A COMPARISON OF ARC-FLASH INCIDENT ENERGY REDUCTION TECHNIQUES USING LOW-VOLTAGE POWER CIRCUIT BREAKERS

William A. Brown, P.E. Member, IEEE Square D/Schneider Electric 1010 Airpark Center Dr. Nashville, TN 37217 USA bill.brown@us.schneider-electric.com

Ron Shapiro Member, IEEE EYP Mission Critical Facilities 81 Main Street White Plains, NY 10601 USA rshapiro@eypmcf.com

1-4244-0336-7/06/$20.00 ©2006 IEEE. 1

Abstract - Increasing industry awareness of arc-flash hazards has led to the development of a number of techniques for the reduction of arc-flash incident energy levels with low-voltage power circuit breakers. This paper describes the system parameters that control arc-flash incident energy levels and how these parameters can be controlled in different ways to achieve reductions in these energy levels when low-voltage power circuit breakers are used. Practical applications of these techniques to power systems utilizing low-voltage power circuit breakers are presented and analyzed. The effects of implementing these techniques vs. system reliability are also discussed.

Index Terms - Arc Burn, Arc Burn prevention, Arc-flash, Circuit Breakers, Industrial Power System Protection, Industrial Power System Reliability, Occupational Health and Safety, Power System Faults, Power System Maintenance, Power System Protection.

I. INTRODUCTION

The reduction of arc-flash incident energy is a topic that continues to generate interest and concern wherever electrical power is utilized and electrical equipment must be maintained. As industry awareness of this subject increases, so too do the number and complexity of methods used to reduce arc-flash incident energies. Because of their inherently fast operation at high fault currents, current-limiting fuses have been the focus of numerous articles and papers on the subject. However, power system design, operation, and maintenance can be improved by the identification of arc-flash incident energy reduction techniques utilizing low-voltage power circuit breakers, and this has become an area of focus. In order to understand these techniques, it is necessary to review the fundamentals of arc-flash incident energy and the parameters which can be controlled in order to reduce it.

II. BACKGROUND

An arc-flash hazard is defined as “a dangerous condition associated with the release of energy caused by an electric arc” [3]. This release of energy is in the form of light and heat, and occurs when the insulation between two conductors (often only air) can no longer withstand the voltage between them, resulting in an insulation breakdown.

Because there is a voltage across an electrical arc, the arcing fault current at a given location in an electrical system will be less than the available bolted fault current at the same point. Maximum energy will be transferred to the arc when the arc voltage is equal to one-half of the system voltage (maximum power transfer), however in practice energy levels for a 600V system reach a maximum of only 79% of this value since the arc voltages at this system voltage do not reach the levels required to produce maximum arc power [4].

The incident energy associated with an arc is defined as “the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event” [3]. By definition, this could be due to both the heat radiated by the arc and the associated pressure-wave effects due to the expansion of air and vaporized metal.

A.) Calculation of Incident Energy using IEEE-1584

IEEE-1584 defines an empirical method to calculate the incident energy from the arc due to heat, which is responsible for the most common effect of an arc-flash event: Burns [3]. This method does not take into account energy associated with pressure-wave effects, such as flying debris. It is applicable only over a given range of system voltages, frequencies, and fault currents.

From the IEEE-1584 empirical method, for a system under 1000V which has an available bolted fault current in the range 700A – 106kA and meets the other applicability criteria the arcing current can be calculated as follows [3]:

bfIlogG00304.0bfIlogV5588.0G000526.0V0966.0bfIlog662.0K10Ia−++++= (1)

where

Ia arcing current (kA);

K = -0.153 for open configurations or -0.097 for box configurations;

Ibf available three-phase bolted fault current (kA RMS sym.)

V system voltage (kV);

G gap between conductors (mm).

2

Given Ia calculated from (1), the incident energy, normalized for an arc duration of 0.2 seconds and a distance of 610mm (24”) is [3]:

G0011.0Ilog081.1KKn

a2110E +++= (2)

where

En normalized incident energy;

K1 = -0.792 for open configurations or -0.555 for box configurations;

K2 = 0 for ungrounded and high-resistance grounded systems or -0.113 for grounded systems;

Ia arcing current from (1);

G gap between conductors (mm).

