but somewhat different is Mounting and Wiring for a future device or adevice to be field installed by the user. Mounting and wiring may be furnished in any of the above units orin an active unit. Mounting and wiring includes the necessary space, physical supports, and primary andsecondary connections to allow easy installation of the future device. This may include temporaryprimary and/or secondary connections or jumpers to allow use of the circuit pending the addition of thefuture device.


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Powell Technical Brief #13Fut ure Use of Space in Powel l Equipment


page 2

Where any of these conditions leave openings in the front door or in isolation barriers required bystandards, the opening must be covered by a temporary cover plate.





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Powell Technical Brief #14Aut ot ransf ormer St ar t ing of Mot ors


April 1, 1991

One of our customers recently experienced failures of two autotransformers used in medium voltagemotor starters. The circuit used was the familiar 3-contactor, 2-coil Korndorfer circuit, which has beenused for many years and appears in textbooks and handbooks on motor control. The primary circuit isshown below:

An investigation of the failed autotransformers by their manufacturer showed that the failure had been asurface flashover from the line end of the winding either to another tap of the winding or to a groundpoint. There was no damage to the winding or the core, and the autotransformers could be easilyrepaired and put back into service.

We consulted with both the autotransformer manufacturer and the manufacturer of the contactors used inthe starter, and found that there had been previous experiences of this problem. The flashovers occurredbecause system transients generated during the starting sequence caused an excessive voltage toappear on the line end of the autotransformer winding. Upon analysis, we found several conditions thatcontributed to this problem:

The starter was located at the end of a rather weak supply line.

During the starting sequence, the user switched in a rather large capacitor bank to minimize theline voltage drop. This bank was switched off automatically, during the starting sequence, whenthe voltage recovered to a fixed point.

The autotransformer was set on the 80% tap.

We are uncertain of the setting of the timer used to transfer from the starting connection to therunning connection.




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page 2

Although the contactors used in this particular installation were vacuum contactors, the manufacturerinforms us that similar problems have been encountered with both air and vacuum contactors. The typeof contactor used doesn't seem to be a factor in the occurrence of the problem.

Further discussions with our suppliers led to several suggestions to minimize the occurrence of thisproblem:

Insulate the transformer connection points, both the taps that are used and the unused taps. Thisshould be done on all future starters of this type.

Use a lower voltage tap on the autotransformer, such as 65% or 50%, if the motor will acceleratesuccessfully on these taps.

For induction motors, be sure that the timer that transfers to the running connection is set at along enough time so that the motor is fully accelerated before changing to the running connection.

Add an instantaneous current relay to the circuit, set to pick up at about 5 A and drop out justbelow that current. This relay will pick up when the motor is started and drop out when it reachesfull speed. Connect the coil of this relay in any phase CT. Use the contact of this relay to bypassthe timing relay contact, insuring that the motor has fully accelerated before the starter istransferred to the running connection. See the control circuit below. In the future, please include

this relay in all starters of this type.




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page 3

In extreme cases, it may be necessary to connect intermediate class surge arresters to the linetaps of the two autotransformer coils.





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Powell Technical Brief #15Direct ional Overcur rent and Direct ional Power Relays


May 24, 1991

From time to time we experience some confusion about the difference between directional overcurrentrelays, ANSI device 67 , and directional power relays, ANSI device 32 . Although there are somesimilarities between these two types of relays, they are really very different in both construction andapplication.

Directional overcurrent relays ( 67 ) respond to excessive current flow in a particular direction in the powersystem. The relay typically consists of two elements. One is a directional element, which determines thedirection of current flow with respect to a voltage reference. When this current flow is in thepredetermined trip direction, this directional element enables ("turns on") the other element, which is astandard overcurrent relay, complete with taps and time dial, as found on a normal non-directionalovercurrent relay. Because these relays are designed to operate on fault currents, the directional unit ismade so that it operates best on a highly lagging current, which is typical of faults in power systems.Directional overcurrent relays are normally used on incoming line circuit breakers on buses which havetwo or more sources. They are connected to trip an incoming line breaker for fault current flow back intothe source, so that a fault on one source is not fed by the other sources. In complex distribution or sub-transmission networks, these relays may be used to improve coordination of the system.

