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White Paper 0600DB130306/2013
Selective Coordination vs Arc Flash RequirementsRetain for
future use.
2013 Schneider Electric All Rights Reserved
Abstract Present industry standards require higher system
performance and protection against arc flash hazards for
individuals exposed to dangerous levels of incidental energy.
However, in most cases, high system performance achieved through
selective coordination, required in changes to the National
Electric Code (NEC), results in increased arc flash energy. This
conflict between selective coordination and arc flash is explained
in this paper through real world examples. The resolution to this
conflict is provided through both existing and future solutions
which achieve a balance between total selectivity and arc flash
hazard levels. This paper also discusses the two levels of
selective coordination commonly employed: 0.1 seconds and total
selectivity; and the affects each has on calculated arc flash
hazards.
Introduction
Selective Coordination Selective coordination refers to the
selection and setting of overcurrent protective devices (OCPDs) in
an electric power system in such a manner so as to cause the
smallest possible portion of the system to be de-energized due to
an overload condition:
Per the 2011 NEC Article 100 Localization of an overcurrent
condition to restrict outages to the circuit or equipment affected,
accomplished by the choice of overcurrent protective devices and
their ratings or settings.This ensures any overcurrent event is
cleared by the smallest circuit breaker in the system before
allowing a larger line-side circuit breaker to operate on the
fault. This limits the service interruption to only the circuit
experiencing the problem and does not shut down a larger portion of
the facility.Specific selective coordination requirements were
first introduced in NEC 1996, Article 620.62 for elevators,
dumbwaiters, escalators, moving walks, wheelchair lifts and
stairway chair lifts. Subsequent articles were added to the NEC: 1.
Emergency and legally-required standby power systems, NEC 2011
Articles 700.27 and 701.27, respectively. 2. Health-care
facilities, NEC Article 517.26, which says that the essential
electrical system should meet the requirements of Article 700.3.
Critical operations power systems (COPS), NEC Article 708.54.While
the rationale for selective coordination is self-evident clearing
and isolating faults as quickly as possible without disturbing the
unaffected portions of the system the methods for judging OCPD to
OCPD selectivity are not as clear. No industry standards exist
which define device-to-device selectivity over their full operating
ranges; no consensus has been developed among protection engineers
or inspecting authorities regarding device-to-device selectivity
thresholds. Discussions continue over the practicable selectivity
criteria overlaying time-current characteristics of OCPDs to
determine selectivity are complicated by examining the
current-limiting interactions of OCPDs at maximum available fault
currents. As a
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result, essentially two interpretations or definitions of
selective coordination have evolved:A. 0.1 seconds and longer This
means that the time-current curves
(TCCs) of OCPDs in series should not overlap above 0.1s.
Selective coordination at 0.1s and longer includes the vast
majority of fault currents, overloads and arcing faults, but not
the highest levels seen in the instantaneous region.
B. Total selectivity In addition to TCC coordination described
for the 0.1s definition, total selectivity takes into account the
current-limiting behaviors and interactions of OCPDs operating on
the highest available fault currents. There are variations on how
total selectivity is described (e.g. 0.01 seconds), but the intent
is selectivity for the OCPDs entire operating range up to the
maximum fault current.
Arc Flash The consideration of arc flash hazards is a relatively
new concern for power system design. However, it is a concern that
is rapidly gaining momentum due to increasingly strict worker
safety standards. A flash hazard is a dangerous condition
associated with the release of energy caused by an electric arc.
The energy impressed on a surface, a certain distance from the
source, generated during an electrical arc event is termed as
incident energy. Key factors which affect the arc flash incident
energy are:A. available fault current at the equipment B. the time
taken by the upstream protective device to clear the fault C.
distance from the arcing sourceIn most cases achieving selective
coordination comes at the cost of increasing circuit breaker frame
size and/or changing circuit breaker type from a molded case to an
electronic trip type with higher short time/instantaneous settings.
Both solutions could result in an increase in total clearing time
of protective devices during an arcing fault, thereby causing an
increase in arc flash incident energy. An example in the next
section further explains the effect of selective coordination 0.1
second and total on arc flash.
