2015 Flashcards by Olamide Habeeb Oseni

State the reason why a residual current device (RCD) used for personal protection is required to switch both the phase and neutral poles?

Any ONE of:
● Transposed connections on the RCD have no effect
● Ensure there are no transient voltages on the neutral
● Removes the danger if the neutral is above earth potential

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A three-phase, delta-connected induction motor is controlled by a DOL starter. The motor is protected by HRC fuses and a thermal overload.
State what would occur if one fuse blew (opened) while the motor was
operating normally under full load.

Any ONE of:
● The current in the two live phases rises.
● The motor will stall
● The thermal overload will operate

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State the reason why a three-pole MCB would be used to protect a threephase final subcircuit instead of three HRC fuses.

Any ONE of:
● To ensure that a fault on one phase will fully disconnect the supply to the load
● Where the load supplied is sensitive to the loss of one phase
● It is easier to reset after a fault
● MCB cost is significantly less than
equivalent HRC fuses

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Describe the characteristics of a thermistor.

A thermistor is a device which changes resistance with temperature

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Where in a motor circuit would a thermistor be located?

It would be located in the windings of
the motor

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New mains have been installed in a 230V, single-phase domestic
installation. During the work the phase and neutral were interchanged at the mains entry box.

State TWO hazardous situations that could arise if the installation was
livened.

Any TWO of:
● A touch voltage hazard between
conductive parts and the mass of earth.
● Protective devices within the installation may not operate under fault conditions.
● The neutral is being switched throughout the installation.
● The protective device for the installation may not operate.

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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.

Explain why the voltage is a d.c. voltage.

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.

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Some protective devices on a switchboard in an electrical installation have a fault duty lower than the prospective short-circuit current rating of the
installation.

State TWO hazards that could occur when a high prospective short-circuit
fault current occurs in the electrical installation.

Any TWO of
● Flash over on switchboard between
protective devices
● A protective device could rupture and expose live parts
● A protective device could rupture and cause a fire

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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.

Explain why a voltage of 500V is applied.

To stress the insulation at a voltage
above 325V a.c. (or peak a.c. voltage.

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100 metres of three-phase mains cable gave an insulation test result of 100MΩ. Calculate the insulation resistance of 500m of the same type of cable.

R2 = R1 x L1 / L2
= 100 x 100 / 500
= 20M

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Define the term ramp-time as it applies to a variable speed drive
controlling a motor.

The time taken for the motor to change from one speed to another.

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State ONE reason why thermistors would be used for motor protection in addition to thermal overloads and HRC fuses.

To protect the motor against excessive temperature in the motor windings.

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State the reason why 6 conductors are required between a star/delta
starter and a three-phase motor

● To enable the motor to be connected in star on starting.

● And to be connected in delta while
running

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A three-phase motor is controlled by a variable speed drive. The motor is
required to operate at low speeds for extended periods.

What additional actions need to be taken in respect to the motor?

● Additional motor cooling is required
● As conventional cooling is inadequate at low speeds for extended periods

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The total clearance time of an HRC comprises the pre-arcing time and the arcing time.

Define the term pre-arcing time.

It is the time taken between the fuse
elements beginning to melt and the
initiation of the arc

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The total clearance time of an HRC comprises the pre-arcing time and the arcing time.

Define the term arcing time

The time between the initiation of the
arc and the extinguishing of the arc.

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Describe what happens in an electrical installation when a fault occurs and there is correct discrimination between protective devices.

Only the protective device electrically
closest to the fault operates.

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State what would occur if a 230V, single-phase capacitor-start motor that was running suddenly experienced a 50% drop in speed due to mechanical
overloading and the electrical protection failed to operate?

Any ONE of:
● The motor would stall and burn out
● The centrifugal switch will re-engage
and the motor will burn out

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State the precaution that needs to be taken when an HRC fuse rated at AC 20, 550V is used for protection in a d.c. final subcircuit.

Any ONE of:
● Ensure that the fuse markings show an equivalent DC rating.
● Obtain correct information from
manufacturer if there is no DC rating.

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A single-phase, split-phase induction motor comprises a stator, a start
winding, a centrifugal switch and a run winding.

The start winding and centrifugal switch form a closed circuit, the run
winding is open-circuited. Explain why the motor does not rotate when it is switched on.

● To start the motor a simulated rotating magnetic field is required

● The field is created by the different
electrical characteristics between the
start and run windings.

● An open-circuit run winding means no rotating magnetic field effect can be produced

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The single-phase split-phase motor in a refrigerator is a sealed unit that
incorporates only the start and run windings.

State how the start winding is disconnected after starting.

By an external current relay

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State ONE reason why a universal motor is generally used in
conjunction with a gear train.

