Choosing Portable Appliance Testing Equipment

When you start looking for a portable appliance tester, the first thing you will notice is the large range you have to choose from. For simplicity we can place the PAT testing equipment in one of three distinct categories. The following is a guide to the different types, and highlights features to look for.

Simple PASS/FAIL types

PASS/FAIL or GO/NO GO, type testers give a simple pass or fail test result allowing no interpretation of the test data. These testers generally only carry out insulation and earth continuity tests. Most PASS/FAIL PAT testers do not have a selectable Earth Continuity test current.

The IEE Code of Practice states that the earth continuity of the appliance can be tested either:

A - With a current between 20-200mA while flexing the lead of the appliance or,
B - A current not less than 1.5 times the rating current of the appliance, and no greater than 25 amps.

If the earth circuit on an appliance is susceptible to corrosion such as those found in a fridge, washing machine, kettle or dryer then you should test these appliances with the higher earth current to ensure any potential corroded earth wires are suitably stressed. To test IT equipment the tester must be able to perform an earth bond test at the lower current of between 20 - 200mA

The disadvantage of this type of tester is that the high current types (usually 25A) are unsuitable for testing IT equipment and the low current types (usually 100mA) are unsuitable for testing general electrical appliances. A further disadvantage with all the PASS/FAIL type of testers is the earth bond pass limit is set, allowing no interpretation of the test result. The IEE Code of Practice requires the earth bond resistance to be no greater than 0.1 Ohms + the resistance of the cable. A tester with a set Earth Continuity limit of 0.1 Ohms will wrongly fail equipment with long leads or low csa that may have a higher resistance. Some testers avoid this by setting the limit higher, usually 0.3 Ohms, but these do not comply with the IEE Code of Practice and may still wrongly fail equipment.

PASS/FAIL testers have the advantage of being easy to use but have a limited practical use.

Manual PAT Testers

Manual PAT testers give much more functionality than the simple PASS/FAIL testers but do require a level of understanding to correctly interpret the test data. As well as the standard Insulation and Earth Continuity tests, many also carry out Earth Leakage and Load tests. Look for testers with selectable Earth continuity test currents enabling the testing of IT equipment.

Downloadable PAT Testers

For testing large amounts of equipment a testers that automates the process and has recording capabilities is more suitable. These testers are able to initiate a pre-programmed test sequence via a shortcut menu. Test data, including the overall PASS or FAIL result, is stored for downloading or printing out. These testers often have a bewildering amount of features, some of the important ones to look for are:

Multiple Earth Paths ability: It is important to choose a PAT tester with the ability to make accurate earth bond measurements, even when multiple earth paths exist. Most PAT testers can measure earth leakage over multiple earth paths, but this is not the same as checking the integrity of an appliance's main earth. For example, consider the common office computer, which often has several other devices attached to it via a screened data cable - for example a printer, monitor or scanner.

An earth return path can still exist even if the earth bond wire in the cord of a computer is faulty.

If you buy a PAT tester that does not automatically cater for earth bond measurement under Multiple Earth Path conditions, the only way to ensure you are actually testing the earth bond of the computer is to disconnect it from all other ancillary equipment. The end result is an increase in test time, and a significant reduction in the number of tests you can do in a day.

On-board help: PAT testers can be complicated instruments and there is nothing worse than having to hunt for that ever-elusive manual. A PAT tester with on-board help that walks you through the tests and displays on-screen connection diagrams when you need them is a definite advantage.

Multiple Voltage Insulation Testing: With the advent of the EMC regulations in Europe, an increasing number of appliances have used filters to manage conducted emission problems. These surge filters can cause inconsistent test results with PAT testers that use a 500V insulation test. Buy a PAT tester with the option of a 240/250V insulation test.

110V Testing: If you intend on testing large amounts of 110V equipment look for a tester that is able to carry out a full Leakage and Load test at 110V. Most testers stating 110V testing are only able to perform sub-leakage tests on 110V equipment.

PAT Equipment manufacturers and suppliers include - Alphatek, Avo, Fluke, Kewtech, Martindale, Megger, Metrel, Seaward, Robin, Transmille

Verification Instruments