Electricity Testing Flashcards
What is an Insulation Resistance Tester?
A unit that is powered by either an internal battery operated power supply or by a hand operated voltage generator.
The insulation tester voltage (500V) is applied between any normally ‘live’ part and the normally earthed, case, frame or exposed metal part of the appliance.
The resistance measured by the tester should not be less than 1 mega ohm.
State TWO reasons why the prove-test-prove method of testing for isolation is used.
To ensure that the instrument is operating correctly.
To ensure that the circuit has been isolated before it is worked on.
State ONE situation where an isolating switch would carry two safety warning tags?
When two or more persons are working simultaneously on the equipment.
You are testing at the supply side of a three-phase main switch to see if the switchboard is isolated.
Between which terminals would you test to clearly establish that isolation has taken place?
Test between each phase terminal and earth.
Test between each phase terminal and neutral.
A three phase, fixed wire electrical appliance that has been operating normally is to be removed for maintenance.
An isolating switch has been turned off. A voltage test was carried out at the appliance terminals to test for isolation.
The testing showed the terminals to be live. State TWO possible causes for this situation.
The isolation switch is damaged or faulty.
The incorrect isolater has been operated.
A thermal overload provides protection for a three-phase motor.
What is the main type of fault the thermal overload is designed to provide protection for?
Overload.
A 400V three-phase motor is supplied from a final sub-circuit protected by MCBs. The motor is required to be isolated but the isolater is not located adjacent to the motor.
Isolation has been confirmed by using the prove-test-prove method. State two actions either of which can be taken to ensure that the motor remains isolated.
Lock the isolater in the off position
Or, open and lock the MCBs.
Disconnect the cables at the load end of the conductor
Or, disconnect the cables at the switchboard.
You have proved that an appliance is isolated and have ensured the continued isolation of the appliance.
The fixed-wired appliance is supplied via a flexible cable from the lockable isolating switch. You have disconnected the flexible cord from the appliance (that is, the flexible cord is still connected to the isolating switch).
State FOUR precautions which you would take to ensure the safety of persons and property after you have disconnected the appliance.
- Each conductor must be insulated and made electrically safe.
- Each conductor should be marked and labelled to permit reconnection to the correct terminals.
- Mechanical protection of the cables.
- Steps taken to prevent access to the cables.
An appliance is supplied via a flexible cable from a lockable isolating switch. The circuit supplying the appliance is protected by HRC fuses.
You have proved that the appliance is isolated using the prove-test-prove method.
Describe FOUR methods, any one of which will ensure the continued isolation of the electrical appliance.
- Lock open the MCBs and attach a danger tag.
- Lock open the isolating switch and attach a danger tag.
- Disconnect the circuit at the switchboard and attach a danger tag.
- Disconnect the circuit at the isolator and attach a danger tag.
You have repaired a 400V, three phase, Class I electrical appliance that incorporates a control switch and now need to test the appliance to ensure it is electrically safe before you can connect it to the electricity supply.
State FOUR tests using test instruments that you would carry out before connecting it to the supply.
Insulation Resistance Test
Earthing Continuity
Continuity of Conductors
Correct Circuit Connections (Polarity)
