Which motors need differential protection comes down to three inputs: rating, voltage and what an unplanned stop costs. In practice 87M is specified on MV and HV motors, and on any LV motor whose failure would halt a process.

Written by Leon Zhang, Protection Application Engineer · Technical review by Li Ming, Electrical Engineer · Last reviewed September 2026

Table of Contents

Which Motors Need Differential Protection?

Not every motor needs it.

Overcurrent protection is fitted everywhere because it is cheap and covers almost everything; 87M is fitted selectively, where a winding fault is expensive enough to justify six CTs.

Five inputs decide it: motor rated power, operating voltage, how critical the motor is to the process, what an internal fault costs, and how the rest of the system is coordinated.

Power alone does not decide it.

No standard fixes a rating above which motor differential protection becomes mandatory. Every configuration is argued from site conditions, and the argument is economic before it is technical.

Which motors need differential protection: rating, voltage and criticality thresholds for 87M (illustrative)

What Is Motor Differential Protection (87M)?

87M motor differential protection compares the current entering the motor winding with the current leaving it, and trips on the difference. That is the whole principle, and it is why the element is blind to overload.

What Does 87M Protect?

87M differential protection focuses exclusively on motor internal faults that standard 50/51 overcurrent protection may fail to detect promptly:

Stator winding inter-turn short circuits
Phase-to-phase faults inside the motor protection zone
Phase-to-ground faults within stator windings
Low-magnitude internal short-circuit faults

On MV and HV machines it is the standard primary protection for the winding.

87M Motor Protection Zone Definition

The 87M protection zone is defined by dual-terminal CT (current transformer) installation at the motor line side and neutral side. The relay continuously compares real-time current values collected from two sets of CTs. Consistent current values indicate normal motor operation or external faults; a measurable current difference confirms an internal motor fault and triggers an immediate trip.

Motor Differential Requirements: Core Factors for 87M Deployment

Motor differential requirements are argued from five inputs, not from a single number. Only two of them have thresholds attached; the rest are judgement calls that have to be written down.

Motor Rated Power

Damage scales with rating, but not linearly.

A small machine is replaced; a large one is rewound, and the rewind is quoted in months rather than days. That gap, not the kW figure, is what the rating stands in for.

Motor Voltage Level

Voltage level is a key basis for 87M configuration:

LV motors (≤1kV): Low fault energy and equipment value, 87M is generally unnecessary
MV motors (3kV–35kV): High fault energy and operational impact, 87M is strongly recommended
HV motors (>35kV): Mandatory evaluation for 87M as standard primary protection

The voltage bands above are the ones most specifications actually use. Treat them as the starting point, not the verdict.

Motor Operational Criticality

Criticality overrides rating.

A 400 kW motor that stops a production line is a better candidate for 87M than a 2 MW motor with a spare standing next to it.

Typical critical motor loads include main process pumps, large compressors, boiler auxiliary equipment, and mining crushing mills.

Consequences of Internal Motor Faults

An internal stator fault does not stay small.

Without 87M it is cleared by whatever element eventually notices it, which is usually the one with the longest delay and the highest pickup.

Size is the easiest number to read and the least useful one to decide on.

System Protection Coordination Requirements

50/51 has to be graded against whatever sits downstream, so its delay is bought with coordination margin. 87M has nothing to grade against, which is why it can clear in about 20–40 ms without disturbing the rest of the system.

Typical Motors Requiring 87M Differential Protection

Four categories account for most 87M applications, and each one is justified by a different input rather than by rating alone:

Large Medium-Voltage Industrial Motors

Large MV induction and synchronous motors are the commonest 87M applications.

What they share is a long rewind lead time, so clearing a fault early is worth more here than anywhere else on the plant.

Core Process Critical Motors

Oil and gas, water treatment and chemical plants run these drives continuously, so a winding fault is a process stop rather than a maintenance item. 87M clears the faulted machine and leaves the rest of the plant running.

Power Plant Auxiliary Motors

Boiler feedwater pumps, ID/FD fans, circulating water pumps and coal mills all carry unit load directly. Losing one of them derates the unit, so 87M is normally specified for the larger auxiliary drives.

