Thermal Overload Protection 49 (PTTR): How It Works

Short answer

The 49 protection estimates the temperature of equipment from the current flowing through it, with a thermal model that has memory. It trips when the calculated heating reaches the limit, not when the current crosses a threshold. It protects motors, transformers, cables and generators against overloads that 51 overcurrent would let through.

What 49 protection is (thermal overload)

In IEEE C37.2, device 49 is the machine or transformer thermal relay: it operates when the temperature of the equipment exceeds a predetermined value. In IEC 61850 it corresponds to logical node PTTR (thermal overload). The functional requirements for electrical thermal relays are in IEC 60255-149.

There are two ways to implement the 49:

Direct temperature measurement, with RTDs in the windings or with the winding temperature indicator of a transformer.
Thermal model (thermal replica): the relay does not measure temperature, it calculates it from the current and a model of the equipment. It is the most common approach in numerical relays and the one this guide covers.

The difference from time overcurrent is memory. The 49 accumulates the heat that goes in and lets it out with the equipment time constant. A motor that is already hot tolerates less overload than a cold one, and the relay knows it.

An overload is current above rating through a healthy path: there is no fault, but the equipment heats up. The damage is cumulative, because heat ages the insulation. So the question the 49 answers is not “how much current is there?” but “how hot is the equipment?”.

Why not every overload should trip

Demand peaks, load transfers and cold load pickup after a long outage are part of normal operation. A power transformer can carry currents above rating for a while as long as its hot spot stays within the limits of its loading guide. Tripping earlier drops customers without protecting anything.

Overload does consume service life. According to IEC 60076-7, for non-thermally upgraded paper the relative aging rate roughly doubles for every 6 K of hot-spot temperature above 98 °C; IEEE C57.91 takes 110 °C as the reference for thermally upgraded paper. Those guides let the owner decide how much temporary overload is acceptable, and for how long, in exchange for what loss of life. That decision belongs to the company that owns the equipment, and the 49 is the tool that enforces it.

Transformer energizing current (inrush), motor starting and cold load pickup should not cause an overload trip either; many relays handle the last one with cold load pickup logic.

49 versus 51 as overload protection

Function	How it acts	Limitation
Thermal model (49)	Calculates temperature from the current and a time constant; remembers the previous load	Needs the time constant and the allowable current of the equipment
Time overcurrent (51) with low pickup	Fixed time-current curve	No thermal memory: treats cold and hot equipment the same
Measured temperature (26 oil, 49 winding)	Transformer temperature indicators	Depends on their calibration
Low-voltage fuses and breakers	Their own thermal element	Little flexibility

Overcurrent 51 is used as overload protection when the relay has no thermal model, on feeders where the cable is the limit, or as backup for the 49. It works, with a compromise: if its pickup is close to the allowable current, it trips on normal peaks; if it is far, it lets long overloads through that heat the equipment. In transformers, direct oil and winding temperature measurement complements the thermal model and reveals what the model cannot see, such as a cooling system failure.

Where it is applied

Medium- and low-voltage motors in industry: the main application. It includes restart blocking until the motor cools down.
Power transformers: overload trip or alarm, complementary to the oil and winding temperature indicators.
Power cables: medium-voltage feeders where the cable ampacity is the limit.
Generators: stator overload.
Reactors and capacitor banks.

How it works

The simplest model treats the equipment as a homogeneous body that heats with I² and cools towards ambient with a time constant τ. The thermal state θ is expressed as a percentage: 100 % is the temperature the equipment reaches at the maximum continuous current.

With constant current, the trip time defined in IEC 60255-149 follows the form:

t = τ · ln[(I² − Ip²) / (I² − (k · IB)²)]

where I is the present current, Ip the prior load current (which represents the hot state), IB the basic current and k the continuous overload factor. With Ip = 0 you get the cold curve; with Ip equal to the prior load, the hot curve.