Any ONE of:
● To limit the no-load speed of the
motor
● To produce a high torque at a
reduced speed

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The single-phase split-phase motor in a refrigerator is a sealed unit that
incorporates only the start and run windings.

At what percentage (%) of full-load speed does the start winding
become disconnected?

60% to 80% of full-load speed

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Describe what could occur if a universal motor was not used in
conjunction with a gear train.

At no-load the motor could attain
excessive speed and self-destruct

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Describe how to reverse the direction of rotation of a single-phase capacitor-start motor.

Any ONE of: ● Reverse the connections to the start winding ● Reverse the connections to the run winding

Describe how to reverse the direction of rotation of a universal motor.

Any ONE of: ● Reverse the connections to the armature ● Reverse the connections to the field windings

Define the following lighting term: Luminous intensity

Any ONE of: ● The power of a light source to emit light in a given direction. ● The luminous flux per unit solid angle in a specified direction. ● A measure of the perceived power of light in a particular direction

Define the following lighting term: Luminous flux

Any ONE of: ● The brightness of a light source ● The light emitted by a source or received by a surface, irrespective of the directions in which I is distributed. ● A measure of the perceived power of light (flux) - emitted in all directions irrespective of the radiation pattern of the light source.

The halogen cycle ensures that a halogen lamp maintains a constant initial output and colour temperature throughout the lamp’s life. Explain why the halogen cycle ensures that a halogen lamp maintains a constant initial output and colour temperature.

● The tungsten is being re-deposited on the lamp filament at the same rate as it is being evaporated ● Therefore, the filament maintains the initial output and colour temperature over its life.

Define the following lighting term: Luminous efficacy

Any ONE of: ● The amount of light that arrives upon a given area of a surface. ● The ratio of the luminous flux emitted by a lamp to the power consumed by it. ● The specific efficiency that a lamp has in converting the power in electrical watts into lumens

Define the following lighting term: Colour rendering

Any ONE of: ● The ability of a light source to reveal the natural colours of an object. ● How accurate a light source renders colour to the human eye.

The halogen cycle ensures that a halogen lamp maintains a constant initial output and colour temperature throughout the lamp’s life. Describe how the halogen cycle operates within the halogen lamp

● Tungsten molecules are evaporated from the filament and convection currents carry the tungsten towards the lamp wall. ● Near the lamp wall the tungsten combines with a halogen and convection currents carries the tungsten halide back to near the filament. ● The tungsten disassociates from the halide and re-deposits on the filament

State the effect on the operation of a discharge lamp if the ballast shortcircuited while the lamp was operating and the control system was inoperative.

The current through the lamp rises rapidly, and the lamp will fail.

Danger tags and out-of-service tags are designed to promote safety in the workplace. Give a brief description of circumstances when a Danger Tag is used.

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

Danger tags and out-of-service tags are designed to promote safety in the workplace. Give a brief description of circumstances when an Out-of-Service Tag is used.

Where equipment is faulty or damaged and using that equipment would cause damage or injury.

Danger tags and out-of-service tags are designed to promote safety in the workplace. State TWO precautions to be taken when attaching a danger tag to an isolating switch.

Any TWO of: ● Make sure the correct isolating switch is tagged. ● Make sure the switch is in the “OFF” position before it is tagged ● Fasten the Danger Tag securely so that it will not come off. ● Test to ensure isolation has taken place. ● Appropriate details are entered on the tag

Define the term switching off as it relates to three-phase equipment.

Means that the electricity ceases to be supplied to the equipment.

Define the term isolation as it relates to three-phase equipment.

● Means the equipment has been deliberately disconnected from the electricity supply ● And precautions have been taken to prevent reconnection of the supply to the equipment.

Describe how the prove-test-prove method of testing is carried out.

● The test instrument is checked to be operating correctly on a known live source. ● The equipment is tested to confirm (or otherwise) that it is isolated. ● The test instrument is again checked on a known live source to ensure is still operates correctly.

A new 230V electrical installation incorporates permanently-connected electrical equipment with MIMS elements. State the permitted test result for an insulation resistance test for the electrical installation with all the permanently-connected electrical equipment disconnected.

1MΩ

0.01MΩ

A three-phase induction motor normally runs on full-load. State ONE reason why this motor should not be run lightly loaded when it normally runs on full load.

Any ONE of: ● The power factor of the motor will be poor ● The motor will be less efficient. ● Running costs rise

A clip-on ammeter is being used to test the integrity of the insulation of a repaired Class I electrical appliance, while the appliance is live. Where on the appliance would the clip-on ammeter be used?

Clamped around the protective earthing conductor

A three-phase induction motor has been operating normally on fullload. The motor starts to over-heat even though it is drawing the normal full load current. State ONE reason why this situation would occur.

