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 TN-S earthing?
Answer: A system where the neutral (N) and protective earth (PE) conductors are separate throughout the installation, with the earth provided by the supply cable sheath
In a TN-S system the neutral and protective earth conductors are completely separate; the protective earth conductor is provided by the metal sheath or armour of the supply cable and the two are only combined at the source.
What is TN-C-S earthing (PME)?
Answer: A system where the neutral and earth are combined in the supply cable (PEN conductor) but separated at the service head — the most common UK domestic earthing system
TN-C-S (Protective Multiple Earthing, PME) combines the neutral and earth in the distribution network (PEN conductor); they are separated at the service head, providing both neutral and earth terminals for the installation.
What is a TT earthing system?
Answer: A system where the installation has no direct connection to the supply earth; instead an earth electrode (rod) at the installation provides the earth — common where overhead lines supply the property
In a TT system, the earth electrode is at the installation (an earth rod), not provided by the supply network; TT earthing is common in rural areas with overhead line supplies and requires an RCD as the main protective device.
Why must an RCD always be used as the main protective device in a TT system?
Answer: Because earth fault loop impedance is high (via the earth rod) so an MCB cannot disconnect quickly enough; the RCD provides reliable disconnection regardless of Zs
In a TT system, the earth fault loop impedance is high (earth rod resistance can be 20–200 Ω), so the fault current may be too small to operate an MCB quickly; an RCD trips on the residual current regardless of loop impedance.
What is the purpose of a main equipotential bonding conductor?
Answer: To connect incoming metallic services (gas, water, oil pipes) to the main earthing terminal, preventing dangerous voltage differences if a fault energises the pipework
Main equipotential bonding connects all incoming metallic services to the main earthing terminal (MET), ensuring they are all at the same potential and preventing dangerous touch voltages if a fault occurs on any service.
What is a residual current device with overcurrent protection (RCBO) rated?
Answer: It has both a rated residual current (e.g., 30 mA) and a rated overcurrent (e.g., 16 A or 32 A)
An RCBO has two ratings: the residual operating current (IΔn, e.g., 30 mA) for earth leakage protection and the rated current (In, e.g., 16 A) for overcurrent protection, combining both functions in one device.