Selective Coordination versus Arc Flash Example
Selective Coordination Through Comparison of Time-Current
Curves
In this section a five bus circuit has been used to explain the
affect of total selectivity on arc flash. Three cases have been
considered as follows: Case 1 Load based coordination where devices
are selected based on
typical thermal-magnetic trips for circuit breakers other than
service mains and prior to implementing NEC Article 100 requirement
of selective coordination.
Case 2 - Selective coordination to 0.1 seconds and longer Case 3
- Total selective coordinationTable 3 on Page 17 compares arc flash
category and incident energy for each case. The results of this
typical example show how selective coordination is achieved at the
cost of increased arc flash incident energy. Figure 1 shows that
the system is fed from two sources:A. normal source fed by a 1000
kVA utility transformer and B. emergency source fed by a 500 kW
generator.
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The protective devices shown are prior to selective coordination
and are based on load requirement only.
Figure 1: Single Line Drawing of Example System
UTILITYSC Contribution 3P 99999 MVAX/R 3P 8.0
UTI XFM1000 kVAPrl 12470 VSec 480 VZ = 5.75%
GEN500 kW625 kVAPF 0.80 Lag
GM1PB800AF / AS / AT
50 ft.4#500
SM1PG1200AF / AS / AT
001 GEN480 V7.508 kA
AFELA400AT
005 SWBD480 V22.507 kAAFN
LA400AT
MTR LD500 hp500 kVAXd 0.25 PU
100 ft.1#500
E N
400A ATS
002L ATS480 V16.753 kA
003 PNL1480 V14.815 kAPB2
HG125AT
100 ft.1#500
50 ft.1#500
PB4EG40AT
50 ft.1#2
004 PNL2480 V11.857 kA
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The cable length and sizes are noted on the one line drawing
shown in Figure 1. The cables are sized per NEC 2011 Edition table
310.15(B) (16). In order to have a worst case fault analysis, the
main switchboard was loaded with (20) 25 hp motors adding up to 500
kVA, half of the rated kVA of the utility transformer. The circuit
breaker TCCs are plotted based on worst case three phase fault
current from an infinite source. For this example it has been
assumed that the entire system consisting of both normal and
emergency sides should be selectively coordinated for each
case:
a. Case 2: 0.1 second and longer and b. Case 3: total
selectivity.
The TCC graphs shown in Figures 2 and 3 show coordination for
Case 1: without more stringent selective coordination requirements.
Without selectivity requirements the coordination achieved in Case
1 is borderline practicable; for fault level currents load-side of
PB4, mis-coordination exists with line-side circuit breaker PB2,
and mis-coordination exists for the highest levels of fault current
for all of the circuit breakers plotted. However, when Case 2 and
Case 3 are considered there are several issues, notably for Case 3.
Selectivity will be achieved by adjusting the circuit breaker TCCs
shown in Figures 2 and 3 and if required by replacing the circuit
breakers with ones ensuring better coordination. Each case includes
circuit breakers at equipment designations PNL1 and PNL2 fed from
normal and emergency source.
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Figure 2: Case 1 - TCC Graph for PNL2 Circuit Based on Normal
Source Fault Current
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In terms of selective coordination, Case 2 is considered first:
selective coordination 0.1 seconds and longer. If Figures 2 and 3
are compared, both have a common OCPD mis-coordination issue which
exists between circuit breakers PB2 and PB4. By replacing circuit
breaker PB2 with a PowerPact circuit breaker HD 125AT trip 5.2A, we
can improve selectivity to 0.1 seconds and longer. The circuit
breakers AFN, AFE, SM1 and GM1 require setting adjustments in order
to maintain selective coordination of 0.1 seconds and longer. The
new TCC graphs are shown in Figures 4 and 5, for normal and
emergency side, respectively. In order to achieve selective
Figure 3: Case 1 - TCC Graph for PNL2 Circuit Based on Emergency
Source Fault Current
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coordination for Case 2 there were few system design changes
comprising of one circuit breaker upgrade and settings adjustment
of existing circuit breakers.
