Motor Differential Protection CT Wiring: 6-CT and Self-Balancing Schemes

Motor differential protection CT wiring is the secondary circuit behind ANSI 87M, the most selective form of motor differential protection for internal stator winding faults in large industrial motors.

Unlike general overcurrent and thermal protection, 87M relies entirely on an accurate current comparison at both ends of the winding. That comparison is what gives it zone selectivity.

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

Improper CT wiring, polarity mismatch, or incorrect scheme selection are the top causes of 87M relay nuisance tripping, protection failure, and commissioning delays. Industrial and EPC engineers primarily adopt two mature CT wiring schemes for 87M protection: the 6-CT discrete scheme and the self-balancing core-balance scheme.

Six CTs give a per-phase comparison, so the relay can name the faulted phase. One core-balance CT gives a single aggregate instead.

Most of the practical differences between the two schemes follow from that.

What Is Motor Differential Protection CT Wiring?

Motor differential CT wiring is the secondary circuit that connects the CT sets at the motor terminal side and the neutral side to the 87M relay. Closing that loop is what lets the relay compare the current entering the winding zone with the current leaving it, and act on the difference.

An 87M element cannot work without matched motor differential protection CT wiring.

Basic overcurrent protection (ANSI 50/51) samples current at one point only, so it cannot separate an internal motor fault from a downstream system fault.

Dual-end sampling is what gives the differential element its zone selectivity.

How CTs Are Used in 87M Motor Protection

Five elements sit in series between the winding and the trip command. Break any one of them and the protection does not operate:

1. Motor winding zone under protection
2. Two matched CT sets, one at each end of that zone
3. Secondary wiring into the 87M relay differential inputs
4. Relay trip output
5. Circuit breaker trip coil

Key CT deployment and circuit rules for 87M protection:

• Motor terminal side CT: Sampling three-phase inbound operating current of the motor
• Motor neutral side CT: Sampling three-phase outbound current after winding operation
• CT secondary circuit: Transmitting standardized 1A/5A secondary current signals to the 87M relay
• Differential current calculation: The relay calculates the vector difference between dual-end currents to judge fault status

What Does the Motor Differential CT Measure?

Motor differential CT focuses on vector comparison of dual-end three-phase currents, with distinct operating characteristics under three working conditions:

• Normal load condition: Inbound and outbound currents are basically balanced, with only tiny differential current generated by CT ratio error, and the 87M relay remains inactive
• External system fault: Dual-end currents change synchronously, the differential loop maintains balance, and the protection does not operate (zone selectivity)
• Internal motor winding fault: Local short-circuit causes current imbalance inside the protected zone, obvious differential current is generated, and the 87M relay triggers tripping

Basic Principle of 87M CT Wiring

Motor differential protection CT wiring is built on one rule: what flows in must flow out. Kirchhoff’s current law applies to the winding zone as a whole, so the relay only has to watch the residue.

Differential Current in a Healthy Motor

When the motor operates normally or bears rated load, the total current entering the motor winding zone is equal to the total current leaving the zone. After secondary conversion by paired CTs at the terminal and neutral sides, the secondary currents offset each other in the relay differential loop.

Under this condition, the residual differential current is far lower than the relay pickup threshold, so the 87M protection is locked and no tripping command is issued.

Differential Current During an Internal Motor Fault

When phase-to-phase short circuit, inter-turn short circuit, or phase-to-ground fault occurs inside the motor stator winding, the current balance of the protection zone is broken. Fault current flows inside the winding, causing inconsistent sampling values of terminal-side and neutral-side CTs.

A large valid differential current is generated in the secondary loop.

Once the current exceeds the 87M relay’s restrained pickup value, the protection acts instantly to cut off the motor power supply and avoid winding burnout.

Why CT Polarity Matters

CT polarity is the primary factor determining the validity of 87M differential wiring and is a key inspection item for on-site commissioning. All protection-grade CTs follow unified polarity marking standards (P1/P2 primary terminals, S1/S2 secondary terminals) per IEEE C57.13 and IEC 61869-2.

Correct wiring requires consistent current vector directions of paired CTs at the terminal and neutral sides.

Reversed S1/S2 wiring or mismatched P1/P2 installation will generate false differential current under normal operating conditions, directly causing nuisance tripping of 87M protection and affecting motor normal operation.

6-CT Motor Differential Scheme: Wiring and CT Placement

A 6-CT motor differential scheme is the default above roughly 3 kV.

Six protection-grade CTs, one pair per phase, let the relay compare each phase on its own and name the phase that faulted.

The figure above shows where the two CT sets sit relative to the winding zone. Everything outside that zone is external as far as the relay is concerned.

