6.1 Types of tests

Every new installation, extension or alteration must be tested before it is energised, and existing installations are tested at intervals afterwards. Testing proves by measurement that conductors are joined properly, that insulation is sound and that no unintended path exists between live conductors. Three basic tests form the foundation of inspection work: earth continuity, insulation resistance and short circuit tests. Each is done with the supply isolated and proved dead, and each answers a different question about the safety of the wiring. Visual inspection always comes first, because a loose terminal or a crushed cable seen by eye saves time before any instrument is connected.

Safe practice common to all three tests

Several rules apply regardless of the test being done, and following them protects both the tester and the installation.

• Prove dead before touching: use a voltage indicator that has been proved on a known live source or proving unit before and after testing the circuit.
• Lock off and tag: keep the isolator under your own control so another worker cannot energise the circuit.
• Test in the right order: continuity of protective conductors first, then insulation resistance, so that earthing is confirmed before insulation tests are interpreted.
• Discharge after insulation tests: the cable acts as a capacitor and may hold charge after the test voltage is removed.
• Keep records: write every reading on the test schedule and note any repairs made, so the next person can compare results.
• Protect sensitive equipment: disconnect electronic loads before applying any test voltage higher than normal operating voltage.

6.1.1 Earth continuity tests

An earth continuity test confirms that the circuit protective conductor (CPC), the main and supplementary bonding conductors and all their joints form an unbroken, low-resistance path to the main earthing terminal. If a live conductor touches a metal enclosure, this path carries the fault current that makes the fuse or circuit breaker operate quickly. A broken or high-resistance CPC leaves the metalwork live and the protective device slow to trip. For example, in a school laboratory a loose earth screw in a socket outlet could leave a metal-cased appliance dangerous even though it works normally.

The resistance of a conductor depends on its material, length and cross-sectional area:

R = ρ × L ÷ A

For a copper CPC of 2.5 mm² and 20 m length, with ρ = 0.0175 Ω·mm²/m:

R = 0.0175 × 20 ÷ 2.5
R = 0.35 ÷ 2.5
R = 0.14 Ω

A reading far above the calculated value suggests a poor joint, a damaged strand or a wrongly sized conductor.

Equipment for earth continuity tests

Procedure for testing earth continuity

1. Inspect the installation visually: check that CPCs are present, sleeved green/yellow where required, and connected at every outlet, accessory and the distribution board.
2. Isolate the supply, prove it dead with a voltage indicator, then lock off and tag the isolator so nobody can switch it on.
3. Zero the instrument: join the test leads together and note their resistance, or use the instrument's null function, so that lead resistance is not counted as circuit resistance.
4. Connect one lead to the main earthing terminal and the other to the CPC at the point being tested. For a long circuit, use the trailing lead to reach the furthest outlet.
5. Press the test button and read the resistance. Record the value against the circuit number on the test schedule.
6. Repeat at every outlet, light fitting and exposed metal part on the circuit, since a break can occur anywhere along the run.
7. Test main bonding conductors to gas and water pipes, and supplementary bonding conductors, from the earthing terminal to the bonded part.
8. Compare each reading with the expected value from the conductor length and size. Investigate and correct any high or open-circuit reading, then retest.

Interpreting earth continuity results

• Very low, steady reading: the CPC is continuous and joints are sound. The value should agree with the calculated resistance for that conductor length and size.
• Reading much higher than calculated: this points to a loose terminal, corrosion or a partly cut conductor. Tighten or remake the joint and retest.
• Open circuit (infinity): the conductor is broken or not connected. Check at the last accessory where a good reading was obtained.
• Unsteady reading: this usually shows a loose connection that varies with movement. Remake the connection rather than accepting a lucky reading.
• Reading lower than expected: the instrument may be measuring through a parallel path such as bonded metalwork. Disconnect the parallel path or check the test set-up.

6.1.2 Insulation tests

An insulation resistance test measures how well the insulation resists leakage current between conductors that should be electrically separate. Insulation deteriorates through heat, moisture, rodent damage, mechanical injury and age. Finding a weak spot with a test voltage is much safer than finding it through a shock or a fire. For example, at a farm irrigation pump house, damp cable insulation may pass a continuity check yet leak dangerously to earth when tested at 500 V.

The test applies a direct voltage, higher than the normal operating voltage, and measures the resulting resistance in megohms (MΩ). The leakage current follows Ohm's law:

I = V ÷ R

With a test voltage of 500 V and an insulation resistance of 1 MΩ (1,000,000 Ω):

I = 500 ÷ 1,000,000
I = 0.0005 A = 0.5 mA

The higher the reading, the smaller the leakage and the better the insulation.

