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

32A

Answer

38A

Answer

45A

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

A remote-controlled circuit breaker operated by smartphone

Answer

A type of surge protection device

Answer

A replacement for the main switch

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

Ib = breaking current, In = nominal voltage, Iz = impedance

Answer

Ib = insulation resistance, In = neutral current, Iz = zone rating

Answer

Ib = bonding current, In = nominal frequency, Iz = zonal impedance

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

Vd = I²R

Answer

Vd = P × I

Answer

Vd = Zs × In

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

To ensure all circuit breakers trip simultaneously during a fault

Answer

To reduce the cost of the installation

Answer

To increase the earth fault loop impedance

Question 6

What is the adiabatic equation used for in electrical installation design?

Accepted Answer

Calculating the minimum cross-sectional area of a protective conductor to withstand fault current without damage

Answer

Calculating the voltage drop in long cable runs

Answer

Calculating the diversity of a circuit

Answer

Determining the number of socket outlets required