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PAT Knowledge Database

PAT Knowledge Database's PAT Knowledge Database is the first point of call for anyone who has questions surrounding the practicalities, requirements and principles governing PAT testing.

Our advice - written by seasoned PAT testing professionals and qualified electricians - covers the most common questions we have been asked since we started offering PAT products, training and services all the way back in 2004. Whether you're a novice or an old hand looking to refresh your memory, our PAT Knowledge Database is the ideal resource for questions related to PAT testing.

  1. Is PAT testing a legal requirement?
  2. Do I Need Insurance to Carry Out PAT Testing?
  3. Do I need my PAT Tester calibrated?
  4. How often should I carry out PAT Testing?
  5. Do I need to PAT test new electrical equipment?
  6. Why do I have to keep a record of PAT test results?
  7. What Should I Look for in my Visual Check?
  8. Do I need to PAT test a laptop PC?
  9. Should I label an IEC lead separately from the computer it powers?
  10. Why does my PAT tester keep failing long extension leads?
  11. Why do appliances keep failing the earth leakage test?
  12. Why has my 110V adaptor failed a polarity check?
  13. How can I download from my tester if my computer does not have an RS232 port?
  14. What is 110V/230V load and run testing and do I need it?
  15. What is flash testing?
  16. What is Microwave Leakage and do I need to test for it?

Is PAT testing a legal requirement?

The requirements of the Electricity at Work Regulations 1989 mean that all potentially hazardous electrical equipment must be safely maintained. However, there is no legal stipulation in the Regulations about what to do, who needs to do it or how often it needs to be done.

Employers are required by the Health and Safety at Work Act 1974 to ensure the 'Health, safety and welfare of persons at work' as well as that of anyone connected to 'the activities of persons at work'.

All employers have a duty to carry out 'provision and maintenance of plant and systems of work that are, so far as is reasonably practicable, safe and without risks to health'.

A further duty imposed on employers by the Act is that of ensuring the health and safety of persons 'not in his employment'. This means that employers, self-employed persons and organisations possessing premises that are open to the public must ensure the safety of anyone working in or using their premises or facilities.

Do I Need Insurance to Carry Out PAT Testing?

Yes, although this depends on whether you are an employee, an employer or self-employed. If you are an employee, your company’s public liability insurance will cover you. If you are an employer, you will need to arrange both public liability insurance and employers’ liability insurance to cover you should anything happen to a member of the public or an employee. If you are self-employed, you need public liability insurance. The potential payout should be at least £2M or even more depending on customer requirements.

Do I need my PAT Tester calibrated?

There is no legal requirement to calibrate a PAT tester but manufacturers usually recommend that instruments are calibrated every 12 months in order to ensure ongoing accuracy and check for any faults that are developing within the tester. Most PAT Testers have a warranty of more than 12 months so calibration can even lead to a significant saving by getting repairs carried out under warranty.

Moreover, an inaccurate PAT tester can lead to safe appliances being wrongly scrapped or unsafe appliances certified as safe and then posing a danger to anyone using them. It is therefore strongly advised that every tester is calibrated in order to prevent financial losses and incorrect labelling of either safe or unsafe appliances. 

How often should I carry out PAT Testing?

The main principle behind determining the correct frequency of testing is that the duty holder (whoever controls the equipment) must make an individual assessment based on the need for testing after taking advice from the competent test person and considering the factors listed below:

  • Type of equipment (portable, hand held, moveable, stationary, IT or fixed)
  • Style of use (continuous, infrequent, rough)
  • Age of the equipment
  • If regularly moved or transported and by what means
  • Type and competence of personnel using the equipment
  • Environment of usage (outdoors, construction sites, hazardous atmospheres etc).
  • Results of previous tests
  • Manufacturer's recommendations
  • Effect of any modifications or repairs to the equipment
  • The equipment construction ie. Class I earthed or Class II double insulated.

The duty holder can make a suitable judgment by undertaking a risk assessment based on the factors above before coming to a final conclusion. A good example would be the need to test a power tool more frequently than a photocopier.

Do I need to PAT test new electrical equipment?

New products should arrive in a safe condition and therefore not require a formal portable appliance inspection or test. A visual inspection is advised, however, in order to spot any obvious damage.

It is also important to consider that a lot of electrical appliances are manufactured overseas and that the manufacturer will only perform 'Batch Testing' whereby one product in every 100, for example, is tested. Items will then be shipped all over the world via various modes of transport before getting to the eventual customer.

Taking this into account, it is therefore recommended to test new products for safety as they can be damaged during shipment. Insurance companies might also require all electrical items to be tested in order to validate cover for fire in a building. New, untested appliances could invalidate a claim.

Additionally, various companies have a policy that any electrical equipment brought onto their sites must be PAT tested before use, even if it is a new product.

Why do I have to keep a record of PAT test results?

