Low-Voltage Trip Unit: L, S, I and G Functions

What it is and where it is

The trip unit (in IEC terms, the overcurrent release) is the protection that sits inside the low-voltage circuit breaker. It measures the current passing through the breaker itself and, when it detects an overload or a short circuit, releases the mechanism. It needs no external relay or separate protection current transformers (CTs). Electronic units are usually self-powered: they take their energy from the current flowing through their own sensors, so they protect without a battery; auxiliary power is used for the display, communication and some additional functions, depending on the model.

It sits on the front of the molded-case (MCCB) or open-frame (ACB) breaker, with its dials or its setting display. In miniature circuit breakers (MCB) it is fixed and chosen by the B, C or D curve. These breakers protect the incomers, feeders and motors of the low-voltage switchboard and MCC.

How it works

Thermal-magnetic

• Thermal: a bimetal heats up with the current, bends and releases the mechanism. Its response time is inverse: more current, less time.
• Magnetic: an electromagnet attracts its armature when the current exceeds a threshold and trips with no intentional delay.

It is simple and needs no power supply, but its settings are limited and depend on the ambient temperature.

Electronic

Current sensors on each pole feed a microprocessor that calculates the RMS value and compares it with the set curves. The basic functions are:

• L, overload: long-time inverse; on most models it cannot be disabled. Some models offer the IEC 60255 curve to coordinate with fuses and medium-voltage protection.
• S, short-time short circuit: adjustable threshold and time, with a constant-energy (I²t) or definite-time curve. It lets the downstream breaker clear first.
• I, instantaneous: trips with no intentional delay.
• G, ground fault: measures the vector sum of the currents (and that of the neutral, if present) with an adjustable delay.

Depending on the model, thermal memory, a dual setting group switchable by an input, voltage, frequency, reverse power and unbalance protection, and energy and power quality metering functions are added.

Zone selectivity

High-end units can exchange signals. Each breaker that sees the fault sends a restraint signal to the one upstream; the one that receives no restraint signal from below is the closest to the fault and trips with a short delay. This removes the need to stagger increasing delays toward the source, and the clearing time near the transformer drops.

Differences by voltage level

The underlying logic is the same, phase and ground overcurrent, but in low voltage everything is in one device. In the medium-voltage breaker those functions move to a separate relay, which opens through the trip coil.

Data and settings

• In or Iu: rated current of the breaker or the sensor; thresholds are expressed as multiples of In.
• L, S, I and G thresholds and times, with their tolerances, which are in the catalog and must be used in the study.
• Category A or B according to IEC 60947-2: B has a short-time withstand current (Icw) and allows the short-circuit trip to be delayed to provide selectivity.
• Communication: optional Modbus RTU, Modbus TCP, Profibus, Profinet and, on some models, IEC 61850 modules.

Wiring with protection and control

The trip unit needs no external wiring to protect. Its accessories are wired: trip signal contacts (per protection function, different from the position auxiliary contacts), the zone selectivity link, the setting-change input, the external neutral sensor, the auxiliary supply for display and communications, and the communication bus to the PLC or SCADA.

Faults and what to check first

• Trip with no apparent cause: the unit’s event log and the function that operated; G setting against the real unbalance; neutral and its sensor.
• No selectivity: compare the loaded settings with the protection coordination study; check the zone link.
• Trip on motor starting or transformer energization: I or S function too low for the starting or inrush current.
• No communication or display: auxiliary supply or communication module.

Testing and maintenance

The unit is tested by secondary injection, with the manufacturer’s test equipment or a general injection source, verifying the L, S, I and G thresholds and times and the zone selectivity. High-current primary injection tests the whole assembly, sensors included. At the end it is checked that the loaded settings match the study and a copy is saved. Before changing any setting it pays to download the event log and the existing configuration: each manufacturer names, groups and stores these parameters differently, with its own software or from the unit’s display, and without that copy it is hard to reconstruct what was set when a trip occurred.

The tests of the complete breaker are described in low-voltage circuit breaker testing, and the difference between the two methods in primary and secondary injection.

Testing and configuration with Tiza Group

At Tiza Group we load the settings from your coordination study into electronic trip units and relays of the main brands, remotely and worldwide. If you need injection tests on site, in Colombia or elsewhere on request, tell us what equipment you have.

Related guides

• Primary and secondary injection: differences
• Low-voltage circuit breaker testing: ACB and MCCB

References

• IEC 60947-2, Low-voltage switchgear and controlgear: circuit-breakers.
• IEC 60898-1, Circuit-breakers for overcurrent protection for household and similar installations.
• ABB, Electrical installation handbook: Protection and control devices.
• ABB, Technical Application Papers No. 1: Low voltage selectivity with ABB circuit-breakers.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers.

Frequently asked questions