2017

Question 1

Explain why it is necessary to have a protective earthing conductor in a
final subcircuit protected by an RCD.

Answer 1

For the RCD to operate, an earth leakage path must exist.

Question 2

A voltage test for isolation is being carried out on a three-phase, 400V oven that may or may not be live.

State the main safety issue that would occur if the only testing done was between each phase.

Answer 2

One phase could still be alive

Question 3

A three-phase PVC flexible cable supplying a 400V, three-phase motor has been replaced with a three-core neutral-screened flexible cable.

State ONE advantage that a neutral-screen flexible cable has when
compared to a PVC flexible cable.

Answer 3

Any ONE of:
● Better mechanical protection.
● Cable damage is confined to within the cable.

Question 4

Each electrical installation has a prospective short-circuit current rating.

State ONE reason why it is important to consider the prospective-shortcircuit current rating when selecting fittings and accessories (such as the main switch and earth and neutral busbars) for the main switchboard of the electrical installation.

Answer 4

The fittings and accessories must be capable of withstanding the PSSC under fault conditions without damage.

Question 5

Each electrical installation has a prospective short-circuit current rating.

State ONE reason why it is important to consider the prospective-shortcircuit current rating when selecting protective devices for the main switchboard of the electrical installation.

Answer 5

The protective devices must be capable of safely clearing the prospective short-circuit current without damage.

Question 6

State ONE reason why a phase-reversal relay would be installed in the final subcircuit supplying a three-phase crane hoist.

Answer 6

Any ONE OF:
● To ensure that up-limit and down-limit of the hoist will operate correctly.
● To prevent the hoist going down when the up button is pushed.
● To prevent the hoist going up when the down button is pushed.
● To prevent the hoist motor operating in incorrect direction of rotation.

Question 7

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

Answer 7

● The thermistor detects a rise in
temperature in the windings

● It opens the control circuit when a preset temperature has been exceeded.

Question 7

A 230V, plug-in heater has an output of 2kW. The supply voltage to the
heater fluctuates between 230V to 240V.

Describe the effect this voltage fluctuation has on the power output of the heater elements.

Answer 7

The power will vary by the square of the change in voltage

Question 8

What do the initials BJT mean when applied to semiconductors?

Answer 8

A bi-polar junction transistor

Question 9

What is a transducer?

Answer 9

A device that changes energy from one form to another

Question 10

Explain why rewireable fuses are classed as coarse excess current
protection.

Answer 10

Any ONE of:
● They have very inaccurate
operating characteristics.
● The rewireable fuse may operate
within 4 hours at 1.5 times the
current rating.

Question 10

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

Answer 10

● 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 11

Explain why HRC fuses and MCBs are classed as close current
protection.

Answer 11

Any ONE of:
● They have very accurate operating
characteristics.
● The HRC fuse or MCB will operate
within 4 hours at 1.5 times the
current rating.

Question 12

Six 230V final subcircuits are supplied from a distribution switchboard. All final subcircuits are protected by 20A HRC fuses.

The submains to the distribution switchboard is protected by a 63A HRC fuse.

Use the terms pre-arcing time and arcing time to explain how correct
discrimination is achieved between the 20A HRC fuse and the 63A HRC
fuse.

Answer 12

The total of the pre-arcing time and arcing time of the 20A HRC fuse must be less than the pre-arcing time of the 63A HRC fuse.

Question 13

Most electrical installations are supplied from an MEN system of supply.

(a) The earth fault loop is an integral part of an MEN system of supply to an electrical installation.

(i) State ONE reason why the earth fault loop through the main neutral
conductor to the star point of the distribution transformer is more
important than the earth fault loop through the main earth and mass
of earth to the star point of the distribution transformer.

Answer 13

It is the part of the loop where the
impedance is consistently low.

Question 14

Most electrical installations are supplied from an MEN system of supply.

(a) The earth fault loop is an integral part of an MEN system of supply to an electrical installation.

(ii) State ONE reason why the main neutral conductor and the main
earthing conductor in an electrical installation could be carrying similar
currents.

Answer 14

There is a high resistance joint in the
main neutral conductor.

Question 15

State TWO factors that determine the level of the prospective short-circuit current in a low voltage electrical installation.

Answer 15

● Capacity (kA) of the supply source
● The impedance of the earth fault loop.

Question 16

One function of the main earthing conductor and the earth electrode in a low voltage MEN electrical installation is to connect protective earthing conductors and equipotential bonding conductors to the mass of earth.

State TWO other functions of the main earthing conductor and earth
electrode.

Answer 16

● To keep the voltage between neutral
and earth to 0V.