For a different arc duration and/or distance from the arc, the normalized incident energy can be converted into the actual incident energy as follows [3]:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛=x

x

nfD

6102.0

tECE (3)

where

E incident energy (cal/cm2);

Cf = 1.0 for voltages above 1kV or 1.5 for voltages at or below 1kV;

En normalized incident energy from (2);

t arc duration (s);

D distance from the possible arc point to the person (mm);

x distance exponent given in [3].

It should be noted that per IEEE-1584 a second arcing current should be calculated as 85% of the value calculated in (1). The analysis with equations (2) and (3) is then repeated, and the larger of the two results is taken as the arc-flash incident energy.

B.) Effects of System Parameter Variation on Incident Energy

Using (1) – (3) it is possible to determine which parameters can be controlled to limit the incident energy for a low-voltage power circuit breaker. The constants K and K1 are dependent upon the physical enclosure of the circuit breaker; because the circuit breaker is, on a practical basis, always mounted in low-voltage switchgear or a switchboard, the values of these two constants are fixed at values of -0.097 and -0.555 for K and K1 respectively. Constant K2 is dependent upon the system grounding; it should be noted that while a solidly-

grounded system will yield a smaller value for K2, and thus smaller values for En and E, changing the system grounding is not always practical. Therefore, the value for K2 will be considered to be fixed at a value of -0.113 (solidly-grounded system). The system voltage V, when considering low-voltage power circuit breakers, is most often 0.48kV and is not usually easily changed, and it is therefore considered to be fixed at this value. The bus gap G and the associated distance exponent x are dependent upon the equipment design, are not necessarily independent of each other, and are not easily changed (although with specifier demand this fact could change); therefore they are considered to be fixed at values of 32 and 1.473 for G and x, respectively, which are the typical values given in [3] for low-voltage switchgear. Finally, the working distance D is constrained by practical working distances; for low-voltage equipment 18” – 24” (457mm - 610mm) is a practical range of working distances, therefore D will be considered to be fixed at a value of 457mm (18”; this value is used throughout the remainder of this paper since it is the value typically used for working distance for low-voltage switchgear). All of the aforementioned parameters may be considered to uncontrollable at this time.

The remaining two parameters are the bolted fault current Ibf and the arc duration t. Equations (1) - (3) may be simplified as functions of only these two parameters by using the aforementioned values for the parameters considered uncontrollable. Equation (1) therefore reduces to:

bfIlog833.0034.0a 10I +−= (4)

Equation (2) reduces to:

aIlog081.1633.0n 10E +−= (5)

Equation (3) reduces to:

⎟⎠

⎞⎜⎝

⎛=

2.0tE295.2E n (6)

Equations (4) - (6) may now be combined into one equation which expresses the incident energy E as a function of Ibf and t, as follows:

( ) ⎟⎠

⎞⎜⎝

⎛= +−

2.0t10295.2E bfIlog901.0670.0 (7)

Note that (7) is valid only for the assumptions above, which were made for a typical power circuit breaker, i.e., a system voltage of 480V, solid system grounding, a circuit breaker mounted in low-voltage switchgear, and a working distance of 18” (457mm).

The sensitivity of E to changes in Ibf and t may now be explored by taking the partial derivatives ∂E/∂Ibf and ∂E/∂t. The results are as follows:

3

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛=

∂∂ +−

bf

Ilog901.0670.0

bf I10t328.10

IE bf

(8)

( )bfIlog901.0670.010476.11tE +−=∂∂ (9)

The incident energy E from (7) vs. Ibf is shown in Fig. 1 for a

60ms arc duration (typical worst-case instantaneous tripping time for a power circuit breaker) and for a 500ms arc duration (typical worst-case short-time tripping time for a power circuit breaker):

0102030405060708090

1.09

8

7.58

1

15.3

53

23.8

75

32.9

46

42.4

60

52.3

47

62.5

57

73.0

55

83.8

11

94.8

03

Available Bolted Fault Current Ibf (kA)

Inci

dent

Ene

rgy

E (c

al/c

m2 )

t=0.06st=0.50s

Fig. 1 Typical low-voltage power circuit breaker incident

energy E vs. Ibf for a 60ms arc duration and a 500ms arc duration, per (7)

Similarly, a plot of ∂E/∂Ibf vs. Ibf for these same two arc durations are shown in Fig. 2:

0

0.2

0.4

0.6

0.8

1

1.2

1.09

8

7.58

1

15.3

53

23.8

75

32.9

46

42.4

60

52.3

47

62.5

57

73.0

55

83.8

11

94.8

03

Available Bolted Fault Current Ibf (kA)

∂E/∂

I bf

[cal

/(cm

2 kA

)]

t=0.06st=0.50s

Fig. 2 Typical low-voltage power circuit breaker ∂E/∂Ibf vs. Ibf for a 60ms arc duration and a 500ms arc duration, per (8)

The average values of ∂E/∂Ibf over the range of Ibf in Fig. 2 are 0.094 cal/(cm2 kA) and 0.786 cal/(cm2 kA) with t = 0.06s and t = 0.50s respectively. A 10kA reduction in the available fault current would therefore reduce the incident energy, on average, by 0.94 cal/cm2 and 7.86 cal/cm2 for t = 0.06s and t=0.50s respectively. While this does illustrate the point that

the effects of reduced bolted fault current are more pronounced if the circuit breaker trips on its short-time delay rather than per its instantaneous characteristic, it must be noted that reductions in the available fault current are often difficult to achieve and carry with them their own sets of issues, which will not be elaborated upon here. Further, it must be noted that it is the arcing current Ia, and not the bolted fault current, which must be compared to the circuit breaker time-current characteristic to find the circuit breaker tripping time, and thus the arc duration t. Reducing the available fault current can lower the arcing current to a level such that the circuit breaker no longer operates in one of the time-current characteristic regions assumed above (short-time or instantaneous), which will tend to increase the circuit breaker tripping time and therefore increase the incident energy per (7). For these reasons, purposefully reducing fault current magnitude is not a considered viable way to reduce the arc-flash incident energy with low-voltage power circuit breakers.

The final parameter to be investigated is the arc duration t. The arc-flash incident energy vs. arc duration is shown in Fig. 3 for three levels of bolted fault current. Per (9), the time rate of change ∂E/∂t for a given value of Ibf is constant, and is equal to 0.045 cal/(cm2 ms), 0.083 cal/(cm2 ms), and 0.120 cal/(cm2 ms) for bolted fault current values of 25kA, 50kA, and 75kA respectively. Therefore, for a bolted fault current of 50kA, for example, every ms arc duration reduction reduces the incident energy by .083 cal/cm2. Because the arc duration is a controllable parameter, it is therefore an important controllable factor in arc-flash incident energy reduction.

0

1

2

3

4

5

6

7

8

16 21 26 31 36 41 46 51 56 61

Arc Duration t (ms)

Inci

dent

Ene

rgy

E (c

al/c

m2 ) Ibf=75kA

Ibf=50kAIbf=25kA

Fig. 3 Typical low-voltage power circuit breaker incident

energy E vs. arcing duration with Ibf = 25kA, Ibf = 50kA, and Ibf = 75kA, per (7)

Of note are the IEEE-1584 simplified circuit breaker equations [3]. These are a function of Ibf only and assume that the circuit breaker is operating in its instantaneous trip region (or in the short-time region if the circuit breaker has no instantaneous function), thus giving a limit upon the lowest value of Ibf for which they are applicable. These are intended for use if no other information is known about a particular circuit breaker. If accurate information is known about a circuit breaker’s trip characteristics the IEEE-1584 general equations [(1), (2), and (3) above] should be used instead of the simplified equations, unless specific manufacturer-

4

supplied simplified equations are available for the circuit breaker in question [4].

The conclusion, then, is that while both the available bolted fault current and arc duration may be reduced in order to reduce the arc-flash incident energy when low-voltage power circuit breakers are utilized, only the arc duration is controllable on a practical basis. All of the methods for incident energy reduction for power circuit breakers described herein, therefore, are predicated upon reducing the arc duration.

C.) Amount of Energy Reduction Required

In order to quantify the level of arc-flash incident energy reduction required to make a practical difference in maintenance practices, the required PPE characteristics [2] must be considered. These are shown in Table I. Also of note is NFPA 70E article 130.7(C)(5), which states that if there is an exposure level above 1.2 cal/cm2 that flame resistant (FR) clothing must be worn [2].

