87M Motor Differential Protection Settings: Pickup, Bias and CT Parameters

87M protection settings are the parameters that decide when a motor protection relay trips: differential pickup, bias slope, CT ratio and class, high-set value and trip delay. Motor differential protection differs from overcurrent protection because the element compares two currents that ought to cancel. A few percent of CT error is therefore the whole problem. Set it wrong in either direction and the motor either trips on load or leaves a turn-to-turn fault invisible.

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

A complete 87M relay configuration covers six parameters: differential pickup threshold, bias and restraint characteristics, CT ratio and accuracy, rated motor current matching, high-set differential value, and protection operating delay. Each one comes from measured data, not from a catalogue default.

The sections below follow the order a commissioning engineer actually works in: what the relay compares, how to choose pickup and bias, what CT data the calculation needs, and how to prove the result on site. Worked examples use real nameplate values so the arithmetic can be checked line by line.

87M Protection Settings for Motor Differential Protection

87M motor differential protection settings refer to a series of configurable relay parameters that govern fault judgment, current calculation, and trip logic for motor winding differential protection. All motor differential relay settings are designed to balance two core engineering goals: capturing minor internal motor faults sensitively and avoiding maloperation during external faults and normal load fluctuations.

Seven items have to be configured before a scheme is complete. The middle column is what each one is for, not how to calculate it.

Setting Parameter	Engineering Purpose
Differential Pickup	Sets the minimum differential current threshold for relay operation to detect internal motor winding short-circuit faults
Bias / Restraint Characteristic	Suppresses false differential current caused by CT saturation and mismatch during high through-current external faults, ensuring protection stability
CT Ratio	Matches relay secondary current measurement with on-site actual CT transformation parameters to ensure accurate current sampling
CT Class / Accuracy	Defines CT error limits and saturation performance, providing basic data for pickup and bias setting calculation
High-Set Differential	Provides instantaneous trip output for severe internal motor faults to minimize equipment damage and fault expansion
Trip Time / Operating Delay	Realizes protection coordination with upstream and downstream devices and avoids maloperation from transient current fluctuations
Phase Assignment	Ensures one-to-one correspondence of inlet and outlet current phases for correct differential current comparison

How Does 87M Differential Protection Determine a Fault?

Every setting on this page comes from one comparison: operating current against restraint current. Fix those two quantities first and the rest follows.

Operating Current and Differential Current

The operating quantity is the differential current, the vector difference between the current entering and the current leaving the zone. The same expression applies to every motor differential scheme:

I_d = | I₁ − I₂ |

Where:

• I₁ = phase current entering the motor protection zone (CT primary inlet current)
• I₂ = phase current leaving the motor protection zone (CT primary outlet current)

In service the differential current says which of three states the motor is in. Healthy: the two currents cancel and I_d sits near zero. External fault: a small spill current appears from CT error alone. Internal winding fault: I_d rises well past the threshold within a cycle or two.

Operating Current vs Restraint Current

A bias restraint mechanism keeps the element stable when a through-fault drives the CTs toward saturation. Two currents decide the outcome:

• Differential/Operating Current: The unbalanced current generated inside the protection zone, which is the action trigger signal
• Bias/Restraint Current: The reference current dynamically calculated based on through-current, which improves the protection threshold proportionally with fault current

The restraint expression is not the same across manufacturers:

No single formula covers every relay. Take the restraint expression from the manufacturer’s manual for the device on site, and compare it to the characteristic curve before you use it.

87M Pickup and Bias: Setting the Differential Pickup

Differential pickup is the first number to fix. Everything else in the 87M pickup and bias group is built on top of it, because the slope only matters once something crosses the pickup.

What Is the Differential Pickup Setting?

Pickup is the differential current at which the element starts to operate, provided the restraint condition is satisfied at the same time. Set it too low and ordinary CT error trips the motor; set it too high and a turn-to-turn fault stays invisible.

Factors That Affect the Pickup Setting

Pickup is set above the worst spill current the motor can produce while healthy. Seven sources feed that number:

• CT inherent accuracy error and parameter mismatch
• CT saturation characteristics under high fault current
• Relay internal measurement and sampling error
• Normal three-phase load imbalance of the motor
• Transient current impact during motor starting
• On-site cable wiring deviation and contact resistance error
• Expected sensitivity requirements for minor winding turn-to-turn faults

How to Calculate the Initial Differential Current Setting

No single percentage works for every motor. The calculation follows one rule:

Minimum Differential Pickup Setting > Maximum Expected Spill Current + Engineering Safety Margin

Maximum spill current is what CT error, measurement deviation and load imbalance add up to at the worst normal operating point, including through-faults. The figure this rule returns is a floor. The relay characteristic curve, the measured CT performance and the project protection philosophy all move it from there.