Figure 4: Case 2 - TCC Graph for PNL2 Circuit Based on Normal
Source Fault Current
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For Case 3, the selectivity table and the online selectivity
tool are used in order to improve coordination by removing the
overlap in the instantaneous region between the circuit breaker
curves as shown in Figures 2 and 3. The analysis starts at the
smallest downstream circuit breaker in PNL2 and subsequently moves
up the system to the main switchboard SWBD. There is an overlap in
the instantaneous region of device PB4 with devices PB2, AFN and
SM1.
Figure 5: Case 2 - TCC Graph for PNL2 Circuit Based on Emergency
Source Fault Current
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Per Schneider Electric data bulletin 0100DB0501 [1] the circuit
breakers HG 125AT [PB2] and EGB 40AT [PB4] are totally selective up
to 1300 A. The available fault current at PNL2 is 11.857kA which is
greater than 1300 A. Hence in case of a fault PB2 may trip along
with PB4 resulting in lack of total selectivity for faults above
1300 A. In order to achieve total selectivity between circuit
breakers PB2 and PB4 the following design options exist: A. Change
the thermal magnetic circuit breaker PB2 to an electronic trip
type
of same size with an adjustable short time and instantaneous
setting. B. Increase the trip and frame size of circuit breaker
PB2, thereby having a
higher instantaneous trip region. Note that the higher trip size
would require an increase in cable size. Increased cable size will
have lower impedance which in turn will increase the fault current
at the panel PNL2.
C. Introduce an isolation transformer between ATS load side and
PNL1. The isolation transformer will reduce the fault current.
D. Redesign the cable lengths to insure lower fault currents by
increasing the cable length and impedance. This is a worst case
option when total selectivity is required and there are no circuit
breaker pairs available. Drawbacks of D are that the building may
not be conducive to the longer cable runs required to reduce the
fault current, or there may be voltage drop issues created by the
long runs. Typically options a), b), and c) are considered first,
in that order, before opting for d).
For our example option A) is chosen which has the least amount
of system design changes. In order to select circuit breakers to
achieve selective coordination the design engineer can refer to
manufacturer published tables. The instantaneous region of the
device bands tend to show an overlap on a TCC (plotted by most
commercially available analysis software programs) for many circuit
breakers because the curves have been based on the standalone
characteristic curves for the maximum three-phase bus fault values.
If dynamic impedance is considered for this region, then the fault
current observed at the upstream circuit breaker may not be high
enough to trip before the downstream circuit breaker reaches its
maximum trip time for the manufacturers tolerances for
instantaneous faults. Different combinations of circuit breakers
can be tested to show coordination at or below certain fault values
even though the software-generated TCC device bands overlap each
other in the instantaneous region. Schneider Electric has published
data bulletin 0100DB0501 - Short Circuit Selective Coordination for
Low Voltage Circuit Breakers to present short circuit selective
coordination data for various combinations of Schneider Electric
low voltage circuit breakers. They were determined by comparing the
current let-through of the downstream circuit breaker with the
minimum instantaneous trip characteristic of the upstream circuit
breaker, taking into account manufacturing tolerances. Thus the
maximum level of selective coordination was determined for various
pairings of upstream and downstream circuit breakers. Table 1 shows
a table in 0100DB0501 for L-frame selectivity with QO and E-frame
circuit breakers shows the option for upstream circuit breaker
(PB2).
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Based on the choices provided in Table 1, the new PowerPact L-W
400AF/125AT mission critical circuit breaker is selected. The
circuit breaker LG-W selectively coordinates with circuit breaker
EGB up to 30 kA at 480 V, which is higher than our available fault
current of 11.857 kA. The LG-W mission critical circuit breaker has
the same circuit breaker curve as LG trip 5.3A shown in our example
in Figures 8 and 9, on Pages 14 and 15, respectively. The
difference between L-Frame and L-Frame mission critical lies in
their tripping mechanism; the L-frame mission-critical circuit
breaker has an inherent 5 ms delay and, for high fault currents,
operates on load-side energy rather than peak current. This
effectively allows the J- and L-frame mission-critical circuit
breakers to distinguish between load-side faults and let-through
currents of load-side circuit breakers operating on faults. This
energy-based tripping improves selectivity and has an arc flash
advantage in that clearing time is not different than other molded
case circuit breakers.