For the parameter assumptions given in Section II. above, the arc-flash incident energy reduction ranges required to move from one hazard/risk category to another, and the resulting arc duration reductions required for a typical low-voltage power circuit breaker with Ibf = 25kA, 50kA, and 75kA, are given in Table II. Note that in this table 0.1 cal/cm2 is the minimum energy reduction considered and 1ms is the smallest incremental time reduction considered. As can be seen, even a small arc duration reduction can bring the hazard/risk category to the next lower level.

TABLE I PPE CHARACTERISTICS FROM [2]

Hazard/Risk Category

Clothing Description Required Minimum

Arc Rating of PPE

(cal/cm2)

0 Non-melting, flammable materials (i.e., untreated cotton, wool,

rayon, or silk, or blends of these materials) with a fabric weight of

at least 4.5 oz/yd2

N/A

1 FR shirt and FR pants or FR coverall

4

2 Cotton underwear – conventional short sleeve and brief/shorts, plus

FR shirt and FR pants

8

3 Cotton underwear plus FR shirt and FR pants plus FR coverall, or

cotton underwear plus two FR coveralls

25

4 Cotton underwear plus FR shirt and FR pants plus multilayer flash

suit

40

TABLE II INCIDENT ENERGY LEVEL REDUCTIONS REQUIRED FOR

VARIOUS HAZARD/RISK CATEGORY REDUCTIONS WITH LOW-VOLTAGE POWER CIRCUIT BREAKERS (ASSUMING

PARAMETERS PER SECTION II. ABOVE) Arc Duration Reduction

Required (ms) Starting Hazard/

Risk Category

Final Hazard/

Risk Category

Incident Energy Reduction Required (cal/cm2) Ibf = 25kA Ibf = 50kA Ibf = 75kA

1 0 0.1 - 2.8 3 - 63 2 - 34 1 - 24

2 1 0.1 - 4 3 - 89 2 - 49 1 - 34

2 0 2.9 - 6.8 65 - 152 35 -82 25 - 57

3 2 0.1 - 17 3 - 378 2 - 205 1 - 142

3 1 4.1 - 21 92 - 467 50 - 254 35 - 175

3 0 6.9 - 23.8 154 - 529 84 - 287 58 - 199

4 3 0.1 - 15 3 - 334 2 - 181 1 - 125

4 2 17.1 - 32 380 - 712 207 - 386 143 - 267

4 1 21.1 - 36 469 - 800 255 - 434 176 - 300

4 0 23.9 - 38.8 532 - 863 288 - 468 200 - 324

III. INCIDENT ENERGY REDUCTION TECHNIQUES WITH LOW-VOLTAGE POWER CIRCUIT

BREAKERS

A.) Optimal Time-Current Curve Selection

Simply stated, this method consists of using the fastest practical time-current characteristic while maintaining selective coordination with upstream and downstream protective devices. For example, consider the system of Fig. 4:

UTILITY SOURCE13.8kV

kA15=bfI

PRIMARY FUSE200E

TRANSFORMER3000 kVA13.8kV:480Y/277V5.75%Z

kA78754 .I bf =

MAIN CBELECTRONIC TRIP LSIG4000A/3P

CB F1ELECTRONIC TRIP LSIG2000A/3P

CB F2ELECTRONIC TRIP LSIG1600A/3P

CB F3ELECTRONICTRIP LSIG800A/3P

MAIN SWITCHGEAR

Fig. 4 Example system

5

This system uses low-voltage power circuit breakers in its main switchgear. The system is selectively coordinated, with protective device time-current curves per Fig. 5. Even though the system is coordinated, the arc-flash incident energy at the switchgear main bus (using the parameter assumptions per Section II. above) is 291 cal/cm2 – above hazard/risk category 4 and not suitable for work on or near energized equipment with exposed live parts under any circumstances. [It must be noted that the main circuit breaker time current characteristics are used for the calculation of the arc-flash incident energy at the switchgear main bus; this is done with the understanding that the fault is considered to be in a feeder section circuit breaker compartment (for example, for arc-flash exposure during racking of a feeder circuit breaker between positions). This is valid if low-voltage metal-enclosed switchgear per the ANSI C37.20.1 standard is used and the switchgear section under consideration is not the same vertical section which houses the main circuit breaker. This is assumed to be the case for all of the switchgear examples given in this paper. If this were not the case (for example, if arc-flash exposure due to racking of the main circuit breaker between positions is the activity under consideration) the upstream device (in this case, the primary fuse) timing characteristics would have to be used; this would have the effect of increasing the arc-flash incident energy level.]

