In short. A breaker, contactor or relay is specified by its function, frame, rated current, breaking capacity, trip characteristic, coil voltage and accessories, not by size or one current marking. A trip is often a correct response to a real fault, so investigate before replacing. A replacement must keep protection coordination with the cables and downstream devices, and changes to rating or settings need technical review.
Three devices, three different jobs
These parts sit side by side in a generator control or distribution panel and are easily confused, so it helps to separate their duties.
| Device | Primary job | Key ratings | Typical failure |
|---|---|---|---|
| Circuit breaker (MCB, MCCB, ACB) | Carry load current and interrupt overloads and short circuits | Rated current, breaking capacity in kA, trip unit type and settings, poles | Nuisance tripping, failure to close, contact damage after heavy faults |
| Contactor | Switch load on and off frequently under remote control | Rated operational current for the utilisation category, coil voltage, auxiliary contacts | Burnt or welded main contacts, coil burn-out, chattering |
| Control relay | Switch small control signals, multiply contacts | Coil voltage AC or DC, contact rating, base type | Contact wear, coil failure, sticking |
| Protection relay | Measure via CTs and VTs and decide when to trip | Functions provided, CT and VT inputs, auxiliary supply, settings | Power supply failure, wrong settings, wiring faults on inputs |
A contactor is not a fault-interrupting device. It must be protected by a breaker or fuses that clear short circuits it cannot handle. A protection relay does not interrupt anything either; it commands a breaker through its trip coil. Measurement, decision, command and interruption are separate steps, and a fault in any one of them can make a healthy-looking device appear to have failed.
Identifying a breaker properly
Collect the maker, model and frame size; the rated current and any adjustable setting; the breaking capacity (often marked Icu and Ics in kA under IS/IEC 60947-2); the trip unit type, thermal-magnetic or electronic, with its current settings; the number of poles; whether it is fixed, plug-in or draw-out; and the accessories fitted, such as shunt trip, undervoltage release, motor operator and auxiliary contacts. A photograph of the front label and of the side showing accessories is worth a paragraph of description.
Breaking capacity must match the fault level
Isc ≈ Ir / X″d
Here Isc is the initial symmetrical short-circuit current from the generator, Ir is the rated current and X″d is the subtransient reactance in per unit from the alternator data sheet. (Illustrative) a 500 kVA, 415 V set has a rated current of about 696 A; with X″d of 0.12 the initial fault current is about 5.8 kA. The breaker on that set must interrupt at least that, and the same panel fed from a large DISCOM transformer may see far more, so breaking capacity is chosen for the highest source that can feed the panel. Fault studies are explained on fault current.
A trip is evidence, not an inconvenience
A breaker that trips repeatedly is telling you something. The cause may be a genuine overload, a motor starting current outside the trip curve, a cable fault, an earth fault, a loose connection heating the thermal element, or a trip unit set lower than the design. Raising the rating without checking the cable size, the downstream equipment and the fault duty can leave a cable unprotected and turn a nuisance trip into a fire. The right sequence is to read the trip indication or the electronic trip unit log, measure the actual load current with a clamp meter, inspect the connections and cable, and compare the setting with the design.
Contactors deserve the same thinking. Welded or badly pitted contacts usually mean the contactor was switching a current or a load type beyond its utilisation category, closing onto a fault, or chattering because the coil voltage was low. Freeing a welded contactor and putting it back is unsafe. Find the reason, then replace the contactor, and replace the contacts in sets where a contact kit exists.
Keeping protection coordination intact
Protection is designed so that the device nearest a fault clears it while upstream devices stay closed. That selectivity depends on the time-current characteristics of every device in the chain. A replacement breaker with a different curve, a different instantaneous setting or a different frame can break coordination, so that a fault on a small feeder trips the generator main breaker and blacks out the whole site. When a protection relay is replaced, its settings must be re-entered from the setting sheet, its CT ratio confirmed, and its trip outputs tested through to the breaker. Typical settings published in guides are starting points; real settings come from the system data and a coordination study. See protection coordination for the method.
Obsolete devices and retrofits
Older panels carry breakers and relays that are no longer made. The options are a direct successor from the same maker, a retrofit kit that fits a modern breaker into the old cradle, or a panel modification. Each must be checked for breaking capacity, dimensions, busbar connection, accessories and coordination. Used or refurbished devices can fill a gap, but their condition, test record and limitations must be disclosed, and they are rarely the right answer for a main generator breaker. Our approach is on used and refurbished parts, and transfer-related parts are on ATS and switchgear parts.
Testing after replacement
- Torque all terminations to the maker’s value and record it.
- Check that trip unit settings match the setting sheet, and photograph them.
- Operate the device manually and electrically, and confirm auxiliary contacts change state.
- Inject a test signal or use the trip unit test function to confirm the trip path works through to the breaker.
- Run on load and check temperature at terminals after an hour.
Related photographs and videos
Common mistakes
- Fitting a higher-rated breaker to stop nuisance tripping; the cable is no longer protected and overheats under the next overload.
- Choosing a breaker with breaking capacity below the site fault level; it fails violently on the first serious short circuit.
- Using a control relay or a small contactor for a duty beyond its utilisation category; contacts weld and the circuit cannot be switched off.
- Replacing a protection relay without re-entering and testing its settings; the generator runs unprotected without anyone knowing.
What to send us
Send photographs of the device label and accessories, the panel nameplate and the single-line diagram if you have it. Tell us where the device sits in the system, what it was doing when it failed, any trip indication or event log entry, and the coil or control voltage. Send a spare parts enquiry.
Questions engineers ask
Can I replace an MCCB with a different brand of the same rating?
Only after checking breaking capacity, trip curve, dimensions, terminal arrangement and accessories, and confirming that coordination with upstream and downstream devices still holds. Same rated current is the start of the check, not the end.
What is the difference between Icu and Ics?
Icu is the ultimate breaking capacity the breaker can interrupt once, possibly needing replacement afterwards. Ics is the service breaking capacity after which it remains fit for use. For important feeders, Ics should cover the expected fault level.
Why does my contactor hum or chatter?
Usually the coil voltage is low, the pole faces are dirty, or the shading ring on an AC coil is broken. Chatter burns contacts quickly, so measure coil voltage and clean or replace the magnet system.
Do I need a coordination study to replace one breaker?
Not always. If the replacement has the same frame, curve and settings, coordination is preserved. If anything changes, check it against the upstream and downstream devices.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.