Although there is no legal requirement to maintain records of PAT test results, it is useful to keep a permanent record - either physical or virtual - so the user can refer back to it when reassessing the most suitable test frequency for an appliance. Records highlight deterioration and trends that may influence such a decision and are therefore an important part of maintaining safety and working efficiently.

Page 56 section 8.3 of the IET Code of Practice states: ‘Although there is no requirement in the Electricity at Work Regulations 1989 to keep records of equipment and of inspections and tests, the HSE Memorandum of Guidance on these regulations advises that records of maintenance including tests should be kept throughout the working life of the equipment. These records are a useful management tool for reviewing the frequency of inspection and testing and without such records duty holders cannot be certain that the inspection and testing have in fact been carried out… These records may be retained on paper or in electronic memory providing reasonable precautions are taken with respect to security.’

What Should I Look for in my Visual Check?

The visual check is of equal importance to all the other tests performed during a PAT test but is often the least understood of the different elements of the test. Our checklist goes through some of the things to look out for.


  • Make sure the appliance or cable does not represent a trip hazard
  • Make sure appliances and cables are being used in an appropriate environment e.g. indoor appliances are not being used outside

Power Cords

  • Visually check for cuts and kinks in the cable
  • Feel for cuts and kinks in the cable
  • Check that connections with the plug and appliance are secure
  • Make sure IEC plugs are not damaged or adapted


  • Check the casing for cracks, cuts and alterations
  • Make sure pins are not bent/broken/loose/missing
  • Check pins are sleeves
  • Check that the cardboard guard around the pins has been removed
  • Check the cord grip is secure

Moulded Plugs Only

  • Check the fuse meets the rating given on the casing of the plug
  • Make sure the cover for the fuse is secure
  • Shake and listen for a rattle indicating loose parts

Re-wirable Plugs Only

  • Check the fuse rating is 3A if appliance power is under 700W or 13A if appliance power is >700W
  • Check that the live and neutral plugs are the right way round
  • Check for signs of burns or excessive exposed copper within the plug
  • Check that earth is present (Class I appliances only)
  • Check there is no earth (Class II appliances only)


  • Check casing for cracks and signs of burns
  • Check connections with power cord

Do I need to PAT test a laptop PC?

A laptop PC does not require PAT testing but it is always worth performing a visual inspection for damage.

Laptops operate at under 120V DC - most typically around 20V DC - so they therefore qualify as a Class III appliance requiring just a visual inspection.

A laptop charger does need to be PAT tested, however, because it operates from a 230V wall socket. It would therefore need to be tested from the plug to the charger. A visual check is required beyond the charger.

Should I label an IEC lead separately from the computer it powers?

Yes. An IEC lead is detachable and is therefore considered a separate appliance that must be tested and labelled separately from the computer. The lead and computer might deteriorate at different rates and can function independently of one another so it is important to define separate testing schedules for each. Page 78 of the IET Code of Practice has more information.

Why does my PAT tester keep failing long extension leads?

According to the IET Code of Practice, a cord set (extension lead) should have an earth continuity resistance of no more than 0.1Ω+R.

'R' is the lead's resistance itself.

'R' can be calculated by working out the resistance per metre in milliohms for the cable's cross-sectional area and then multiplying that by the lead length in metres. (A useful table is available on page 134 of the IET Code of Practice.)

Every PAT tester has a preset fail limit for earth continuity. If a particularly long extension lead is under test, the increased resistance might be higher than the fail limit on the tester which would therefore show a 'Fail'.

A calculation of 'R' would then be needed in order to check whether a reading falls above or below the actual maximum limit. If the reading is permissibly low, the extension lead can pass despite a 'Fail' being indicated by the tester. 

Why do appliances keep failing the earth leakage test?

As PAT testers have pre-set safe limits for earth leakage current stored within their firmware, they will provide a simple PASS/FAIL indication based on the results of tests performed by the user. However, these limits are typically a one-size-fits-all affair with an understandable safety-first approach so it is important to take into account the type of appliance under test because different appliance types have varying safe limits for earth leakage current. Those limits are:

  • 0.75mA for handheld Class I appliances
  • 3.5mA for other Class I appliances
  • 0.75mA or 0.75mA per kW for electrical heating appliances, whichever is greater

It is therefore important to correctly identify the appliance under test and consider the actual reading shown on the tester for earth leakage current. If it is under the maximum limit defined on p96 of the IET Code of Practice 4th Edition then it is OK to certify the appliance as safe as long as it meets all other PAT test criteria, even if the PAT tester is indicating a FAIL result.

Why has my 110V adaptor failed a polarity check?

All our 110V adaptors are deliberately wired incorrectly. This is to prevent them being used to connect 110V appliances to a 230V mains supply as this is dangerous and will irreparably damage the appliance. With incorrect wiring, power cannot pass through the adaptor and therefore nothing will happen if someone attempts to power a 110V appliance from a 230V socket using the PAT adaptor as a mains adaptor.

