June2013

Question 1

List TWO types of earth electrode that may be used to connect an MEN
system to the mass of earth.

Answer 1

Any TWO of:
● Earth rod
● Buried electrode
● Metallic reinforcing
● Earth mat

Question 2

State TWO main functions of a ballast (choke) in a fluorescent light fitting.

Answer 2

To limit the current flowing through the
electrode and tube.
● To produce a high voltage when the
starter opens to start the tube glowing

Question 3

An alternator has 48 poles and is to feed into the MEN distribution system.
Calculate the speed of rotation of the alternator to enable it to feed into the
distribution system.

Answer 3

125 rpm

Question 4

A RCCB protects a 230V final subcircuit and a Class I, fixed-wired electrical
appliance. The RCCB has operated (tripped).
State what has occurred to cause the RCCB to operate (trip).

Answer 4

Any ONE of:
* There is an imbalance between the
neutral current and phase current
* There is earth leakage current in
the circuit
● Greater than the residual tripping current
of the RCCB

Question 5

What do the initials BJT mean when applied to semiconductors?

Answer 5

A bi-polar junction transistor

Question 6

What is a transducer?

Answer 6

A device that changes energy from one
form to another

Question 7

State the main reason why it is important to consider prospective-shortcircuit current when selecting a protection device for installation on the
switchboard of a low voltage electrical installation.

Answer 7

To ensure that the protective device can
safely clear the prospective short-circuitcurrent of the installation.

Question 8

A low voltage domestic installation is required to have a main earthing lead.
(i) To which parts of the electrical installation are the ends of the main
earthing conductor connected?Except for the parts stated in (g)(i), state:
(A) What is required to be attached at one end of the main earthing
lead? (B) At which end of the main earthing lead is the part stated in (ii)(A)
to be attached?

Answer 8

(i) ● The earth bar (½ mark)
● The earth electrode (½ mark)
(ii) Any ONE of:
● (A) A notice warning against
disconnection
(½ mark)
(B) At the earth electrode (½ mark)
● (A) A notice at the switchboard
stating
(B) The position of the earth
electrode

Question 9

Define the term cut off time as it applies an HRC fuse.

Answer 9

This is the time it takes the fuse to
interrupt the current flow

Question 10

Define the term total clearing time as it applies an HRC fuse.

Answer 10

This is the time it takes to interrupt the
flow of current and extinguish the arc.

Question 11

State how a thermistor protects a three-phase induction motor when a
mechanical overload occurs.

Answer 11

● The thermistor detects a rise in
temperature in the windings
(1 mark) How the motor is
stopped must be
● It opens the control circuit when a pre- described
set temperature has been exceeded.

Question 12

State the formula used to determine the power consumed by:
(i) An inductive three-phase load.

Answer 12

(i) P = √3 x VL x IL x pf

Question 13

State the formula used to determine the power consumed by:(ii) A resistive single-phase load.

Answer 13

(ii) P = V x I