PAT Testing Leakage Current Test

The leakage test can be used as an alternative to the insulation test. It's a useful test to perform where an appliance has an electronic switch that requires mains power to operate, or for equipment containing surge protection circuits which often give low insulation resistance readings.

Most modern PAT instruments carry out a leakage test as part of their auto test sequences, however it’s not a test required by the IET Code of Practice, rather it is intended to be used as an alternative, or in addition to the insulation test.

For Class I equipment, the leakage test measures the current flowing through the protective conductor (earth wire) whilst powered at the supply voltage. A leakage test on Class I equipment is therefore referred to as a Protective Conductor Current Test. In practice, most test instruments measure the difference between the current flowing through the line and neutral conductors. Any difference can be considered as leaking through the protective conductor. Therefore it is often described by test instrument manufacturers as a Differential Leakage Test.

For Class II equipment, the leakage test is made by attaching the test lead to any exposed metal parts of the equipment and measuring the current flowing through the test lead. The test instrument uses a 2kΩ resistor to represent the current that would typically flow through the human body. The leakage test on Class II equipment is referred to as a Touch Current Test.

Many test instruments also have a substitute or alternative leakage test. The substitute leakage test applies a reduced ac voltage, typically 40 - 60 Vac, between the line and neutral joined together and the earth conductor, or test lead for Class II equipment. The instrument then calculates the leakage current that would be expected if the appliance was operating at the supply voltage. The substitute leakage test was initially used for testing computers to prevent them from powering up for just a few seconds which could cause errors when rebooting. More recently, with the introduction of battery-operated test instruments, the substitute leakage test is often included in the standard test sequences for Class I and Class II equipment. Because it does not measure leakage at mains voltage, it should not be used as a direct substitute for an insulation resistance test. Some battery-operated models can be connected to the mains supply to carry out a true leakage test at mains voltage.

Leakage Test Fail

The most common reason why equipment often appears to fail a leakage test is that an incorrect limit is used. The 5^th edition of the IET Code of Practice introduced a single leakage current limit of 5 mA for Class I and Class II equipment, replacing the previous appliance-specific limits used in earlier editions. Many PAT instruments remain preset to the older limits. If these presets are used without adjustment, equipment may be incorrectly recorded as a fail. Test instruments should therefore be configured in accordance with the current edition of the IET Code of Practice. Where reprogramming is not possible, results must be interpreted against the 5 mA limit to avoid unnecessary failures.

Appliance Type	Old Limit	5th Edition Limit
Class I Handheld & Portable	0.75mA	5mA
Class I IT, Movable, Stationary & Fixed	3.5mA	5mA
Class I Heating & Cooking	0.75 mA or 0.75 mA per kW, whichever is greater, with a maximum of 5 mA	5mA
Class II All Types	0.25mA	5mA

Some equipment is designed to operate with a leakage higher than the 5mA limit. Because there is a risk of an electric shock if the protective conductor becomes disconnected, extra precautions are required. They must be permanently wired to the fixed installation or connected with an industrial plug and socket. They should also have a label warning of the high leakage current and the protective conductor must be at least 1.0mm². For equipment with a leakage current greater than 10mA, additional precautions are also required.

If you want to learn more about electrical testing, we regularly run PAT testing courses at venues across the country. Our experienced trainers will guide you through the process of conducting visual inspections, earth continuity tests, insulation resistance tests, leakage current tests and more. At the end of the course, you will have a thorough understanding of the principles of testing, as well as the skills and knowledge needed to perform these tests competently and confidently. Our courses are suitable for anyone who is responsible for the testing and maintenance of electrical equipment, including facilities managers, electricians, and health and safety professionals.