What is PAT testing?

The Seaward Primetest 350 PAT meter as used by P A Testers
The Seaward Primetest 350 PAT meter as used by P A Testers

Portable appliance testing (commonly known as PAT or PAT Testing) is a system of formal, recorded visual checks and combined inspection and testing using a PAT testing meter, like the one shown here. This is usually carried out on any equipment which has a plug, whether or not it is portable. Portable hand-held equipment is likely to need more frequent PAT testing than equipment which tends to stay in the same place.

At periodic intervals, it is essential to test the portable appliances to measure that the degree of protection to ensure that it is adequate. At these intervals, a formal visual inspection is carried out and then followed by PAT testing with a meter.

The visual inspection discovers potential problems before they can cause any danger. For example, if the power cable is frayed or the plug is cracked, users need to be advised NOT TO USE the appliance and report the fault to a supervisor.The insides of plugs MUST be checked unless they are moulded or there is an unbroken seal covering the screws (bad internal wiring or an unsuitable fuse would cause the item to be classed as dangerous).

The interval between tests depending on both the type of appliance and the environment it is used in. The resulting evidence of Portable Appliance Testing is clearly visible to staff in the form of ‘Passed’ , ‘Tested For Electrical Safety’ and ‘DO NOT USE after’ labels affixed to various parts of the electrical equipment they use.

The tests that must be carried out to declare an item electrically safe is dependent on the class of construction (shown below). Some of these tests are:

  • Earth Continuity
  • Insulation Resistance
  • Polarity of Wiring
Construction Classes

Class 1 – Single insulated wiring, which requires an earth connection. There is no symbol for a Class I product so if a rating plate has no symbol on it then it is usually Class I.
Class 2 – Double insulated wiring, therefore no need for a earth lead. Class II is indicated by a double box symbol (shown right) displayed on the outside of the equipment.

Class 3 – These are appliances that are supplied at a low voltage (usually called Safety Extra Low Voltage) which must be less than 50V. These appliances are supplied with a transformer supply that is also marked.

It is recommended that persons conducting PAT Tests attend a course related to the subject matter. There can be much confusion on what needs PAT Testing, what class an appliance is (and therefore which areas to test), and how often appliances legally need to be tested. Proof of a company’s competence in PAT Testing is usually found in the form of a course certificate or qualification.

Advanced PAT Testers

These are PAT Testers designed to display more information than just pass or fail. As well as giving the Pass / Fail results it will give readings for the tester to interpret. Some of these include:

  • Earth Continuity Resistance (aka Earth Bond) tests with a measurement range of 0 – 1.99Ω at high test currents (usually 8A, 10A or 25A) and lower test currents (in the range 20mA to 200mA), enabling a complete range of appliances including personal computers to be tested.
  • Insulation Resistance tests at test voltages of 500Vdc or 250Vdc.
  • Protective Conductor/Touch Current measurement (sometimes referred to as Earth Leakage tests on some older PAT units).
  • Fuse test.
  • Lead Polarity
  • Some units incorporate an RCD test function
  • Testing both 230V and 110V appliances
Computerised PAT Testers

Some advanced PAT testers, such as those used by P A Testers, have the capability of downloading the information to a PC for recording. This allows a specialist to audit the results and to present comprehensive PAT reports for the client.

Calibration of PAT Testers

As PAT Testers are sophisticated instruments, it is important to make sure that they are continuing to measure correctly. If a company fails to check and maintain calibration, it could face difficulty substantiating any measurements in the event of a claim. It is usually recommended that calibration is carried out annually on a PAT testing unit and our company complies with this recommendation.

When a PAT Tester is calibrated it is re-configuring it to match the original specification. This includes:

  • Calibrating the unit back to National Standards. This is best performed by the product manufacturer (if they offer a calibration service) or a laboratory accredited by UKAS
  • If it is mains powered, then a safety test must be carried out
  • A calibration certificate should be issued to prove the PAT unit has been electrically tested

See also:

  1. Electrical Safety Regulations and Guidance
  2. PAT Testing Advice

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Portable Electrical Appliance Testing