Part P Circuit Design and Protection 2

Question 1

An electrician is designing an electric shower circuit. The shower is rated at 9.5kW at 230V. What is the design current (Ib)?

Accepted Answer

41.3A

Answer

The design current is calculated using Ib = P/V = 9500/230 = 41.3A. This means a 45A MCB and appropriately rated cable (typically 10mm² twin and earth) would be required. The cable must be rated to carry at least the MCB rating (In) after applying all relevant correction factors for the installation conditions.

Question 2

What is an RCBO and what advantage does it offer over separate MCB and RCD protection?

Accepted Answer

A combined MCB and RCD in a single device providing individual circuit RCD protection

Answer

An RCBO (Residual Current Circuit Breaker with Overcurrent protection) combines the functions of an MCB and RCD in a single module-width device. Each circuit has its own independent RCD protection, so a fault on one circuit does not trip other circuits. This eliminates the nuisance of a shared RCD tripping and affecting multiple circuits simultaneously.

Question 3

When selecting a cable for a circuit, the relationship Ib ≤ In ≤ Iz must be satisfied. What do these symbols represent?

Accepted Answer

Ib = design current, In = rated current of protective device, Iz = current-carrying capacity of cable

Answer

This fundamental inequality ensures that: the design current (Ib, the actual current the circuit will carry) does not exceed the rating of the protective device (In, the MCB/fuse rating), and the protective device rating does not exceed the current-carrying capacity of the cable (Iz, the maximum safe continuous current for the cable in its installation conditions).

Question 4

A 6mm² twin and earth cable is installed in a 20m run from the consumer unit to a cooker switch. The design current is 28A. What formula is used to calculate voltage drop?

Accepted Answer

Vd = (mV/A/m × Ib × L) / 1000

Answer

Voltage drop is calculated using Vd = (mV/A/m × Ib × L) / 1000, where mV/A/m is the voltage drop per ampere per metre from the cable tables (BS 7671 Appendix 4), Ib is the design current in amps, and L is the cable length in metres. The result in millivolts is divided by 1000 to convert to volts for comparison with the permitted limits.

Question 5

What is the purpose of discrimination (selectivity) between protective devices in a domestic installation?

Accepted Answer

To ensure only the protective device closest to the fault operates, leaving other circuits unaffected

Answer

Discrimination ensures that during a fault, only the device nearest to the fault trips, while upstream devices remain closed. In a domestic consumer unit, this means a fault on one circuit should trip only that circuit's MCB, not the main switch or RCD protecting other circuits. This is achieved through appropriate device selection and time-current coordination.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Circuit Design and Protection 2