Example of Differential Pickup Calculation

Worked below with a mid-voltage machine. Substitute your own nameplate data.

Basic Field Parameters:

• Motor rated current: 250 A
• CT ratio: 300/5 A
• Maximum CT mismatch error: 3%
• Relay measurement error: 2%
• Engineering safety margin: 5%

Calculation Process:

Maximum combined spill current error = 3% + 2% = 5% of rated secondary current

Minimum pickup threshold = 5% + 5% margin = 10% of CT secondary rated current

Final selected differential pickup setting: 0.5 A (10% × 5 A secondary rated current)

Note: Example values are for illustration only. All field settings must be verified against actual device and equipment parameters.

87M Bias and Restraint Settings

Bias is what keeps the element stable while the motor is healthy. Without it, every external fault would leave a small differential current from CT error, and the relay would have to choose between tripping on load and being blind to a real fault.

Bias as a Dynamic Threshold

Bias, or restraint, moves the operating threshold up as through-current rises. The reason is that CT error grows with current: at twice rated current a 5P20 CT is still inside its composite error, but at ten times rated it is not. Raising the threshold in step keeps that spill current away from the trip level.

Why Is Bias Needed for 87M Protection?

The point of bias is to separate the two fault directions:

• Internal Motor Fault: The current difference inside the protection zone increases sharply, the actual differential current exceeds the bias-adjusted threshold, and the relay operates correctly to trip
• External System Fault: The through-current increases sharply, CT saturation generates unbalanced spill current, and the rising bias threshold offsets the false differential current to prevent maloperation

Single-Slope vs Dual-Slope Differential Characteristics

Most industrial 87M relays offer two characteristic shapes. The choice depends on how wide the gap is between normal load current and the worst through-fault current.

• Single-Slope Characteristic: Applies to conventional low-power motors. The bias ratio is fixed in the full current range, with simple configuration and stable performance
• Dual-Slope Characteristic: Adapts to medium and large critical motors. It sets a low slope for low-current regions to ensure minor fault sensitivity and a high slope for high-current regions to resist CT saturation interference, with a set transition current point for slope switching

How to Select the 87M Bias Setting

Factors Used to Select Bias

Five inputs set the bias ratio:

• Maximum through-fault current of the motor branch
• CT saturation limit curve and accuracy class
• CT secondary burden resistance and lead wire resistance
• Maximum allowable CT mismatch error
• Built-in algorithm characteristics of the 87M relay

Higher Bias vs Lower Bias

Bias trades sensitivity for stability. Both ends of the range have a cost:

Bias Setting	Advantage	Potential Risk
Lower Bias	Higher sensitivity to minor internal winding faults	Small stability margin, prone to maloperation under severe CT saturation
Higher Bias	Excellent stability for external faults and strong anti-interference ability	Reduced sensitivity, possible missed detection of minor turn-to-turn faults

Core Principle: bias is a trade, not a value to optimise. Raise it until external faults stop producing spill trips, then confirm that a 5% turn-to-turn fault still crosses the operating characteristic.

87M CT Parameters: Ratio, Class and Burden

The 87M CT parameters are the data every other setting is calculated from. Ratio, class and burden decide how much spill current the element has to tolerate, and a mismatch between the relay record and the CT nameplate is the most common reason a scheme misoperates on the first day. Wiring and scheme choice are covered in the motor differential protection CT wiring article; this section stays on the numbers.

CT Ratio

Industrial motor CTs are commonly 100/5 A, 200/5 A or 400/5 A, but the figure that matters is the one on the nameplate of the CT actually installed. The ratio entered in the relay has to match it exactly. A ratio that is off by one step converts every sample wrongly and leaves a standing differential current that looks like a permanent internal fault.

CT Class and Accuracy

Protection-grade CTs, usually 5P20 or 10P20, are mandatory here. The class fixes the composite error at the rated accuracy limit factor, and that figure sets the floor for both pickup and bias. A measurement-grade CT cannot be used: its accuracy is quoted at rated current only, and it saturates well before the through-fault current this element has to ride through.

CT Secondary Current

CT secondaries come in 1 A and 5 A. The relay input module and its configuration have to match what is installed. Mixing the two, say a 1 A setting on a 5 A circuit, scales every reading and the differential current by five. That is not a subtle error.

CT Burden and Lead Resistance

Saturation depends on the secondary burden, and the burden is not just the relay. Lead resistance, terminal-block contact resistance and any other load in the loop all add to it, and all of them grow the CT error once the through-current is high. Measure the loop on site rather than reading it off the drawing, and put the measured value into the bias calculation.

CT Polarity

Reversed polarity leaves a standing differential current as soon as the motor is loaded, and that is usually the first thing a commissioning team sees. The check itself is a wiring task rather than a setting one. The method is set out in motor differential protection CT wiring, together with the self-balancing scheme alternative.

