ECS Earthing and Bonding 3

Question 1

Which system of earthing has the supply neutral conductor connected directly to earth at the source, with no separate earth conductor run to the consumer?

Accepted Answer

TN-C-S (PME) system

Answer

In a TN-C-S (PME) system, the neutral and protective functions are combined in a single PEN conductor from the source, with the earth provided at the consumer's installation by connection to this combined conductor.

Question 2

What danger can arise if main protective bonding is not carried out on incoming metal water pipes?

Accepted Answer

The pipe could become live due to a fault elsewhere, causing electrocution to someone touching it and another earthed part simultaneously

Answer

Without bonding, an electrical fault could energize an incoming metal pipe, creating a dangerous touch voltage between the pipe and any earthed metalwork or ground.

Question 3

When testing earth fault loop impedance, the measured value must not exceed the maximum value (Zs max) for the circuit's protective device — why?

Accepted Answer

To guarantee the protective device disconnects the supply within the required time under fault conditions

Answer

If Zs exceeds Zs max, the fault current will be too low for the protective device to disconnect quickly enough, leaving dangerous touch voltages present for longer than the permitted time.

Question 4

What is the purpose of a 'combined protective and neutral' (PEN) conductor in a TN-C system?

Accepted Answer

To carry both the normal return current and provide the protective earth function in a single conductor

Answer

A PEN conductor serves both as the return path for normal load current and as the protective earth conductor, which reduces cable quantity but introduces certain hazards if the PEN conductor breaks.

Question 5

Under what circumstances might an electrician need to install supplementary bonding even though the installation already has main bonding?

Accepted Answer

In locations of increased shock risk, such as bathrooms, where the basic protection requirements alone may not be sufficient

Answer

Supplementary bonding is required in special locations like bathrooms where simultaneous contact with multiple conductive parts is likely and body resistance is reduced by moisture.

ECS - Electrotechnical Certification Scheme Practice Test

ECS - Electrotechnical Certification Scheme Practice Test

ECS Earthing and Bonding 3