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
Use = 3 Phase, 440V – Fitted in MSB / PDB
(6) MCCB: Moulded Care Circuit Breakers (100-1000A)
Use = 3 Phase, 440V – Fitted in MSB / PDB / Motor Switch Board / Battery Switch Boards
(7) MCB: Miniature Circuit Breakers (upto 100A)
Use = Single Phase, 220 V – Fitted in Lighting Switch Board
Short Circuit Fault – is said to have taken place when the current is not confined to its normal path of flow but diverted through alternate path(s)
Short Circuit Fault causes High Fault Current
This Short Circuit/High Fault Currents may result in excessive Thermal Stresses, Mechanical Stresses & Arcing leading to the damage of the electrical installation
Short Circuit Trip / Relay (Instantaneous)
It is used to protect the Generator / Equipment against Heavy Fault Current caused by a Short Circuit Fault
When Short Circuit Fault happens – this Trip / Relay Trips the Circuit Breaker – thus the Faulty Circuit (Generator / Equipment) is Isolated from the Supply
It consists of:
An Electromagnet (Iron Core surrounded
by a Current Coil (Load Current Coil))
An Armature (Trip Lever) held by a spring
When a Short Circuit happens:-
Short Circuit / Fault Current is produced
This Current flows through the Coil
A Magnetic Field is produced by the Electromagnet (Iron Core & Coil)
Electromagnet (acts as a Magnet) Pulls (Attracts) the Armature against the spring
When the Short Circuit / Fault Current is enough to pull the Armature against the Spring Force – A contact is made with the Trip Circuit
Circuit Breaker is tripped
(Old type electrical door bells use the similar mechanism – a circuit is made and broken several times to produce the buzzing sound)
Overload Trip / Relay
It is used to protect the Generator / Equipment against Overload (High Load Currents)
When Overload happens – this Trip / Relay Trips the Circuit Breaker – thus the Generator / Equipment is Isolated from the Bus Bar / Supply
An overload trip should not trip the Generator / Equipment –
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 – high current is needed only during motor starting
Hence a Time Delay (15 seconds) is incorporated in the Trip/Relay – dashpot type trip
It consists of:
A Plunger and a Solenoid
Plunger is attached to a Piston
Piston has a small hole
Piston is placed in a Silicone Fluid
(viscosity does not vary with temperature)
Solenoid is connected to the Load Current
When Overload occurs / happens:
Load Current increases excessively
Plunger is attracted to the solenoid
Plunger / Piston moves upwards, as silicone fluid is displaced from top to bottom through the small hole in the piston
Time delay / lag depends upon hole size and viscosity of the silicone fluid
If the hole is big – time delay / lag is less and vice versa
Load Current Setting for Trip is about 25 % above maximum (but should not exceed 50 %)