Equipment for insulation tests

Preparing for the insulation test

• Isolate the supply: switch off, prove dead and lock off the main switch so the test voltage is never applied to a live circuit.
• Remove lamps and disconnect equipment: electronic devices, dimmers, surge protectors, televisions and computers can be damaged by the test voltage or give false low readings.
• Close all switches and circuit breakers on the circuit: this makes sure the whole length of the wiring is included in the test.
• Select the correct test voltage: 500 V DC is normally used for circuits up to 500 V, such as 230/400 V systems. Extra-low-voltage circuits use a lower test voltage.
• Check the tester: confirm the battery is good and the leads read near zero when shorted on the continuity or check setting.

Procedure for testing insulation resistance

1. Complete the preparation above and confirm that the circuit is dead.
2. Connect the line and neutral conductors of the circuit together at the distribution board, or test each conductor separately if you want to identify a fault.
3. Connect one tester lead to the joined live conductors and the other to the circuit protective conductor or earth terminal.
4. Press and hold the test button until the reading settles, usually a few seconds, and read the value in MΩ.
5. Release the button and allow the instrument to discharge the circuit before disconnecting any lead.
6. Separate the line and neutral conductors from each other (disconnect the neutral from the neutral bar), then test between line and neutral.
7. For three-phase circuits, test each pair of live conductors, and each live conductor to earth.
8. Record each reading on the test schedule, reconnect everything, and then refit lamps and equipment.

Interpreting insulation resistance results

Readings are compared with the minimum values stated in the wiring regulations in force; for 230/400 V circuits tested at 500 V, the minimum is commonly taken as 1 MΩ. In practice a healthy new installation gives readings of many megohms, often above the instrument's upper limit.

• High reading, well above the minimum: insulation is in good condition.
• Reading just above the minimum: the installation may be acceptable, but investigate if it is much lower than similar circuits, since moisture or ageing may be starting.
• Reading below the minimum: the circuit fails. Split the circuit into sections and retest to find the damaged cable, joint or accessory.
• Zero or near-zero reading: a direct fault exists between the tested conductors. Look for trapped cable, a nail through a conductor or a wet junction box.
• Reading that climbs slowly: this is normal for long cables because of capacitance. Wait for the value to settle before recording.

6.1.3 Short circuit tests

A short circuit test checks that there is no unintended low-resistance connection between line and neutral conductors (or between phases) in a circuit that is switched off and disconnected from the supply. A short circuit allows very large current to flow, which overheats cables, trips protective devices or starts fires. For example, in a hotel guest-room wing, a lighting circuit that trips the breaker every time it is switched on may have a nail or staple pressing through the cable, creating a short between line and neutral.

The test is carried out with the circuit isolated, all loads removed and a resistance-measuring instrument connected across the conductors. A healthy circuit shows a very high or open-circuit reading between line and neutral. A low reading shows a short.

Equipment for short circuit tests

Procedure for testing for a short circuit

1. Isolate the supply, prove it dead and lock off the isolator.
2. Switch off or unplug all loads, and remove lamps, so that no lamp filament or appliance winding gives a false low reading.
3. Close all switches in the circuit under test so that the full length of the wiring is included.
4. Disconnect the circuit's line and neutral conductors from the distribution board terminals so that other circuits cannot influence the reading.
5. Connect the instrument leads across the line and neutral conductors at the board end.
6. Take the reading and note whether it is open circuit, high resistance or low resistance.
7. For three-phase circuits, repeat the test between each pair of phases, and between each phase and neutral.
8. If a low reading is found, divide the circuit at junction boxes or accessories and test each section until the faulty part is isolated, then repair and retest.

Interpreting short circuit test results

• Open circuit or very high resistance: no short exists between the tested conductors. The circuit is satisfactory in this respect.
• Low resistance, a few ohms or less: a short circuit is present, or a load has been left connected. Check that all lamps and appliances were removed before concluding the cable is faulty.
• Zero resistance: a solid short, often caused by cable damage, crossed conductors at a terminal or flooded accessories.
• Reading that changes when a switch is operated: a fault lies in the part of the circuit controlled by that switch, such as a damaged switch drop or fitting.

Prospective short-circuit current

The size of current that would flow in a short circuit can be estimated from the supply voltage and the loop impedance:

I = U ÷ Z

For a 230 V supply and a fault loop impedance of 0.46 Ω:

I = 230 ÷ 0.46
I = 500 A

The protective device must be able to interrupt this current safely, so its breaking capacity must be higher than the largest prospective fault current at its point of installation.