The main circuit breaker short time pickup and delay, however, can be lowered without sacrificing coordination, as shown in Fig. 6. The lowered short time settings for the main circuit breaker lower the arc-flash incident energy level to 28.9 cal/cm2 – hazard/risk category 4. The arcing fault current Ia is calculated to be 22.07Ka; the principle is to insure that the circuit breaker operates in the short-time region at the arcing fault level. From inspection it can be seen that this is not the case with the time-current coordination of Fig. 5, but is the case with the time-current coordination of Fig. 6. It should be noted that the 22.07kA value for Ia is 85% of the value calculated per (4); this is in accordance with IEEE-1584 since this smaller value for Ia results in a larger arc duration, and thus a larger value for E.

The key to successfully implementing this incident energy reduction method is the ability to lower the short-time or instantaneous trip settings in order to lower the arc duration at the calculated arcing current without sacrificing coordination with downstream devices. In this case the instantaneous function for the main circuit breaker has been disabled. While the use of the instantaneous function for the main circuit breaker would lower the arc-flash incident energy at the switchgear main bus even further, it is not possible to use the instantaneous function in this case while maintaining coordination with the feeder circuit breakers.

100

100

1K1K

10K

10K

100K

100K

1M1M

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN SECONDS

PRIMARY FUSE

TRANSFORMER

MAIN CB

CB F1

PRIMARY FUSE

TRANSFORMER

MAIN CB

CB F1

Current Scale X 10^0 Reference Voltage: 480

Fig. 5 Time-current coordination for the system of Fig. 4

100

100

1K1K

10K

10K

100K

100K

1M1M

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN SECONDS

PRIMARY FUSE

TRANSFORMER

MAIN CB

CB F1

PRIMARY FUSE

TRANSFORMER

MAIN CB

CB F1

Current Scale X 10^0 Reference Voltage: 480

Fig. 6 Revised time-current coordination for system of Fig. 4

using the Optimal Time-Current Curve Selection method

B.) Zone-Selective Interlocking (ZSI)

This method involves logic between circuit breakers to minimize the tripping time if a fault is known to be within a

6

circuit breaker’s protective zone. The system of Fig. 4 can be re-configured for Zone-Selective Interlocking, as shown in Fig. 7:

UTILITY SOURCE13.8kV

kA15=bfI

PRIMARY FUSE200E

TRANSFORMER3000 kVA13.8kV:480Y/277V5.75%Z

kA78754 .I bf =

MAIN CBELECTRONIC TRIP LSIG4000A/3P

CB F1ELECTRONIC TRIP LSIG2000A/3P

CB F2ELECTRONIC TRIP LSIG1600A/3P

CB F3ELECTRONICTRIP LSIG800A/3P

MAIN SWITCHGEAR

ZSI

Fig. 7 Example system utilizing Zone-Selective Interlocking

The use of Zone-Selective Interlocking requires circuit breakers with electronic, ZSI-capable trip units. Functionally, the interlocking is accomplished via wired connections between the trip units. If a feeder circuit breaker detects a fault it sends a restraining signal via this wired connection to the main circuit breaker. The main circuit breaker will then follow its time-current characteristic curve and therefore serve as a backup to the downstream breaker for feeder faults. Thus, the main and feeder circuit breakers must still be coordinated by conventional time-current coordination techniques. However, if the main circuit breaker detects a fault but the feeder circuit breakers do not, the main circuit breaker will trip with no intentional delay. The maximum unrestrained delay varies by manufacturer and circuit breaker type, but a representative value is 80ms. This has a large effect on the arc-flash incident energy.

Taking into account the Zone-Selective Interlocking yields the time-current coordination of Fig. 8 for a fault on the switchgear bus. Note that an additional 20ms worst-case time delay has been added to the instantaneous characteristic of the circuit breaker to take into account the Zone-Selective Interlocking logic. This delay is built in to insure that the tripping scheme does not produce an unrestrained trip on the main circuit breaker for a fault downstream of a feeder circuit breaker. The value of this delay will vary from one manufacturer to another. The arc duration for a fault on the switchgear bus is substantially reduced, yielding an arc-flash incident energy level of 7.22 cal/cm2 – hazard/risk category 2.