Directional power relays ( 32 ) measure real power , so they operate best at a high powerfactor. Various degrees of sensitivity and speed of operation are available in various models ofdirectional power relays. There are three typical uses of these relays:

Connected to measure power flow into a generator, the relay will operate to trip the generatorbreaker if the generator begins to draw power from the system and act as a motor. This is usuallydue to loss of prime mover power.

Connected to measure power flow into a transformer from the secondary side, a very sensitivedirectional power relay can measure core loss power input to the transformer, detecting loss ofthe primary source to the transformer. The transformer can then be disconnected from thesystem.

A directional power relay can be used to limit power flow in a circuit. The relay may trip a breakeror initiate control action to change the system configuration. By using quadrature potentialconnections or a phase shifting transformer, these relays can be made to measure vars

. A typical use would be to limit the real or reactive power drawn from a utility sourceto a contractual level.




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Powell Technical Brief #15Direct ional Overcur rent and Direct ional Power Relays


page 2

Neither the functions ( 67 and 32 ) nor the actual relays are interchangeable. Be sure to use the functionand the hardware which fit the application.





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Powell Technical Brief #16Prevent ing Condensat ion in Medium Volt age Mot ors


June 12, 1991

Condensation or other accumulation of moisture can be very damaging to the windings and mechanicalparts of a motor, especially a medium voltage motor. This is not usually a problem for a motor that isrunning, as the windings generate enough heat to prevent condensation. When the motor is stopped,however, supplementary heat is often required to keep the motor dry.

One way of providing the required heat is to install heaters in the motor. Another way is to energize themotor windings from a low voltage source. The one-line diagram below shows the connections for thismethod of heating the windings. This method may be preferable to the use of heaters, as it actually heatsthe windings instead of relying on the transmission of heat from a separate heater.

When using this method of heating, several precautions must be observed:




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Powell Technical Brief #16Prevent ing Condensat ion in Medium Volt age Mot ors


page 2

The heating contactor must be a full line voltage contactor, as the motor winding side of thiscontactor is energized at line voltage when the motor is running.

The running contactor and the heating contactor must be mechanically and electrically interlockedso that only one of them can be closed at any time.

There needs to be a time delay between the opening of the running contactor and the closing ofthe heating contactor, to allow the residual voltage on the motor to decay before the motorwindings are connected to the low voltage source. Since it is not critical to apply the heatingcircuit immediately, it is recommended that this time delay be in the order of 2 to 5 minutes.

Tests show that there is an open circuit time of approximately 75-80 milliseconds when therunning contactor is picked up by a "b" contact of the heating contactor. The user should considerwhether this is an adequate time period to prevent unwanted system problems. If not, a timedelay of a few seconds can be inserted in the pickup circuit of the running contactor to be surethat the heating contactor has cleared before the motor is energized by the operating voltage.

The voltage applied to the motor windings must be carefully selected to produce the properheating. This value must be selected by the user, based on input from the motor manufacturer.

Baldwin Bridger, P.E.

Technical Director




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Powell Technical Brief #17Ground Lead Disconnect or s on Dist r ibut ion-Class Surge

Arresters


July 18, 1991

Many current models of zinc oxide distribution or riser pole arresters come equipped with ground leaddisconnectors. This is a device which is mounted on the ground end of the arrester and which looksabout like a small hockey puck. The enclosure is black, blue or green plastic, a couple of inches indiameter and an inch or so tall.

The normal failure mode of these arresters is a short circuit to ground, causing ground fault current toflow. This current will cause the arrester body to fail if it is not stopped quickly. The first function of theground lead disconnector is to disconnect the ground lead of the surge arrester in case of an internalfailure of the arrester, preventing explosive failure of the arrester body. The ground lead disconnectorcontains a cartridge in series with a gap. The gap is shunted by a resistor. As the current rises, thevoltage across the gap increases until the gap flashes over, creating an arc which ignites the cartridge,blowing the ground lead free.

The ground lead disconnector is not a fault current interrupter. The arc drawn by the ground lead as itseparates from the body of the arrester may or may not go out on its own. If it does not go out, a circuitbreaker, recloser or fuse must operate to extinguish the arc. The ground lead disconnector is expected tocreate a gap which will not reignite when power is reapplied to the circuit, but the gap which will becreated is a function of the length and flexibility of the ground lead.

The second function of the ground lead is to give a visible indication of arrester failure for arrestersmounted on overhead distribution lines. If a lineman sees an arrester with its ground lead hanging inmidair, he knows that he has a failure which must be replaced.