● To ensure the voltage in the electrical
installation does not rise above 230V
under fault conditions.

Question 17

State FOUR components of an MEN system that are found in a MEN low
voltage electrical installation.

Answer 17

Any FOUR of:
● The main earth bar
● The main earthing conductor.
● The earth electrode
● The MEN connection (link)
● The main neutral bar
● The main neutral conductor
● Protective earthing conductors
● Equipotential bonding conductors

Question 18

AS/NZS 3000 requires that a test voltage of 500V d.c. is used for an
insulation resistance test of a low voltage electrical installation.
State ONE reason why the voltage is a d.c. voltage.

Answer 18

Any ONE of:
● To ensure capacitive or inductive
reactance does not influence the test
result.
● A constant maximum voltage is a more stringent test than the momentary peaks of an a.c. waveform.

Question 18

State ONE reason why a polarity test is carried out on the mains of a
single-phase, low voltage electrical installation.

Answer 18

To ensure that no phase/neutral
transposition has occurred.

Question 19

An earth loop impedance test at the main switchboard of a single-phase low voltage electrical installation shows a result of 50Ω

There is an MEN link between the neutral busbar and earth bus bar.
State ONE reason why such a high test result would be obtained

Answer 19

There is a high impedance somewhere in the
earth fault loop.

Question 20

A single-phase low voltage, MEN electrical installation incorporates only one MEN switchboard.

In the MEN switchboard, the MEN link between the main neutral bar and main earth bar is missing.

State ONE hazard that could occur if there was an earth fault in the
electrical installation.

Answer 20

Any ONE of:
● Under fault conditions, the protective
devices will not operate within the
required time.
● The voltage may rise or fall.
● Dangerous touch voltages may appear
of bonded conductive or extraneous
parts.

Question 21

State ONE reason why a current transformer is dangerous when its
secondary winding is left in the open-circuit condition.

Answer 21

A high voltage can be present at the
secondary terminals.

Question 21

In an office, a 40A RCCB and 20A MCB protect a 230V final subcircuit supplying two permanently-connected electrical appliances. The MCB has tripped. When the MCB is reset, it trips again when the supply is restored to the final subcircuit. The fault is either in the fixed-wiring or in a permanently-connected electrical appliance. The final subcircuit has been isolated at the switchboard. If the fault is in the fixed wiring. The test instrument to be used is a multi-function test instrument with insulation resistance (with 500V and 250V ranges), resistance, voltage and temperature functions. (i) State the action taken before the testing. (ii) State the function and range selected on the test instrument. (iii) Describe how testing to locate the fault is carried out. (iv) State the test result when the expected fault is found.

Answer 21

(i) ● Disconnecting the fixed wired appliances ● Disconnecting the final sub circuit at the switchboard (ii) ● Selecting the insulation resistance function ● Selecting the 500V range (½ mark) (iii) Testing between the phase and neutral of the final subcircuit at the switchboard. (iv) An extremely low reading on the insulation resistance tester.

Question 22

If the fault is in a permanently-connected electrical appliance. The test instrument to be used is a multi-function test instrument with insulation resistance (with 500V and 250V ranges), resistance, voltage and temperature functions. (i) State the action taken before the testing. (ii) State the function and range selected on the test instrument. (iii) Describe how testing to locate the fault is carried out. (iv) State the test result when the expected fault is found.

Answer 22

(i) ● Disconnecting the fixed wired appliances (ii) ● Selecting the resistance function ● Selecting the lowest range or auto-range (iii) Testing between the phase and neutral of each electrical appliance. (iv) Much less than the resistance of the appliance in normal service

Question 23

Define the term impedance as it applies to an a.c. circuit.

Answer 23

It is the total opposition in Ω to the current flow in an a.c. circuit.

Question 24

Impedance is comprised of three components. State TWO of those components.

Answer 24

Any TWO of: ● Resistance (R) ● Inductive reactance (Xl) ● Capacitive reactance (XC)

Question 25

On a switchboard of a low voltage electrical installation, protective devices were installed that have a rupturing capacity lower than the prospective short circuit current rating of the installation. State what could occur if there was a short-circuit fault on the electrical installation.

Answer 25

Any ONE of: ● A flash-over could occur on the switchboard due to inadequate kA rating of protective devices ● The protective devices could be damaged.

Question 26

Why is a 500 kVA transformer able to operate at a heavier load when used in a low ambient temperature environment, than when it is used in a high ambient temperature environment

Answer 26

Because the transformer in the low ambient temperature environment can dissipate more heat than the transformer in the high ambient environment.

Question 27

(i) What test instrument is used to determine whether protective devices on a switchboard have the correct rupturing capacity. (ii) Where in the electrical installation would the test instrument stated in (i) be used.

