Can PAT Testing Damage the Equipment Being Tested?

Portable Appliance Testing (PAT) combines a visual inspection with electrical tests to confirm that equipment is safe to use. When the correct tests are selected and carried out properly, PAT testing should not cause damage. Problems usually arise when the wrong test is applied to the wrong type of equipment (for example, high-current bond testing on sensitive electronics, or powering the wrong voltage during a leakage test).

Key Points

  • Damage is uncommon when test selection matches the equipment type and construction.
  • Most risk comes from high-current earth bond testing, 500V insulation testing on older electronics, or powered leakage tests where the appliance is energised.
  • The IET Code of Practice (5th edition) explains test selection and when alternative tests are appropriate.

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.



Selecting the correct test current

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.


A weak connection failing

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.


IT equipment should be tested with a low current

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.





IEC 62368-1

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.





Boot error caused by powering on for just a few seconds

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.





PAT tester with flash testing function

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.


Common Causes of Damage (In Practice)

  • Using high-current earth bond on sensitive IT/electronic equipment.
  • Applying a 500V insulation test to older electronics that are better suited to 250V.
  • Running a powered leakage test on equipment that should not be energised (or energised at the wrong voltage).
  • Using a flash test as part of routine PAT testing.

A Simple “Safe Test Selection” Guide

  • IT / electronic equipment: low-current earth bond; consider 250V insulation if appropriate; 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.

Summary

Selecting the correct tests is the key to avoiding damage, especially when using older mains-powered instruments or powered leakage tests. This is why competence and training matter. For a wider view of setting up a safe system of inspection and testing, see managing PAT testing in the workplace, and for common errors that lead to misleading “passes”, see common PAT testing mistakes to avoid. Our PAT testing training courses also cover correct test selection, safe procedures, and interpreting results.