PAT Testing In Detail

Visual Inspection of a BS1363 13amp plug
Visual Inspection of a BS1363 13amp plug

Inspection Intervals

The environment in which electrical appliances are used is an important factor in determining how often they should be inspected and tested. The IET code of practice gives basic guidance on intervals and should be the starting point in making a decision. As time goes on and a few cycles of inspection and testing have elapsed, intervals can be adjusted if necessary. It is beneficial (and officially recommended) to keep records of test and inspection results; they provide evidence of a responsible approach to health and safety and they can be used to assess wear and tear.

Visual Checks

The first step in visual checking is to examine the appliance’s label to ascertain its voltage, power consumption (or load current) and electrical class; earthed equipment is Class I, double-insulated Class II. Next the plug is checked, it should have insulated live and neutral pins and no cracks, damage or signs of over-heating. If possible it is opened and checked for correct fitting of the cord-grip, correct terminal wiring, correct fuse rating and no signs of over-heating. Both plug and fuse are only passed if they bear the correct British Standard (BS) numbers. The cable is checked for cuts, twisting, excess length and the correct thickness of the conductors for the expected electrical load.

The appliance’s casing is examined, it should be free from cracks, corrosion and signs of overheating and there should be no access to live parts. Appropriate safety-guards should be present and switches should function correctly.

A Note on Fuse Ratings

In an effort to standardise and simplify the use of BS1362 fuses (the type fitted to 13 Amp plugs), the IET recommends 3A fuses for appliances rated 700W and below, and 13A fuses for those rated above 700W. Manufacturers may choose differently rated fuses and these ratings should be adhered to where the appliance comes from a reputable supplier.

Electrical Tests

The tests described below are industry-recognised tests, however, their implementation depends to some extent on the particular testing equipment being used. The following tests are described as performed by Megger test equipment.

Earth Continuity and Earth Bond Testing

Continuity and bond testing are both used to confirm the existence of a safety earth return path in a Class I appliance. However, they differ in how they work and both have their benefits and drawbacks.

1. Continuity (low current or “Soft”) testing

This test uses a 200 mA test current at 4 to 5 Vdc. The test current is low so that there is no risk of damaging earth connections that may only exist for functional, rather than safety reasons.

2. High current (Bond) testing

Uses a 10 A or a 25 A AC test current at 8-9 V.
This test is used where concerns that an earth may be maintained by a few strands of wire or where poor surface contact by the probes or clips could give a misleading reading, this test is frequently the preferred test by many organisations.

3. In situ bond testing (200mA)

In situ bond testing is the practice of measuring the earth integrity of a Class I asset that is “hard wired” to the electrical supply. There would be no convenient method to connect the asset to the tester without disconnecting the equipment from the supply. Instead the tester is connected to an adjacent electrical outlet and a bond test conducted to the case of the asset. The measured resistance will be:
The resistance of the asset to the earth circuit (wall plate or fused spur)
The resistance of the fixed wiring through the building to the earth point into which the tester is connected. This could be a few metres or 10’s of metres. Allowance should be made for the additional building (fixed) wiring resistance when deciding if the asset is safe to use.

Insulation testing

This is used to confirm there is separation between the live conductors (live and neutral) and any accessible conductive parts. The instrument will connect together the live and neutral conductors and then apply a 500V or 250V insulation test between the temporarily connected live/neutral and the earth. Typically a test voltage of 500V is used. However, on sensitive equipment or especially where there are surge protection devices fitted, 250V can be used. For equipment with no return earth (Class II) the use of an external probe is required to provide the return.

Leakage testing
1. Differential earth leakage test (IDIFF).

Also called protective conductor current test.
Test voltage:
230Vac 50 Hz 19.99mA
110Vac 50 Hz 19.99mA
The differential earth leakage test determines whether any current is flowing to earth. Normally appliances should have no, or very little, earth leakage current. Class II appliances can exhibit earth leakage through their mountings or by operator contact. During the test the actual mains voltage is also measured at the appliance socket. In order to ensure that the equipment is safe even when the mains supply rises to its maximum permitted value (253V or 121V) the tester calculates and displays the leakage current that would flow at this value.

2. Substitute leakage test (IPE).

Also called alternative Earth Leakage or Equivalent Earth Leakage
Test voltage:
40Vac 50Hz 19.99mA
For this test the appliance has its phase conductors joined together within the tester and a 40V supply is applied between both phase conductors and the protective conductor connection of the equipment under test in the case of Class I equipment. In the case of Class II appliances the test probe is connected to the tester and is applied to any conductive parts on the appliance. The actual voltage is measured at the appliance socket. From these readings the earth leakage current of the appliance is calculated and scaled depending upon appliance supply voltage (240 V or 110 V).

3. Touch current Test (IF).

Also called absence of potential
Test voltage:
230Vac 50Hz 9.99mA
110Vac 50Hz 9.99mA
During this test the appliance is powered from its normal mains supply and the test probe is connected to the tester terminal and is applied to any conductive parts on the appliance. The tester performs a current measurement with respect to earth.

Load testing

Also called Operation or VA
Measuring the load (VA) of an appliance is a good indication of its operating condition, an excessive load suggests faulty operation. The Load VA limit is usually set based on the fuse rating in the appliance or in the mains connector. However, some equipment can draw very high inrush currents when starting. This should be considered when assessing pass or fail.

Residual Current Device (RCD) Testing

RCDs are used to protect the user from electrical faults on individual items of electrical equipment. The tests ensure correct operation and that the RCD does not trip at too low a current (< 30mA). It must also trip within 300ms for small fault currents and trips within 40ms for large fault currents.

Power Lead and Extension Lead Testing

Power cords and extension leads are tested in a similar way to Class I appliances. Extension leads or power leads with surge protection should be tested with an Insulation test voltage of 250 V. This prevents the surge protection from operating and artificially failing the lead.
Extension leads fitted with RCDs should be tested as per an ordinary plug-in RCD (see above). In addition the following tests can be used:

Bond test (continuity test)

Differential leakage test (The RCD must be switched on manually during the test).
The Insulation test will not test beyond the RCD and consequently has little value.

Cross Sectional Area of Cores (square mm)                                Maximum length (m)
1.25                                                                                                12
1.5                                                                                                  15
2.5                                                                                                 25

Any lead longer than the recommended length should be fitted with a Residual Current Device (RCD) with a rated operating current not exceeding 30mA, as the earth resistance can be high enough that the operation of standard protective devices cannot be guaranteed.