Care must be used when applying Zone-Selective Interlocking where a large motor contribution to the arcing fault is involved. The short-time pickup of each feeder circuit breaker for the system of Fig. 7, for example, must be above the level of its motor contribution to a fault on the switchgear main bus. The reason for this is because the Zone-Selective Interlocking logic uses the short-time pickup level to determine whether a feeder circuit breaker detects the fault; a fault on

the main bus must not be detected by the feeder in order for the Zone-Selective Interlocking to function properly. Also, if multiple paralleled power sources can supply a fault at the location in question differential relaying is recommended for that location. Differential relaying is beyond the scope of this paper but is an alternative means of reducing arc-flash incident energy.

100

100

1K1K

10K

10K

100K

100K

1M1M

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN SECONDS

PRIMARY FUSE

TRANSFORMER

MAIN CB

PRIMARY FUSE

TRANSFORMER

MAIN CB

Current Scale X 10^0 Reference Voltage: 480

Fig. 8 Time-current coordination for system of Fig. 7 for a fault

on the switchgear bus, with Zone-Selective Interlocking

C.) Adjustment of Time Current Curve for Maintenance (Alternate Settings)

A logical extension of the previous two methods is the adjustment of the circuit breaker’s time current curve prior to maintenance. This has the potential for a reduction in arc-flash incident energy, but several items must be considered for successful implementation.

A secure and reliable means of lowering the settings and positive feedback to the personnel doing the work that the lower settings are in effect must be provided. As a time current coordination study is essential for all considerations, an engineering study is required to determine the values of the standard and lowered settings. A method of switching must be provided to activate the appropriate settings. This switching means may be a simple key-operated switch, a sophisticated software scheme utilizing communications with the circuit breaker trip unit, or other methods as appropriate for the facility in question. In all cases, however, positive feedback must be given to the operator that a lowered set of settings are in effect. In no case should operating personnel manually lower the circuit breaker settings; the risk (for nuisance tripping due to settings that are lowered too far, nuisance tripping due to failure to restore the settings to the

7

original levels after maintenance is complete, or lack of coordination due to restoring the settings after maintenance is complete but to different levels than the original settings) is too great.

The marking of equipment must be accurate considering the activation process of the alternate group of settings. One current practice is to field-label equipment with the arc-flash incident energy so as to facilitate selection of PPE when the equipment must be maintained without being put into an electrically safe working condition. Two different arc-flash labels should be considered with this method, with signage that directs which label to use based upon the positive feedback from the alternate setting activation.

The key to implementing this method is to enable the instantaneous pickup of the circuit breaker at a pickup level below the arcing fault current level. Care must be used when determining the lower setting values as this may result in a loss of selectivity with downstream devices. Where multiple sources of power can contribute fault current to a given system location (for example, a switchgear main bus), the Alternate Settings Method would require alternate settings to be enabled for each circuit breaker which connects a source of fault current to that location. Differential relaying is the recommended method to use where multiple paralleled sources of power are involved.

It will be noted that the maximum time delay for the Zone-Selective Interlocking method from Fig. 8 is 80ms, which is longer than the maximum instantaneous trip time of 60ms for a typical low-voltage power circuit breaker. The reason that the instantaneous characteristic is 20ms faster is due to the logic operation required for the Zone-Selective Interlocking. The reduced arc duration delay carries with it a reduced arc-flash incident energy level – in the system of Fig. 4/Fig. 7, 5.42 cal/cm2 for the Alternate Settings method vs. 7.22 cal/cm2 for the Zone-Selective Interlocking method. However, both of these values fall within hazard/risk category 2, and thus both methods construe the same practical benefit in this case. This is to be expected from the analysis given in Section II above. Depending upon the working distance and available bolted fault current, results for the Zone-Selective Interlocking and Alternate Settings method may not fall into the same PPE category, however. Analysis for each individual case will determine which of the two methods presents the greatest practical benefit.

D.) Low Arc-flash Circuit Breaker Design

Low arc-flash circuit breaker design is a developing field. It involves the use of new design techniques to make the circuit breaker operate more quickly than traditional designs in the instantaneous region.

