ECS Electrical Supply Systems and Distribution 3

Question 1

What is the maximum permitted voltage drop from the supply terminals to any point in a final circuit for lighting loads under NEC guidelines?

Accepted Answer

3%

Answer

The NEC recommends a maximum voltage drop of 3% for branch circuits (final circuits), with a combined drop of no more than 5% from the service to the final outlet.

Question 2

In electrical distribution, what is meant by 'prospective short circuit current' (PSCC) at a given point?

Accepted Answer

The maximum current that would flow if a zero-impedance short circuit occurred at that point

Answer

Prospective short circuit current is the theoretical maximum fault current that would flow if a bolted (zero impedance) short circuit occurred, used to select appropriately rated protective devices.

Question 3

What is the purpose of power factor correction capacitors in an electrical distribution system?

Accepted Answer

To reduce reactive power and improve the power factor of the supply

Answer

Power factor correction capacitors supply reactive power locally, reducing the reactive current drawn from the supply and improving the overall power factor, which reduces distribution losses.

Question 4

Which instrument would an electrician use to measure the impedance of an earth fault loop at a point in an installation?

Accepted Answer

An earth fault loop impedance tester

Answer

An earth fault loop impedance tester injects a test current around the fault loop path and measures the resulting impedance, used to verify protective devices will operate within required disconnection times.

Question 5

What is the significance of the 'short circuit capacity' (SCC) rating marked on a circuit breaker?

Accepted Answer

The maximum fault current the breaker can safely interrupt without damage

Answer

The short circuit capacity (also called interrupting or breaking capacity) is the maximum prospective fault current the circuit breaker can safely break without being damaged or causing an arc flash.

ECS - Electrotechnical Certification Scheme Practice Test

ECS - Electrotechnical Certification Scheme Practice Test

ECS Electrical Supply Systems and Distribution 3