There is an additional data bulletin for transformer protection
0100DB0902 - Guide to Low Voltage Transformer Protection and
Selective Coordination [2]. For a quick check Schneider Electric
has an online selective coordination tool: click here. It does up
to three levels of total selectivity look-up with user-input fault
values or it can do simple fault calculations. The next
coordination issue exists between circuit breakers PB4 and PB2 with
AFN. The online selective coordination tool is used for selecting
circuit breaker AFN as shown in the screenshots in Figures 6 and
7:
Table 1: Schneider Electric Selective Coordination Table for
L-Frame Low-Voltage Circuit Breakers
Circuit Breaker Voltage Current One-Line DiagramMain Branch
L-W, 250 AQO(B0QO(B)-HQO(B)-VHQH
1060 A240 V
18
70125 A 10
L-W, 400 AL-W, 600 A
QO(B0QO(B)-HQO(B)-VHQH
15150 A 240 V 30
L-W, 250 AL-W, 400 AL-2, 600 A
E-Frame 115-125 A240 V 30 kA
480Y/277 30 kA
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Figure 6: Screenshot of Schneider Electric Online Selective
Tool
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For this example the fault currents are manually filled in at
three zones starting with Zone 1 22.507 kA at SWBD, Zone 2 14.816
kA at PNL1 and Zone 3 11.857 kA at PNL2, as shown in Figure 6.
Based on the available fault current and circuit breaker types -
EGB340 (PB4), LGUW3400-125AT (PB2), there are two options for
circuit breaker AFN a) PG3400 and b) PK3400, as shown in Figure 7.
Both options selectively coordinate with downstream circuit
breakers up to 21.6 kA which is higher than our available fault
current at Zones 1 and 2. However, option a) is selected based on
lower short circuit withstand rating. The short circuit withstand
rating of PG circuit breaker at 35 kA is adequate for the available
fault current of 22.507 kA at Zone 1.
Figure 7: Result of Online Selectivity Tool for 3 Tier
System
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The only circuit breaker which is lacking selectivity on the
normal side is the main circuit breaker SM1 in switchboard SWBD.
Table 2 shows the table in 0100DB0501 for UL 480 Vac 400 A
Selective Coordination. Based on Table 2, PowerPact RG 1200AT is
selected as the main circuit breaker SM1. The RG circuit breaker
has total selectivity with circuit breaker AFN-PG 400AT. With the
help of Schneider Electric data bulletin 0100DB0501 and the online
selectivity tool, total selectivity is achieved for the normal side
of the system as shown in Figure 8.
Table 2: Schneider Electric Selective Coordination Table for 400
A / 480 Vac Downstream Circuit Breaker
Upstream Circuit Breaker Downstream Circuit Breaker -
Type/kAIRMaximum Level of Selective Coordination Shown in kA
Maximum Continuous
Current Rating
kAIR TypeLA LA-MC LH
LH-MC DG DJ LD LG LJ LL LC LE LX LI LXI PG PJ PL
30 30 35 35 35 65 18 35 65 100 65 65 65 200 200 35 65 100
1200 A
35PG 21.6 21.6 21.6 21.6 35.0 35.0 18.0 35.0 35.0 35.0 35.0 35.0
35.0 35.0 35.0 21.6 21.6 21.3RG 30.0 30.0 35.0 35.0 35.0 35.0 35.0
35.0 35.0 35.0 35.0 35.0 35.0 35.0 35.0 35.0 35.0 35.0
50NT-NH 1200 A 30.0 30.0 35.0 35.0 35.0 50.0 18.0 35.0 50.0 50.0
50.0 50.0 50.0 50.0 50.0 35.0 35.0 35.0PK 21.6 30.0 35.0 35.0 35.0
50.0 18.0 35.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 21.6 21.6
21.6
65
NT-L1 1200 A 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0
9.0 9.0 9.0 9.0 9.0NW-N 2000 A 30.0 30.0 35.0 35.0 35.0 65.0 18.0
35.0 65.0 65.0 65.0 65.0 65.0 65.0 65.0 35.0 35.0 35.0PJ 9.0 9.0
9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0RJ
30.0 30.0 35.0 35.0 35.0 65.0 18.0 35.0 65.0 65.0 65.0 65.0 65.0
65.0 65.0 35.0 40.8 40.8RK 30.0 30.0 35.0 35.0 35.0 65.0 18.0 35.0
65.0 65.0 65.0 65.0 65.0 65.0 65.0 35.0 65.0 51.3 NT-L 1200 A 9.0
9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0
9.0
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After ensuring total selectivity for the normal side the
emergency side of the system is evaluated. When fed from an
emergency source, the panel PNL2 has an available fault current of
6 kA which is lower than 11.857 kA from normal source. Hence the
devices selected for total selectivity for normal source will
continue to have total selectivity when fed from the generator.
