Switchgears

 • SWITCHGEAR :

 In an electric power system switchgear is the combination of electrical disconnect switches, fuses or circuit breaker used to control protect and isolate electrical equipments.

Switchgear is used both to de-energise equipment to allow work to be done and to clear faults downstream.

• LOW VOLTAGE SWITCHGEAR : 

1) Switches : 

Device that can carry and interrupt normal load  current and then disconnect portions of the network.

2) Recloses : 

Device similar in function to circuit breaker, except they also have the ability to reclose after opening. Open again and reclose again, repeating this cycle a predetermined number of times until they locked.

3) Fuses : 

Device that can carry a defined load current without deterioration and interrupt a defined short circuit current.

4) Circuit breaker :

Device that can carry and interrupt normal load current like Switches, in addition they interrupt short circuit ( faults) current.

Circuit breaker are always paired with a relay which sences short circuit condition using potential transformer and current transformer.

Types of circuit breaker : 

• MCB : ( Miniature Circuit Breaker)

 

Rated current not more than 100A

Trip characteristics normally not adjustable thermal (or) thermal magnetic operating.

Rating : Single pole (SP)

               Double pole (DP)

               Triple pole (TP)

               Four pole (FP)

Size : 10A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A.

• MCCB : (Moulded Case Circuit Breaker)

Rated current not more than 1000A

Trip characteristics normally not adjustable thermal (or) thermal magnetic operating.

Rating : Triple pole (TP)

               Four pole (FP)

Size : 10A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, 125A, 160A, 200A, 250A, 320A, 430A, 500A, 630A, 800A, 1000A.

• ELCB : ( Earth Leakage Circuit Breaker)

- Phase (line) , Netural and earth wire connected through ELCB . It's rating are same as MCCB.

- ELCB is working based on earth leakage current.

• Operating time of ELCB :

- The safest limit of current which human body can withstand is 30 ma sec.

- Suppose human body Resistance is 500Ω and Voltage to group is 230 volts.

- The body current will be 500/230=460MA.

- Hence ELCB must be operated in 30 ma sec/ 460ma= 0.65/sec.

• RCD : (Residual current Device)

- Phase(line) and Netural both wire connected through RCD.

- It's trips the circuit when there is earth faults current.

- The amount of current flow through to phase should return through Netural.

- A 300/500 ma RCCB may be used where only fire protection is required.

• MEDIUM VOLTAGE PROTECTION : 

• ACB : ( Air Circuit Breaker)

- Rated current upto 10,000A

- Trip characteristics often fully including configurable trip thresholds and delay.

- Often used for main power distribution in large industrial plants where to breaker are arranged in draw-out encloses for case of maintenance.

Rating : 

TP and FP

Size : 

630A, 800A, 1000A, 1250A, 1600A, 2000A, 2500A, 3200A, 4000A, 5000A, 6300A, 8000A, 10,000A.

• Isolator : 

It's is a device use only for ON/OFF, No protection.

• HIGH VOLTAGE PROTECTION : 

- VCB ( Vaccum Circuit Breaker)

- OCB ( Oil Circuit Breaker)

- SF6  ( Sulpher flexachloride)

Therefore, this CB are used only in 11 kv or 33kv above.