These explosive ground lead disconnectors are not suitable for use in metal-enclosed equipment. We do not want the explosion and subsequent uncontrolled arc inside equipment, where the clearancesare not nearly as great as on overhead lines, and where secondary damage from the arc is much morelikely to occur. The visible indication function of the disconnector is useless if the device is mountedwithin an enclosed equipment.

All surge arresters used in Powell's equipments should be of the type without ground lead disconnectors.If a user requests that we include a surge arrester with a ground lead disconnector, we should offer anequivalent model without the disconnector.





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Powell Technical Brief #18Operat ing Times of PowlVac Circuit Breakers


July 19, 1991

We are frequently asked about the actual operating times of PowlVac circuit breakers. The followingvalues may be used in application studies for these circuit breakers.

Closing Time

For all current production models of PowlVac circuit breakers, the time from energizing the closing coilwith rated control voltage until the primary contacts touch is 80 milliseconds or less. Typical values are inthe 44 to 45 millisecond range.

Opening Time

Opening times vary with the model of PowlVac breaker, as shown in the following table. All times arefrom energizing of the trip coil with rated control voltage until the primary contacts part.

Breaker Model "Dash 2" "Dash 3"

Vacuum Interrupter Mitsubishi General Electric

Opening Time, milliseconds

Design Limits

Typical Test Values

25-35

26 or 27

40-50

48 or 49

"S" (asymmetry) Factor 1.2 1.1

All of these breakers are rated 5 cycles interrupting time in accordance with the preferred ratings found inTable 1 of ANSI C37.06-1987, even though they may be faster. The "Dash 2" breaker, in particular, isvery nearly a 3 cycle breaker.





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Powell Technical Brief #19Use of Powl Vac Cir cuit Br eaker s f or Cont inuous Curr ent s

Above 3000 Amperes


August 26, 1991

In accordance with ANSI/IEEE Standard C37.06, the highest continuous current rating of our standardline of PowlVac circuit breakers is 3000 A. For systems that require continuous current ratings above3000 A, we can offer two possible solutions.

First, we can offer our standard 3000 A circuit breaker with cooling fans. We have a design that has beensuccessfully tested at 3750 A, and the results of that test indicate that the fan-cooled breaker may beapplied at 4000 A without overheating. This design requires a unit somewhat wider than the standard 36-inch switchgear unit to include the necessary air ducts. The standard fan control equipment includes acurrent-actuated control to start the fans at about 2500 A and an alarm circuit which uses air flowswitches to detect and alarm loss of cooling air at currents above this level. A completely redundantsecond set of fans can be furnished if desired. Fan cooling is our preferred method of obtaining highercontinuous current ratings.

A second method of providing for high continuous currents is to parallel two circuit breakers. Using thisapproach, we can provide for continuous currents of about 3500 A by paralleling two 2000 A breakersand about 5000 A by paralleling two 3000 A breakers. When breakers are paralleled, the interruptingrating is neither increased nor decreased. Precise timing in closing or opening the two paralleledbreakers is not critical, as whichever breaker closes first can carry the continuous current for the fewmilliseconds until the second breaker closes, and the last breaker to open has the capability ofinterrupting the full fault current. Paralleling of breakers does require special circuitry to balance thecurrents between the two breakers and individual overcurrent protection for each breaker as well ascombined overcurrent protection for the entire circuit. Main bus construction must also be very carefullybalanced to insure equal impedance in both legs of the circuit. Parallel breakers should only be used fora user who refuses to use fan cooled circuit beakers.

Regardless of which breaker uprating method is used, special attention must be given to the design ofany portions of the switchgear bus which are rated over 3000 A. If the main bus exceeds 3000 A,standard PowlVac bus cannot be used, and the required special bus design limits the switchgear toone-high construction.





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Powell Technical Brief #20Appl icat ion of Dummy Circui t Breakers in Met al-Clad

Swi t chgear


August 27, 1991

Dummy circuit breakers are used in metal-clad switchgear to provide a method of disconnecting andisolating a circuit or circuits without using a circuit breaker. A common use of a dummy circuit breaker isas a temporary connection in a switchgear cell where a circuit breaker will be installed as part of aplanned future expansion. Another use might be to isolate one end of a tie bus or cable from aswitchgear bus.

