Electric Wiring Testing & Inspection Instruments Requirements

In order to carry out the verification process proficiently, the person conducting the inspection and test must be in possession of suitable test instruments. In this article, we look at some of the key requirements for electric wiring testing and inspection instruments.

The commonly used instruments for electric wiring testing and inspection include:

• Continuity tester (low ohms).
• Insulation resistance tester.
• Loop impedance tester.
• Residual current device (RCD) tester.
• Prospective fault current (PFC) tester.
• Approved test lamp or voltage indicator.
• Proving unit.

Nowadays we have dual or multifunction instruments, so you may have continuity and insulation resistance in one unit, loop impedance and PFC tests in one unit and loop impedance, PFC and RCD tests in one unit, and so on.

Continuity Tester (Low Resistance Ohmmeter)

A continuity tester should have a no-load source voltage between 4 to 24 V and capable of delivering an AC or DC short circuit (not less than 200 mA). It should have a resolution, that is, detectable difference in resistance of at least 0.05 m𝝮.

Insulation Resistance Tester

An insulation resistance tester must be capable of delivering 1 mA when the required test voltage is applied across the minimum acceptable value of insulation resistance.

Accordingly, an instrument selected for use on a low-voltage system should be capable of delivering 1 mA at 500 V across a resistance of 1 m𝝮.

Loop Impedance Tester

A loop impedance tester works by creating, in effect, an earth fault for a short moment, and is connected to the circuit via a plug or by ‘flying leads’ connected separately to line, neutral and earth.

This instrument should only allow an earth fault to exist for a maximum of 40 ms and a resolution of 0.01 𝝮 is sufficient for circuits up to 50 A. Above this circuit rating, the ohmic values become too small to give such an accuracy using a standard instrument and more specialized equipment may be needed.

Residual Current Device (RCD) Tester

RCD tester is usually connected by the use of a plug, although flying leads are required for non-socket-outlet circuits. This instrument allows a range of out-of-balance currents to flow through the RCD to cause its operation within specified time limits.

This instrument should not be operated for longer than 2s and it should have 10% accuracy across the full range of test currents.

Prospective Fault Current (PFC) Tester

This instrument measures the prospective line-neutral or line-earth fault current at the point of measurement using the flying lead. It is normally one half of a dual, loop impedance /PSCC tester.

An Approved Test Lamp or Voltage Indicator

A typical approved test lamp is illustrated below:

The leads and probes associated with test lamps, voltage indicators, voltmeters are generally recommended to have the following characteristics:

• The leads should be adequately insulated and ideally fused.
• The leads, where separate, should be easily distinguished from each other by colour.
• The leads should be flexible and adequately long for their purpose.
• The probes should incorporate finger barriers to prevent accidental contact with live parts.
• The probes should be insulated and have a maximum of 2 mm of exposed metal, but preferably have spring-loaded enclosed tips.

Proving Unit                          

This is an optional item of test instruments, in that test lamps should be proved on a known supply which could be say an adjacent socket or lighting point. Nonetheless, to prove a test lamp on such a known supply may involve entry into enclosures with the associated hazards that could come up.

A proving unit is a compact device, capable of electronically developing 230 V DC across which the test lamp may be approved. The exception to this are test lamps incorporating 230 V lamps which will not activate from the small power source of the proving unit.

Test lamps must be proved against a voltage similar to that to be tested. Therefore, proving test lamps that incorporate an internal check such as shorting out the probes to make a buzzer sound is not acceptable if the voltage to be tested is higher than that delivered by the test lamp.

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