High-Power Synchronous Motors

Large synchronous motors on constant-speed drives have more complex windings and cost more to repair. 87M sits alongside the thermal model and the negative-sequence element, and none of the three covers what the other two miss.

Scenarios Where 87M Differential Protection Is Unnecessary

87M costs six CTs, two cable runs and a commissioning day. On the following duties that spend buys nothing measurable.

Small Low-Voltage Non-Critical Motors

Small LV motors for auxiliary ventilation, ordinary conveying, and auxiliary lighting loads only need 50/51 overcurrent protection, thermal overload protection, and locked-rotor protection. The fault impact is limited, and 87M brings no obvious engineering benefit.

Small motors are normally covered by a low-voltage motor protection relay, which carries the thermal model and 50/51 in one device.

Easily Replaceable Non-Critical Loads

Where a motor is cheap, stocked and not on the critical path, overcurrent backup is enough. Six CTs and a relay cost more than the machine they protect.

Cost-Sensitive Conventional Projects

87M needs matched CTs at both ends, two cable runs and a commissioning day. On a low-risk project with a hard capex ceiling, that spend is hard to defend and the simpler scheme is the right answer.

87M Differential Protection vs 50/51 Overcurrent Protection

87M and 50/51 protections are complementary rather than alternative solutions for industrial motors, forming a complete motor protection system.

Core Functions of 50/51 Overcurrent Protection

50/51 covers external short circuits, overload and anything else on the motor branch, and it is the element that trips if 87M is out of service. It does that without a zone, so it cannot tell an internal fault from one on the cable. The two are compared in overcurrent protection relay terms on the dedicated page.

Unique Advantages of 87M Differential Protection

87M closes the gap that overcurrent protection leaves: a zone, and a threshold that does not depend on how large the fault current grows. Everything outside that zone is still 50/51’s job.

Core Conclusion: For most large and critical MV/HV motors, 87M primary differential protection + 50/51 backup overcurrent protection is the standard industrial protection configuration.

Motor Differential Protection vs Overcurrent Protection

Motor 87M Protection Engineering Decision Table

The table is the short answer to which motors need differential protection, with the threshold that decides each row. The kW figures are indicative; the deciding input is what losing the motor costs.

Motor duty	Threshold that decides it	87M
LV motor, ≤ 1 kV, non-critical duty	Replacing the machine costs less than the CTs	Not normally
LV motor, large, process duty	Outage cost against six CTs and a commissioning day	Case by case
MV motor, 3–35 kV	Fault energy and rewind lead time	Recommended
MV/HV motor, roughly 2 MW and above	Winding repair cost and spare availability	Recommended
HV motor, > 35 kV	Treated as standard primary protection in most utility specs	Evaluate as standard
Compressor or pump on a critical process	Cost of a full process stop, not of the motor	Recommended
Power plant auxiliary (BFP, ID/FD fan, coal mill)	Whether losing it derates the unit	Recommended
Motor with a spare standing beside it	Switch-over time against fault-clearing time	Usually not

Key Engineering Checks Before 87M Relay Selection

When deploying a motor differential relay, four checks come before the model is chosen.

CT Matching and Arrangement

Protection-grade CTs belong on an 87M zone, not metering-grade ones: 5P20 is the usual minimum, and 0.2S or 0.5S are metering classes that will saturate during a motor start or an external fault. Both ends need the same ratio, class and polarity, and the kneepoint has to be checked against the starting current.

CT mismatch and saturation cause most 87M misoperations.

Motor Electrical Parameters

Take rated voltage, rated current, starting current, system frequency and earthing arrangement off the nameplate before setting anything. The starting current is the one that decides whether the CTs survive the first start.

Complete Protection Function Matching

87M covers the winding and nothing else.

The relay still has to carry 50/51, an earth-fault element, the thermal model and a negative-sequence element, because none of those can be replaced by adjusting the differential element.

Existing System Protection Coordination

Check upstream breaker protection logic, backup protection setting values, and switchgear configuration to avoid protection dead zones and mis-coordination.

