HomeCan PAT Testing Damage the Equipment Being Tested?
PAT testing should not damage equipment when the correct tests are selected and carried out properly. However, using an unsuitable test or applying the wrong voltage can damage electrical components. Proper training helps testers understand which tests are appropriate, how to use their instrument correctly and when an appliance requires additional care.
Electrical Tests
The tests used depend on the equipment type, but the most common are the earth continuity (earth bond) test and the insulation resistance test. Some testers also offer leakage testing (powered or substitute methods) and, on older instruments, a dielectric strength (flash) test.
Earth Continuity (Earth Bond) Test
An earth continuity test confirms that a Class I appliance has a low resistance path to earth if a fault occurs. The test passes a current through the protective conductor and measures resistance. There are two approaches: high-current and low-current earth bond testing.
High-current earth bond testing uses a current up to around 25A. It can be useful for identifying weak or corroded connections because heating can reveal poor joints (or cause an already-failing connection to open circuit). However, high-current testing is not suitable for some IT and electronic equipment where the protective earth may be connected into circuits used as a 0V reference or for screening. Applying a large test current through those paths can cause damage.
Low-current earth bond testing typically uses around 20mA to 200mA. It does not stress connections in the same way as a high-current test, but it is far less likely to cause any issues with sensitive electronics and is the preferred method for many electronic and IT products.
Test selection matters. As a rule of thumb, IT and electronic equipment should be tested with a low-current earth bond. High-current testing is more appropriate for items without sensitive electronics (such as extension leads, heaters, kettles and many simple appliances). High-current testing can also give more stable readings and is less affected by probe/contact resistance.
Many modern battery-powered PAT testers only perform low-current earth continuity testing, which reduces the likelihood of damage during this part of the test sequence.
Insulation Resistance Test
An insulation resistance test applies a DC voltage (commonly 500V) between live conductors and accessible metalwork (Class I to earth, Class II to accessible parts). Some older electronic equipment can be more susceptible to damage when subjected to a 500V insulation test, particularly equipment that predates modern ITE safety design.
Modern Information Technology Equipment (ITE) is generally designed with insulation systems that can tolerate a 500V DC insulation test, and newer products are covered under IEC/EN 62368-1 (which replaced the legacy ITE and AV standards IEC/EN 60950-1 and IEC/EN 60065). The industry transition away from those older standards completed when they were withdrawn in December 2020. For older equipment (particularly pre-1990s), a 250V insulation test is often the safer option where a reduced-voltage test is appropriate.
Leakage Test (Powered and Substitute Methods)
Leakage testing measures current leaking to earth. Some leakage tests power the appliance briefly (often only a few seconds), which introduces different risks compared with “dead” tests. Potential problems include:
- Powering 110V equipment at 230V (for example site tools that should be supplied via a step-down transformer). This can cause serious damage very quickly.
- Short power-cycling of certain IT equipment, which can sometimes trigger boot errors or file-system issues.
- Mechanical risk where equipment starts unexpectedly (fans, pumps, motors), especially if the tester is unfamiliar with the appliance.
Where powering-up is undesirable, many testers offer a substitute (sub) leakage test, which performs a similar measurement using a reduced voltage (often around 40–60V). It is not a direct equivalent of a powered leakage test, but it avoids the risks associated with applying full mains voltage. For more background, see our article on leakage current testing.
Flash Test (Dielectric Strength Test)
The dielectric strength test (often called a “flash test”) applies a high voltage (commonly around 1,500V) and measures any breakdown current across insulation. This is primarily a production test or a test used after repair. It is not part of routine in-service inspection and testing, and it can be destructive if insulation is weak or compromised.
Some older PAT instruments include a flash test function. If used incorrectly, it can damage insulation or components. High-voltage testing should only be performed by trained personnel on equipment designed to withstand the applied test voltage.
Key Considerations When Selecting Tests
The points below highlight some considerations when choosing tests. They should be used alongside your training and the instructions for both the appliance and the PAT tester.
- IT / electronic equipment: low-current earth bond; consider a 250V insulation test for sensitive electronic equipment; avoid powering during tests unless necessary.
- Simple appliances (kettles, heaters, extension leads): high-current earth bond (if suitable and available); 500V insulation is usually appropriate.
- 110V equipment: take extra care with powered tests—ensure the correct supply arrangement is used before any leakage test that energises the appliance.
Why Proper Training Matters
Preventing damage involves more than knowing which buttons to press. The person carrying out PAT testing needs to understand what each test does, recognise when it may be unsuitable and follow any relevant equipment manufacturer’s instructions. An automatic test sequence does not replace these decisions.
Our PAT testing courses combine practical training with an explanation of the tests and their limitations. Delegates learn how to select suitable tests, connect equipment correctly and interpret the results, with supervised practice to help build their confidence.
For further guidance, see our articles on managing PAT testing in the workplace, and common PAT testing mistakes to avoid.