CT Parameters and Differential Current Calculation

Before you can judge whether a setting is right, you have to reproduce the numbers the relay sees. Two conversions do that.

Converting Primary Current to CT Secondary Current

Standard primary-secondary current conversion formula for CT:

Matching CT Parameters With the Relay

The matching chain runs from the motor terminals to the trip output:

1. Actual primary current at the motor terminals
2. Conversion through the on-site CT ratio
3. Standard secondary current at the relay input
4. Relay sampling and differential calculation
5. Comparison against the preset pickup and bias thresholds
6. Fault decision and trip output

Calculating Differential Current From CT Inputs

A worked check on sampled data:

Inlet CT secondary current reads 2.45 A; outlet reads 2.42 A.

I_d = | 2.45 − 2.42 | = 0.03 A

At 0.03 A the element sits far below any usable pickup, so this motor is healthy. Push the imbalance past the pickup at that restraint level and the same arithmetic produces a trip.

Recommended 87M Settings for Different Motor Applications

There is no single 87M setting that suits every motor. Rated current, fault level and how critical the drive is all move the pickup and the slope, so the three cases below are starting points rather than answers.

Medium-Voltage Industrial Motors

These are the easiest motors to set, because both the fault level and the starting duty are moderate. Pickup and bias sit near the middle of the usable range. The setting that needs attention is time coordination with the upstream overcurrent element, so the 87M clears an internal fault before the upstream device sees it.

Large Critical Motors

Power plant main motors and process-critical drives justify the tighter end of the range. Pickup comes down to catch minor faults, the dual-slope characteristic keeps stability at the high end, and the CTs have to be 5P-class with a measured burden. Where an unintended trip stops the plant, duplicated trip logic is usually worth the extra hardware.

Motors With High Starting Current

Starting restraint works differently from overcurrent protection. A motor drawing six times rated current pushes six times rated current through both CT sets at once, so the element is asked to stay stable at six times its normal burden. Work the numbers before accepting a setting.

Take a 280 A motor with 6.5 times starting current and 300/5 A CTs. Starting primary current is 1,820 A, or 30.3 A on the secondary side. If one CT reads 3% low and the other 3% high, the spill current is 6% of 30.3 A, which is 1.82 A. A pickup of 0.5 A would trip on every start. The bias characteristic has to lift the threshold above 1.82 A at that restraint level, or the pickup has to be raised to suit. Check that against the manufacturer’s slope curve rather than assuming it.

87M Protection Settings Example

One complete case, carried from the nameplate through to the settings sheet.

Motor and CT Basic Data

Parameter	Example Value
Motor Rated Voltage	6.6 kV
Motor Rated Current	280 A
CT Ratio	300/5 A
CT Secondary Rating	5 A
CT Class	5P20
Maximum Branch Fault Current	20 kA

Final 87M Setting Table

Setting Parameter	Selected Value	Setting Basis
Differential Pickup	0.5 A	CT combined error + engineering safety margin
Slope 1 (Low Current)	10%	Ensure minor fault detection sensitivity
Slope 2 (High Current)	25%	Resist high-current CT saturation interference
High-Set Differential	5 A	Instant trip for severe short-circuit faults
Trip Delay	0 s	Fast clearing of internal faults

Disclaimer: Actual settings must be calculated and verified against the specific relay manufacturer’s application manual and the motor protection study report of the project.

87M Settings for Internal and External Faults

The three states below are how you tell whether 87M protection settings are correct, by reading the differential current rather than the settings sheet.

Normal Motor Operation

Three-phase current is balanced, the differential current sits inside the CT error band, and the element stays quiet. This is the state the whole bias design has to hold.

External Fault

Through-current rises sharply and one CT starts to saturate, so a spill current appears. The bias threshold rises with it and stays above the spill, and that is the only thing keeping the element from tripping on a fault outside its zone.

Internal Motor Fault

A winding short circuit unbalances the zone. The differential current crosses both the pickup and the bias-restrained threshold, and the relay trips. There is no ambiguity in this case, because the two currents stop cancelling altogether.

Common Mistakes When Setting a Motor Differential Relay

These six mistakes account for most 87M commissioning problems we see, roughly in the order they occur.

• Overly Low Pickup Setting: Excessively sensitive threshold amplifies CT mismatch and measurement error, leading to frequent nuisance tripping during normal operation and motor starting
• Overly High Pickup Setting: Excessively conservative threshold reduces protection sensitivity, resulting in failure to detect minor turn-to-turn faults and latent equipment hazards
• Ignoring CT Saturation Characteristics: Fixed bias setting without considering high-current saturation leads to external fault maloperation
• Mismatched CT Ratio Configuration: Inconsistent relay parameters and on-site CT actual parameters cause long-term false differential current
• Neglecting CT Polarity Verification: Wrong polarity leads to current superposition error, triggering abnormal protection action
• Blind Copy of Existing Settings: Motor rated current, CT parameters, and system fault level vary in different projects. Copied settings cannot adapt to on-site conditions

How to Test and Verify 87M Protection Settings

No setting is confirmed until it has been injected and observed. The four tests below move from static checks to a full trip, and each one depends on the previous.

