IN-SERVICE INSPECTION AND TESTING OF ELECTRICAL EQUIPMENT
City and Guilds 2377 - 200 On page 13 of the Code of Practice the definition for a Competent Person has a part in it that seems to be least understood and applied - adequate understanding of the system to be worked on and practical experience of that class of system. If you dont understand complex IT or electronic equipment power supplies and filters, beware just going ahead with the usual standard tests, when in fact - dont test! You will encounter equipment where you wont be sure what values or scales you should test at and furthermore wont understand the results. dont pass as fit what you dont understand and dont test complex equipment that you might ruin. British Standard specialist publications, PAT forums and manufacturers are good sources to help you make the right informed decisions, just know your limits.
Basic PAT Testing Mistakes you can easily make
1. Not doing the tests in the required order If on a class 1 appliance you do the insulation test first, you dont know if the conductive earth path is good. (In class 1 appliances PATs use the earth wire as the return signal path, which is why you dont need to use a probe for the insulation test). If the earth path isnt tested and proven good, you cant rely on the insulation test. 2. Forgetting to switch the appliance on Think about the situation where the live wire is broken inside the appliance and touching the outer casing. When you do the insulation test, everything inside the appliance should be at 500V. The tester detects (via a probe or the earth wire) any voltage escaping to the appliance housing. But if you havent switched it on the voltage cant get through the live wire, nothing will escape and you will have an incorrect pass of a dangerous appliance. 3. Not checking the fuse is conducting (see 2 above). You dont have to do a separate fuse test the operation test will prove that the appliance is both on and the fuse is okay. 4. Touching the appliance The truth is most appliances pass the insulation test, so you get into the habit of holding drills etc to make it easier to hold the probe in contact. Then one day the appliance insulation fails and you get a 500V shock. 5. Only doing one earth bond test Many of the most experienced testers fall for this. (Thats because some of the earliest downloading PATs had standard test sequences that only expected you to do 1 earth test). If an appliance has multiple, apparently isolated, outer parts with separate earthed paths, each one needs to be separately measured.
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IN-SERVICE INSPECTION AND TESTING OF ELECTRICAL EQUIPMENT
City and Guilds 2377 - 200 6. If you have multiple earth paths Dont take the average reading its the worst case that matters. 7. Only doing one insulation test For the reason in 5 above this is a common fault. At how many places on a power tool could the insulation break down? Obviously at the chuck, but what about the ventilation slots, the trigger, the assembly screws, the speed control. One idea is to wrap the device in kitchen foil and test once on the foil (you can buy special conductive bags, but foil is cheaper). 8. Failing to do an operation test ie functional test (powering it up) Some entry level PATs dont have the ability to power up the appliance. Plug the appliance into the wall socket (after PAT testing) and make sure it works properly. Putting your initials on a PASS label for an appliance that doesnt work or is obviously faulty is a bit embarrassing. 9. Don't try to do an earth bond test on a class II appliance No, it's bound to fail! Class II appliances have no earth. This is the single most common problem found for incorrect fails. 10. Mains input filter capacitors give misleading results (see number 14 below) The insulation test applies 500-600V d.c. between the live and neutral conductors and earth. If the appliance is fitted with mains input filter capacitors, this can stress these components. The fitting of filter capacitors may also give misleading results - under these circumstances, the manufacturers advice should be sought. 11. Remove or scrape away oxidized material to bare metal before testing When testing electric kettles, the test lead should be clipped to the kettle element. In hard water areas, this will require you to scrape away scale until the bare metal can be seen. This is true of the preparation for many types of equipment. 12. Mains earth point or signal/functional earth point? When testing computers, ensure that the earth bond test point is a genuine mains earth and not a signal earth point. If there is any doubt, simply complete a visual inspection and skip the tests. A note should be made on the Certificate of Inspection. 13. Surge protected extension leads and insulation testing at 250vdc It is easy to get a load of extensions to test and ignore which ones are surge protected when doing insulation testing. You can destroy an inexpensive or expensive surge protected lead by not lowering the insulation resistance test voltage to 250vdc. Remember investigate its not always a good idea testing at 500vdc!
IN-SERVICE INSPECTION AND TESTING OF ELECTRICAL EQUIPMENT
City and Guilds 2377 - 200 14. Testing Class 1 appliances where there is no access to earthed metalwork. This is generally equipment such as: Kettles that have a flat plate which covers the element Shredders Desktop Fans "Henry" hoovers Monitor power supplies Flat screen / LCD monitors which have no accessible metalwork The most important thing to remember is that the following applies to Class 1 equipment where there is NO accessible metalwork. If you come across a standard Class 1 appliance then you should always try to obtain an earth connection but with some of these types of appliances above it is impossible. In these cases the only option is to omit the earth bond test. This would not normally be acceptable but the thoughts on the subject are if you cannot touch metalwork on the appliance with your earth bond clip then a user cannot touch any metalwork which may become live as a result of a fault condition. However I have come across desk and floor fans that seemed to have no accessible metal part until I came across a single small metal nut that was the main fixing for the motor casing or rotor which would become live under earth fault conditions and dangerous. In this case you would connect your earth bond clip to the metal part of an insulated screwdriver which you would use to maintain contact with the nut during the earth bond test. The main question that people ask is "Shall I test it as a Class II appliance?" The short answer is NO! The appliance is always Class 1 and therefore you should always apply Class 1 PASS marks. You therefore should carry out tests as follows: Visual Inspection Earth Bond test if you can find any metal to touch OR Insulation test with a Pass mark of 1.0Mohm or substitute leakage/leakage test with a PASS mark of 3.5mA (0.75mA for hand held or portable)
IN-SERVICE INSPECTION AND TESTING OF ELECTRICAL EQUIPMENT
City and Guilds 2377 - 200 15. Variable Speed Drives - Examples include fitness machines for use in gymnasiums and potters wheels used in schools and colleges Insulation testing may not be the best choice here because of the filters present, though you should still test at least at 250vdc and record the result. Equipment employing a VSD will, in most cases, have a leakage current greater then 3.5mA (6mA) due to the filters at the input and hence will be unsuccessful when performing a touch current test. Although this does not show the equipment to be unsafe if this test is performed, the value of current should be recorded since an increased current in any future tests can be due to deterioration of components within the equipment. The engineer performing the tests will then have to decide if the equipment is still safe for service and will make his judgement based on whether the protective conductor current increased from the previously recorded value or from the manufacturers advised maximum current. The engineer must then verify that such equipment with a protective conductor current designed to exceed 3.5mA complies by the presence of the following: 1. proper warning labels, 2. the use of commando type industrial BS EN 60309-2 plugs, 3. minimum 1.0 mm2 protective conductor size. This can be a complex and involved test and require careful recommendations! Again the best advice you can give yourself is dont pass as fit what you dont understand and dont test complex equipment that you might ruin.