Even though circuit breakers PB2 and PB4 have total selectivity
amongst
Figure 8: Case 3 - TCC Graph for PNL2 Circuit Based on Normal
Source Fault Current
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themselves there is still cause for concern as circuit breaker
AFE (LA 400AT) still lacks coordination with PB2 and PB4 as shown
in Figure 3 on Page 6. The selection process for AFE is simplified
by using the same circuit breaker as AFN. Both AFE and AFN are
feeding the same load and as stated above the emergency source has
lower fault current, so that what works for the normal side will
continue to work for the emergency side.
Figure 9: Case 3 - TCC Graph for PNL2 Circuit Based on Emergency
Source Fault Current
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The final coordination issues for the generator fed system are
between circuit breaker GM1 and downstream devices. In Figure 3 on
Page 6, the GM1 (PG 800AT) circuit breaker does not overlap with
the downstream circuit breakers. The same does not hold true after
the upgrade of downstream circuit breakers. We have to increase the
short time and instantaneous settings of GM1 in order to avoid
mis-coordination at the short time and instantaneous region. Figure
9 shows the new time current coordination graph for the emergency
side of the system having total selectivity between the protective
devices.
Arc Flash Analysis Arc flash analysis can be performed only
after the protective devices have been adjusted for best possible
coordination. With the help of a computer analysis through SKM
Power Tools we are able to calculate the arc flash incident energy
and categories at each piece of equipment. This is the most
efficient way to calculate the incident energies and flash
protection boundaries where multiple sources exist which must be
taken into account (such as generators and motors). The SKM tool
uses the National FIre Protection Association (NFPA) 70E 2012 Annex
D.7 [3] and Institute of Electrical and Electronics Engineers
(IEEE) 1584 [4] standards to determine the incident energy and arc
flash boundaries.An arc flash analysis was performed for both
normal and emergency side of the system with protective device
settings as per Case 1 Load-based coordination where devices are
selected based on
typical thermal-magnetic trips for circuit breakers other than
service mains and prior to implementing NEC Article 100 requirement
of selective coordination.
Case 2 Selective coordination to 0.1 seconds and longer Case 3
Total selective coordinationBased on the IEEE 1584 requirement for
arc flash analysis both minimum and maximum three phase fault
current have been considered. Arcing current is significantly lower
than bolted current and based on typical calculations it can be as
low as 52% of bolted fault current at 480 V [5]. Hence, to expect
that total selectivity achieved at maximum three phase bolted
current will yield optimized arc flash results at arcing current of
minimum three phase bolted current is not reasonable, as shown in
Table 3.
Table 3 shows a comparison between the arc flash results for
Case 1, Case 2, and Case 3. From the table it is clear that, for a
totally selectively coordinated system, the incident energy levels
are significantly higher for the transfer switch 002L ATS, panels -
003 PNL1 and 004 PNL2. However, the arc flash incident energy
remained the same for most equipment in Case 2 except for an
increase in panel PNL1. The new instantaneous settings in both the
cases resulted in higher trip time for the protective devices and
thereby higher arc flash energy. This difference in incident energy
and risk category in Case 2 and 3, is one of the reasons for
preferring selective coordination to 0.1 seconds and longer, over,
total selectivity. The increase in arc flash energy may pose a
greater threat to personnel and could increase the amount of
equipment damage in an arc flash event.