What Is a 6-CT Differential Protection Scheme?

The 6-CT configuration deploys three independent CTs at the motor terminal side (A/B/C three phases) and another three matched CTs at the motor neutral side, forming a 6-CT dual-end sampling system in total. All CTs are protection-grade with consistent ratio and accuracy class, defining an independent and complete motor winding protection zone.

How the 6-CT Scheme Works

The scheme adopts phase-by-phase independent current comparison logic:

1. Terminal-side CTs sample three-phase inbound current of the motor
2. Neutral-side CTs sample three-phase outbound current of the motor winding
3. Secondary currents of CTs with the same phase are connected to the corresponding differential input terminals of the 87M relay
4. The relay compares real-time current vectors of each phase; unbalanced current triggers differential protection action

6-CT Differential CT Connection Standard

Five rules cover almost every motor differential protection CT wiring error seen on site.

All five are checkable with a meter before energisation, and cheap to check.

• Phase A terminal CT matches Phase A neutral CT, connected to relay IA differential terminals
• Phase B terminal CT matches Phase B neutral CT, connected to relay IB differential terminals
• Phase C terminal CT matches Phase C neutral CT, connected to relay IC differential terminals
• Unified S1 as the positive terminal and S2 as the negative terminal for all CT secondary wiring
• Single-point grounding of CT secondary loop to avoid ground loop interference

Self-Balancing Differential Scheme: Core-Balance CT Wiring

A self-balancing differential scheme passes all three phase conductors through one window-type core. There is no second CT set to match, so the whole class of ratio and polarity mismatch disappears.

The trade-off is what one core can measure.

It sees only the sum of the three phase currents, so a fault that leaves that sum at zero is invisible to it.

The self-balancing motor differential protection relay is built on this arrangement.

What Is a Self-Balancing CT Scheme?

An integrated window-type core-balance CT sits around all three phase conductors.

It is not the same device as a zero-sequence CT on a feeder: this one is the only measuring element in the zone, and it replaces six discrete CTs entirely.

There is no phase-by-phase secondary wiring to get wrong, and no polarity to check. One core, one circuit.

How Self-Balancing Differential Protection Works

• Normal operating condition: the three phase currents are balanced, their fluxes cancel inside the core, and the secondary output sits close to zero with no differential action
• Internal earth fault: current finds a path to earth, the sum of the three phase currents is no longer zero, and the residual flux drives a secondary current the relay can act on
• Phase-to-phase fault without earth: the sum of the three phase currents stays at zero, so a single core-balance element has no operating quantity for it

Self-Balancing CT Connection Specifications

• Standard penetrating layout of three-phase motor conductors through the CT window
• Unified secondary lead wiring to 87M relay differential signal terminals
• Strict single-point grounding of CT secondary circuit to prevent induced voltage interference
• Fixed CT installation position to avoid conductor offset affecting magnetic balance accuracy

6-CT vs Self-Balancing CT Scheme: Full Engineering Comparison

Every row below follows from the same cause: how many measuring elements sit inside the zone. Neither column is better in general.

The 6-CT scheme has to tolerate a few percent of ratio mismatch between its two CT sets, and the bias characteristic is what absorbs it; the core-balance scheme has no mismatch to absorb, but also no second measurement to cross-check.

Feature	6-CT Scheme	Self-Balancing Scheme
Number of CTs	6 discrete protection-grade CTs	1 integrated core-balance CT
CT Installation Location	Motor terminal side + neutral side	Around motor three-phase conductors
Measurement Logic	Phase-by-phase independent current differential comparison	Overall three-phase magnetic balance detection
Wiring Complexity	High, multiple secondary circuits and phase matching	Low, simplified secondary wiring structure
Installation Workload	Large, requiring dual-side CT fixed installation	Small, integrated penetrating installation
Protection Accuracy	High, supports precise single-phase fault location	Medium, only judges overall winding faults
Commissioning Difficulty	Strict polarity and phase sequence verification required	Low, focuses only on CT installation position
Maintenance Cost	High, requiring inspection of 6 CT circuits	Low, simple integrated structure maintenance
Applicable Scenarios	Large/medium-voltage critical industrial motors	Compact layout and conventional medium motors

87M CT Wiring Diagram: Standard Connection Points

An 87M CT wiring diagram has to carry four things a single-line diagram leaves out: CT primary direction, secondary polarity, which relay terminals each pair lands on, and where the secondary is earthed.