Answer 27

(i) An earth-fault loop impedance tester. (ii) At or close to the switchboard.

Question 28

In which type of motor is the direction of rotation changed by reversing the connections to the start winding.

Answer 28

Any ONE of: ● Split-phase motor ● Capacitor start motor ● Cap-start, cap-run motor

Question 29

In which type of motor is the direction of rotation changed by reversing the connections to the field windings.

Answer 29

Any ONE of: ● Universal motor ● Series motor

Question 30

State TWO safety reasons why the secondary of a CT metering circuit must be short-circuited before disconnecting the meters.

Answer 30

● To prevent a high voltage developing in the circuit. ● To prevent damage to the CT winding insulation

Question 31

Explain the purpose of electrical interlocks in a star/delta starter

Answer 31

To prevent the star and delta contactors closing simultaneously

Question 32

The flexible cable connecting the motor to the starter is 2.5 mm2 and is 3 metres long. An earth continuity test carried out between the end of the protective earthing conductor of the cable and the frame of the motor showed a result of 0.8 Ω (i) State the reason why this test result is not acceptable. (ii) State the reason why this test result was obtained.

Answer 32

(i) The test result is too high for the length of the flexible cable. (ii) There is a high resistance joint in the connection to the motor frame.

Question 32

State the effect of the electrical interlocks failing in a star/delta starter.

Answer 32

Any ONE of: ● It could prevent the motor starting. ● It could cause a phase-to-phase short.

Question 33

When the load on a transformer changes from no-load to full-load, there is a voltage drop at the secondary terminals. State ONE technical reason why this voltage drop occurs.

Answer 33

● The windings have an impedance. ● When the increased current flows through the impedance. ● A voltage drop occurs across the windings ● So, the transformer terminal voltage is lower.

Question 34

A three-phase 400V, 10 kW induction motor in an industrial machine is controlled by a star-delta starter. The motor was repaired and is ready to be reconnected. The flexible cable (6 phase conductors and an earth conductor) that connects the motor to the star/delta starter is connected to the motor. (a) Describe how to test the motor and cable to identify the motor windings. Identify the type of instrument used, and state the expected test results.

Answer 34

The description must cover: ● Using an ohmmeter. ● Testing between the ends of the phase conductors. ● A low resistance between two phase conductors indicates a winding. ● Each winding will be a similar resistance.

Question 35

An insulation resistance test has to be carried out on the motor and flexible cable. For this test: ● State the type of test instrument used. ● State the test voltage used. ● Describe how the test is carried out. ● State a test result that will permit the motor to be returned to service.

Answer 35

● Use an insulation resistance tester ● A test voltage of 500V d.c. ● Test between each of the three windings. ● Test between each of the three windings and the motor framework ● A test result test of 1MΩ or more for each test.

Question 36

State ONE function of the earth electrode in a low voltage electrical installation supplied from an MEN distribution system.

Answer 36

Any ONE of: ● To ensure that the voltage cannot rise above 230V. ● To ensure that the potential difference between the neutral and earth is 0 V. ● To connect earthing conductors and exposed conducive parts of the installation to earth.

Question 37

New mains have been installed in a 230V, single-phase domestic installation. The installation has been livened. (i) A voltmeter and two other pieces of equipment are used to test whether a phase/neutral transposition has occurred on the mains. State the two other pieces of equipment. (ii) Using the equipment stated in (i) describe how to test for a phase/neutral transposition on the mains.

Answer 37

(i) ● Remote (independent) earth ● Trailing lead (ii). ● Insert the remote earth stake in the ground a reasonable distance away from the installation earth ● Connect trailing lead between the remote earth and the voltmeter. ● Use the voltmeter to test at the supply side of the main switch. ● Use the voltmeter to test at the earth/neutral bar.

Question 37

New mains were installed in a 230V, single-phase domestic installation and the phase and neutral conductors were transposed at the mains entry box. State TWO hazardous situations that would occur if the installation was livened.

Answer 37

Any TWO of: ● The main fuse will not operate. ● Exposed conductive parts will be live at up to 230V. ● Opening the main switch does not make the installation safe. ● Internal components of equipment are live with control switch in the off position.

Question 38

New mains have been installed in a 230V, single-phase domestic installation. The installation has been livened. (iii) State the test results if there is no transposition on the mains.

Answer 38

(iii) ● At the supply side of the main switch – about 230V. ● At the earth/neutral bar – about 0 V.

Question 38

New mains have been installed in a 230V, single-phase domestic installation. The installation has been livened. (iv) State the test results if there is a transposition on the mains.