Because a dummy circuit breaker is really a set of three jumper bars mounted on a breaker carriage, ithas absolutely no current interrupting rating. If an attempt is made to withdraw the dummy circuit breakerwith current flowing, arcing will occur at the primary disconnect fingers. This may result in operator injury,equipment damage, or both. Therefore, dummy circuit breakers normally are interlocked with otherswitching devices so that the dummy cannot be withdrawn until the other devices are opened, insuringthat no current is flowing in the dummy.

A particular application that can be troublesome is isolating a tie cable that has been opened by a circuitbreaker at the other end. If the cable is still attached to an energized bus through the dummy breaker,cable charging current will flow through the dummy. It only takes a few hundred feet of 15 kV cable todraw a charging current of as much as half an amp. This highly capacitive current is difficult to interrupt.It is recommended that the interlocking for any circuit involving power cable and a dummy circuit breakerbe arranged so that the cable is completely deenergized before the dummy circuit breaker is removed toisolate the cable.

Deenergizing the unloaded bus of a lineup of metal-clad switchgear by withdrawing a dummy circuitbreaker is an acceptable application. The limited length and very low capacitance of a switchgear busstructure keeps the charging current low enough to be successfully interrupted by withdrawing a dummycircuit breaker.





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Powell Technical Brief #21Swit ching Capabil i t y of Rol lout or Ti l t out Carr i ages


December 3, 1991

We are often asked about the switching capability of the rollout or tiltout carriages used in mediumvoltage switchgear to mount voltage transformers, small control power transformers, and fuses for largercontrol power transformers. This question usually takes the form "How large a CPT can you handle withfuses mounted in a rollout or tiltout?"

There is no industry standard to measure this switching capability, and no test data is available to certifythis performance. The switching capability will vary with the details of the design, and to some extent willdepend on the operator, since the speed of opening a rollout or tiltout depends on the individual openingthe device.

Within these restraints, however, our experience with 5 kV and 15 kV equipments over the years has ledus to adopt the following limits:

Voltage transformers: A set of three wye connected VT's or two open delta connected VT's canbe switched with a rollout or tiltout without any interlocking of the secondary circuit.

Control power transformers: A CPT up to 50 kVA single phase or 75 kVA three phase can beswitched with a rollout or tiltout provided the carriage is interlocked so that the CPT must beunloaded before opening the primary device. The CPT may be mounted on the rollout or tiltout, orthe rollout or tiltout may contain only the fuses for a stationary mounted CPT. Larger CPT's mustbe switched with some other mechanism, such as a load break disconnect switch.

Capacitors: Rollouts or tiltouts must not be used to switch capacitors.

Any other application should be reviewed by Powell's engineering department.





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Powell Technical Brief #22Short Ci r cui t Current s Crest , rms Symmet r ical and rms

Asymmet r ical


December 4, 1991

The figure below shows a typical short circuit current wage form and defines the various componentparts of this wave. At the moment of initiation of a short circuit the ac current wave, which is normallysymmetrical about the zero axis BX is offset by some value, creating a waveform which is symmetricalabout another axis, CC'. The degree of asymmetry is a function of several variables, including theparameters of the power system up to the point of the short circuit and the point on the ac wave at whichthe short circuit was initiated. In a 3-phase circuit, there is usually one phase which is offset significantlymore than the other two phases.

It is convenient to analyze this asymmetrical waveform as consisting of a symmetrical ac wavesuperimposed on a dc current. CC' represents the dc current, and the value of that current at any instantis represented by the ordinate of CC'. The dc component of the current normally decays rapidly, andreaches an insignificant value within 0.1 s in most power systems. The rate of decay is a function of thesystem parameters. When the initial value of the dc current is equal to the initial peak value of the accurrent, the resulting waveform is said to be fully offset, or to have a 100% dc component. It is possible,

in some power systems, to have an offset in excess of 100%, which may result in a waveform that hasno current zeros for one or more cycles of the ac power frequency.

The ac component of the short circuit current will also decay, at a rate dependant on the systemparameters. In general, the closer the fault is to generators or other large rotating machinery, the fasterthe decay will be.




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Powell Technical Brief #22Short Ci r cui t Current s Crest , rms Symmet r ical and rms

Asymmet r ical


page 2

In the figure, I MC is the crest, or peak, value of the short circuit current. It is the maximum instantaneouscurrent in the major loop of the first cycle of short circuit current.

