Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Thoery2 EW > 2016 > Flashcards

2016 Flashcards

1
Q

State the test voltage used for carrying out an insulation resistance
test on three-phase, 400V, electrical equipment incorporating MIMS
elements.

A

● 500 V
● d.c.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

State the minimum permitted test result for the insulation resistance
test on the electrical equipment.

A

0.01MΩ

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

State what is meant by the term pull-out torque as it applies to an
a.c. induction motor.

A

The maximum value of torque the motor can produce.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

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.

State ONE reason why the rewireable fuses need to be replaced.

A

Because rewireable fuse have a maximum rupturing capacity of 1 kA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

State what occurs to an a.c. induction motor if the load torque
exceeds the pull-out torque.

A

The motor will stall

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

A single-phase induction motor is being installed in a circular saw bench.

State ONE reason why a capacitor-start motor is used rather than a splitphase motor.

A

This type of motor develops higher starting torque under load.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Measurements from a single-phase light industrial electrical installation
show the following:

● 235V
● 70A
● 15.2 kW
Calculate the power factor of the circuit.

A

235 x 70 = 16450 VA
= 15200 / 16450
= 0.9241

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

A single-phase induction motor is being installed in a ventilation fan for a ducted air conditioning system.

State ONE reason why a capacitor-start capacitor-run motor is used rather than a split-phase motor.

A

Any ONE of:
● It runs a lot quieter
● It develops a higher running torque
● It develops a higher starting torque

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Define the term current rating as it applies to HRC fuses.

A

This is the maximum current that the
fuse is designed to carry continuously
without deterioration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Define the term breaking (or rupturing) capacity as it applies to
HRC fuses.

A

The maximum fault current the fuse can safely interrupt.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

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

A

● To limit the current flowing through the electrode and tube.

● To produce a high voltage when the
starter opens to start the tube glowing.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

(i) State ONE precaution that could be taken when using an insulation
resistance tester, set on the 500V d.c. range, to carry out an
insulation resistance test on electrical equipment containing semiconductor devices.

State ONE reason why you would take the precaution stated in (i).

A

(i) Any ONE of:
● Disconnect semi-conductor devices
● Bridge out between phase and
neutral

(ii) The voltage of the test instrument may be damage the semi-conductor devices.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Define the term direct light distribution as it applies to a light fitting.

A

Any ONE of:
● The light from the fitting is thrown
directly onto the surface being
illuminated.
● 90% to 100% of the light is directed in a specific direction.
● There is no obstruction or shading
between the light source and the surface being illuminated.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe the method by which a rotating magnetic field effect is
created in a single-phase split-phase induction motor.

A

● The start winding and the run
winding are physically
displaced.

● The start winding and the run
winding have different
impedances.

● This causes a phase
displacement between the
start winding current and the
run winding current

● Creating a rotating magnetic
field effect.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Explain why a single-phase capacitor-start induction motor has a higher starting torque when compared to a single-phase split-phase induction motor?

A

● The capacitor increases the
difference in the impedances
between the start winding and
run winding.

● This causes a larger phase
displacement between the
start winding current and the
run winding current

● Creating a stronger rotating
magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

If an external starting relay is to be used for a single-phase, split-phase
induction motor, which winding will it switch?

A

The start (or auxiliary) winding.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

State the reason why transformers are rated in kVA.

A

Because kVA is the maximum current
rating stated regardless of the pf of the load to be connected.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Explain the reason why a 500 kVA transformer at Scott Base, Antarctica
can be operated more heavily loaded than an identical 500 kVA transformer installed in Wellington.

A

The transformer at Scott Base can
dissipate more heat than the
transformer in Wellington because the ambient temperature is lower.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

A three-phase, 10 kW pump motor needs repairing. The motor has been
isolated and cables disconnected. The motor cables are still connected to the DOL starter.

State TWO precautions which must be taken to ensure the safety of
persons and property while the work site is left unattended.

