PAT Testing Basic knowlage

Plymouth Pat TestingWhilst many employers think that their responsibility towards the PAT Testing is to bring in a PAT Testing company, they fail to realise that they and their employees have an additional role – the User Check. The PAT Testing Contractor may visit the site every year or so to formally inspect and test the appliances, but who should determine whether the appliances are safe to use between scheduled PAT Testing visits? The PAT Tester won’t be on hand 24 hours a day, 7 days a week to keep an eye on the appliances, so the job falls to the appliance user.

A User Check is designed to help identify any external damage or abnormalities on an appliance prior to it being used. The user should be trained to carry out a basic external inspection of the appliance, with it switched off, and to report any issues to the Responsible Person e.g. Line Manager. If no issues are found, no record of the user check needs to be made. Click here for more information

User Check

The full name for PAT Testing is In-service Inspection and Testing of Electrical Equipment. The PAT Test equipment we use these days is very good, but it has its limitations. The tests we perform are designed to look for faults in specific areas of the appliance e.g. the Earth Continuity Test will establish the presence and quality of the earth path in a Class I appliance, it won’t however let us know if one of the controls has broken off the appliance.

The Formal Visual Inspection is designed to identify many things, including; any external damage or abnormalities; the correct fuse is being used; the appliance has been made to the correct European Standard; the Class of the appliance; whether the plug and fuse carry the correct BS numbers; the suitability of the appliance for the environment; ingress protection rating (where necessary) and many more. Due to the technical nature of the Formal Visual Inspection, it is normally carried out by a suitably trained member of staff or PAT contractor. The results of a Formal Visual Inspection are to be recorded, irrespective of the results obtained i.e. whether the appliance has passed or failed.

If carried out correctly, the Formal Visual Inspection will identify up to 95% of appliance failures, whereas the electrical tests will only find around 5% of appliance problems. Click here for more information

Formal Visual Inspection

The PAT Tests we carry out are designed to measure the quality of the safety devices built into appliances and their ability to protect the user from harm. One specific safety feature is an earth path. Some appliances have exposed metal parts which may become ‘live’ should the appliance develop a fault. This in turn could lead to an electric shock for the user if they touch these parts. To help prevent this, an extra wire is added to the appliance by the manufacturer, which connects the metal parts of the appliance to the protective earthing conductor in the fixed installation. If the appliance develops a fault and the metal parts become live, the fault current should pass from the metal parts down the earth wire into the mains wiring of the building and to safety.

An appliance with a protective earthing conductor (earth wire) is known as Class I.

The Earth Continuity Test uses a test current which is applied to exposed metal parts of the appliance – thus simulating a fault in the appliance. If the earth wire in the appliance is present and in good condition, the test current should go from the metal parts down the earth wire to the PAT Test equipment, which displays the test results as a resistance value measured in Ohms (Ω). The engineer should use their skills and known values to determine whether the test results are acceptable for the appliance.

Typical results are between 0.01Ω and 0.10Ω for an appliance with a short lead, rising to values in excess of 0.30Ω for longer extension leads. Maximum values do apply and must be observed at all times. Click here for more information

Earth Continuity

Insulation is described as any non-conductive material being used to keep the appliance user away from live parts within the appliance – thus reducing the risk of electric shock. It can be anything from modern materials such as plastic or silicone, through to more traditional materials such as wood or even air. The quality of the material being used needs to be very good for obvious reasons, but over time it may deteriorate with age and use.

The Insulation Resistance Test is designed to measure the quality of the insulation surrounding live parts within the appliance and therefore its ability to protect the equipment user from an electric shock. A test voltage of between 250Vdc and 500Vdc is used to help identify any weakness or breakdown in the appliance insulation between the live parts and the appliance body.

The PAT Test equipment will display the test results as a resistance value measured in Megaohms (MΩ). The engineer should use their skills and known values to determine whether the test results are acceptable for the appliance.

Typical test results are generally very high if the appliance is in good condition (between 9.99MΩ and 299.99MΩ depending upon the test equipment being used). Minimum values do apply and must be observed at all times. Click here for more information

Insulation Resistance

When we talk about polarity in electric circuits, we tend to think about the direction that the current flows around the circuit. This is very important when we look at direct current (DC) as incorrect polarity can affect how certain components work e.g. a motor could run backwards. However, when we look at alternating current (AC) which is supplied to homes and businesses, the current reverses direction in the circuit 50 times a second (50Hz), so first it passes up the live wire into the appliance, then it reverses direction and passes up the neutral wire. It continues to do this constantly, changing direction every 50th of a second.

A bit of trivia – AC is used because it is cheaper to produce; easier to transport long distances; allows the use of simple transformers to manage voltage levels; and is much safer for the end user.

If the wires to the appliance are reverse polarity i.e. the live wire and neutral wire are connected to the wrong terminals in the plug, will the appliance still work? As the current changes direction anyway, the appliance will normally be unaffected, but we will have reduced the safety of the appliance. Under normal circumstances, the current will go to the appliance down the live wire first – having passed through the fuse in the plug. If the appliance has a fault and draws too much current, the fuse would detect this and blow in order to cut the current flow to the appliance, therefore protecting the user. If the live and neutral wires are crossed over, the current will pass down the neutral first – which doesn’t have a fuse – so the equipment user could be at risk if the appliance has a fault.

A polarity test cannot be performed on an appliance (as we need access to both ends of the live wire), but it must be performed on detachable power leads and extension leads, because if they are incorrectly wired, they pass this problem onto any appliance they are used with. Click here for more information


When an appliance is running, current is flowing through the appliance and back into the mains supply. If there is a fault in the appliance (normally due to insulation breakdown), some of the current in the appliance could ‘leak’ out of insulated areas into the appliance body. If the user was to touch the affected area of the appliance, they could get a shock. Alternatively, the leakage current could pass down the earth path (if Class I) and into the earthing provision of the building. Whilst modern mains circuits with RCDs would trip and cut the mains supply, making the appliance safe, older circuits may allow the fault current to pass into other appliances. This can mean non-faulty appliances can become ‘live’ to the touch because of a faulty appliance somewhere else in the building.

Current Leakage testing

comes in 3 forms; the Current Leakage Test; the Substitute Leakage Test; and the Touch Current Test.

Whilst they all measure current, they do it in a slightly different way and the test used depends upon the appliance being tested.

Results are displayed as milliamps (mA) and typical results are less than 0.15mA if the appliance is in good condition. Maximum values do apply and must be observed at all times.

that completes PAT Testing Basic knowlage