Earth Leakage – PAT Testing
The aim of this page is to clear up some confusion about the earth leakage test – to explain what it is, why you might need to carry it out, and also why, in many cases, the test is NOT necessary.
Many PAT Testing machines have the facility to carry out a leakage test, and leakage tests are often built into ‘default sequences,’ especially on computerised PAT Testers such as the Seaward Europa or Megger PAT420. Some battery powered machines such as the Seaward Primetest 100 and the Kewtech KT72 carry out a substitute leakage test as part of their test sequences.
Some PAT Testing machines carry out a leakage test every time, even when it is not needed, and sometimes even throws up an incorrect ‘FAIL’ indication – which can, if the operator is not experienced enough, lead to items being taken out of use unnecessarily.
The test was originally called the ‘earth leakage test’ although nowadays it is correctly called the ‘protective conductor current test’ or ‘touch current test’ and this page will explain why!
Why Might We Need to Measure Earth Leakage?
The easiest way to think of an earth leakage test is as an alternative to the insulation test i.e. it is designed to detect a breakdown of insulation. If the insulation was faulty on a Class 1Toaster, for example, current would flow to the outside of the case and the user might get a shock when touching it. In this case a 500V DC insulation test is a perfectly satisfactory way of checking the insulation of an appliance, and so a leakage test (which is checking the same thing) would not be needed. However, a leakage test may be of benefit if the insulation test gives suspect results, or if the appliance has an electronic ‘switch’ which is only closed when the appliance is energised.
Some appliances cannot be considered to be ‘fully on’ until they are actually operating, and so a leakage test (which applies mains voltage to the appliance during the test) may be useful for finding faults which could not be detected by a normal insulation test.
Earth Leakage (Protective Conductor Current Test)
This test is designed to to see if there is any leakage current flowing from the live wire to the earth wire (protective conductor) of a Class 1 appliance. This current might be due to a breakdown of insulation, or components between Live and Earth (such as in surge protection devices,) or a combination of both. This test is done at full mains voltage i.e. at 230V. The appliance is plugged into the tester and while 230V mains is being applied, the tester measures the current flowing through the Earth wire. As the earth wire is known as the Circuit Protective Conductor, this test is now referred to as the “Protective Conductor current” test.
As this test is carried out at full mains voltage, the appliance will start operating during the test. This could be problematic or even dangerous! Great care should be taken to make sure the appliance can operate safely.
As this test measure the current flowing through the Earth wire, it is important to ensure that the appliance is not connected to anything else during the test. Example – if a computer was connected to a monitor during the test, some current could flow through the video cable and through the monitor’s earth wire – thus giving false readings.
Note that it is NOT necessary to connect the test lead (earth lead) from the PAT Tester to the case – the case is already connected to the earth pin on the plug, and this should have been checked during the earth continuity test.
Touch Current Test
The touch current test is identical to the Earth Leakage test except for one important aspect. Since Class 2 appliances do not have an earth connection, the test lead is connected to an exposed metal part. It is called the Touch Current test as it measures the current that would flow through a person if they were touching the appliance. It is obviously very important to attach the test lead in a way that does not cause danger – for example if it were to be connected to the ‘chuck’ of a drill, danger would arise when the drill started to operate.
Substitute Earth Leakage
The substitute earth leakage (sometimes referred to as ‘sub leakage’ or ‘I leak’ has appeared recently on a number of battery powered testers, and probably gives rise to more confusion than any other test. A ‘normal’ earth leakage test requires full mains voltage to be applied to the appliance under test, and obviously, battery powered PAT testers cannot provide this test.
However some battery powered testers provide a similar test by applying a voltage of 40V AC. The resulting leakage current measured by the tester is multiplied by 6 to give the current that would be flowing if 240V mains were to be used. This is intended to give an ‘approximation’ of the actual leakage, although it does not always give reliable results.
The most important point to make about the substitute leakage test is that it does not power up the appliance. If the appliance has an electronic switch, it will not be fully energised during the substitute leakage test, and it is possible that a fault might not be shown up by this test. However, it can safely be carried out on items such as electric drills without the risk of the appliance starting up.
Carrying Out the Tests
Most PAT Testing machines will only have one button or setting for the Earth Leakage test, and this will check for earth leakage on both the flying lead and the earth pin on the socket – in other words it will carry out both a protective conductor test and a touch current test simultaneously. On other testers, the operator will need to select either Touch Current or Protective Conductor current. Some testers also carry out a Load Test, which is used to measure the current drawn by the appliance – this is often done at the same time as the leakage test, depending on the design of the tester. It is not unusual to see buttons labelled as load/leakage on some machines.
Care will need to be taken to select the correct test – remember that a substitute leakage test will NOT power up the appliance, but a full leakage test will!
Earth Leakage Tests on High Power and 3 Phase Appliances
Because the appliance needs to be ‘powered up’ during a leakage test, it is especially difficult to carry out a leakage test on a large industrial appliance – especially one which operates on 3 phase. No currently available PAT Testing machines are able to power up these appliances directly. However, one method which works reliably is to use a simple earth leakage clamp meter to measure the current flowing in the earth wire. It may be necessary to make up a special lead with the earth wire separated from the other wires. There is a special adaptor available for Seaward PAT testers, although this only works with their machines (Primetest 250 and Apollo)
Pass Values for Leakage Tests
|Portable or Hand-Held Class 1||0.75mA or less|
|Class 1 Heating Appliances||0.75mA per kW, maximum 5mA|
|Other Class 1 (including IT)||3.5mA or less|
|Class 3 equipment||0.5mA|
Why does my PAT Tester sometimes ‘Fail’ IT equipment on the Sub Leakage Test?
It will be seen from the above that the maximum allowed value for leakage on Class 1 IT equipment is 3.5mA. Many IT appliances, including computers and monitors often have a protective conductor current of up to 1mA or more, due to the components used in the power supply. This can cause confusion as some PAT Testers (such as the Seaward Primetest 100 or Kewtech KT72) are set to a pass value of 0.75mA or less. It is important to check the actual value obtained, and compare it to the table above. However, it should also be remembered that the substitute leakage test is not the same as a full earth leakage test, and so any fail reading on the sub leakage test should be viewed with suspicion. If you obtain a satisfactory earth continuity and insulation reading on an IT appliance then it is usually safe to ‘pass’ the item even if the sub leakage test comes up as a ‘Fail.’
Higher maximum values for Protective Conductor Currents (up to 10mA) are allowed in special circumstances – the appliance should be permanently connected to the supply or connected via an industrial plug and socket (e.g. Ceeform), and suitably labelled. For more information on this, see the IET CoP and BS 7671.