Verify CT Parameters

Check ratio, secondary rating, accuracy class, polarity and wiring continuity against the nameplate and the drawing. Errors found here invalidate every test that follows.

Secondary Injection Test

Inject current into the secondary loop and read back three things: phase sampling accuracy, the differential value the relay calculates, and where the pickup and the slope actually operate. Comparing the operated points to the characteristic curve is what confirms the settings, not the reading on its own.

Differential Protection Stability Test

Drive both inputs with the same high current and confirm the element does not operate. This is the check that catches a bias set too low.

Differential Protection Trip Test

Drive one input only and confirm the element operates, the trip contact closes, the breaker trips, and the event record shows the currents that caused it.

87M Protection Settings Checklist

Three groups of checks, in the order a commissioning team normally runs them. Anything unresolved in the first group invalidates the other two.

CT Data Verification

• CT ratio configuration consistent with site actuals
• CT secondary current specification (1A/5A) matched
• CT accuracy class meets protection requirements
• CT polarity fully verified and correct
• CT secondary burden and lead resistance qualified

Relay Setting Verification

• Differential pickup value calculated and set correctly
• Bias/slope parameters matched with fault level
• High-set differential threshold configured reasonably
• Trip delay and phase assignment accurate

Commissioning Test Verification

• CT polarity test passed
• Secondary injection calibration completed
• External fault stability test qualified
• Internal fault pickup and trip test qualified

Frequently Asked Questions About 87M Settings

What are the main 87M protection settings?

Six, and they work as a set: differential pickup, bias or restraint slope, high-set differential value, trip delay, CT ratio and class, and phase assignment. Together they fix the fault judgement logic and the operating time.

How is the differential current setting calculated?

Start from the worst spill current a healthy motor can produce, which is CT error plus measurement deviation plus load imbalance, and add a margin. There is no universal percentage. The figure is then corrected against the measured CT performance and the relay characteristic.

What is bias in motor differential protection?

Bias raises the operating threshold as through-current rises, so the spill current from CT saturation never reaches the trip level during an external fault. Without it, the element would have to be set blunt enough to ignore that spill, which would blind it to real internal faults.

How do you select the pickup setting for a motor differential relay?

Pickup has to sit above the total spill current from CT accuracy, saturation, relay measurement error, load imbalance and starting transients, with a margin on top. Then check that the value still catches the smallest fault the scheme is required to detect.

What CT parameters are required for 87M protection?

Ratio, secondary rating (1 A or 5 A), accuracy class, accuracy limit factor, measured secondary burden, and polarity. Each of them feeds straight into the pickup or the bias calculation.

Why is CT saturation important for 87M settings?

A saturating CT stops transforming correctly, so the two currents the relay compares no longer match even though the motor is healthy. The resulting spill current is a false differential current, and only the bias characteristic stands between it and a spurious trip.

Can the same 87M settings be used for different motors?

No. Rated current, CT ratio, fault level and starting duty all differ, and pickup and bias are derived from them. A copied setting sheet is either too sensitive for the new CTs or too blunt for the new fault level, and neither failure shows up until the motor faults.

Conclusion

87M protection settings come down to three decisions: where the pickup sits above the worst spill current, how far the bias slope rises above that, and whether the CT data used for both matches the nameplate. Set them from measured values, then prove them with a stability test and a trip test before the motor is energised.

87M Motor Differential Protection Relay

A differential element is only as good as the current measurement behind it. What matters at the relay level is a configurable CT ratio and class, a switchable single or dual-slope characteristic, and stored event records that let you see the sampled currents from the trip.

Our 87M motor differential protection relay covers the settings discussed here: configurable CT ratio and class, single or dual-slope bias, and event records that show the sampled currents behind each trip. It is built for medium and large MV motors in plants and pumping stations. Product data and ordering information are on the product page.

Technical References and Data Sources

Every figure above traces back to one of these documents. Where a manufacturer’s manual and a standard disagree, the manual wins for that relay.

Related reading: motor protection relay setting calculation.

Source	What it covers	Type	Edition
IEC 60255-187-1	Functional requirements for differential protection, including motor schemes	Standard	2021
IEC 61869-2	Current transformers: accuracy class, composite error and burden	Standard	2012
IEEE C37.96	Guide for AC motor protection, including differential applications	Guide	2012
IEC 60255-1	Measuring relays and protection equipment, common requirements	Standard	2022
ANSI/IEEE C37.2	Device function numbers, definition of the 87M function	Standard	2022