87M Application Guide: Typical Industrial Scenarios

This 87M application guide section lists the industries where it turns up most often. The pattern is the same in each: continuous process, expensive stop.

Power Generation Industry

Typical duties: boiler feed pumps, cooling water pumps, induced draft fans and coal mill drives.

Oil & Gas Industry

Typical duties: process compressors, pipeline pumps and offshore platform drives.

Water & Wastewater Industry

Typical duties: high-power supply and drainage pumps, and pumping station main drives.

Mining & Heavy Industry

Typical duties: ore crushers, mills, and conveyor and ventilation fan drives.

Precision Manufacturing Industry

Typical duties: high-power constant-speed drives on core production lines.

Practical 87M Protection Engineering Application Case

Project background: a 10 kV cooling water pump motor on a thermal unit, classified as core auxiliary plant for the cooling circuit. Losing it derates the unit.

Protection challenge: the pump runs continuously, and a low-magnitude winding fault stays below the 51 pickup for long enough to turn a repair into a rewind.

Deployed scheme: 87M as primary protection, with 50/51, earth fault and thermal overload alongside it.

Application Effect: internal faults clear in about 20–40 ms instead of waiting on the 51 curve, so a winding fault becomes a repair rather than a rewind.

The 50/51 element stays in place for overload and cable faults.

87M Motor Rating Threshold: Is There a Fixed MW Limit?

No standard fixes a rating above which motors need differential protection.

Neither IEC 60255 nor IEEE C37.96 sets a kW figure, and any number quoted in a project spec is a house rule rather than a requirement.

The 87M motor rating threshold that a plant actually uses is derived, not quoted: it is the rating at which a rewind plus the outage costs more than six CTs and a relay.

Derive it once per plant and write it into the design basis.

FAQ: Motors Requiring Differential Protection

Q1. Which motors require differential protection?

A: MV and HV machines, drives that stop a process, and power plant auxiliary motors. Rating matters less than what losing the motor costs.

Q2. Do all MV motors need 87M protection?

A: No. Non-critical medium-power MV motors can adopt simplified protection schemes according to project cost and risk assessment.

Q3. At what motor size is differential protection required?

A: There is no global MW figure.

Which motors need differential protection is decided by voltage, criticality and rewind cost, and the rating only enters as a proxy for those.

Q4. Is 87M necessary for all large motors?

A: Size puts a motor on the list. Criticality decides whether it stays there.

Q5. What is 87M used for in motor protection?

A: Internal stator winding faults, including ones too small for an overcurrent element to see. Nothing outside the winding.

Q6. Can 50/51 overcurrent protection replace 87M differential protection?

A: No. Overcurrent protection has no zone, and it only responds once the fault current passes its pickup.

Internal faults below that level are invisible to it.

Q7. What CT requirements apply to 87M differential protection?

A: Matched protection-grade CTs at both winding ends, same ratio, same class, same polarity. If the neutral end cannot be reached, a 6-CT zone cannot be built at all.

Q8. Is 87M widely used for medium-voltage motors?

A: Yes, on core MV drives.

It is not fitted on every MV motor; the ones that get it are the ones with no spare and a long rewind.

Conclusion

Which motors need differential protection is settled by rating, voltage and criticality together, and by what a stop costs. 87M delivers its value where a winding fault has to be cleared before it becomes a rewind, which means MV and HV machines and the process-critical LV drives.

87M and 50/51 are specified together, not instead of each other: 87M takes the winding, 50/51 and the thermal model take everything the differential element cannot measure.

Once the duty is decided, the choice becomes a model question: see relay selection, or the 87M motor differential relay for the differential element itself.

Technical References

Source	What it covers	Type	Edition
IEC 60255-187-1	Functional requirements for differential protection of motors, generators and transformers	Standard	2021
IEEE C37.96	Guide for AC motor protection	Guide	2012
IEEE C37.110	Guide for the application of current transformers used for protective relaying	Guide	2023
IEC 61869-2	Instrument transformers – additional requirements for current transformers	Standard	2012
IEC 60255-1	Measuring relays and protection equipment – common requirements	Standard	2022
ANSI/IEEE C37.2	Device function numbers, acronyms and contact designations	Standard	2022

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