6-CT 87M Wiring Diagram Key Marks

The standard 6-CT diagram shows every node the commissioning team will actually touch:

• Motor main body and A/B/C three-phase primary loop
• Terminal-side three-phase CT group and neutral-side three-phase CT group
• CT primary P1/P2 direction and secondary S1/S2 polarity terminals
• Secondary wiring correspondence between CT groups and 87M relay differential terminals
• Differential current transmission path and secondary single-point grounding position

Self-Balancing CT Wiring Diagram Key Marks

• Integrated CT core and three-phase conductor penetrating layout
• CT secondary signal output terminals
• Wiring path from CT secondary to 87M relay
• Unified secondary grounding protection point

CT Secondary Wiring to the 87M Relay: Standard Specifications

• Strictly distinguish phase sequence labels to avoid cross-phase connection
• Standard relay terminal numbering matching, no random wiring crossover
• CT secondary circuit equipped with test shorting terminals for convenient commissioning
• Shielded secondary wiring for high-interference industrial environments
• Forbid open-circuit operation of CT secondary loop to prevent high-voltage breakdown

CT Polarity and Phase Sequence in Motor Differential Protection

Polarity and phase-sequence mistakes cause more 87M commissioning failures than any other wiring fault, and neither shows up on a continuity check.

Three rules settle them.

Correct CT Polarity Configuration

All paired terminal-side and neutral-side CTs must maintain consistent polarity direction: P1 faces the power supply side, P2 faces the motor load side; secondary S1 is the positive sampling terminal, S2 is the negative terminal.

The polarity configuration of corresponding phase CTs must be completely unified. A six-CT scheme has three phase pairs, and reversing either end of one pair produces a differential current of twice the load current in that phase, so polarity is verified pair by pair with the motor isolated rather than inferred from a load reading.

Phase Matching Rules

Absolute one-to-one correspondence of three-phase wiring must be guaranteed: Phase A CT group matches relay A-phase differential input, Phase B corresponds to B-phase, Phase C corresponds to C-phase.

Cross-phase connection will completely destroy differential current balance logic.

Fault Consequences of Reversed CT Polarity

Reversed CT polarity generates continuous false differential current under normal motor load. Wrong polarity trips on load.

When the current exceeds the relay threshold, the 87M protection will trigger nuisance tripping. In severe cases, it will cause commissioning failure and long-term unstable motor operation.

How to Test Motor Differential CT Wiring Before Commissioning

Every test below is finished and recorded before the motor is energised.

The CTs are checked to IEC 61869-2 and IEEE C57.13; the relay measurement chain is checked against IEC 60255-1.

CT Ratio Verification

Compare the ratio on the CT test certificate with the nameplate, not with the drawing. Both ends have to match.

A 5P20 CT is the usual minimum for an 87M zone; below that, transient performance during motor starting has to be checked separately. Ratio error at rated current should sit inside 1% for a protection class CT.

CT Polarity Test

Use a professional polarity tester to verify primary and secondary polarity consistency of each CT, confirm S1/S2 wiring correctness, and ensure unified vector direction of paired CTs.

Continuity and Insulation Checks

Check continuity first, then insulation to earth, and add the lead resistance into the CT burden before assuming the declared class still holds.

Lead length is part of the burden.

A long run back to the relay panel can push a correct CT outside its accuracy class on paper.

One earth point for the whole secondary circuit.

Secondary Injection Test

Inject a known current into each phase pair and read back the differential current the relay reports.

With the same current injected on both sides of one phase, the differential reading should sit close to zero.

A reading near twice the injected current means one CT is reversed.

Functional Trip Test

Simulate internal motor fault signals and verify relay pickup, alarm output and breaker trip linkage. Trip testing is the only stage that proves the whole chain end to end, because a correct relay output that never reaches the breaker trip coil leaves the motor just as unprotected as a relay that never picked up.

Common Motor Differential CT Wiring Problems & Symptoms

Fault Type	Core Symptoms	Engineering Hazards
Reversed CT Polarity	High stable differential current under normal load	Frequent nuisance tripping
Incorrect Phase Connection	Unbalanced differential current, irregular fluctuation	Protection failure or misoperation
CT Ratio Mismatch	Residual differential current exceeds standard range	Reduced protection sensitivity
Open CT Secondary Circuit	Abnormal relay sampling, no current signal	High-voltage breakdown, equipment damage risk
Multiple CT Grounding	Interference differential current generation	Unstable protection operation
Wrong Relay Terminal Connection	Phase sampling disorder, invalid differential data	Complete protection function failure

How to Choose Between 6-CT and Self-Balancing Scheme

When to Adopt 6-CT Scheme

• Medium/high-voltage large-capacity critical motors (above 3kV)
• High-value core process motors requiring precise fault location
• Projects with reserved dual-side CT installation conditions
• Industrial scenarios requiring high protection stability and anti-interference

When to Adopt Self-Balancing Scheme

• Medium and small-sized conventional industrial motors
• Compact equipment layout with limited installation space
• Projects requiring simplified wiring and reduced maintenance workload
• General industrial scenarios without precise single-phase fault location requirements

Key Pre-Selection Check Items

Match the scheme to the switchgear that already exists.