A

Any TWO of:
● The cable must be made electrically
safe.
● Protect the cables to prevent damage
● Prevent access to the cables.
● Prevent access to the cables.
● Replace the danger tag with an out-ofservice tag.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Describe how isolating three-phase electrical equipment is different from switching off that three-phase electrical equipment.

A

Isolating means:
● that the motor has been deliberately
disconnected from the electricity supply

● and precautions taken to prevent
reconnection

Switched off means:
that the electricity ceases to be supplied to the motor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Describe THREE methods of preventing the reconnection of the electricity supply to an isolated fixed-wired, three-phase machine that is being worked on.

A

Any THREE of:
● Lock open the isolating switch.
● Lock open the MCBs or fuses.
● Disconnect the circuit supplying the
electrical equipment.
● Use an access permit or “hold card”
system.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Describe the circumstances where a danger tag is used.

A

Where there is a possibility of personal
danger through the supply being restored.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

A voltage test using the prove-test prove method is being carried out on the supply side of a three-phase main switch to see if a switchboard is isolated.

What tests need to be made to clearly establish that isolation has taken place.

A

● Test between each phase and earth.

● Test between each phase and neutral.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

A three-phase, 400V workshop has been completely rewired. The work included all final subcircuits including a fixed wired, single-phase 2 kW water heater with a MIMS element. There was no change to the mains, main earthing system or
switchboard.

(a) An insulation resistance test is required to be carried on the new work and the installation has been isolated.

(ii) The water heater has been disconnected before testing of the
electrical installation commences.
State the THREE other actions that need to be taken before insulation
resistance testing can commence.

A

● Disconnect the MEN link
● Disconnect the main neutral
● Turn all switches and circuit breakers to the ON position

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

A three-phase, 400V workshop has been completely rewired. The work included all final subcircuits including a fixed wired, single-phase 2 kW water heater with a MIMS element. There was no change to the mains, main earthing system or
switchboard.

(a) An insulation resistance test is required to be carried on the new work and the installation has been isolated.

(i) For the insulation resistance test state:
● The type of instrument used.
● The test voltage – if applicable

A

● An insulation resistance tester
● 500V d.c.

25
Q

A three-phase, 400V workshop has been completely rewired. The work included all final subcircuits including a fixed wired, single-phase 2 kW water heater with a MIMS element. There was no change to the mains, main earthing system or
switchboard.

(a) An insulation resistance test is required to be carried on the new work and the installation has been isolated.

(iii) Describe how to carry out the insulation resistance test on the shop. Include the permitted minimum value of any test results.

A

The description has to include:
● Any ONE of:
* Bridging each phase and neutral in
turn.
Testing between the bridge and earth.
* Testing between each phase and earth
Testing between each neutral and earth

● A minimum of 1 MΩ test value for each test

26
Q

The required testing has been completed and the installation has been relivened. An earth-fault loop impedance test has been carried out on the main switchboard

(i) What other earth fault loop impedance test needs to be carried out?

(ii) State ONE reason why the earth fault loop impedance test stated in (i) is carried out.

A

(i) An earth fault loop impedance test in the socket outlet furthest from the switchboard.

(ii) To ensure the protection on the final subcircuit operates within the required time.

27
Q

An electrical installation has been livened but the MEN link was not
installed in the main MEN switchboard.

State what occurs when the electrical installation operates normally.

A

The installation operates normally

27
Q

A three-phase, 400V workshop has been completely rewired. The work included all final subcircuits including a fixed wired, single-phase 2 kW water heater with a MIMS element. There was no change to the mains, main earthing system or
switchboard.

(a) An insulation resistance test is required to be carried on the new work and the installation has been isolated.

(iv) Describe how to carry out the insulation resistance test on the water
heater. Include the permitted minimum value of any test results.

A

The description has to include:
● Testing between phase and earth
Testing between neutral and earth
● A minimum of 10,000 Ω test value

28
Q

An electrical installation has been livened but the MEN link was not
installed in the main MEN switchboard.