Answer 38

(iv) ● At the supply side of the main switch – about 0 V. ● At the earth/neutral bar – about 230 V.

Question 39

State ONE reason why the protective device for the 230V single-phase domestic installation will not operate if a phase/neutral transposition occurs.

Answer 39

The impedance of the fault path between to earth stake and the star point of the distribution transformer is too high.

Question 40

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The phase and neutral were transposed at the supply side of the motor isolating switch. The motor was turned on. (ii) State the reason why the RCCB does not operate - even though there is a phase/neutral transposition.

Answer 40

(ii) The current detected between the phase and the neutral is still balanced so the RCCB will not operate.

Question 41

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The phase and neutral were transposed at the supply side of the motor isolating switch. The motor was turned on. (i) State ONE reason why the motor still operates. (ii) State the reason why the RCCB does not operate - even though there is a phase/neutral transposition. (iii) State the reason why the RCCB continues to provide protection to the motor final subcircuit - even though there is a phase/neutral transposition. (iv) State the reason why the MCB does not operate - even though there is a phase/neutral transposition. (v) State the reason why the MCB continues to provide protection to the motor final subcircuit - even though there is a phase/neutral transposition. (vi) State ONE hazard that exists because of the phase/neutral transposition.

Answer 41

(i) The motor will operate normally regardless of the phase/neutral polarity of the supply.

Question 42

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The phase and neutral were transposed at the supply side of the motor isolating switch. The motor was turned on. (iv) State the reason why the MCB does not operate - even though there is a phase/neutral transposition.

Answer 42

(iv) There is no overcurrent or shortcircuit between phase and neutral so the MCB will not operate.

Question 42

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The phase and neutral were transposed at the supply side of the motor isolating switch. The motor was turned on. (iii) State the reason why the RCCB continues to provide protection to the motor final subcircuit - even though there is a phase/neutral transposition.

Answer 42

(iii) Any earth leakage in the motor final subcircuit means that the phase and neutral currents are out of balance so the RCCB will operate.

Question 43

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The phase and neutral were transposed at the supply side of the motor isolating switch. The motor was turned on. (v) State the reason why the MCB continues to provide protection to the motor final subcircuit - even though there is a phase/neutral transposition. (vi) State ONE hazard that exists because of the phase/neutral transposition.

Answer 43

(v) Any overcurrent or short-circuit between phase and neutral will still cause the MCB to operate.

Question 44

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The phase and neutral were transposed at the supply side of the motor isolating switch. The motor was turned on. (vi) State ONE hazard that exists because of the phase/neutral transposition.

Answer 44

(vi) The motor is live with the isolating switch in the off position.

Question 45

A RCCB and MCB protect a final subcircuit supplying a 230V, single-phase wash-down pump. The neutral and earth were transposed at the supply side of the motor isolating switch. What would occur when the pump was turned on? State a reason to support your answer.

Answer 45

● The RCCB will trip. ● The phase and neutral currents would be unbalanced as the PEC would carry return current.

Question 46

State TWO tests that would detect the interchange of the phase and neutral conductors before the final subcircuit is livened.

Answer 46

● Circuit connections test (polarity) ● Visual check

Question 47

A low voltage electrical installation has an earth fault loop impedance of 0.1 Ω when measured at the switchboard. Calculate the prospective short circuit current of the electrical installation.

Answer 47

I = V / Z = 230 / 0.1 = 2300 A

Question 48

A low voltage, three-phase industrial electrical installation has a prospective short circuit current of 15 kA at the main switchboard. On the main switchboard a final subcircuit with a load of 57 A is protected by HRC fuses. (i) State the current rating of the HRC fuses selected for the final subcircuit. (ii) State the voltage rating of the HRC fuses selected for the final subcircuit. (iii) State the rupturing capacity of the HRC fuses selected for the final subcircuit.

Answer 48

(ii) 63 A (ii) Any voltage rating of 400V a.c. and above. (iii) Any rating in excess of 15 kA

Question 49

Define the term cut off characteristic as it applies to an HRC fuse

Answer 49

This is the value of fault current that cuts off (operates) the fuse that is less than the prospective short-circuit current.

Question 50

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

Answer 50

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

Question 51

An electrical installation has a prospective short-circuit current rating of 3.7 kA when measured at the switchboard. There are rewireable fuses on the switchboard. (i) State ONE reason why the rewireable fuses on the main switchboard need to be replaced. (ii) State TWO types of protective devices that can be used to replace the rewireable fuses.

Answer 51

(i) Because rewireable fuse have a maximum rupturing capacity of 1 kA (ii) ● MCBs ● HRC fuses