Overcurrent Coordination Setting Guidelines Motors

  
     
  The information presented in this application guide is for review, approval, interpretation and application by a registered professional engineer only. SKM disclaims any responsibility and liability resulting from the use and interpretation of this information.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM Systems Analysis Inc.

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 motors to meet the objectives listed above.  
  MV Motor Switchgear Feeder Unit  
  Industry standard overcurrent protection schemes for MV induction and synchronous motors fed from switchgear circuit breakers include an instantaneous overcurrent relay (device 50/51). The 50/51 relay characteristics are plotted on a phase time-current curve (TCC) along with the motor starting and damage curves, and the feeder damage curve.

The purpose of the 50/51 relay is to allow the motor to start and run, and to protect the motor and cable from overloads and faults. To accomplish this, the relay curve must be above and to the right of the motor starting curve, and to the left and below the rotor, stator and cable damage curves, and the amp rating of the cable.

Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of nuisance trips.		  
     
 
Device Function Recommendations Comments
CT Size 125-150% of FLA  
51 Pickup 115-125% of FLA Set below motor stator damage curve.
      Set at or below cable ampacity.
51 Time Dial 2-10 seconds above knee of motor curve Set below motor rotor damage curve.
      Set below cable damage curve.
50 Pickup 200% of LRA Set below cable damage curve.
      Cable damage curve must be above the maximum fault current at 0.1 seconds.
 
     
   
  Fig. 1 MV motor switchgear feeder unit - one line  
   
  Fig. 2 MV motor switchgear feeder unit - phase TCC  
  MV Motor Fused Starter Feeder Unit

Industry standard overcurrent protection schemes for MV induction and synchronous motors fed from fused starters include an overcurrent relay (device 51), and a set of R-rated fuses (device 50). R rated fuses melt at 100 times the R rating and 20 seconds. Both the fuse and relay characteristics are plotted on a phase TCC along with the motor starting and damage curves, and the feeder damage curve.

The purpose of the fuse-relay combination is to allow the motor to start and run, and to protect the motor and cable from overloads and faults. To accomplish this, the fuse-relay combined curve must be above and to the right of the motor starting curve, and to the left and below the rotor, stator and cable damage curves, and the amp rating of the cable.

Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of nuisance trips. 		 
     
 
Device Function Recommendations Comments
CT Size 125-150% of FLA  
51 Pickup 115-125% of FLA Set below motor stator damage curve.
      Set at or below cable ampacity.
51 Time Dial 2-10 seconds above knee of motor curve Set below motor rotor damage curve.
      Set below cable damage curve.
50 Fuse Size RRating > 1.1*LRA/100 Set below cable damage curve.
    AMPRating > FLA Cable damage curve must be above the maximum fault current at 0.01 seconds.
 
   
  Fig. 3 MV motor fused starter feeder unit - one line  
   
  Fig. 4 MV motor fused starter feeder unit - phase TCC  
     
  LV Motor Circuit Breaker Feeder Unit

Industry standard phase overcurrent functions purchased with circuit breakers serving LV motors include long time pickup, long time delay and instantaneous pickup. This applies to PCBs, ICCBs, MCCBs and TMCBs. Short time pickup and short time delay functions are not used. The CB characteristics are plotted on a phase TCC along with the motor starting curve and safe stall point, and the feeder damage curve.

The purpose of the CB is to allow the motor to start and run, and to protect the motor and cable from overloads and faults. To accomplish this, the CB curve should be above and to the right of the motor starting curve, and to the left and below the motor safe stall point, cable damage curve and amp rating. Note it is not always possible to be below the cable amp rating due to breaker tolerances.

Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of nuisance trips. 		 
 
Device Function Recommendations Comments
LV CB LTPU 125% of FLA Set at or below cable ampacity.
LV CB Time Dial 2-10 seconds above knee of motor curve Set below motor safe stall point.
LV CB INST 200% of LRA Set below cable damage curve.
      Cable damage curve must be above the point defined by the maximum fault current and the PCB instantaneous clear curve.
 
   
  Fig. 5 LV motor circuit breaker feeder unit - one line  
   
  Fig. 6 LV motor power circuit breaker feeder unit - phase TCC  
     
  LV Motor MCP Starter Feeder Unit

Industry standard phase overcurrent protection is provided in MCP starter units by two discrete components, a thermal overload relay and a motor circuit protector (MCP). The MCP is a circuit breaker with the thermal element removed. The overload and MCP characteristics are plotted on a phase TCC along with the motor starting curve and safe stall point, and the feeder damage curve.

The purpose of the overload-MCP combination is to allow the motor to start and run, and to protect the motor and cable from overloads and faults. To accomplish this, the overload-MCP combined curve should be above and to the right of the motor starting curve, and to the left and below the motor safe stall point, the cable damage curve and amp rating. Note it is not always possible to be below the cable amp rating due to overload tolerances.

Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of nuisance trips.		  
 
Device Function Recommendations Comments
OL Pickup 125% of FLA if SF > 1.15
115% of FLA if SF = 1.00		 Set at or below cable ampacity.
OL Time Dial Fixed assume Class 20 Set below motor safe stall point.
MCP Size 125-160% of FLA Set below cable damage curve.
MCP Pickup 200% of LRA  
      Cable damage curve must be above the point defined by the maximum fault current and the PCB instantaneous clear curve.
 
   
  Fig. 7 LV motor MCP starter feeder unit - one line  
   
  Fig. 8 LV motor MCP starter feeder unit - phase TCC  
     
  LV Motor Fused Starter Feeder Unit

Industry standard phase overcurrent protection is provided in fused starter units by two discrete components, a thermal overload relay and a fuse. The overload and fuse characteristics are plotted on a phase TCC along with the motor starting curve and safe stall point, and the feeder damage curve.

The purpose of the overload-fuse combination is to allow the motor to start and run, and to protect the motor and cable from overloads and faults. To accomplish this, the overload-fuse combined curve should be above and to the right of the motor starting curve, and to the left and below the motor safe stall point, the cable damage curve and amp rating. Note it is not always possible to be below the cable amp rating due to overload tolerances.

Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of nuisance trips.		  
 
Device Function Recommendations Comments
OL Pickup 125% of FLA if SF > 1.15
115% of FLA if SF = 1.00		 Set at or below cable ampacity.
OL Time Dial Fixed assume Class 20 Set below motor safe stall point.
Fuse Size 175% of FLA Set below cable damage curve.
      Cable damage curve must be above the point defined by the maximum fault current and 0.01 seconds
 
   
  Fig. 9 LV motor fused starter feeder unit - one line  
   
  Fig. 10 LV motor fused starter feeder unit - phase TCC  
     
  References  
  •   Other Application Guides offered by SKM Systems Analysis at www.skm.com
•   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 620, IEEE Guide for the Presentation of Thermal Limit Curve for Squirrel Cage Induction Machines
•   IEEE Std C37.96, IEEE Guide for AC Motor Protection
•   NEMA MG-1, Motors and Generators 		 
     
     
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