General

Question 1

What does ‘double insulated’ mean?

Answer 1

In case of the appliance having accessible metal parts, that supplementary insulation is provided in addition to the normal basic insulation in order to protect against electric shock in case of a breakdown of the basic insulation.

Question 2

What is arcing?

Answer 2

Means maintaining a flow of current through the air between two terminals .

Question 3

State two reasons why the neutral in the MEN system is double earthed.

Answer 3

To establish a parallel path to the star point at the distribution transformer.

Ties the voltage between phase and earth to 230 V

Minimises the potential difference between neutral and earth.

Question 4

A 230V Class II plug-in heating appliance is supplied from a socket outlet and is operating at full load.

State the main effect on the appliance if it was supplied via an extra-long extension cord while operating at full load.

Answer 4

The power output of the appliance will drop.

The voltage at the appliance terminal will drop.

Question 5

The three-phase and neutral electricity supply to the three-phase fixed-wired electrical appliance is controlled by only one isolating switch. You have replaced the isolating switch because it is faulty and relivened the circuit.

When the isolating switch is in the off position, you find that one terminal on the phase is live.

State two reasons why one appliance terminal was live.

Answer 5

One phase and the neutral conductor have been transposed at the switch.

One of the phase contacts has failed to open in the switch.

Question 6

An electrical service technician intends to isolate a faulty fixed-wired three-phase electric motor, but has withdrawn the wrong fuses for that motor on the factory switchboard.

State two dangerous conditions that could occur because the electrical service technician withdrew the wrong fuses.

Answer 6

Other equipment could fail or stop during an essential operation (lift or elevator control circuit)

The faulty motor remains energised.

Motor could draw an arc if fuse withdrawn on load.

Question 7

HRC fuses provide protection for a three-phase motor circuit.

What is the main type of fault the HRC fuses are designed to provide protection for?

Answer 7

Short-circuit current

Question 8

State one situation where an isolating switch would carry two safety warning tags?

Answer 8

When two or more persons are working simultaneously on the same equipment.

Question 9

HRC fuses protect the final subcircuit supplying a three-phase motor.

Describe how you would carry out a test, using an ohmmeter to find out whether the protective earthing conductor of the three-phase motor is connected to the earthing system (that is, the protection will operate if a fault occurs).

Answer 9

Test between the frame of the motor

And, a known earth other than the motor circuit.

Question 10

The NZ single phase 230V supply operates at a frequency of 50 Hz. Define the meaning of the term Hz.

Answer 10

The number of complete cycles per second.

Question 11

State two reasons explaining how a protective earthing conductor contributes to the electrical safety of a Class I electrical appliance when a fault occurs.

Answer 11

Under fault conditions, touch voltages are maintained at safe levels.

It provides a low resistance path for the fault current.

Ensures sufficient fault current can flow to operate the protection.

Question 12

State two reasons why a neutral conductor is required in the cable supplying a three-phase electrical appliance which has heating loads that draw different values of current on each of the phases.

Answer 12

The final subcircuit neutral conductor provides the return path from the loads back to the distribution transformer for the resultant ‘out of balance’ current from three phases.

The neutral conductor is required to ensure that the potential across each single phase load is 230V with respect to earth.

Question 13

Define the term tripping factor.

Answer 13

It is the ratio of the MCB minimum tripping current to the MCB current rating.

Question 14

Describe the internal operation of a combined thermal/magnetic MCB when an overload occurs on the final subcircuit it protects.

Answer 14

The overload current heats a bi-metal strip.

The bi-metal strip bends when heated and trips the MCB.

Question 15

Describe the internal operation of a combined thermal/magnetic MCB when a short circuit occurs on the final subcircuit it protects.

Answer 15

The short-circuit current induces a magnetic field in an armature coil.
Which attracts a metal strip and trips the MCB.

Question 16

Define the term inverse-time-current characteristic in relation to gG type HRC fuses.

Answer 16

As current through the HRC increases, the time taken to operate decreases.

Question 17

When selecting a flexible cord for fitting to a single-phase electrical appliance it is necessary to consider its length and cross-sectional area.

Lost four other physical factors that may need to be considered in selecting the cord.

Answer 17

Number of cores

Colour coding

Sheath suitable for the environment

Cord construction

Flexibility

Question 18

When connecting a three-phase motor to the electricity supply it is necessary to connect the three supply conductors to the motor windings. It is also necessary to securely connect the motor to earth by means of a protective earthing conductor.

State one reason why the protective earthing conductor is required.

Answer 18

To maintain the machine framework at earth potential under fault conditions.

OR, to ensure the protection device operates in the event of a fault.

Question 19

A three phase, fixed-wired electrical appliance that has been operating normally is to be removed for maintenance.

An isolating switch has been turned off. A voltage test was carried out at the appliance terminals to test for isolation.

The testing showed the terminals to be live. State TWO possible causes for this situation.

Answer 19

The isolating switch is faulty.

The incorrect isolator has been operated.

Question 20

A 400V, three phase, fixed-wired appliance is supplied by a flexible cable from an adjacent isolating switch. The appliance has been isolated and the flexible cable disconnected at the appliance terminals. State TWO precautions which must be taken to ensure the safety of persons and property after the isolation and disconnection has taken place.

