17. overcurrent coordination setting guidelines motors

8/22/2019 17. Overcurrent Coordination Setting Guidelines Motors

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Overcurrent Coordination Setting Guidelines Motors

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

Reproduction of this material is permitted provided proper acknowledgement is given to SKMSystems 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 isrequired. 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 thedevices. Therefore, compromises are inevitable.

There are three fundamental objectives to overcurrent coordination that engineers should keep inmind while selecting and setting protective devices.

The first objective is life safety. Life safety requirements are met if protective devices arerated to carry and interrupt maximum available load currents, as well as, withstand andinterrupt maximum available fault currents. Life safety requirements are never

compromised.

The second objective is equipment protection. Protection requirements are met ifovercurrent devices are set above load operating levels and below equipment damagecurves. Feeder and transformer damage curves are defined in applicable equipmentstandards. Motor and generator damage curves (points) are machine specific, and are

normally provided in the vendor data submittal package. Based on system operating andequipment sizing practices equipment protection is not always possible.

The last objective is selectivity. Selectivity requirements are met if in response to asystem fault or overload, the minimum area of the distribution system is removed fromservice. 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). The50/51 relay characteristics are plotted on a phase time-current curve (TCC) along with the motorstarting 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 motorand 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 cabledamage 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 CommentsCT 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.


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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 fedfrom fused starters include an overcurrent relay (device 51), and a set of R-rated fuses (device50). R rated fuses melt at 100 times the R rating and 20 seconds. Both the fuse and relay

Cable damage curve must be above themaximum fault current at 0.1 seconds.


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characteristics are plotted on a phase TCC along with the motor starting and damage curves, andthe feeder damage curve.

The purpose of the fuse-relay combination is to allow the motor to start and run, and to protectthe 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 therotor, 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 motorand cable while reducing instances of nuisance trips.

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

Device Function Recommendations Comments

CT Size 125-150% of FLA

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


51 Time Dial 2-10 seconds aboveknee 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 themaximum fault current at 0.01 seconds.


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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 motorsinclude 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. TheCB 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 cablefrom overloads and faults. To accomplish this, the CB curve should be above and to the right ofthe motor starting curve, and to the left and below the motor safe stall point, cable damagecurve and amp rating. Note it is not always possible to be below the cable amp rating due to

breaker tolerances.



LV CB LTPU 125% of FLA Set at or below cable ampacity.LV CB Time Dial 2-10 seconds above

knee of motor curveSet 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.


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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 discretecomponents, 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 ona phase TCC along with the motor starting curve and safe stall point, and the feeder damage

curve.


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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-MCPcombined 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 alwayspossible 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.

Fig. 7 LV motor MCP starter feeder unit - one line


OL Pickup 125% of FLA if SF > 1.15

115% of FLA if SF = 1.00


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

MCP Size 125-160% of FLA Refer to manufacturers recommendedsizes.

MCP Pickup 200% of LRA Set below cable damage curve.

Cable damage curve must be above thepoint defined by the maximum fault currentand the MCP instantaneous clear curve.


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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 discretecomponents, 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 feederdamage curve.

The purpose of the overload-fuse combination is to allow the motor to start and run, and toprotect the motor and cable from overloads and faults. To accomplish this, the overload-fusecombined 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 alwayspossible to be below the cable amp rating due to overload tolerances.



OL Pickup 125% of FLA if SF > 1.15

115% of FLA if SF = 1.00


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 thepoint defined by the maximum fault currentand 0.01 seconds


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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


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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 Industrialand Commercial Power Systems (IEEE Buff Book)

IEEE Std 620, IEEE Guide for the Presentation of Thermal Limit Curve for Squirrel CageInduction Machines

IEEE Std C37.96, IEEE Guide for AC Motor Protection

NEMA MG-1, Motors and Generators

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17. overcurrent coordination setting guidelines motors

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