Undercurrent and underpower protection 37 (PTUC)

Short answer

Protection 37 (IEEE C37.2: undercurrent or underpower relay) operates when the current or the active power of a motor drops below a set value for a set time. It detects loss of load: a pump running dry or cavitating, a broken conveyor belt, a fan with its damper closed. In IEC 61850 it is modeled with PTUC (undercurrent) or PDUP (underpower).

What the 37 protects

In IEEE C37.2, device number 37 is the undercurrent or underpower relay: it operates when the current or the power flow drops below a predetermined value. It does not react to an electrical fault but to an abnormal process condition: the motor is still healthy, but the machine it drives no longer demands work.

That loss of load almost always announces mechanical damage on the way:

Pumps running dry or cavitating. Without liquid, mechanical seals and product-lubricated bearings heat up quickly. Submersible well pumps have the same problem, and the equipment is out of sight.
Conveyors with a broken belt or a loose coupling.
Fans and compressors with the suction damper or valve closed, or with a broken drive belt.
Broken couplings or shafts on any driven machine.

The 37 is part of the motor protection relay, together with thermal overload 49 and locked rotor protection 51LR and 48. On generators, forward underpower is used in sequential tripping logic; that case, together with reverse power, is covered in reverse power protection 32.

Where it is applied

Equipment	LV	MV	HV and EHV	Notes
Pump motor	Yes, in motor relays and electronic starters	Yes, in the motor relay of the MCC or switchgear	Not applicable	Main application: dry running and cavitation
Submersible well pump	Yes, often in the drive	Depends on design	Not applicable	The equipment cannot be seen; the 37 is the early warning
Conveyor, fan, compressor	Yes	Yes	Not applicable	Broken belt, closed damper
Generator	Rarely	Yes, as forward underpower	Yes	Sequential tripping with the 32
Feeder, transformer, line	Not usual	Not usual	Not applicable	Low current does not indicate damage on this equipment

It is an industrial function: mining, cement, water and wastewater, oil and gas, pulp and paper, and food.

How it measures

Current-based 37. The relay takes the phase currents from its current transformers (CTs) and compares the fundamental value with the threshold. Depending on the manufacturer it uses the lowest of the three phases, the average or the positive sequence. It needs only CTs.

Power-based 37. The relay calculates three-phase active power from the voltages of its voltage transformers (VTs) and the phase currents. It needs CTs and VTs.

The difference matters. The no-load current of an induction motor is largely magnetizing current, so between loaded and unloaded running the current drops less than one would expect, especially in lightly loaded motors or motors oversized for their pump. Active power does drop clearly when the load disappears. That is why power-based 37 is the more reliable option when the margin between the minimum normal load and no load is narrow.

Blocking. With the motor stopped the current is zero, which is below any threshold. The function is armed only when the motor is running: through the breaker or contactor status, or a minimum «motor stopped» threshold below which the relay does not operate. It is also blocked during starting, so the pump has time to prime and the process to stabilize.

The characteristic

The characteristic is definite time, on the current (or power) versus time plane:

A normal operating band, between the minimum and maximum process load.
A low threshold: if the quantity drops below it and stays there longer than the time delay, the function operates.
A motor-stopped threshold: below it, the relay considers the motor stopped and does not operate.
A starting block window, counted from the moment the start is detected.

Many relays offer two stages: alarm with a short delay and trip with a longer one. Reset hysteresis prevents the function from chattering when the load hovers around the threshold.

Setting criteria

Threshold: below the minimum normal process load and above the no-load or dry-running load, measured or estimated from pump and motor data. Ideally it is confirmed with real operating records.
Time delay: long enough to ignore process transients (valve operations, level changes) and shorter than the time the machine tolerates without load.
Starting block: at least the starting and priming time.
Motors on drives: the threshold must hold over the whole speed range, or supervision is left to the drive.
Restart: if the process needs a wait before restarting (for example, for a pump to refill), it is coordinated with the relay’s starting logic.