A retrofit with no room at the neutral end cannot take six CTs, and no setting will fix that. Where both ends are accessible, the deciding inputs are motor rating, the CT class available, and whether the operator needs the faulted phase identified.

Engineering Application Example: 6.6kV 6-CT 87M Protection Scheme

Illustrative values, not a fixed design. Confirm against the motor nameplate and the CT test certificates.

• Protected object: 6.6kV industrial large AC motor
• Protection scheme: Standard 6-CT phase-separated differential configuration
• CT configuration: 6 sets of unified ratio 5P20 protection-grade CTs
• Core logic: Dual-end current vector comparison, restrained differential protection
• Normal operating state: Residual differential current ≤ 0.02In, protection locked
• Fault state: Internal winding short circuit generates obvious differential current, 87M relay trips instantly to isolate faults

Motor Differential Protection CT Wiring Commissioning Checklist

Before Wiring

• Confirm CT ratio and protection accuracy class compliance
• Verify CT installation position and dual-side layout scheme
• Unify CT polarity marking standards
• Sort out three-phase phase sequence identification labels

During Wiring

• Strict A/B/C one-to-one phase matching, no cross connection
• Standard S1/S2 polarity wiring for all CTs
• Accurate relay terminal docking, clear wiring marks
• Standard single-point grounding of secondary circuit
• Keep CT test shorting terminals available

Before Energization

• Complete CT polarity and ratio verification test
• Secondary circuit continuity and insulation test
• Secondary injection functional verification
• Relay differential current sampling calibration
• Trip circuit linkage function test

FAQs About 87M CT Wiring

1. How many CTs are required for motor differential protection?

Two configurations are available: 6 discrete CTs for standard phase-separated differential protection, and 1 integrated self-balancing CT for compact differential protection, selected according to motor voltage level and importance.

2. What is 87M CT wiring?

87M CT wiring is the secondary connection that lets an ANSI 87M relay compare winding current at both ends of the motor. Without it there is no differential quantity to measure.

3. Where are CTs installed for 87M protection?

In the 6-CT scheme one set sits at the motor terminal side and the matched set at the neutral side. The self-balancing scheme has no second set: all three phase conductors pass through one core at the terminal box.

4. Why adopt 6-CT differential configuration?

The 6-CT scheme compares each phase on its own, so a single-phase fault can be located rather than just detected. That matters on high-value motors where the operator wants to know which phase failed.

5. What are the consequences of reversed CT polarity?

Reversed polarity produces a false differential current even with the motor running normally. Once it passes the pickup threshold the relay trips on load, so the motor cannot be started.

6. What is a self-balancing CT for motor protection?

It is one window-type CT with all three phase conductors passing through it. It responds to the sum of the three currents, so the wiring is simple, but the blind spot described above still applies.

7. How to test 87M CT wiring on site?

Work through five checks in order: ratio verification, polarity test, continuity and insulation, secondary injection, then trip linkage. The order matters, because each one assumes the previous one passed.

Relay Inputs for Both CT Schemes

Whichever scheme the plant can physically build, the relay has to accept it. Six independent differential inputs cover the 6-CT arrangement; one core-balance input covers the self-balancing arrangement, on the same device.

Overcurrent, thermal overload, locked-rotor and earth-fault elements sit alongside the differential element, so a motor does not need a second relay for the faults 87M is not meant to see.

Both schemes are supported on the 87M motor differential protection relay.

Conclusion

Motor differential protection CT wiring comes down to one question: can the plant fit two CT sets?

If it can, the 6-CT scheme buys per-phase detection and is the safer choice above about 3 kV.

If it cannot, a core-balance scheme is the only realistic option, and its blind spot has to be covered by a separate earth-fault element.

Whichever route is taken, four checks catch most failures: ratio, polarity, phase correspondence, single-point earthing. Do them before energisation and record the results.

Technical References

Source	What it covers	Type	Edition
IEC 60255-187-1	Functional requirements for differential protection of motors, generators and transformers	Standard	2021
IEC 61869-2	Instrument transformers – additional requirements for current transformers	Standard	2012
IEEE C57.13	Standard requirements for instrument transformers	Standard	2016
IEEE C37.96	Guide for AC motor protection	Guide	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

Related reading: 87M protection settings.