Types of Circuit Breakers
[Based on Arc Quenching (Cooling) Media]
1. ACB: Air Circuit Breakers
2. ABCB: Air Blast Circuit Breakers
3. OCB: Oil Circuit Breakers
4. VCB: Vacuum Circuit Breakers
5. SF6CB: Sulphur-Hexa-Fluoride Circuit Breakers
Usage: 3 Phase, 440V – Fitted in MSB / PDB
6. MCCB: Moulded Case Circuit Breakers (100-1000A)
Usage: 3 Phase, 440V – Fitted in MSB / PDB / Motor Switch Board / Battery Switch Boards
7. MCB: Miniature Circuit Breakers (up to 100A)
Usage: Single Phase, 220V – Fitted in Lighting Switch Board
A short circuit fault occurs when the current is diverted through alternate paths instead of its normal flow. Short circuit faults cause high fault currents, leading to excessive thermal stresses, mechanical stresses, and arcing that can damage the electrical installation.
A short circuit trip/relay (instantaneous) is used to protect the generator/equipment against heavy fault currents caused by a short circuit fault. When a short circuit fault occurs, this trip/relay activates the circuit breaker, isolating the faulty circuit (generator/equipment) from the power supply.
The short circuit trip/relay consists of:
– An electromagnet (iron core surrounded by a load current coil)
– An armature (trip lever) held by a spring
During a short circuit fault, the fault current flows through the coil, creating a magnetic field produced by the electromagnet (iron core and coil). The electromagnet attracts the armature against the spring force. When the fault current is sufficient to pull the armature against the spring force, contact is made with the trip circuit, tripping the circuit breaker.
(Old type electrical doorbells use a similar mechanism, where a circuit is made and broken several times to produce the buzzing sound.)
An overload trip/relay is used to protect the generator/equipment against high-load currents. When an overload occurs, this trip/relay activates the circuit breaker, isolating the generator/equipment from the bus bar/supply.
An overload trip should not trip the generator/equipment in the following scenarios:
– If the increase in load is only for a short duration (momentary current surge)
– When the load is shared by an incoming generator
– When high-load motors are operated, as high current is only needed during motor starting
To account for this, a time delay (15 seconds) is incorporated in the trip/relay, known as a dashpot type trip.
The overload trip/relay consists of:
– A plunger and a solenoid
– The plunger is attached to a piston
– The piston has a small hole
– The piston is placed in silicone fluid (viscosity does not vary with temperature)
– The solenoid is connected to the load current
During an overload, the load current increases excessively, attracting the plunger to the solenoid. The plunger/piston moves upwards as the silicone fluid is displaced from top to bottom through the small hole in the piston. The time delay depends on the size of the hole and the viscosity of the silicone fluid. A larger hole results in a shorter time delay, and vice versa.
The load current setting for the trip should be about 25% above the maximum but should not exceed 50%.