Fig. 9 shows a traditional “blow-closed” low-voltage power circuit breaker design [5]. The pivot point of the contact assembly is offset toward the contact, leveraging magnetic forces to keep the contact closed as the current increases. This creates a high withstand and close and latch rating for the circuit breaker.

Fig. 9 Traditional “blow-closed” low-voltage power circuit breaker design, from [5]

Low arc-flash circuit breaker designs use a different approach, as shown in Fig. 10 [5]. This is a blow-open design. The current path is denoted by the shaded areas. The blow-open terminal is designed so that there is a reverse current loop at the moving arm. The magnetic force, proportional to the current, forces the contacts open when the current is high enough. The unlatching mechanism and stored energy from the opening springs are not required to open the contacts at high currents, resulting in very fast opening speeds which lower the arc-flash incident energy at high current levels.

Low arc-flash design circuit breakers are usually manufacturer-tested for arc-flash performance. This results in simplified empirical equations for the arc-flash incident energy and flash-protection boundary. These equations show better performance than using the IEEE-1584 general equations with the time-current curves and are similar in form to the IEEE-1584 simplified circuit breaker equations.

Fig. 10 Low arc-flash circuit breaker design, from [5]

Consider a continuation of the system from Fig. 4, with the 800A feeder circuit breaker CB-F3 feeding a distribution panel. This arrangement is shown in Fig. 11. For the particular circuit breaker used in this example, the manufacturer-published arc-flash incident energy equations are:

)kA42IkA15(I064.0067.0E bfbf ≤≤×+= (10)

8

)kA100IkA42(I017.016.2E bfbf ≤<×+= (11)

where

E incident energy (cal/cm2)

Ibf available bolted fault current (kA RMS sym.)

The resulting time current coordination is as shown in Fig. 12. The arc-flash incident energy at the distribution panel is 5.37 cal/cm2 – hazard/risk category 2 [it should be noted that the parameters G and x from (1) - (3) have changed to 25 and 1.641 respectively due to the change in equipment type]. With circuit breaker CB-F3 replaced by a low arc-flash design circuit breaker the time-current coordination is shown in Fig. 13. The arc-flash incident energy at the distribution panel is 2.98 cal/cm2 – hazard/risk category 1.

It must be noted that the manufacturer-specific arc-flash incident energy equations for the particular circuit breaker in this example apply only for applications at 480V, a working distance of 18” (457mm), and the instantaneous function enabled. For applications at other voltages, or outside the fault-current ranges given in (10) and (11), (1) - (3) would have to be used a to determine the arc-flash incident energy. This would yield higher arc-flash incident energies than the use of the manufacturer-specific equations. Also, the circuit breakers used in this example are only available up to 2000A and as switchgear feeder circuit breakers only. Another fact to be noted is that selectivity between CB-F3 and CB-P1 above approximately 23.7kA cannot be determined from the time-current plot in Fig. 13; above this level selectivity must be determined by test.

It may be concluded that low arc-flash design circuit breakers are quite useful in reducing arc-flash incident energy where the manufacturer-specific incident energy equations can be used, the time-current characteristics of the circuit breaker are acceptable for coordination with downstream devices, and the low arc-flash design circuit breaker is available for the ampacity and application required.

E.) Use of Current-Limiting Fuses In Series with the Circuit Breaker

Because of their fast operation at high current levels, the use of current limiting fuses in series with the circuit breaker seems to be a logical method for reducing arc-flash energy while still retaining the flexibility of a circuit breaker. However, mainly due to the fuse ampacities required, the current limiting characteristics of fuses are such that they are not particularly suited to assist in lowering arc-flash incident energy levels with power circuit breakers as they are often operating outside of their current-limiting range. Indeed, in many cases circuit breakers have lower arc-flash incident energy characteristics than current-limiting fuses, depending upon the available bolted fault current and the current-limiting characteristics of the device [4].

