Circuit Protection Flashcards
6 cards from real PART P practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Circuit Protection flashcards as text
What is the main difference between a semi-enclosed fuse (BS 3036) and a cartridge fuse (BS 88)?
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
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.
Why is the use of BS 3036 semi-enclosed fuses discouraged in modern installations?
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
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.
What Type of MCB is most commonly used for a ring main circuit in a domestic installation?
Answer: Type B 32 A MCB
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.
What is the maximum floor area that can be served by a single 32 A ring main circuit in a domestic dwelling?
Answer: 100 m² (any floor, or individual floors above 100 m² require additional circuits)
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.
What is a radial circuit, and how does it differ from a ring main?
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
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.
What is a 'dedicated circuit' and give a common example in domestic installations?
Answer: A circuit serving only one appliance or outlet — e.g., a cooker circuit, shower circuit or electric vehicle charging point
A dedicated circuit serves a single appliance, providing protection sized specifically for that load — common examples include electric cookers (typically 32–45 A), showers (typically 40–45 A) and EV charging points.