Protection Chain — Contactors, Breakers, Overload vs Short Circuit

1. Brains and Muscle — Relay vs Contactor

The relay example uses a small control signal of about 12 V and 0.5 A; the contactor switches the heavy load, while the breaker interrupts fault current.

Relay — the brain

Light-duty switch and fault watcher. A small solenoid or electronic sensor detects abnormal current, voltage or sequence and signals a local or remote breaker to trip.

Contactor — the muscle

Electrically controlled heavy switch for power circuits: motors, lighting, heating, capacitor banks. Same idea as a relay, built for far higher make-and-break currents.

Protective relays extend the family: over-current, negative-phase-sequence and dedicated single-phasing relays identify the fault signature, then isolate only the connected load — larger motor relays arrive with single-phasing protection already fitted.

2. Breaker Family — ACB, MCCB, NFB and the Fuse

ACB (air circuit breaker) guards 400 V high-current incomers; MCCB (moulded-case) guards 400 V medium feeders; NFB (no-fuse breaker) covers small circuits around 75 A. All three are isolating switches that also act as fuses — rated for a normal working current plus an overload rating with time delay — and all three reset where a fuse cannot.

The fuse caps the current a circuit may draw by melting open past its pre-set limit, in milliseconds, against both overload and short circuit. It protects circuit and surroundings from fire — then gets renewed.

3. Overload Is a Slow Cook, Short Circuit an Explosion

Overload — over the design current

Momentary (motor starting current, ignored) or sustained (too many appliances, jammed rotor, failed bearings). Fuses, breakers and overload devices disconnect on excess — tripping in seconds at 1.1–1.5× normal.

Short circuit — conductors joined by fault

Phase-to-phase through failed insulation: cut wires, fire damage, crushed cable. Enormous current — tripping in milliseconds.

The thermal overload relay is the slow-cook specialist: load current heats a bi-metallic strip, the strip bends with heat, and the bend trips the breaker — set at 90–100% of rated current, sometimes 105%. After a trip it must be reset (switched off before on), and sometimes the strip needs its cooling time before it will reset at all.

Warning — Available power is fixed — the breaker setting is the load limit.

Voltage fixed, power fixed, so the fuse or breaker setting at normal load current is what caps the load. Past rating, it opens — that is the whole load-limiting scheme.

4. Discrimination, Sequential Start and Load Sharing

Discrimination is a protection system's promise to disconnect only faulty circuits and keep healthy ones alive. It is built by coordinating fuse current ratings and over-current relay time settings from generator to load: devices nearest the load take the lowest rating and shortest time, nearest the generator the highest rating and longest time. A pump-motor fault then trips its own branch — never the generator breaker.

Sequential starting

Automatic staged restart of essentials when power returns after failure — steering gear and other critical loads cut in step by step instead of slamming the bus at once.

Load sharing

Paralleled generators hold an equal balance of load through every load change — no set loafs while its neighbour labours. Before paralleling, the incoming set runs slightly fast (frequency a touch above the bus) so it takes load on closing — a slow incoming set would motorize, drag the bus, and risk tripping the healthy sets.

Preference tripping

The mirror image under overload: non-essential load (galley, laundry, accommodation, deck kit) trips at set intervals — first group near 5 s, next near 10 s — while the alarm calls the engineer.