On top of that each relay adds what the equipment needs:

Two time constants (copper and iron, or winding and oil) to follow the heating of a transformer or motor more closely.
A different cooling constant with the motor stopped, because without ventilation it cools more slowly.
Negative-sequence weighting in motors: unbalanced current heats the rotor more and is weighted in the model.
Ambient temperature compensation, measured with an RTD or set as a fixed value.
Alarm before the trip, and restart blocking in motors.

The measurement can use true RMS, which includes harmonics, or the fundamental only. For heating, RMS is the right one.

Setting criteria

Basic current: the rated current of the equipment in secondary amperes, not the CT rating. It is the most common mistake.
k factor: the continuous overload the equipment allows according to its nameplate, service factor or loading capability.
Time constants: from the motor, transformer or cable manufacturer. For transformers, IEC 60076-7 and IEEE C57.91 cover loading and heating; for cables, ampacity depends on the installation.
Design and actual ambient temperature.
Coordination with the motor starting curve and with the 51, so the 49 does not operate on a normal start. It is defined in the coordination study.

Time constants and factors are not universal: they come from the equipment data and the relay manual.

How each manufacturer implements it

Siemens SIPROTEC offers thermal overload protection for general equipment and specific models for motors and transformers; it is set in DIGSI.
SEL includes thermal models in motor, feeder and transformer relays, some with its own motor model; they are set in AcSELerator QuickSet.
Relion (Hitachi Energy 670/650; ABB 615/620) has separate thermal blocks for feeders, motors and transformers in PCM600.
GE Vernova (Multilin) uses a thermal model with its own curves and an unbalance bias factor in its motor relays; it is set in EnerVista.
Schneider MiCOM / Easergy offers the thermal model with one or two time constants; it is configured in MiCOM S1 or Easergy Studio.

What changes is the base of the thermal percentage, the number of time constants, how the memory is reset and whether the model accounts for negative sequence or ambient temperature. The same motor needs different settings in two brands.

Why it trips or blocks

Real overload: a jammed process, a pump in poor condition, a transformer loaded above its capability.
Wrong basic current setting: taking the CT current instead of the equipment current leaves the 49 too sensitive or useless.
Short time constant: it trips on long starts or on normal load peaks.
High thermal memory after repeated starts, after a trip or after testing.
Voltage unbalance: in motors with negative-sequence weighting the thermal state rises faster.
Harmonics from loads with drives, when the model uses RMS and the settings did not account for it.
Loss of cooling in transformers with forced cooling, which the model does not reflect unless it receives the signal.

What to check first: the thermal state recorded before the event, the actual load current, and the basic current and time constant settings.

How it is tested

By secondary injection with a relay test set.

The test checks:

The trip time at several currents, starting from the cold state, compared with the theoretical curve of the setting.
Hot behaviour: a prior load is injected until it stabilizes, then the overload.
The alarm threshold and restart blocking.
Cooling: the decay of the thermal state when the current is removed.

These tests are long, because time constants can be many minutes. At the end the thermal memory is reset before the relay goes back into service.

Did your relay trip on the 49?

Did your relay trip or block on this function and the reason is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or write to us with the brand and model of the device.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
IEC 60255-149, Electrical thermal relays: functional requirements.
IEC 61850-7-4, logical nodes (PTTR).
IEEE C37.96, Guide for AC Motor Protection.
IEEE C37.91, Guide for Protecting Power Transformers.
IEC 60076-7 and IEEE C57.91, loading guides for oil-immersed transformers.
J. Lewis Blackburn and Thomas J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why did the 49 trip at a lower current than the previous trip?

Because the thermal model has memory. If the equipment was already hot from an earlier load, the relay needs less time to reach the limit. Check the thermal state recorded before the event.

Does the 49 replace the 51 protection?

No. The 51 protects against short circuits and coordinates with other protection; the 49 protects against heating from sustained overload. They are used together, each for its own phenomenon.

Why would the motor not start after relay testing?

Injection testing often leaves the relay's thermal memory high and the restart block active. Before the relay goes back into service, the thermal state is reset following the manufacturer's procedure.