Part P Circuit Protection 2

Question 1

What is the main difference between a semi-enclosed fuse (BS 3036) and a cartridge fuse (BS 88)?

Accepted Answer

A BS 3036 fuse uses a replaceable fusewire and has less accurate characteristics; a BS 88 fuse is a sealed cartridge with precise characteristics and higher breaking capacity

Answer

BS 3036 rewirable fuses use fusewire and have relatively imprecise characteristics and a low breaking capacity; BS 88 cartridge fuses have precise, reproducible characteristics and a much higher breaking capacity.

Question 2

Why is the use of BS 3036 semi-enclosed fuses discouraged in modern installations?

Accepted Answer

They have unpredictable characteristics, can be incorrectly rewired with wrong-gauge wire, and have a low breaking capacity compared to modern MCBs or cartridge fuses

Answer

BS 3036 rewirable fuses are prone to being rewired with incorrect gauge wire (either too small causing nuisance blowing, or too large causing fire risk), and their characteristics are imprecise compared to MCBs.

Question 3

What Type of MCB is most commonly used for a ring main circuit in a domestic installation?

Accepted Answer

Type B 32 A MCB

Answer

A Type B 32 A MCB is standard for domestic ring main (ring final) circuits, providing overload and short circuit protection with a trip characteristic suited to the resistive loads of socket outlets.

Question 4

What is the maximum floor area that can be served by a single 32 A ring main circuit in a domestic dwelling?

Accepted Answer

100 m² (any floor, or individual floors above 100 m² require additional circuits)

Answer

BS 7671 Appendix 15 recommends that a single 32 A ring main circuit should not serve more than 100 m² of floor area, to limit the number of socket outlets and connected load.

Question 5

What is a radial circuit, and how does it differ from a ring main?

Accepted Answer

A radial circuit runs from the consumer unit to a series of outlets and terminates at the last outlet; a ring main returns from the last outlet back to the consumer unit

Answer

A radial circuit runs from the CU and terminates at the last outlet; a ring main runs out from one CU terminal, connects all outlets in a loop, and returns to a second terminal of the same circuit breaker.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Circuit Protection 2