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Overcurrent Coordination Setting Guidelines Motors

The information presented in this application guide is for review, approval, interpretation and application by a registered professional en

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 re

s required. The electrical engineer is always responsible for this analysis. It is an unfortunate fact of life that many times the engineer w

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 prote

The first objective is life safety. Life safety requirements are met if protective devices are rated to carry and interrupt maximum ava

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 l

curves. Feeder and transformer damage curves are defined in applicable equipment standards. Motor and generator damage curves (p

normally provided in the vendor data submittal package. Based on system operating and equipment sizing practices equipment protecti

The last objective is selectivity. Selectivity requirements are met if in response to a system fault or overload, the minimum area of t

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

50/51). The 50/51 relay characteristics are plotted on a phase time-current curve (TCC) along with the motor starting and 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 a

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 rat

Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of n

Device Function Recommendations Comments

CT Size 125-150% of FLA

51 Pickup 115-125% of FLA Set below motor stator damage

Set at or below cable ampaci

51 Time Dial2-10 seconds above knee of

motor curveSet below motor rotor damage c

Set below cable damage curv

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50 Pickup 200% of LRA Set below cable damage curv

Cable damage curve must be abo

maximum fault current at 0.1 se

Fig. 1 MV motor switchgear feeder unit - one line


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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 overcurre

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 p

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

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 damag



CT Size 125-150% of FLA

51 Pickup 115-125% of FLA Set below motor stator damage

Set at or below cable ampaci

51 Time Dial2-10 seconds above knee of

motor curve Set below motor rotor damage c

Set below cable damage curv

50 Fuse Size RRating > 1.1*LRA/100 Set below cable damage curv

AMPRating > FLACable damage curve must be abo

maximum fault current at 0.01 se

Fig. 3 MV motor fused starter feeder unit - one line


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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 del

PCBs, ICCBs, MCCBs and TMCBs. Short time pickup and short time delay functions are not used. The CB characteristics are plotted on a

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 accomplis

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

amp rating due to breaker tolerances.



LV CB LTPU 125% of FLA Set at or below cable ampaci

LV CB Time Dial2-10 seconds above knee of

motor curveSet below motor safe stall poi


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LV CB INST 200% of LRA Set below cable damage curv

Cable damage curve must be above

defined by the maximum fault curre

PCB instantaneous clear curv

Fig. 5 LV motor circuit breaker feeder unit - one line


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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 an

s a circuit breaker with the thermal element removed. The overload and MCP characteristics are plotted on a phase TCC along with the

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

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 sta

rating. Note it is not always possible to be below the cable amp rating due to overload tolerances.



OL Pickup125% of FLA if SF > 1.15

115% of FLA if SF = 1.00Set at or below cable ampaci

OL Time Dial Fixed assume Class 20 Set below motor safe stall poi


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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 a

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

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 sta

rating. Note it is not always possible to be below the cable amp rating due to overload tolerances.



OL Pickup125% of FLA if SF > 1.15

115% of FLA if SF = 1.00Set at or below cable ampaci

OL Time Dial Fixed assume Class 20 Set below motor safe stall poi

Fuse Size 175% of FLA Set below cable damage curv


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Cable damage curve must be above

defined by the maximum fault curren

seconds

Fig. 9 LV motor fused starter feeder unit - one line


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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

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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