State TWO situations that could occur if an earth fault occurs in the
electrical installation

A

Any TWO of:
● The voltage to earth could rise above 230V
● The protection may not operate
● There may be a shock hazard on
exposed conducive parts or extraneous conductive parts.

29
Q

What characteristic is described by the term as current through the
protective device increases, the time taken to operate
decreases?

A

An inverse time-current characteristic

30
Q

What test would be carried out to determine if the rupturing capacity
of an HRC fuse is compatible with the prospective short circuit current
of an electrical installation.

A

An earth fault-loop impedance test.

31
Q

Three-phase, fixed-wired electrical equipment has been operating normally and has been isolated from an adjacent isolating switch.
Two phases were found to be live when voltage testing was carried out at the equipment terminals.

State what fault has occurred.

A

The contacts in the isolating switch have failed to open.

32
Q

A three-phase, 400V commercial oven has 3 MIMS elements connected in star. Each element is rated at 6 kW.

Calculate the phase current drawn by the oven.

A

I = P / V
= 6000 / 230
= 26.01 A.

33
Q

State ONE reason why the resistance of the protective earthing conductor
of a flexible cord for a Class I electrical appliance is set at a maximum of 1 Ω.

A

Any ONE of:
● Under fault conditions, it prevents the touch voltages rising to dangerous levels.
● To ensure sufficient fault current flows to operate the protective device.

34
Q

State ONE reason why an earth fault-loop impedance test is carried out on the socket outlet furthest from the main switchboard in a low voltage electrical installation.

A

Any ONE of:
● To confirm that the electrical protection for the final subcircuit will operate within the required time.
● To confirm that the electrical protection all other final subcircuits supplied from the main switchboard will operate with the required time.

35
Q

State ONE hazard that could occur when disposing of low-pressure or highpressure sodium-vapour lamps.

A

Any ONE of:
● Fire risk (sodium + moisture produces hydrogen)
● Risk of chemical burns (sodium + water produces sodium hydroxide (caustic
soda))

35
Q

State ONE reason why having a protective earthing conductor resistance of less than 1 Ω in a Class I electrical appliance effectively prevents electric shock hazards under fault conditions.

A

Any ONE of:
● Under fault conditions, it prevents the touch voltages rising to dangerous levels.
● To ensure sufficient fault current flows to operate the protective device.

36
Q

A three-phase induction motor is controlled by a star/delta starter that
incorporates thermal overloads.

State ONE reason why HRC fuses are installed at the origin of the final
subcircuit supplying the induction motor.

A

To provide short-circuit fault protection to both the final subcircuit and the motor.

37
Q

When the load on a transformer changes from no-load to full-load a voltage drop occurs at the secondary terminals.

(i) Is this voltage drop normal?

(ii) State a reason to support your answer in (i).

A

(i) Yes

(ii) Because of the current flow through the
impedance of the windings.

38
Q

Building alterations have been completed. A new correctly-rated flexible cable has been connected to the new hot water cylinder.

State TWO instrument tests that must be carried out on the new flexible cable and hot water cylinder to ensure that they are safe to connect to the electricity supply.

A

● Earth continuity test.
● Insulation resistance test.

39
Q

The electrician connecting the new cylinder to the contactor has found there is no lock or danger tag on the isolator.

State the action that the electrician, connecting the cylinder to the
contactor, must take to ensure that it is safe to connect.

A

Ensure the circuit is still isolated by using the prove-test-prove method.

40
Q

The new hot water cylinder has been connected to the contactor.

Describe how to test whether the hot water cylinder is effectively connected to the MEN earthing system. State the test instrument used.

A

● Use an ohmmeter

● Test between the frame of the hot
water cylinder and earth reference
(other than a bonded extraneous earth path) that is separate from the hot water cylinder final subcircuit.

41
Q

The elements in the existing hot water cylinder were rated at 1500 W each.

The elements in the new hot water cylinder are rated at 6000 W each.

State TWO factors relating to the existing final subcircuit that need to be considered because of the increase in load.