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The arc flash incident energies of generator panel 001 GEN and
main switchboard 005 SWBD do not change. This is because the
results for both were based on maximum trip time of 2 seconds for
all three cases. For the rest of the equipment (ATS, PNL1 and
PNL2), there is considerable increase in incident energy as we go
from load based coordination to selective coordination at 0.1
seconds and above and finally to total selective coordination.
Hence, solely from an arc flash perspective, it is best to opt for
selective coordination to 0.1 seconds and longer.
Conclusion In order to ensure reliability as well as safe
working conditions, optimized selective coordination and arc flash
mitigation have to work in tandem. The NFPA has already taken a
step in that direction through NFPA 99-2012 [6] for health care
facilities. NFPA 99-2012 requires that the essential electrical
system be coordinated to 0.1 seconds for all types of fault current
generated by the alternate source. In emergency, legally required,
and critical operation power systems, NFPA 110-2010 [7] requires
that the OCPDs feeding the automatic transfer switch(s) be
selectively coordinated to the extent practicable. The IEEE color
books Brown [8], Buff [9] and Orange [10], recognize the difficulty
in achieving these opposing goals, and recommend selective
coordination as far as practicable. The draft International
Electrotechnical Commission (IEC) technical report [11] states
that, Selectivity over the whole range of fault current up to the
prospective fault current at the point of installation is not
always possible or necessary. A more economic solution may be found
in many cases by accepting a limited selectivity, particularly
taking into account the low probability of a high short-circuit
fault current.Schneider Electric recognizes the importance of
worker safety by reduced incident energy and uninterrupted supply
by improved selectivity, hence extensive research has been done to
ensure both, and improve OCPD performance. One technique developed
by Schneider Electric in each case a) in practice and b) in process
of finalization, is as follows:A. Zone selective interlocking (ZSI)
allows electronic trip devices to
communicate with each other so that a short-time trip or ground
fault will be isolated and cleared by the nearest upstream circuit
breaker with no intentional time delay. Devices in all other areas
of the system (including upstream) remain closed to maintain
service to unaffected loads. Without ZSI, a coordinated system
results in the circuit breaker closest to the fault clearing the
fault, but usually with an intentional delay. With ZSI, the device
closest to the fault will ignore its preset short-time and/or
ground fault delays and clear the fault with no intentional delay.
Zone-selective interlocking eliminates intentional delay, without
sacrificing coordination, resulting in faster tripping times. This
limits fault stress by reducing the amount of let-through energy
the system is subjected to during an overcurrent. At low voltage
(600 V and below), Schneider Electric
Table 3: Arc Flash Comparison Table
Bus NameArc Flash Incident Energy in cal/cm2
(Case 1)
Required Protective Arc Rated Clothing Characteristics
(Case 1)
Arc Flash Incident Energy in cal/cm2
(Case 2)
Required Protective Arc Rated Clothing Characteristics
(Case 2)
Arc Flash Incident Energy in cal/cm2
(Case 3)
Required Protective Arc Rated Clothing Characteristics
(Case 3)001 GEN 12.44 Category 3 12.44 Category 3 12.44 Category
3002L ATS 0.51 Category 0 0.51 Category 0 11.91 Category 3003 PNL1
0.46 Category 0 9.80 Category 3 11.68 Category 3004 PNL2 0.38
Category 0 0.62 Category 0 2.37 Category 1005 SWBD 80.00 Dangerous
80.00 Dangerous 80.00 Dangerous
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MasterPact low-voltage power circuit breakers, offer increased
arc-flash protection due to faster clearing times, especially at
higher current levels. In addition to superior arc-flash protection
inherent in their design, MasterPact and PowerPact circuit breakers
can be configured in a variety of Zone Selective Interlocking
schemes to further enhance protection with no impact on
selectivity. At higher voltages, SepamTM overcurrent relays can
also be applied in ZSI solutions. (AF protections using ZSI is
achieved only with the ST-ZSI option.)