Any ONE of: ● The motor ventilation is restricted in some manner. ● The cooling fan has become detached from the motor shaft

A clip-on ammeter is being used to test the integrity of the insulation of a repaired Class I electrical appliance, while the appliance is live. State the maximum test result that would allow the appliance to be returned to service.

5 mA

Explain what is meant by the term inverse time-current characteristic in relation to protective devices?

● As the current through the protective device increases. ● The time taken to operate decreases.

A clip-on ammeter is being used to test the integrity of the insulation of a repaired Class I electrical appliance, while the appliance is live. State TWO components that are typically used to smooth the d.c. output of a rectifier.

Any TWO of: ● Capacitors ● Inductors ● Resistors

The output circuit of an autotransformer must be insulated for the highest input or output voltage of the autotransformer. State ONE reason for this requirement.

An auto transformer can fail in such a way so the primary voltage appears across the secondary.

What do the initials BJT mean when applied to semiconductors?

A bi-polar junction transistor

State the function of a transducer.

A device that changes energy from one form to another

Any ONE of: ● Voltage test ● Load test

One HRC fuse protecting the final subcircuit supplying a concrete-making vibrating plant has blown (operated). The plant is driven by a permanently-connected, 400V, three-phase, 1.5 kW induction motor controlled by a DOL starter. The motor and starter are controlled by an adjacent isolator. The motor has been isolated and the isolator has been locked off and tagged. Initial testing shows that there is no fault between the switchboard and the load side of the DOL starter. An insulation resistance test is to be carried out on the cables supplying the motor and the motor itself. Describe how this testing is carried out and include: ● What cables/windings need to be disconnected or separated. ● The test instrument used. ● The test voltage applied. ● The test result expected for the fault.

The description has to include: ● Disconnecting the cables at the motor ● Separating the windings of the motor ● Use of an insulation resistance tester. ● Applying a test voltage of 500V d.c. ● Testing the cable Testing between each phase conductor. Testing between each phase conductor and the protective earthing conductor. ● Testing the motor Testing between each winding. Testing between each winding and the frame of the motor. ● A insulation resistance test result of much less than 1 MΩ

One HRC fuse protecting the final subcircuit supplying a concrete-making vibrating plant has blown (operated). The plant is driven by a permanently-connected, 400V, three-phase, 1.5 kW induction motor controlled by a DOL starter. The motor and starter are controlled by an adjacent isolator. The motor has been isolated and the isolator has been locked off and tagged. Initial testing shows that there is no fault between the switchboard and the load side of the DOL starter. The fault has been rectified and the motor has been tested and reconnected to the DOL starter but not livened. Testing has to be carried out to ensure the motor is effectively connected to the MEN earthing system. (i) Describe how this testing is carried out and include: ● The test instrument used ● The test result that will allow the motor to be re-commissioned (ii) State what the test carried out in (i) verifies.

(i) The description has to include: ● Use of an ohmmeter (½ mark) ● Testing between the frame of the motor and earthing point independent of the motor circuit ● A test result of 1Ω or less. (½ mark) (ii) Any ONE of: ● That the protective devices will operate under earth fault conditions. ● It proves the motor is effectively earthed

Describe the basic operation of a gas discharge lamp.

Light is produced by an electric discharge by passing current through a gas or vapour in the discharge tube

Name TWO types of gas discharge lamps.

Any TWO of: ● Mercury vapour ● Metal halide ● Sodium vapour ● Fluorescent light ● Neon light

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

There is no certainty that the fuse with operate within 4 hours at 1.5x its rated current

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

The HRC fuse or MCB will operate with certainty within 4 hours at 1.5x its rated current

State TWO requirements for good illumination.

Any TWO of: ● Provides clear vision without eye strain ● Sufficient light intensity ● Minimum glare ● Uniform light intensity throughout the illuminated area ● Suitable colour rendering

The total time taken for a HRC fuse to operate is defined as the total operating (or clearance) time. The total clearance time comprises the prearcing time and the arcing time. Define the term pre-arcing time.

The pre-arcing time is the time taken for a fault current large enough to operate the fuse

The arcing time is time taken from the initiation of the arc Until the fuse elements have opened and the arc extinguished

Describe how correct discrimination is achieved between protective devices in an electrical installation.

Only the protective device closest to the fault operates

A MEN, low voltage electrical installation operates at standard low voltage. What type of switchboard must be electrically closest to the point of supply?

An MEN switchboard

A MEN, low voltage electrical installation operates at standard low voltage. What is required to be attached at one end of the main earthing conductor?

A tag warning against disconnection

A MEN, low voltage electrical installation operates at standard low voltage. To which parts of the electrical installation are the ends of the main earthing conductor connected?

● The earth bar ● The earth electrode

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

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

State the THREE points in an MEN system that are earthed.

● The star point of the distribution transformer ● The distribution line neutral ● The consumers installation

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2015 Flashcards

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