SKM Modeling Arc Flash Mitigation Techniques

Technical Guide

Category: Arc Flash Subcategory: Mitigation

Modeling Arc Flash Mitigation Technique Using the SKM Power*Tools for Windows Software

Lowell L. Oriel

Arc flash is one of the most dangerous workplace hazards. It could cause serious injuries and

fatalities. Furthermore, it could cost companies millions in damage to equipment as well as

worker’s compensation. An arc flash incident occurs when electric current passes between two

conducting metal through ionized air. When this phenomenon happens, a large amount of heat

(incident energy) is released that can severely burn human skin and set clothing on fire. Besides

the intense heat that could reach up to 5000F, electrical arc flash also produces high-pressure blast

with molten metal and shrapnel, as well as deaf dying sounds.

Equations developed by electrical safety standards (IEEE 1584 and NFPA 70E) to calculate the

potential incident energy caused by an arcing fault indicates that the faster the arcing fault is

cleared, the lower the incident energy. Therefore, one of the best and efficient ways to mitigate

arc flash incident energy is to clear the arcing fault as fast as possible.

There are various ways to clear the arcing fault. One way is by simply modifying the existing

settings of protective devices. Another way is to apply new technologies that have been developed

to clear the arcing fault instantly during an arc flash incident. Finally, another way is by taking

advantage of alternative protection schemes such as differential protection and zone interlocking,

since these types of schemes clear the fault instantaneously.

The various arc flash mitigation techniques mentioned above will be illustrated and modeled using

the SKM Power*Tools for Windows software.

Technical Guide


Changing Instantaneous and STPU setting

The easiest way to reduce clearing time of arc fault is by reducing the instantaneous setting or the

short time pickup (STPU) setting of a protective device. The following four figures illustrate this.

Figure 1 shows a partial single line modeled in SKM software along with its corresponding time

current curve (TCC). From the figure, we can see that if the fault happens on the “E SWBD”(Critical

Switchboard), the arcing current of 15.95 kA going through, “CSWB MCB” will clear the fault at

0.215 seconds. This will produce a total incident energy of 11.3 cal/cm2.

CR HVAC FDR

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS

CSWBD FCB-3 - Phase

CSWBD FCB-1 - Phase

CSWBD FCB-2 - Phase

CSWBD MCB - Phase

SIEMENS SHND6, Sensitrip III SHND6 Trip 1200.0 APlug 1200.0 ASettings Phase LTPU (20-100% x P) 80 % (960A) LTD (2.2-27 Sec.) 4 STPU (1.5-10 x LTPU) 2 (1920A) STD (0.05-0.0 Sec.) 0.2 Sec (I^2t In) INST (2-40 x LTPU) 20 (19200A)

CR HVAC

CR HVAC FDR

CSWBD FCB-3 - Phase

CSWBD FCB-1 - Phase

CSWBD FCB-2 - Phase

CSWBD MCB - Phase

SIEMENS SHND6, Sensitrip III SHND6 Trip 1200.0 APlug 1200.0 ASettings Phase LTPU (20-100% x P) 80 % (960A) LTD (2.2-27 Sec.) 4 STPU (1.5-10 x LTPU) 2 (1920A) STD (0.05-0.0 Sec.) 0.2 Sec (I^2t In) INST (2-40 x LTPU) 20 (19200A)

CR HVAC

CR HVAC FDR

15.95 kA Arcing Current @ 0.215 seconds


CRITICAL SWITCHBOARD

COMPUTERROOM HVAC

40ft

E SWBD

11.3 Cal/cm^2

@ 18 inches

PPE Category 3CSWBD FCB-1 CSWBD FCB-2 CSWBD FCB-3

CSWBD MCB

TCC: AF_E SWBD Current Scale x 10 Reference Voltage: 480 October 13, 2011

Figure 1

Technical Guide


Now, if the instantaneous setting of the “CSWB MCB” device is changed from 20 to 10, as in Figure

2, it will then clear the fault at 0.018 seconds. This will then produce a total reduced incident

energy of 1.0 cal/cm2.