There are two solutions to dealing with the incorrect wiring in our 110V adaptors. If there is a danger someone might mistake it for a mains adaptor, we recommend that the user simply ignores the failed polarity check and focuses on the readings for earth bond continuity and insulation resistance. If it will only be used by one person and kept safely in that individual's possession, it is possible to rewire the adaptor so that the polarity is correct.

How can I download from my tester if my computer does not have an RS232 port?

Although many PAT testers now have a USB connection or even wireless connectivity, many still rely on the slightly more old-fashioned option of the RS232 serial port. This can sometimes present a problem as many new computers, particularly laptops do not feature an RS232 port, thus rendering the download leads supplied with some testers incompatible. In such cases, the ideal option is a USB to RS232 Serial Lead.

What is 110V/230V load and run testing and do I need it?

As you will see on our category pages, a number of our PAT Testers can perform load and run testing at either 110V or 230V. Neither of these tests is usually required for a standard PAT test but can be useful in certain circumstances.

Run testing for 110V and 230V appliances comprises the load test and the earth leakage and touch leakage tests. The leakage tests measure the current leaking through the insulation in Class I and Class II appliances which always occurs and can be caused by inevitable small imperfections in insulation as well as by transient voltage spikes which have a range of different causes. In Class I appliances this will leak to earth via the protective conductor (earth wire) whilst in Class II appliances it will get to earth when touched by a conductor. Almost all our PAT testers can perform an earth and touch leakage test and this is recommended for when an insulation resistance test gives an unusual result, or when there is a risk that the test might damage an appliance, for example with IT equipment.

Safe limits for earth/touch leakage are given on page 96 of the IET Code of Practice 4th Edition. It is important to note that the instrument must be turned on to perform a run test so it is sensible to take various precautions to safeguard the testing operative and the appliance. For example, drill bits should be removed from a power drill and it should be firmly secured without attaching the earth bond lead to the chuck. It is also advised to fill a kettle with water to prevent damage to the heating element.

The load test measures how much current an appliance is drawing which is important for handheld appliances as overdrawing or underdrawing current may indicate a fault and represent a danger to the user. Worn bearings in a drill for example may increase the amount of current being drawn and therefore make the drill overpowered. The load test is usually performed at the same time as the earth leakage test and also requires the appliance to be powered up. By using the formula P (power in watts) = VI (voltage in volts x current in amps) to work out the appropriate current draw from a 110V or 230V mains socket, the user can determine the whether current is too high or too low, which may indicate a fault.

For example:

The user has a 2kW kettle that is being powered from a 230V standard UK mains plug. By inserting these values into the formula P = VI the user obtains:

2000W (P) = 230V (V) x I

To isolate the current, simply divide P by 230 to get:

2000W ÷ 230V = I

The value of I is therefore:

2000W ÷ 230V = 8.7A = I

If the load test returns a current reading of roughly 8.7A, this shows that the appliance is not drawing too much or too little current indicating that it is in good working order. It is important to recognise, however, that this test is not enough on its own to show an appliance is safe - the user must perform a regular PAT test for that - and that it is only recommended for tests on handheld appliances.

What is flash testing?

Flash testing, also known as dielectric strength testing or hi-pot testing, uses an extremely high test voltage (usually ≥1kV) on an appliance whilst short circuiting phase and neutral to acquire a leakage current value, thereby measuring the effectiveness of the appliance’s insulation. It should not be used on in-service electrical equipment and is therefore not needed by the majority of PAT testing operatives.

However, several instruments available from can perform flash testing and these are recommended for repair and restoration workshops or electrical equipment manufacturers. This is because the flash test is a common production test so new or ‘as new’ appliances should be subjected to a flash test before being declared safe to use.

What is Microwave Leakage and do I need to test for it?

Microwave leakage is the radiation from a microwave oven. Dangerously high levels can lead to serious health issues including a weakened immune system, burns and cancer.

The 3rd edition of the IET Code of Practice states that:

'Microwave leakage should be checked at appropriate intervals'

The 4th edition (2012) states:

'Microwave leakage testing is not within the remit of the Code of Practice, because it does not directly relate to "Electrical Safety" and for that reason microwave in service leakage testing was removed.'

It does not state, however, that it is not required. Considering that the person carrying out microwave leakage testing is most often the person carrying out appliance testing, it makes sense that he/she tests microwave leakage to ensure health and safety compliance. 

Recommended limits are stated in BS5175.

It is important to visually inspect interlock switches and door seals for signs of damage in order to be certain that they are safe and secure.

To test a microwave oven, the inspector must use a microwave leakage detector to ensure radiation falls under the 5mW/cm2 limit . A microwave emission label can then be attached to certify that radiation levels are acceptable for safe use.

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