Often ZSI is specified only within the main distribution board,
though a more likely location for a fault occurrence is on a feeder
circuit leaving the switchboard, or even lower in the system. To
maximize the protection offered by using ZSI, as many levels of the
system as possible need to be interlocked. This way, devices at the
lower levels of the system will trip without any intentional delay,
when necessary, without sacrificing coordination. This provides
true selective coordination and maximum protection against fault
stress. Additionally in certain areas of the system it is necessary
to self-restrain a circuit breaker to maintain the delay before
tripping during a fault condition. This results in the circuit
breaker always introducing a time delay before tripping on a short
circuit or a ground fault (the time delay is always activated).
Cases where self-restraint should be applied are: The interlocked
device is feeding a non-interlocked device
downstream (or a number of non-interlocked devices in a panel).
A time delay is desired for short-circuit and/or ground-fault
occurrences (usually to avoid false tripping during transients
and inrushes).
Minimal tripping time would compromise coordination.For more
information on ZSI and self-restraint refer to data bulletin
06000DB0001 [12].
B. Energy based discrimination The new mission critical
PowerPact J- and Lframe circuit breakers developed by Schneider
Electric have energy based discrimination. The energy based method
with its consistency allows the line-side circuit breaker to
effectively distinguish between load-side faults and let-throughs
of load-side circuit breakers operating on faults further
downstream. This method for achieving selectivity uses supplemental
trip systems in conjunction with specially designed primary trip
systems. The primary trip system will not trip during the first
half-cycle of a fault regardless of the fault current. The
intentional delay that allows the reflex tripping to see load-side
energy does not reduce overall clearing time, resulting in higher
levels of selective coordination without necessarily unleashing
higher levels of fault energy, including arc flash incident
energy.For more information on energy based tripping refer to
papers [13] and [14].
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References [1] Short Circuit Selective Coordination for Low
Voltage Circuit Breakers, 0100DB0501R01/12-03/12.[2] Guide to Low
Voltage Transformer Protection and Selective Coordination,
0100DB0902R04/11-04/2011.[3] NFPA 70E-2012, Standard for Electrical
Safety in the Workplace. [4] IEEE Std. 1584-2002, IEEE Guide for
Performing Arc-Flash Hazard Calculations. [5] Arc Flash Hazard
Calculations: Myths, Facts, and Solutions, H. Wallace Tinsley III,
Michael Hodder, and Aidan M. Graham, IEEE Industry Applications
Magazine, Jan/Feb 2007, originally presented at the 2006 IEEE/IAS
Pulp and Paper Industry Conference, pp. 59, 60.
[6] NFPA 99-2012, Health Care Facilities Code.
[7] NFPA 110-2010, Standard for Emergency and Standby Power
Systems. [8] IEEE Recommended Practice for Protection and
Coordination of Industrial and Commercial Power Systems, IEEE Std
242-2001 (Buff Book), pp. 3-5, 607.[9] IEEE Recommended Practice
for Power Systems Analysis, IEEE Std 399-1990 (Brown Book), pp.
367.[10] IEEE Recommended Practice for Emergency and Standby Power
Systems for Industrial and Commercial Applications, ANSI/IEEE Std
446-1987 (Orange Book), pp. 175.[11] Draft IEC/TR 61912-2 Ed.1.0:
Low-voltage switchgear and controlgear Overcurrent protective
devices Selectivity under overcurrent conditions, International
Electrotechnical Commission, March 23, 2007, committee draft
updated after Copenhagen, pp. 11.[12] Reducing Fault Stress with
Zone-Selective Interlocking, 0600DB0001R11/11-04/12.[13]
Energy-based discrimination for low-voltage protective devices,
Marc Serpinet and Robert Morel, Cahier Technique n 167, March
1998.[14] Energy Based Tripping and Its Effect on Selective
Coordination, John Carlin & Josh Allen, Schneider Electric, May
2013.
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Selective Coordination vs Arc Flash Requirements 0600DB1303White
Paper 06/2013
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AbstractIntroductionSelective CoordinationArc Flash
Selective Coordination versus Arc Flash ExampleSelective
Coordination Through Comparison of Time-Current CurvesArc Flash
Analysis
ConclusionReferences