Interaction with other functions

48 and 51LR: start supervision defines when the start ends and, therefore, when the 37 is armed.
49 and 66: a 37 trip does not heat the motor, but the restart inhibit of the 66 and the thermal image of the 49 still apply to the next start.
VT supervision (60): if a voltage is lost, the calculated power drops and power-based 37 would see a false loss of load. Loss-of-potential supervision must block it, as explained in loss of potential alarm from a blown VT fuse.
27: on undervoltage, the motor is protected by undervoltage 27; the 37 should not be the function that acts.
32 on generators: forward underpower and reverse power are combined in sequential tripping.
Outputs: the alarm goes to the process control system; the trip goes to the motor contactor or breaker, usually without going through the lockout relay 86, because the motor has no fault.

How each manufacturer implements it

ABB Relion 615: the REM615 offers it as loss of load supervision, with the LOFLPTUC logical node.
SEL: motor relays include load-loss elements; the IEC 61850 conformance statement of the SEL-849 declares the PTUC and PDUP nodes.
Siemens SIPROTEC, GE Vernova (Multilin) and Schneider (MiCOM, Easergy): undercurrent or underpower elements, with different names, measured quantities and blocking logic.

The practical differences are which quantity each element uses (lowest phase, average, positive sequence or power), how it detects a stopped motor and from when it counts the starting block.

IEC 61850 logical node

IEC 61850-7-4 has two nodes that fit the 37:

PTUC, undercurrent protection.
PDUP, directional underpower protection: it operates when power in the set direction drops below the threshold.

Both publish the start (Str) and operate (Op) signals. The operate signal goes through PTRC, which conditions the trip to the switching device node (XCBR), by hardwiring or by GOOSE. The first-stage alarm usually goes to the control system as one more motor signal, by report or by GOOSE.

How it is tested

In general, secondary injection is used to verify:

The pickup and reset, lowering the quantity from the normal band.
For power-based 37, the threshold at different angles between voltage and current.
The time delay of each stage.
That the function does not operate with the motor stopped (current below the motor-stopped threshold, or breaker open).
That it does not operate during the starting window and does operate after it.
The loss-of-potential block, if the function is power based.

During commissioning, the setting is compared with the actual motor load at the minimum process operating point.

Typical mistakes

Threshold below the no-load current: the function never operates and the pump runs dry.
Threshold above the minimum normal load: trips whenever the process reduces production.
Using current on a lightly loaded motor, where the difference between no load and load is small.
Missing motor-stopped block, so the alarm or trip appears with the motor stopped, or a block that prevents the next start.
CTs sized only for starting current, which measure the no-load current at the bottom of their range, with poor accuracy.
Reversed CT or VT polarity in power-based 37: the relay calculates negative power and operates with the motor loaded.
Settings copied from another pump with a different operating point.

We configure your motor relay

Is your 37 tripping for no apparent reason, or did it miss a pump running dry? We review the events and settings of your motor relay remotely, worldwide, and set them to the real operating point of the process. Tell us the brand and model through remote relay configuration or contact us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (device 37).
IEEE C37.96, Guide for AC Motor Protection: protection of induction and synchronous motors.
IEC 61850-7-4, Compatible logical node classes and data object classes: PTUC, PDUP and PTRC nodes.
ABB, REM615 Application Manual: LOFLPTUC loss of load supervision.
SEL, IEC 61850 conformance statement (MICS) of the SEL-849: PTUC and PDUP nodes.

Frequently asked questions

Why is power-based 37 preferred over current-based 37?

Because the no-load current of an induction motor is largely magnetizing current and does not drop much when the load is lost, especially in lightly loaded or oversized motors. Active power, on the other hand, drops clearly. Power-based 37 needs voltage transformers in addition to current transformers.

Why does the 37 trip when the motor is stopped?

Because with the motor stopped the current is zero, which is below any threshold. The relay must block the function when the motor is stopped, using the breaker or contactor status or a minimum current threshold, and during the starting window. If that block is missing or badly set, the alarm or trip appears with the motor out of service.

Does the 37 work on a motor fed by a variable frequency drive?

A relay installed upstream of the drive sees the drive input power, not the motor current at variable frequency. Underload supervision is usually done in the drive itself, which knows speed and torque. If the incoming relay is used, the threshold must be studied over the whole speed range.