10. Switchgear

Question 1

What are the 4 general types of protective device:

Answer 1

• BS 88 HRC/HBC Fuse
• BS 1361 Cartridge fuse
• BS 3036 Semi-enclosed fuse
• Circuit Breakers

Question 2

Features of a BS 88 Fuse:

Answer 2

High breaking capacity fuses.

• Have 7 elements.
• Designed to rupture at high currents.

May be gG (General)
May be gM (Motor rated)

Question 3

Features of a BS 1361 Fuse

Answer 3

• Single Element
• Good blast containment
• Breaking capacity up to 16kA.

Question 4

Features of a BS 3036 Fuse

Answer 4

BS 3036 Semi-enclosed (rewireable fuses)

• Only the carrier acts as blast containment.
• Low breaking capacity (1kA)
• Still recognized

Question 5

Features of Circuit Breakers

Answer 5

Have 2 elements:

1. Thermal to protect against overloads.
2. Electro-magnetic to protect against short circuit.

• Have more accurate tripping current.
• Resettable

Question 6

What are the types of circuit breaker and where are they used:

Answer 6

Type B:
Used for domestic installations and general purpose protection.

Type C:
Used for small inductive loads such as discharge lighting and small motors.

Type D:
Used for large inductive loads such as large motors, welding equipment, x-ray machines.

Question 7

What are the tripping characteristics of the types of circuit breaker

Answer 7

Trip instantaneously…

Type B = 5x their current rating

Type C =10 times their current rating

Type D = 20 times their current rating.

Question 8

How should isolators be opperated

Answer 8

Isolators should be switched under NO LOAD.

Using an isolator to switch on load will lead to damage to the contactors. This can result in a high resistance contact leading to fires.

Question 9

What is a Switch Fuse?

Answer 9

This is an extension of the isolator, in a way that the load side of the supply is interrupted by a fuse in the line conductor.

Question 10

What is a Fuse Switch

Answer 10

Fuse switches are used on three phase systems.
The fuse forms part of the moving operating blade.