Answer 20

Prevent access to the cables.

Attach an out-of-service tag.

Mechanically protect cables.

Question 21

In a bakery a 400V, three-phase oven incorporates three single-phase star connected 6kW MIMS elements. The 400V final subcircuit supplying the oven is controlled by HRC fuses.

The oven is faulty. One HRC fuse blew and, a week later, another HRC fuse blew. One element in the oven still operates.

Describe how you would isolate the oven so it is safe to locate the faults.

Answer 21

Remove the fuses. Test the circuit for isolation using the prove-test-prove method. A voltage test using a voltmeter would determine isolation between the load side of the fuses and the supply side of the contactor.

Question 22

State ONE unsafe effect a phase and neutral transposition could create in an electrical appliance.

Answer 22

Most of the internal wiring will be alive at 230V to earth with the switch in the OFF position

Question 23

An RCD is live. The test button is pushed, but the RCD does not trip.

State TWO conditions, any one of which would prevent the RCD from tripping.

Answer 23

Open circuit in the sensing coil.

Test button circuit is faulty.

Tripping mechanism is frozen.

Electronic circuit is open circuited.

Question 24

State the TWO circumstances in which 230V single-phase electrical appliances must not be deliberately connected to earth.

Answer 24

In a Class II appliance or double insulated.

When they are supplied from an isolating transformer.

Question 25

A new three-pin plug has been fitted to a three-core PVC sheathed flexible cord supplying a Class I electrical appliance.

State the visual inspections that need to be made BEFORE fitting the cover to the plug.

Answer 25

Check that:

There is no damage to the conductor cores.

The basic insulation is up to the terminals.

The conductors are tightly terminated.

The protective earthing conductor is longer than other conductors.

The conductors are terminated at the correct terminals.

Question 26

A Class I, 230V, 600W plug-in faulty washing machine was put back into service without being repaired. It was plugged into a live socket outlet and turned on and the homeowner received an electric shock.

The outlet is protected by a 20A HRC with a utilisation category of 1.5.

Tests showed the following;

• protective earthing conductor resistance of 9 ohms.
• phase to frame fault of 8 ohms.
• the washing machine still operated normally when switched on.
  a) Use calculations to show why the HRC fuse did not operate, when the washing machine was operating.
  b) Use calculations to show why the homeowner received an electric shock.

Answer 26

a) Iph = P/V
= 600/230
= 2.6 A

I fault = 230/9+8
           = 13.52 A 

I total = 16.12 A 

Fusing current = 30A

The fault current is too low to operate the fuse.

b) Touch Voltage = Vd across protective earthing conductor.

Vde = I * R
= 13.52 * 9
= 121.68V

The homeowner received a shock because of the 121.68 touch voltage.

Question 27

A 230V, Class I pottery kiln is supplied by a circuit that has RCCB and MCB protection at the switchboard.

The kiln is sitting on the tiled section of a wooden floor.

a) The kiln isolator has been replaced. During the work the PHASE and NEUTRAL were accidentally transposed at the SUPPLY side of the isolating switch. When the isolating switch was turned on:

What effect would this transposition have on the operation of the kiln?

b) What effect would this transposition have on the operation of the protective devices under FAULT CONDITIONS?
c) What effect would this transposition create?
d) If the NEUTRAL and EARTH were accidentally transposed state what would occur when the isolating switch was turned on.
e) What would happen when switched on if the PHASE and EARTH were transposed?
f) What effect would this have on the protective devices under FAULT CONDITIONS?
g) What danger would this transposition create?

Answer 27

a) The kiln will operate normally.
b) Neither the MCB or RCCB will be affected.
c) The kiln circuitry would be live when the isolating switch is in the ‘OFF’ position.
d) The RCCB will trip as the PEC would carry the return current.
e) The kiln will not operate.

f) The MCB will operate
The RCCB will operate

g) The kiln framework will be alive at 230V to earth.

Question 28

When replacing an HRC fuse which has blown, the replacement must have FOUR electrical characteristics that are similar to the original fuse.

Answer 28

Category of Duty (Rupturing Capacity)

Fusing factor

Current rating

Voltage Rating

Question 29

A three-phase induction motor, controlled by a DOL starter has been operating normally and has been switched off. When restarted using the start button, the contactor closes, but as soon as the start button is released again the contactor drops out of the circuit.

Note: The voltage level is normal
The protection is correctly rated
and is NOT faulty.

State TWO causes for this kind of fault.

Answer 29

The hold in contacts are stuck in the open position.

There is foreign matter preventing the core from closing.

Question 30

What type of protection device is installed on three-phase final subcircuit a to SPECIFICALLY provide protection for

i) The loss of one phase on the electricity supply
ii) The reversal of phase rotation on the electricity supply.

Answer 30

i) Phase-failure relay

ii) Phase reversal protection.

Question 31

It is necessary to have a reliable protective earthing conductor for a Class I electrical appliance.

State TWO reasons why a protective earthing conductor contributes to the electrical safety of a Class I appliance.

Answer 31

It maintains the voltage of the frame of the appliance to about 0 volts.

It provides a low resistance path for the fault current.

Ensures sufficient fault current can flow to operate the protective devices.

Question 32

What does MIMS stand for?

Answer 32

Mineral Insulated Metal Sheath