Protective Device Introduction

What is protective device introduction?

Electrical distribution systems are often quite complicated. They  cannot be absolutely fail-safe. Circuits are subject to destructive overcurrents. Harsh environments, general deterioration, accidental damage or damage from natural causes, excessive expansion or overloading of the electrical distribution system are factors which contribute to the occurrence of such overcurrent.

Reliable protective devices prevent or minimize costly damage to transformers, conductors, motors, and the other many components and loads that make up the complete distribution system. It is essential to avoid the severe monetary losses which can result from power blackouts and prolonged downtime of facilities. It is the need for reliable protection, safety, and freedom from fire hazards.

Ok….Now i listed some basic for LOW VOLTAGE PROTECTIVE DEVICE common  in use: –

1) Fuses

2) MCB ( Miniature Circuit Breaker )

3) MCCB ( Moulded Case Cuircuit Breaker )

4) RCB ( Residual Circuit Breaker )

5) RCCB ( Residual Current Circuit Breaker )

6) ACB ( Air Circuit Breaker )

7) OCB ( Oil Circuit Breaker )

8) VCB ( Vacuum Circuit Breaker )

9) EFR ( Earth Fault Relay )

10) OCR ( Over Current Relay )

I will explain detail about ten ( 10 ) of  protection device on my next post…

LOW VOLTAGE PROTECTIVE DEVICE is intended to protect from 3 main points

  • over current,
  • short circuit
  • overload


  • act fast
  • variable rating
  • high sensitivity
  • high reliability
  • high stability

What is over current protection?

Ib < = In <= Iz

Ib = current design

In = nominal current

Iz = current capacity in the cable

From this formula we summary that :-

  • nominal current of not less than the current design
  • nominal current does not exceed current capacity in the cable
  • I2 =1.45 x current capacity in the cable ( Iz )

What is difference between the over current  and over load?

Over current

a) circuit damage

b) High ampere ( Kilo Ampere )

c) Should be trip  immediately

Over load

a) Not a circuit damage

b) Current value in the limit of 150%

c) Trip time may be delayed

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