Overcurrent Coordination Setting Guidelines Conductors

  
     
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Introduction

The proper selection and coordination of protective devices is mandated in article 110.10 of the National Electrical Code. To fulfill this requirement an overcurrent coordination study is required. The electrical engineer is always responsible for this analysis. It is an unfortunate fact of life that many times the engineer who specified and purchased the equipment will not set the devices. Therefore, compromises are inevitable.
There are three fundamental objectives to overcurrent coordination that engineers should keep in mind while selecting and setting protective devices. 		 
  •   The first objective is life safety. Life safety requirements are met if protective devices are rated to carry and interrupt maximum available load currents, as well as, withstand and interrupt maximum available fault currents. Life safety requirements are never compromised.
•   The second objective is equipment protection. Protection requirements are met if overcurrent devices are set above load operating levels and below equipment damage curves. Feeder and transformer damage curves are defined in applicable equipment standards. Motor and generator damage curves (points) are machine specific, and are normally provided in the vendor data submittal package. Based on system operating and equipment sizing practices equipment protection is not always possible.
•   The last objective is selectivity. Selectivity requirements are met if in response to a system fault or overload, the minimum area of the distribution system is removed from service. Again, based on system operating and equipment selection practices selectivity is not always possible. 		 
  Purpose  
  The purpose of this guide is to provide overcurrent protective device setting guidelines for conductors to meet the objectives listed above.  
  MV Switchgear Feeder Unit  
  Industry standard overcurrent protection schemes for MV feeders fed from switchgear circuit breakers include an instantaneous overcurrent relay (device 50/51). The 50/51 relay characteristics are plotted on a phase TCC along with the feeder damage curves.

The purpose of the phase overcurrent relay is to allow for full use of the feeder, and to protect the feeder from overloads and faults. To accomplish this, the relay curve should be at or below the feeder ampacity and to the left of the damage curve.

Suggested margins are listed below that have historically allowed for safe operation of the feeder while reducing instances of nuisance trips.

NFPA 70: National Electrical Code (NEC) does allow MV circuit breaker trip settings up to 600% of the ampacity of the conductors. However, settings in this range will not protect the conductors. 		 
     
 
Device Function Recommendations Comments
CT Size 100% of ampacity  
51 Pickup < ampacity  
  Time Dial No specific rule Set below the damage curve.
Set at or above the load-side device.
50 Pickup Set below through-fault current level Set below cable damage curve.
Cable damage curve must be above the maximum fault current at the maximum breaker clearing time.
 
     
   
  Fig. 1 MV switchgear feeder unit - one line  
   
  MV Fused Switch Feeder Unit

E-rated power fuses are typically used in MV fused switch feeders. Fuses rated 100E or less must trip in 300 seconds at currents between 200 and 240% of their E ratings. Fuses above 100E must trip in 600 seconds at currents between 220 and 264% of their E ratings. The fuse characteristics are plotted on a phase TCC along with the feeder damage curve.

The purpose of the fuse is to allow for full use of the feeder, and to protect the feeder from faults. To accomplish this, the fuse continuous amp rating should be at or below the feeder ampacity and the fuse curve should be to the left and below the damage curve. The fuse curve will always be above the cable ampacity at 1000 seconds.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.

NFPA 70: National Electrical Code (NEC) does allow MV fuse sizes up to 300% of the ampacity of the conductors. However, settings in this range will not protect the conductors. 		 
     
 
Device Function Recommendations Comments
51 Fuse Size < ampacity The size is the continuous amp rating of the fuse.
Cable damage curve must be above the maximum fault current at 0.01 seconds.
 
     
  Fig. 2 MV switchgear feeder unit - phase TCC  
   
  Fig. 3 MV fused switch feeder unit - one line  
   
  Fig. 4 MV fused switch feeder unit - phase TCC  
     
  LV CB Feeder Unit

Industry standard overcurrent protection schemes for LV circuit breaker feeder circuits include breakers equipped with long-time, short-time and instantaneous functions. The circuit breaker characteristics are plotted on a phase TCC along with the feeder damage curve.

The purpose of the circuit breaker is to allow for full use of the feeder, and to protect the feeder from overloads and faults. To accomplish this, the circuit breaker long time pickup should be at or below the ampacity and the breaker curve should be to the left of the damage curve. Depending upon the breaker long time pickup tolerance characteristics the breaker curve may be above the feeder ampacity at 1000 seconds.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.		  
 
Device Function Recommendations Comments
51 LTPU < ampacity  
  LTD, STPU &
STD		 No specific rules  
50 Instantaneous Set below through-fault current level Set below cable damage curve.
Cable damage curve must be above the maximum fault current at the CB total clear curve.
 
   
  Fig. 5 LV CB feeder unit - one line  
   
  Fig. 6 LV CB feeder unit - phase TCC  
     
  LV Fused Switch Feeder Unit

Class J (<600A) and Class L (>600A) fuses are typically used in fused switch feeder circuits. The fuse characteristics are plotted on a phase TCC along with the feeder damage curve.

The purpose of the fuse is to allow for full use of the feeder, and to protect the feeder from faults. To accomplish this, the fuse continuous amp rating should be at or below the feeder ampacity and the fuse curve should be to the left and below the damage curve. The fuse curve will always be above the cable ampacity at 1000 seconds.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.		  
 
Device Function Recommendations Comments
50 Fuse Size Size < Ampacity The size is the continuous amp rating of the fuse.
    No specific rules Cable damage curve must be above the maximum fault current at 0.01 seconds.
 
   
  Fig. 7 LV fused switch feeder unit - one line  
   
  Fig. 8 LV fused switch feeder unit - phase TCC
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  References  
  •   Other Application Guides offered by SKM Systems Analysis at www.skm.com
•   Aluminum Electrical Conductor Handbook, The Aluminum Association Inc., Washington, D.C., 3rd edition, 1989
•   Electrical Transmission and Distribution Reference Book, ABB Power T&D Company, Raleigh, North Carolina, 1997
•   Protective Relaying Theory and Applications, 2nd Edition, Marcel Dekker, New York, 2004 		 
  The latest revision of:  
  •   IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
•   IEEE Std C37.113, IEEE Guide for Protective Relay Applications to Transmission Lines 		 
     
     
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