The rms symmetrical value of the short circuit current at any instant, such as EE', is the rms value of the

ac portion of the current wave. Its value is equal to , and it is shown graphically by the distancefrom CC' to DD'. The rms asymmetrical value of the short circuit current is the rms value of the combinedac and dc waves, and it is calculated by the formula:





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Powell Technical Brief #23Using Design Test s t o Quali f y Several Rat ings of Equipment


December 5, 1991

The many variations in construction and ratings encountered in the typical switchgear or motor controlproduct line make the planning of design and conformance test programs quite complex at times. Ofcourse, it is possible to run every test on every possible rating of equipment but such an extensiveprogram is very expensive and is seldom required to fully document the performance of a product line.

The ANSI standards for switchgear recognize this complexity and provide for the qualification of a pieceof equipment for all lower ratings provided test results show it to be qualified for the highest rating forwhich it is used. Some of the conformance test standards in the ANSI C37.50 series discuss theprinciples of testing to qualify multiple ratings. These standards also give guidance in the grouping ofequipment ratings for testing.

A typical example of qualifying multiple ratings by a single test is the bus structure used in PowlVac metal-clad switchgear. This bus structure is the same for all voltage and short circuit ratings, varying onlyfor continuous current ratings. To demonstrate the momentary and short-time current ratings of this busstructure, tests are performed on the bus with the lowest continuous current rating, 1200 A, which usesthe smallest, weakest bars of any continuous current rating of PowlVac bus. The tests are performed atthe maximum momentary current, 132 kA crest, and the maximum short-time current, 49 kA rms,required for any rating of PowlVac switchgear. It is fairly obvious that passing these tests qualifies the1200 A bus for this rating and for all lower momentary and short-time current ratings. What may not bequite so obvious is that successful tests on the 1200 A bus also qualify higher continuous current ratings,such as 2000 A and 3000 A. These higher bus ratings are covered because they use larger bus bars,which are mechanically stronger and which have greater thermal capacity than the bus bars used in the1200 A bus.

The grouping of ratings and the selection of which rating to test requires a thorough knowledge not onlyof the standards but also of the particular product line being tested. The grouping of ratings may differ fordifferent tests. It also may differ for different products, or different manufacturers offerings in the sameproduct line. The example given in the previous paragraph is true for PowlVac switchgear, but may notnecessarily be true for other manufacturers' similar products.

Although Powell and many other manufacturers have used these principles in performing their designtests for many years, not everyone in the industry understands the concept. To aid in this understanding,all future Powell test reports will document the additional ratings covered by any test.





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Powell Technical Brief #24Sizing Bus Bar s in Swi t chgear and Mot or Cont rol




February 7, 1992

We occasionally get questions about how we select the size of bus bar for various continuous currentratings in Powell equipments. The answer is that we use temperature rise as the basic criterion. All of theANSI, IEEE and NEMA standards for switchgear and motor control have requirements for the maximumoperating temperature of various parts of the equipment. For bus bars, the requirement is generally for atemperature rise of no more than 65C, although this may vary for different classes of equipment. Theserequirements are designed to prevent overheating the insulation supporting and enclosing the bus bars,since excessive temperature shortens the life of the insulation.

A number of factors affect the temperature rise of bus bars. Some of the major ones are:

Size and material (copper or aluminum) of the bus bar.

Whether the bar is insulated. Surprisingly, a bus bar covered with insulation generally runs coolerthan an equivalent bare bus bar, because the usually darker color of the insulating material is abetter radiator of heat than the shiny surface of a bare bus bar.

Size and material (magnetic or non-magnetic) of the enclosure around the bus.

Flow of ventilating air past the bus bars or the bus enclosure.

Proximity of other conductors and other heat-producing devices.

The complex interaction of these and other factors makes it nearly impossible to calculate temperaturerise, and leads to the requirement in all applicable standards for continuous current tests to determinethe temperature rise of a bus design.