CR HVAC FDR

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS

CSWBD MCB - Phase

Name CSWBD MCB ManufacturerSIEMENS Type SHND6, Sensitrip III Frame/Model SHND6 Trip 1200.0 APlug 1200.0 ASettings Phase LTPU (20-100% x P) 80 % (960A) LTD (2.2-27 Sec.) 4 STPU (1.5-10 x LTPU) 2 (1920A) STD (0.05-0.0 Sec.) 0.2 Sec (I^2t In) INST (2-40 x LTPU) 10 (9600A)

CSWBD FCB-3 - Phase

CSWBD FCB-1 - Phase

CSWBD FCB-2 - Phase

CR HVAC

CR HVAC FDR

CSWBD MCB - Phase

Name CSWBD MCB ManufacturerSIEMENS Type SHND6, Sensitrip III Frame/Model SHND6 Trip 1200.0 APlug 1200.0 ASettings Phase LTPU (20-100% x P) 80 % (960A) LTD (2.2-27 Sec.) 4 STPU (1.5-10 x LTPU) 2 (1920A) STD (0.05-0.0 Sec.) 0.2 Sec (I^2t In) INST (2-40 x LTPU) 10 (9600A)

CSWBD FCB-3 - Phase

CSWBD FCB-1 - Phase

CSWBD FCB-2 - Phase

CR HVAC

CR HVAC FDR



CRITICAL SWITCHBOARD

COMPUTERROOM HVAC

40ft

E SWBD

1 Cal/cm^2

@ 18 inches

PPE Category 0

CSWBD FCB-1 CSWBD FCB-2 CSWBD FCB-3

CSWBD MCB

TCC: AF_E SWBD Current Scale x 10 Reference Voltage: 480 October 13, 2011

Figure 2

Technical Guide


An example of how changing the STPU of a protective could also reduce the incident can be seen in

Figure 3 and 4. Figure 3 shows a partial single line modeled in SKM software along with its

corresponding time current curve (TCC). From Figure 3, we can see that if the fault happens on

bus “MCC#1A”, the arcing current of 9.38 kA going through, “52-SUB3A-MCC1A” will clear the fault

at 0.24 seconds. This will produce a total incident energy of about 10.4 cal/cm2.

50/51-MSB1-SUB3A - Phase

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

T3A


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

T3A

9.38 kA Arcing @ 0.24 seconds9.38 kA Arcing @ 0.24 seconds Fault

52-MSB1-SUB3A

CBL-012

89-T3A-PRI

S

PT3A

B-T3A-PRI

52-SUB3A-MN

52-SUB3A-MCC1A

CBL-013

50/51-MSB1-SUB3A

MCP BLWR#2

CBL-0075

BLWR#2

O/L BLWR#2

52-TIE-3A&3B

MCC#1A10.4 Cal/cm^2

@ 18 inches

PPE Category 3

SUB-3A

MSB-1

TCC: MCC#1A_bus Current Scale x 10 Reference Voltage: 480 October 13, 2011

Figure 3

Technical Guide


Now, if he STPU setting of the “52-SUB3A-MCC1A” device is reduced from 0.2 to 0.1, as in Figure 4,

it will then clear the fault at 0.15 seconds. This will then produce an arc-flash incident energy of 6.7

cal/cm2.


50/51-MSB1-SUB3A - DT 3000

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

50/51-MSB1-SUB3A - DT 3000

T3A


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

50/51-MSB1-SUB3A - DT 3000

T3A

9.38 kA Arcing @ 0. 15 seconds9.38 kA Arcing @ 0. 15 seconds

Fault

52-MSB1-SUB3A

CBL-012

89-T3A-PRI

S

PT3A

B-T3A-PRI

52-SUB3A-MN

52-SUB3A-MCC1A

CBL-013

50/51-MSB1-SUB3A

MCP BLWR#2

CBL-0075

BLWR#2

O/L BLWR#2

52-TIE-3A&3B

MCC#1A6.7 Cal/cm^2

@ 18 inches

PPE Category 2

SUB-3A

MSB-1

TCC: MCC#1A_bus Current Scale x 10 Reference Voltage: 480 October 13, 2011

Figure 4

Note that special care must be taken when changing the STPU or the instantaneous settings of

protective devices to mitigate arc flash. One should not just haphazardly lower the settings to

reduce the tripping time of devices. One must be careful that no overlapping of TCC or mis-

coordination is mistakenly achieved on other part of the system when the settings are lowered.

Mis-coordination could cause nuisance tripping or even increase the incident energy on the other

part of the system.

Technical Guide


Maintenance Switch and Multiple Settings Group

Another effective way of lowering the arc-flash incident energy is by temporarily over-riding the

breaker’s or relay’s delay function to trip without intentional delay whenever a fault is detected.

This can be achieved by applying maintenance switch or multiple settings group during

maintenance mode of operation.

An ARMS (Arc Resistance Maintenance Switch) is device that you can retrofit with certain existing

trip unit, such that when the ARMS is switched on, the tripping time of the unit is very fast when a

fault is detected. When a person wants to perform maintenance, the maintenance switch is turned

on. The breaker’s delay functions are automatically over-ridden and the breaker then trips

instantaneously if a fault is detected. When maintenance task is completed, the switch is turned

off and all previous trip unit settings are reactivated.

Multiple settings group works in a similar fashion as the ARMS. Here, you configure two relays in

series, such that when you turn a switch on “maintenance” you have two curves on your TCC. One

curve is for the normal operation and the other curve is instantaneous curve such that when

there’s a fault, the relay sends a signal to trip really fast.

The next three figures illustrate how the ARMS device works and modeled in the SKM software.

Figure 5 shows a partial single line modeled in SKM software along with its corresponding time

current curve (TCC). From the figure, we can see that if the fault happens on the line side “52-

CRITICAL-MN”, the arcing current of 16.02 kA going through, “52-SUB2A-UPS#1” will clear the fault

at 0.216 seconds This will produce a total incident energy of 11.7 cal/cm2.

Technical Guide



TX Inrush

T2A

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS

52-SUB2A-MN - Phase

52-SUB2A-UPS#1 - Phase


T2A

52-SUB2A-MN - Phase



T2A

16.020kA Arcing Current @ 0.216 seconds

16.020kA Arcing Current @ 0.216 seconds Fault

52-MSB1-SUB2A

CBL-006

S

PT2A

52-SUB2A-MN

52-SUB2A-UPS#1

CBL-0019

50/51-MSB1-SUB2A

E N

ATS-#1

CBL-0058

52-CRITICAL-MN11.7 Cal/cm^2

PPE Category 3

52-DPNL-CRT1

MSB-1

SUB-2A

CRITICAL-LD

52-SUB2-TIE

TCC: CRIT LD (N) Current Scale x 10 Reference Voltage: 600 October 13, 2011

Figure 5

Technical Guide


To model the ARMS device for the “52-SUB2A-UPS#1” component, we can create another function

named “ARMS” for this device. We then assign this function the ARMS device from the static trip

category of the protective device library. To activate this function in arc flash, make sure that the

“Use in Arc Flash” check box is checked. See Figure 6.

Figure 6

Once the ARMS device has been properly modeled and activated for arc flash study, the new result

can be seen in Figure 7.

From Figure 7, we can see that if the fault happens on the line side “52-CRITICAL-MN”, the arcing

current of 16.02 kA going through, “52-SUB2A-UPS#1-ARMS” will now clear the fault at 0.05

seconds. This will produce a total incident energy of 2.8 cal/cm2.

Technical Guide



TX Inrush

T2A

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS

52-SUB2A-MN - Phase



52-SUB2A-UPS#1 - ARMS

T2A

52-SUB2A-MN - Phase



52-SUB2A-UPS#1 - ARMS

T2A



Fault

52-MSB1-SUB2A

CBL-006

S

PT2A

52-SUB2A-MN

52-SUB2A-UPS#1

CBL-0019

50/51-MSB1-SUB2A

E N

ATS-#1

CBL-0058

52-CRITICAL-MN2.8 Cal/cm^2

PPE Category 1

52-DPNL-CRT1

MSB-1

SUB-2A

CRITICAL-LD

52-SUB2-TIE

TCC: CRIT LD (N) Current Scale x 10 Reference Voltage: 600 October 13, 2011

Figure 7

Technical Guide


The next two figures illustrate how the multiple settings group (using DT 3000) works and modeled

in the SKM software. Figure 8 shows a partial single line modeled in SKM software along with its

corresponding time current curve (TCC). From the figure, we can see that if the fault happens on

the line side “52-SUB3A-MN”, the arcing current of 1.048 kA going through, “50/51-MSB1-SUB3A”

will clear the fault at more than 2.0 seconds. This will produce a total incident energy of 49.7

cal/cm2.


0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

T3A


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

T3A

1.048 kA Arcing @ more than 2 seconds1.048 kA Arcing @ more than 2 seconds

Fault

52-MSB1-SUB3A

CBL-012

89-T3A-PRI

S

PT3A

B-T3A-PRI

52-SUB3A-MN49.7 Cal/cm^2

PPE Category Dangerous!

52-SUB3A-MCC1A

CBL-013

50/51-MSB1-SUB3A

MCP BLWR#2

CBL-0075

BLWR#2

O/L BLWR#2

52-TIE-3A&3B

MCC#1A

SUB-3A

MSB-1

TCC: MCC#1A Current Scale x 10 Reference Voltage: 480 October 13, 2011

Figure 8

Multiple settings can be modeled in a similar fashion as the ARMS device mentioned above. This

time we assign the new function the “DT 3000” from the relay category of the library. We then

modify the setting of this new function such that the curve is an “L” shape curve. Once the

multiple settings group has been properly modeled and activated for arc flash study, the new result

can be seen in Figure 9.

Technical Guide


From the Figure 9, we can see that if the fault happens on the line side “52-SUB3A-MN”, the arcing

current of 1.048 kA going through, “50/51-MSB1-SUB3A-DT 3000” will clear the fault at more than

0.2 seconds. This will produce a total incident energy of 7.4 cal/cm2.


50/51-MSB1-SUB3A - DT 3000

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

50/51-MSB1-SUB3A - DT 3000

T3A


52-SUB3A-MN

52-SUB3A-MCC1A

O/L BLWR#2

50/51-MSB1-SUB3A - DT 3000

T3A

1.048 kA Arcing @ 0. 2 seconds1.048 kA Arcing @ 0. 2 seconds

Fault

52-MSB1-SUB3A

CBL-012

89-T3A-PRI

S

PT3A

B-T3A-PRI

52-SUB3A-MN

7.4 Cal/cm^2

PPE Category 2

52-SUB3A-MCC1A

CBL-013

50/51-MSB1-SUB3A

MCP BLWR#2

CBL-0075

BLWR#2

O/L BLWR#2

52-TIE-3A&3B

MCC#1A

SUB-3A

MSB-1

TCC: MCC#1A Current Scale x 10 Reference Voltage: 480 October 13, 2011

Figure 9

Other Arc Flash Instantaneous and Protection Schemes

Optical relay and Arc Vault are recent new technologies that had been developed to help with

mitigating arc-flash incident energy. In a nutshell, these two devices clear or extinguish the arc

flash in a matter of milliseconds when it is detected.

Optical relays works with light sensors that detect the sudden increase in light intensity. It also

works with fault detector to prevent false tripping. When the optical sensor detects a sudden

increase in light and the fault detector detects a fault, the optical relay sends a trip signal to the

circuit breaker to trip in as fast as 2 milliseconds.

Technical Guide


Arc Vault is a protection system that can be retrofitted with most low voltage switchgear, motor

control centers, switchboard, etc. When this system detects an arc flash or spike in current, the

containment dome is initiated to create a secondary arc to transfer and extinguish the original arc

in the dome. The main breaker is also called upon trip to de-energize the system. All this happens

is less than 8 milliseconds.

Also, alternative protection schemes (bus differential protection, zone interlocking, etc.) are

gaining popularity, since these types of schemes clear the fault instantaneously.

For bus differential protection, the current going in and out of the bus are measured. If they are

equal no action is taken. However, if the current are not equal, then the bus breakers are called

upon to trip instantaneously.

Zone selective interlock (ZSI) is composed of a main breaker communicating with downstream

breakers. In the event of a downstream fault, a signal is sent to the main breaker to hold, and the

feeder breaker nearest the fault would trip. However, if a fault happens on the bus directly below

the main breaker, the main breaker would be called upon to clear the fault instantaneously.

The protection schemes mentioned above do not really rely on a time over current device to clear

an arc fault. The arc fault is cleared or extinguished instantaneously when certain criteria are met.

To model this in SKM, the user could check the “Available” checkbox in the “Arc Flash

Instantaneous Protection” in the” Equipment & Arc Flash” sub view of the said bus. See Figure 10.

Once this is done, and the arc study has been re-run, the user can then enter his own trip delay

time and breaker opening time. The incident energy is then recalculated based on the time that

the user specified. See Figure 11.

Figure 10

Figure 11

The next two figures show how using the arc flash Instantaneous works and modeled in the SKM

software. Figure 12 shows a partial single line modeled in SKM software along with its

corresponding time current curve (TCC). From this figure, we can see that if the fault happens on

Technical Guide


the “SWBD” bus, the arcing current of 17.11 kA going through, “SWDB MCB” will clear the fault at

around 0.36 seconds. This will produce a total incident energy of 22.5 cal/cm2.

0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS

MCC MCP-1

MCC MCP-2

MCC MCP-3

SWBD FCB-1 - Phase

SWDB MCB - Phase

SWBD ATS CB - Phase

MCC MCP-1

MCC MCP-2

MCC MCP-3

SWBD FCB-1 - Phase

SWDB MCB - Phase

SWBD ATS CB - Phase



SW

BD

22.5

Cal

/cm

^2

@ 1

8 i

nch

es

PP

E C

ateg

ory

3

SWDB MCB

SWBD FCB-1

SWBD FCB-2

SWBD FCB-3

SWBD FCB-4

SWBD FCB-5

SWBD ATS CB

TCC: AF_SWBD Current Scale x 10 Reference Voltage: 480 October 5, 2011

Figure 12

From the TCC, we can see that coordination is quite tight. To reduce the incident energy, changing

the protective device settings may not be a feasible solution, since it may require redoing the

coordination of the system. Let’s assume that retrofitting the switchboard with an Arc Vault was

selected to mitigate arc flash for this system.

Once the “Arc Flash Instantaneous Protection” has been properly modeled by following the steps

above and setting the trip delay time to be 8 milliseconds, the new result can be seen in Figure 11

and 13.

From the Figure 11, we can see that if the arc flash happens on the “SWDB”, the Arc Vault will

extinguish it in 8 milliseconds (specified by the user). Based on this time, new total incident is now

0.55 cal/cm2.

Technical Guide


0.5 1 10 100

1K

10K

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIME IN SECONDS

MCC MCP-1

MCC MCP-2

MCC MCP-3

SWBD FCB-1 - Phase

SWDB MCB - Phase

SWBD ATS CB - Phase

MCC MCP-1

MCC MCP-2

MCC MCP-3

SWBD FCB-1 - Phase

SWDB MCB - Phase

SWBD ATS CB - Phase

SW

BD

0.6

Cal

/cm

^2

@ 1

8 i

nch

es

PP

E C

ateg

ory

0

SWDB MCB

SWBD FCB-1

SWBD FCB-2

SWBD FCB-3

SWBD FCB-4

SWBD FCB-5

SWBD ATS CB

TCC: AF_SWBD_4 Current Scale x 10 Reference Voltage: 480 October 5, 2011

Figure 13

Conclusion

Arc flash is one of the most dangerous workplace hazards. Serious injuries and death could be

cause by this hazard. One of the best and efficient ways to mitigate arc flash is to clear the arcing

fault as fast as possible. There are various ways to achieve this. Modifying the existing settings of

protective devices, applying new technologies such as ARMS and ARC Vault, and taking advantage

of alternative protection schemes such as differential protection and zone interlocking, since these

types of schemes clear the fault instantaneously, are some of the options available.

No one method works all the time. What works in one location may not work effectively in another

location. The various arc flash mitigation techniques mentioned above was illustrated and

modeled using the SKM Power*Tools for Windows software.

Documents

SKM Modeling Arc Flash Mitigation Techniques