Selective Coordination vs Arc Flash Requirements

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Selective Coordination vs Arc Flash Requirements Selective Coordination vs Arc Flash Requirements Selective Coordination vs Arc Flash Requirements Selective Coordination vs Arc Flash Requirements

Text of Selective Coordination vs Arc Flash Requirements

  • 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.


    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


    005 SWBD480 V22.507 kAAFN


    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


    100 ft.1#500

    50 ft.1#500


    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

  • 0600DB1303 Selective Coordination vs Arc Flash Requ