Specifications will sometimes call for bus sized by current density, a favorite requirement being 1000 Aper square inch for copper bus. This may be a good way to choose bus sizes for the mythical "singleconductor in free air", but it isn't a satisfactory way to design buswork in practical equipments. Considerthe following chart, based on bus sizes used in our PowlVac metal-clad switchgear:

Switchgear Bus Rating 1200 A 2000 A 3000 A

Number of bus bars per phase 1 1 2

Size of bus bar, inches 1/4 x 4 1/2 x 6 1/2 x 6

Cross section area of bus, square inches 1 3 6

Current density, amps per square inch 1200 667 500

Maximum temperature rise, from test data 60C 59.7C 59.5C


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Powell Technical Brief #24Sizing Bus Bar s in Swi t chgear and Mot or Cont rol


page 2

The last line of the chart shows that the temperature rises of the three bus ratings are almost identical inspite of the 2.4:1 ratio of the current densities.





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Powell Technical Brief #25Appl icat ion of Met al-Enclosed Swi t chgear at High Alt i t ude




February 11, 1992

Both low- and medium-voltage metal-enclosed switchgear and the circuit breakers used in theseequipments depend on air for both cooling and insulation. At high altitudes, the less dense air is lessefficient both as in insulator and as a heat transfer medium. Because of this, the ANSI standards requirederating when these equipments are used at high altitudes. The following tables show the altitudecorrection factors taken from the ANSI standards.

Low Voltage Switchgear and Breakers

Altitude (ft)* Voltage Current

6600 (2000 m) (and below) 1.00 1.00 8500 (2600 m) 0.95 0.99

13,000 (3900 m) 0.80 0.96

Medium Voltage Switchgear and Breakers

Altitude (ft)* Voltage Current

3300 (1000 m) (and below) 1.00 1.00

5000 (1500 m) 0.95 0.99

10,000 (3000 m) 0.80 0.96

* Intermediate values may be obtained by interpolation.

You will notice that there are different altitudes given for low voltage and medium voltage. I have neverbeen able to get a reasonable answer as to why this is true, and I understand that the committeeresponsible for the standards is reviewing these values with the idea of reconciling them.

In all cases, the current correction factor is applied to the continuous current rating of the switchgear andthe circuit breakers. This does not usually present a problem, as we seldom design a system with loadcurrents over 95% of the equipment rating. The current derating does not apply to interrupting current orany of the other high-current ratings of the breakers.


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Powell Technical Brief #25Appl icat ion of Met al-Enclosed Swi t chgear at High Alt i t ude


page 2

For low voltage equipments, the voltage correction factor applies to the low frequency withstand (hipot)rating of both the breaker and the equipment. It also applies to the rated maximum voltage of the circuitbreaker. When derating the rated maximum voltage, the short circuit rating of the circuit breaker cannotexceed the rating at the voltage before derating. For instance, if a breaker is used on a 480 V system, asmost of those in Powell equipment are, with a 0.95 rating factor the short circuit rating at 480 V may beused, since the rated maximum voltage for that system nominal voltage is 508 V, and 0.95 x 508 is 482.6V, slightly above the 480 V service voltage. However, if this same system required a 0.80 rating factor,the breaker short circuit rating at 600 V must be used, since 0.80 x 508 is only 406 V, less than theservice voltage, but 0.80 x 635 is 508 V, comfortably above the service voltage.

For medium voltage equipments, the voltage correction factor applies to the low frequency withstand(hipot) rating and the impulse withstand (BIL) rating of both the breaker and the equipment. It alsoapplies to the rated maximum voltage of the circuit breaker unless a sealed interrupter, such as avacuum interrupter, is used. The use of surge arresters to protect the equipment should be consideredfor all such high altitude installations.





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Powell Technical Brief #26Volt age Rat ings of Surge Ar r est ers


April 13, 1992

Surge arresters (formerly known as lightning arresters) are applied to electrical power distributionsystems to protect the equipment and the circuits from damaging overvoltages caused by lightning orother surges. It is important that surge arresters of the correct voltage rating be used. The proper voltagerating depends on the system line-to-line voltage, the method of system grounding, and the type of surgearrester used.

Older designs of surge arresters generally consist of silicon carbide resistor blocks in series with airgaps. These arresters carry no current in the normal state. Each arrester model has a single voltagerating. For solidly (effectively) grounded systems, the next higher arrester rating above the system line-to-neutral voltage is used. For resistance grounded or ungrounded systems, a ground fault on one phasecan raise the other two phases to line-to-line voltage above ground, so the next higher rating above thesystem line-to-line voltage is used. Except for a few special conditions, application seems quite simple.