TO MAIN SWITCHGEAR BUS

kA78754Ibf .=

CB P1THERMAL-MAGNETIC400A

CB P2THERMAL-MAGNETIC400A

CB P3THERMAL-MAGNETIC300A

DISTRIBUTION PANEL

CB F3ELECTRONICTRIP LSIG800A/3P

MAIN SWITCHGEAR

(3) 350kcmil THHN PER PHASE AND NEUTRAL + (3) #1/0 GND, 50', RIGID STEEL CONDUIT

kA97547 .I bf =

Fig. 11 Continuation of the example system of Fig. 4, with CB-

F3 feeding a distribution panel

10

10

100

100

1K1K

10K

10K

100K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN SECONDS

CB F3

CB-P1

CB F3

CB-P1

Current Scale X 10^0 Reference Voltage: 480

Fig. 12 Time-current coordination for the system of Fig. 11

utilizing standard-design low-voltage power circuit breaker for CB-F3

9

10

10

100

100

1K1K

10K

10K

100K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN SECONDS

CB F3

CB-P1

CB F3

CB-P1

Current Scale X 10^0 Reference Voltage: 480

Fig. 13 Time-current coordination for the system of Fig. 11

with CB-F3 replaced with a low arc-flash design circuit breaker

IV. CONCLUSION

Of the five methods presented, only four are viable methods for reducing arc-flash incident energy with low-voltage power circuit breakers.

The Optimal Time-Current Curve Selection with Maintained Coordination method is useful when downstream overcurrent protection devices will allow the short-time settings of the circuit breaker to be lowered to a level that lowers the arc duration at the arcing fault current level without sacrificing coordination, and when the resulting arc-flash incident energy reduction is acceptable.

The Zone-Selective Interlocking method has the potential for tremendous decreases in arc-flash incident energy levels without sacrificing system selectivity. It requires the interwiring between ZSI-capable trip units for implementation, i.e., no additional equipment.

Although the Alternate Settings method is a relatively new method for low-voltage power circuit breakers, it does have merit so long as operational considerations are taken. It can provide slightly lower arc-flash incident energy levels than the Zone-Selective Interlocking method. However, during the maintenance period system selectivity may be compromised. This method is most useful when neither the Optimal Time-Current Curve Selection nor the Zone-Selective Interlocking methods can be used, and the compromised system selectivity can be tolerated for the duration of the maintenance period. It is also useful in the case that the Zone-Selective Interlocking method produces an arc-flash incident energy level slightly above the threshold for the next lower hazard/risk category.

If multiple paralleled power sources are involved for a given system location under consideration, differential relaying for that location is recommended .

The use of reduced arc-flash design circuit breakers can inherently limit the arc-flash incident energy within the circuit breaker’s protective zone. The amount of arc-flash reduction can be substantial. The low arc-flash circuit breaker is most effective in lowering arc-flash incident energies when the system parameters are within the range of its manufacturer-published incident energy equation applicability limits and the circuit breaker is available for the application and ampacity required. Care must be used to insure that system selectivity is retained when using this type of circuit breaker, and in some situations selectivity may not be possible.

Good engineering judgment favors a balanced approach, rather than the exclusive use of any of the above methods. By a judicious use of these methods, systems employing low-voltage power circuit breakers for overcurrent protection can have reduced arc-flash incident energy levels, while at the same time retaining the advantages of circuit breakers.

V. REFERENCES

[1] NFPA 70, 2005 National Electrical Code, Quincy, MA: NFPA

[2] NFPA 70E, 2004 Standard for Electrical Safety in the Workplace, Quincy, MA: NFPA

[3] IEEE Standard 1584-2002, IEEE Guide for Performing Arc-flash Hazard Calculations, New York, NY: IEEE

[4] A. C. Parsons, Arc-flash Application Guide -- Arc-flash Energy Calculations for Circuit Breakers and Fuses, Austin, TX: Square D/Schneider Electric Engineering Services, 2004.

[5] Square D/Schneider Electric, 2003, Arc-flash Protection with Masterpact® NW and NT Circuit Breakers, Data Bulletin 0613DB0202R603.

VI. VITAE

William A. Brown is a staff engineer with the Square D/Schneider Electric Power Systems Engineering group. Mr. Brown received a BSEE degree from Tennessee Technological University in 1994. Over the past 10 years he has held various engineering positions with Square D/Schneider Electric in the areas of electric power systems, switchgear design, and power system automation and control. He is a registered engineer in the State of Tennessee.

Ron Shapiro is an electrical engineer

with EYP Mission Critical Facilities, Inc. Mr. Shapiro received a BSEE from Syracuse University in 1991. Mr. Shapiro designs electrical infrastructure and ancillary systems for data centers throughout the USA.