2013 Flashcards
List TWO types of earth electrode that may be used to connect an MEN
system to the mass of earth.
Any TWO of:
● Earth rod
● Buried electrode
● Metallic reinforcing
● Earth mat
State TWO main functions of a ballast (choke) in a fluorescent light fitting.
● To limit the current flowing through the
electrode and tube.
● To produce a high voltage when the
starter opens to start the tube glowing.
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.
N = 60f / P
= 60 x 50 / 24
= 125 rpm
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).
● 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
What do the initials BJT mean when applied to semiconductors?
(i) A bi-polar junction transistor
What is a transducer?
(ii) A device that changes energy from one
form to another
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.
To ensure that the protective device can
safely clear the prospective short-circuit current of the installation.
A low voltage domestic installation is required to have a main earthing lead.
To which parts of the electrical installation are the ends of the main
earthing conductor connected?
● The earth bar
● The earth electrode
A low voltage domestic installation is required to have a main earthing lead.
Except for the earth bar and electrode:
(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 (A)
to be attached.
Any ONE of:
● (A) A notice warning against
disconnection
(B) At the earth electrode
● (A) A notice at the switchboard
stating
(B) The position of the earth
electrode
Define the term cut off time as it applies an HRC fuse.
This is the time it takes the fuse to
interrupt the current flow
There are various protection devices protecting the motor and motor final
subcircuit.
Complete the following table and state the type of device that is designed primarily to deal with the following faults.
Type of fault Type of protection device
(i) Short circuit
_______________________________
(ii) Over-temperature
_______________________________
(iii) Phase polarity
______________________________
● Short circuit:
HRC fuse or MCB
● Over-temperature:
Thermistor
● Phase polarity:
Phase reversal relay
Define the term total clearing time as it applies an HRC fuse.
This is the time it takes to interrupt the
flow of current and extinguish the arc.
State how a thermistor protects a three-phase induction motor when a
mechanical overload occurs.
● The thermistor detects a rise in
temperature in the windings
● It opens the control circuit when a pre- described
set temperature has been exceeded.
The motor has a protection rating of IP 53.
(i) What does the term IP mean?
(ii) What is indicated by the first number?
(iii) What is indicated by the second number?
(i) Any ONE of:
● International protection
● Ingress protection
(ii) Protection against the ingress of solid
objects
(iii) Protection against the ingress of
liquids.
A three-phase 400V, 3kW induction motor in an industrial machine is controlled by a star-delta starter.
The motor was been removed from the machine for cleaning and checking.
The motor has been returned, and checks and tests must be carried out to ensure the motor is safe to reconnect.
(a) The cable supplying the motor is still connected to the motor.
Describe how to test the motor and attached cables to identify the motor
windings. Identify the type of instrument to be used, and state the
expected test results.
Use an ohmmeter.
● Test between the cable ends. A low
resistance indicates a winding.
(1 mark)
● Each winding is of a similar resistance.
A three-phase 400V, 3kW induction motor in an industrial machine is controlled by a star-delta starter.
The motor was been removed from the machine for cleaning and checking.
The motor has been returned, and checks and tests must be carried out to ensure the motor is safe to reconnect.
The cable supplying the motor is still connected to the motor.
Two tests using test instruments need to be carried out on the motor and attached cables to ensure it is safe to connect to the electricity supply.
(i) State the first test that needs to be carried out.
(ii) For each of the two tests state:
● The type of test instrument used.
● The test voltage, if applicable
● How the test is carried out.
● A test result value that will permit the motor to be returned to
service.
(i) An earth continuity (protective earthing conductor) test.
(ii) ● The earth continuity (protective
earthing conductor) test
* Use an ohmmeter (½ mark)
* Test between the protective
earthing conductor at
beginning of the cable and
the frame of the motor.
* A test result of 1Ω or less
● The insulation resistance test
* Use an insulation resistance
tester
* 500V d.c. test voltage (½ mark)
* Test between each of the three windings.
* Test between each of the
three windings and the motor
framework
* A test result for each test of
1MΩ or more.
In an office, a 40A, RCCB and 20A, MCB protect a 230V final sub-circuit that supplies:
● 6 double socket outlets supplying various electrical appliances.
● 2 fixed-wired electrical appliances supplied from permanent connection
units. When operating normally each appliance has a resistance of
8.54Ω.
The MCB has tripped. All the plug-in electrical appliances were disconnected.
When the MCB is reset, it trips again when the supply is restored to the final subcircuit.
The MCB is not faulty and is correctly rated for the circuit. Overloading is not an issue.
The fault is in the final subcircuit or in one of the fixed-wired appliances.
(a) What do you need to do to ensure it is safe to locate the fault?
● Conduct a voltage test using the prove test-prove method.
● To confirm that the final subcircuit is isolated.
In an office, a 40A, RCCB and 20A, MCB protect a 230V final sub-circuit that supplies:
● 6 double socket outlets supplying various electrical appliances.
● 2 fixed-wired electrical appliances supplied from permanent connection
units. When operating normally each appliance has a resistance of
8.54Ω.
The MCB has tripped. All the plug-in electrical appliances were disconnected.
When the MCB is reset, it trips again when the supply is restored to the final subcircuit.
The MCB is not faulty and is correctly rated for the circuit. Overloading is not an issue.
The fault is in the final subcircuit or in one of the fixed-wired appliances.
(b) All testing to locate the fault will be carried out at the switchboard.
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) The first test at the switchboard will determine whether or not the
fault is in the final subcircuit. For this test state:
● The action taken before the
testing
● The function selected on the test instrument
● The range selected on the test instrument
● How the testing is carried out
● The test result if the fault is in the final subcircuit.
Testing the final subcircuit
● Turn off both fixed-wired
appliance
● Select the insulation resistance
function
● Select 500V range (½ mark)
● Testing between load side of the
MCB and the neutral bar.
● Near 0Ω for the fault
In an office, a 40A, RCCB and 20A, MCB protect a 230V final sub-circuit that supplies:
● 6 double socket outlets supplying various electrical appliances.
● 2 fixed-wired electrical appliances supplied from permanent connection
units. When operating normally each appliance has a resistance of
8.54Ω.
The MCB has tripped. All the plug-in electrical appliances were disconnected.
When the MCB is reset, it trips again when the supply is restored to the final subcircuit.
The MCB is not faulty and is correctly rated for the circuit. Overloading is not an issue.
The fault is in the final subcircuit or in one of the fixed-wired appliances.
(b) All testing to locate the fault will be carried out at the switchboard.
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.
(ii) You have found that the fault was not located in the final subcircuit.
The second test at the switchboard will determine whether or not the
fault is in one of the fixed-wired appliances. Only one appliance can
be tested at a time.
For testing one appliance state:
● The action taken before the testing
● The function selected on the test instrument
● The range selected on the test instrument
● How the testing is carried out
● The test result if the fault is in an appliance.
Testing the fixed-wired-appliances:
● Turn on the fixed-wired appliance
● Selecting the resistance function
● Selecting the lowest range or the
auto range
● Testing between load side of the
MCB and the neutral bar.
● Much less than 8.54Ω.
In New Zealand, the standard distribution system has the neutral
conductor connected to the earth at the main switchboard on all low
voltage installations. What name is given to this type of system?
Multiple Earthed Neutral or MEN system
State TWO technical reasons why the neutral conductor is earthed.
Any TWO of:
● Limits the voltage to 230V to earth under
fault conditions.
● Limits the voltage between neutral and
earth to about 0V.
● Ensures that under fault conditions, the
voltage between any earthed metal and the
mass of earth never rises above 0V
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.
Any TWO of:
● The MCB is unlikely to clear the fault.
● The MCB may be destroyed.
● A current path may be created through the MCB
In an electrical installation the rupturing capacity of an MCB on the
switchboard is over-rated for the fault level of the installation.
State TWO situations that could occur if a short-circuit fault occurs on the final subcircuit that the MCB protects.
The MCB will interrupt the fault as intended
State the TWO conditions that occur when the secondary circuit of a CT is
opened when the CT is live.
Any TWO of:
● A high voltage is induced in the
secondary circuit
● The flux density in the CT rises.
● The CT could be damaged
State what is meant by the term current rating as it applies to an HRC
fuse?
This is the maximum current that the fuse is designed to carry continuously
Describe what occurs in an electrical installation when a fault occurs and
there is correct discrimination between protective devices.
● The protective device nearest the fault operates
● Before any other protective device.