About a decade ago, the metal oxide surge arrester was introduced to the industry. It consists of anumber of blocks of a variable resistance material, usually zinc oxide, with no gaps. It does carry someslight current at all times. It has many advantages as a surge protector, but it is somewhat morecomplicated to apply correctly. Instead of one voltage rating, it has three: a nominal voltage, a maximumcontinuous operating voltage, and a one-second temporary overvoltage capability. Although there is aslight variation with the nominal rating, the maximum continuous operating voltage is about 85% of thenominal rating and the one-second temporary overvoltage capability is about 120% of the nominal rating.For times other than one second, the temporary overvoltage capability is established by curves suppliedby the surge arrester vendor. Care must be taken to avoid overstressing the arrester.

As an example, let's consider a 13.8 kV system. For a solidly grounded system, the continuous operatingvoltage is 13,800 divided by the square root of 3, or 7970 V. This is above the MCOV of 7,650 V for anarrester rated 9 kV. Depending on the value and expected duration of system overvoltages, it may benecessary to use a 10 kV arrester with an MCOV of 8.4 kV or a 12 kV arrester with an MCOV of 10.2 kV.For an ungrounded 13.8 kV system, the 12.7 kV MCOV of a 15 kV arrester is not adequate. It isnecessary to use an 18 kV arrester with an MCOV of 15.3 kV. Finally, for a resistance-grounded 13.8 kVsystem, the choice will be between arresters rated 12 kV, 15 kV and 18 kV, depending on the timeneeded to relay ground faults off the system.





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Powell Technical Brief #27Test ing of Swi t chgear and Mot or Cont rol Equipment




April 14, 1992

Although each particular product line is governed by its own industry standards, switchgear and motorcontrol equipment of the types built by Powell are generally subject to three major categories of tests. Asdefined in ANSI/IEEE C37.20.2-1987 for Metal-Clad and Station-Type Cubicle Switchgear, thesecategories are:

Design Tests: Tests made by the manufacturer to determine the adequacy of the design of a particulartype, style or model of equipment or its component parts to meet its assigned ratings and to operatesatisfactorily under normal service conditions or under special service conditions if specified, and may beused to demonstrate compliance with the applicable standards of the industry.

Production Tests: Tests made for quality control by the manufacturer on every device or onrepresentative samples, or on parts, or materials required to verify during production that the productmeets the design specifications and applicable standards.

Conformance Tests: Conformance tests demonstrate compliance with the applicable standards. Thetest specimen is normally subjected to all planned production tests prior to the initiation of theconformance test program.

Typical design tests for equipment and circuit breakers will include continuous current (heat runs),momentary and short time current, low-frequency withstand (hipot), impulse withstand (BIL) for medium-voltage equipment, and mechanical tests to demonstrate the effectiveness of interlocks. In addition,circuit breakers are subjected to a series of interrupting tests to demonstrate their ability to interruptcurrents of various magnitudes, operational life tests, and several types of timing tests. Many of thesetests are somewhat destructive, and therefore they are run on manufacturer's prototypes, not onproduction equipment which is supplied to customers.

Conformance tests generally include certain of the design tests, chosen to demonstrate compliance withthe standards. These tests are frequently used for third-party certification of a design.

Production tests include hipot to demonstrate insulation integrity and mechanical and control circuit teststo demonstrate proper operation. In addition, circuit breakers receive timing tests to show proper closingand opening speed. Records of these tests, which Powell furnishes to customers on request, can beused as baseline data for future maintenance programs.


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Powell Technical Brief #27Test ing of Swi t chgear and Mot or Cont rol Equipment


page 2

Each type of test, and each test within a given type, has a particular part to play in the overall process ofproducing quality equipment properly rated for a user's needs. No single test demonstrates the properdesign and operation of switchgear or motor control equipment. It takes a combination of tests to do the

job properly.





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Powell Technical Brief #28Shor t Ci r cui t Current Levels Used t o Test Various Types of

Ci rcui t Breakers


August 25, 1992

When applying interrupters of various types, it is important that we understand the meaning of theinterrupting rating given to these devices. Consider, for instance, the methods of making interruptingtests on various types of circuit breakers. As the breakers get smaller and less costly, the test methods inthe industry standards generally get less demanding.

When testing the interrupting capability of a high-voltage (over 1000V) circuit breaker, the currentmeasured is the actual fault current flowing through the circuit breaker at the moment of the interruption.To rate a breaker of this class as a 25kA interrupter, it must actually interrupt 25kA. Momentary andshort-time current requirements of the switchgear are also based on actual current flowing during thetest. The reference standards are ANSI/IEEE C37.04, C37.06 and C37.09 for the circuit breakers andC37.20.2 for the switchgear.

For low voltage circuit breakers, this requirement changes to rating by prospective current. The testterminals of the laboratory source are short-circuited, as indicated by point A in Figure 1, and therequired current flow is established. That short circuit is then removed and the equipment to be tested isconnected to the test source. A short circuit is then applied to the equipment and the test made. Thelocation of the short varies with the type of circuit breaker or equipment being tested:

Figure 1: Fault Locations for Testing Low Voltage Equipments

(A) Low Voltage Power Circuit Breakers(B) Molded Case Circuit Breakers(C) Low Voltage Motor Control Centers




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Powell Technical Brief #28Shor t Ci r cui t Current Levels Used t o Test Various Types of

Ci rcui t Breakers


page 2

For a low voltage power circuit breaker, the fault is placed at the load terminals of the breaker, atpoint B in Figure 1. The reference standards are ANSI/IEEE C37.13 for the circuit breakers andC37.20.1 for the switchgear equipment.

For a molded case circuit breaker, the fault is also placed at the load terminals of the circuitbreaker, at point C in Figure 1. However, 4 feet of appropriately-sized conductor may be includedbetween the test station terminals and the line terminals of the circuit breaker under test. Thereference standard is UL 489.

For a typical combination motor starter unit in a motor control center, the fault is placed at the endof 4 feet of appropriately-sized conductor connected to the load terminals of the starter unit, atpoint D in Figure 1. The reference standard is UL 845.

At each step of this chain, impedance is added to the test circuit, reducing the actual fault current thecircuit breaker is required to interrupt. Several papers presented at recent IEEE conferences have raisedquestions about the adequacy of equipment certified to some of these test standards to interrupt allpossible faults downstream of the circuit breaker. At least two IEEE subcommittees are discussing thismatter.





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Powell Technical Brief #29Int erchangeabil i t y of Drawout Ci r cui t Breaker s in

Swi t chgear Assemblies




August 28, 1992

One of the key features of switchgear assemblies using drawout circuit breakers is the interchangeabilityof circuit breakers within an assembly. This feature allows users to make use of spare circuit breakers toreplace circuit breakers which must be taken out of service for maintenance, minimizing down time whena circuit breaker problem occurs.

The ANSI standard for Metal Enclosed Low Voltage Power Circuit Breaker Switchgear, ANSI/IEEEC37.20.1, addresses interchangeability in 6.11.4. This section requires that "All removable elements ofthe same type and rating on a given assembly shall be physically interchangeable in the correspondingstationary housings. This need not include electrical interchangeability of electrical control circuits."Switchgear of this type and the circuit breakers used in it typically have mechanical interferencemechanisms for breakers of the same physical size but of different ratings. These mechanisms typicallyprevent interchanging breakers if either the frame size (maximum continuous current rating) or theinterrupting rating differ. Trip device characteristics and ratings and electrical accessories available onthis class of circuit breaker are so numerous and changeable that no attempt is made to preventinterchangeability of breakers with differences in these features.

The ANSI standard for Metal Clad Switchgear, ANSI/IEEE C37.20.2, addresses interchangeability in6.2.5. This section requires that "All removable elements of the same type and rating on a givenassembly shall be physically and electrically interchangeable. Removable elements not of the same typeand rating shall not be interchangeable." Since the breakers used in this class of switchgear are notprovided with variable trip devices or very many optional electrical features, this is seldom a problem.

Occasionally, however, a user desires to have some electrical accessory on some but not all breakers ofa given rating in a particular assembly. Most commonly, this is an undervoltage device, which may berequired on motor feeder breakers but not on other breakers of the same rating. When this occurs, theeasiest way to solve the problem is to furnish the required modification on all breakers of that rating inthe assembly. If this is not acceptable to the user, it may be necessary to make specific modifications tothe control circuitry of the breaker with the accessory to prevent breaker interchangeability.


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Powell Technical Brief #29Int erchangeabil i t y of Drawout Ci r cui t Breaker s in

Swi t chgear Assemblies

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