2018 Flashcards
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 the test voltage used for carrying out an insulation resistance test on three-phase, 400V, electrical equipment incorporating MIMS elements.
- 500 V
- d.c.
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.
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.
State one factor that determines the voltage drop in a 400V, final subcircuit TPS cable.
Any one of:
* Type of conductor material
* Cross-sectional area
* Load
* Length of cable
An isolator for 230V electrical equipment has been replaced.
During the work the phase (active) and neutral were accidentally transposed at the supply side of
the isolating switch.
What potential danger would this transposition create?
The internal circuitry of the electrical equipment would be live when the
isolating switch is in the OFF position.
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.
One phase could still be alive.
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 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.
State one reason why an RCD tester is used to test a PRCD.
Any one of:
* To verify that the PRCD operates within specified residual current
limits.
* To verify that the PRCD operates within specified time limits.
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.
To provide short-circuit fault protection to both the final subcircuit and the motor.
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 it
still operates correctly.
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.