A

Any TWO of:
● Is the current rating of final subcircuit cable consistent with the increase in load.
● Are the HRC fuses correctly rated for the final subcircuit.
● Is the current rating of the isolator
consistent with the increase in load.
● Is the current rating of the contactor
consistent with the increase in load.

42
Q

State the purpose of the Zener diode?

A

To provide a stable, constant voltage supply to the connected load.

43
Q

What main advantage does a Zener diode have when compared to a
normal rectifier diode?

A

A Zener diode is designed to carry significant reverse current but a normal rectifier diode cannot.

44
Q

State the main purpose of the resistance R?

A

Any ONE of: (2 marks)
● It limits the current through the Zener diode.
● It drops the supply voltage to the voltage rating of the Zenor diode.

45
Q

State ONE reason why rectified three-phase a.c. is easier to smooth with filters, than rectified single-phase a.c.?

A

Any ONE of:
● The ripple amplitude is less
● The ripple frequency is greater.

46
Q

State ONE component that may be used as part of a circuit to filter a
rectifier output.

A

Any ONE of:
* Inductor
* Capacitor
* Resistor

47
Q

In an electrical installation the rupturing capacity of an MCB on the
switchboard is under-rated for the fault level of the installation.
State TWO situations that could occur if a short-circuit fault occurs on the final subcircuit the MCB protects.

A

Any TWO of:
● The MCB may be destroyed.
● The MCB may not clear the fault before damage occurs to the installation.
● Unwanted operation of upstream
devices
● A fire could be initiated

48
Q

An earth fault loop impedance test at the main switchboard of an electrical installation shows that prospective short-circuit current level is 3 kA.

(i) State ONE reason why the rewireable fuses on the main switchboard
need to be replaced.

A

The rewireable fuses are rated for
maximum fault currents of 1 kA

49
Q

An earth fault loop impedance test at the main switchboard of an electrical installation shows that prospective short-circuit current level is 3 kA.

State TWO types of protective devices that can be used to replace the rewireable fuses.

A

Any TWO of:
● MCBs
● HRC fuses
● RCBOs

50
Q

An HRC fuse has terms 63A - 440V - AC40 marked on it.
Define each of these terms.

63A: ______________________________________

440V: ______________________________________

AC40: ______________________________________

A

63A: This is maximum continuous
current.
the fuse is designed to carry.

440V: This maximum open-circuit
voltage.
the fuse is designed to withstand.

AC40: 40,000A is the maximum
prospective short circuit current.
The fuse can safely interrupt.

51
Q

An electrical installation comprises an MEN main switchboard and three
distribution boards.

State how correct discrimination occurs between protective devices if a fault occurs on a final subcircuit supplied from a distribution board.

A

Only the protective device protecting that final subcircuit operates.

52
Q

State ONE reason why an HRC fuse would be installed up-stream of the
MCBs on a switchboard in an existing single-phase commercial installation.

A

Any ONE of:
● The breaking capacity of the MCBs is too low for the PSSC of the installation.
● The PSSC level of the installation has increased.

53
Q

State the maximum permitted rated residual current of an RCD
installed for personal protection of children in a kindergarten.

A

10 milliamps.

54
Q

State the maximum permitted rated residual current of an RCD
installed for the protection of property.

A

300 milliamps.

55
Q

State the reason why Type AC RCDs are prohibited from being used in New Zealand.

A

Because Type AC RCDs do not have a residual pulsating d.c. function

56
Q

The phase and neutral on the supply side of a single-phase RCD have
been transposed.

Would the RCD operate if it detected an earth leakage fault on the
circuit it protects? State a reason to support your answer.

A

Yes

The RCD detects the imbalance regardless of
the polarity of the supply.

57
Q

Would an RCD operate if there was a short between neutral and earth
on the final subcircuit it protects? State a reason to support your
answer.

A

Yes

The RCD would detect and imbalance between
the phase and neutral currents.

Thoery2 EW (7 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions