Post on 30-Oct-2014
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
Category: Arc Flash Subcategory: Mitigation
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
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
Category: Arc Flash Subcategory: Mitigation
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
15.95 kA Arcing Current @ 0.018 seconds
15.95 kA Arcing Current @ 0.018 seconds
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
Category: Arc Flash Subcategory: Mitigation
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
50/51-MSB1-SUB3A - Phase
52-SUB3A-MN
52-SUB3A-MCC1A
O/L BLWR#2
T3A
50/51-MSB1-SUB3A - Phase
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
Category: Arc Flash Subcategory: Mitigation
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 - Phase
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
50/51-MSB1-SUB3A - Phase
52-SUB3A-MN
52-SUB3A-MCC1A
O/L BLWR#2
50/51-MSB1-SUB3A - DT 3000
T3A
50/51-MSB1-SUB3A - Phase
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
Category: Arc Flash Subcategory: Mitigation
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
Category: Arc Flash Subcategory: Mitigation
50/51-MSB1-SUB2A - Phase
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
50/51-MSB1-SUB2A - Phase
T2A
52-SUB2A-MN - Phase
52-SUB2A-UPS#1 - Phase
50/51-MSB1-SUB2A - 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
Category: Arc Flash Subcategory: Mitigation
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
Category: Arc Flash Subcategory: Mitigation
50/51-MSB1-SUB2A - Phase
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
50/51-MSB1-SUB2A - Phase
52-SUB2A-UPS#1 - ARMS
T2A
52-SUB2A-MN - Phase
52-SUB2A-UPS#1 - Phase
50/51-MSB1-SUB2A - Phase
52-SUB2A-UPS#1 - ARMS
T2A
16.020kA Arcing Current @ 0.05 seconds
16.020kA Arcing Current @ 0.05 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-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
Category: Arc Flash Subcategory: Mitigation
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.
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
50/51-MSB1-SUB3A - Phase
52-SUB3A-MN
52-SUB3A-MCC1A
O/L BLWR#2
T3A
50/51-MSB1-SUB3A - Phase
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
Category: Arc Flash Subcategory: Mitigation
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 - Phase
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
50/51-MSB1-SUB3A - Phase
52-SUB3A-MN
52-SUB3A-MCC1A
O/L BLWR#2
50/51-MSB1-SUB3A - DT 3000
T3A
50/51-MSB1-SUB3A - Phase
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
Category: Arc Flash Subcategory: Mitigation
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
Category: Arc Flash Subcategory: Mitigation
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
17.11 kA Arcing Current @ 0.36 seconds
17.11 kA Arcing Current @ 0.36 seconds
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
Category: Arc Flash Subcategory: Mitigation
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.