all over we look in our everyday lives, from our bench to our bedroom, there are the ubiquitous electrical cords of mains-powered appliances. We don’t give our electrical devices a second thought, but in addition to their primary purpose they all carry out the function of keeping us safe from the harmful mains voltages delivered from our wall sockets.

Of course, we’ve all had appliances that have become damaged. how typically have you seen a plug held together with electrical tape, or a cord with some of its outer sheath missing? It’s something that we shouldn’t do, but it’s likely numerous readers are guiltily shuffling a particular piece of equipment out of the way at the moment.

In many countries there are electrical policies which impose some level of electrical safety on commercial premises. Under those regulations, all appliances should be frequently tested, and any appliances that fail the tests should be either repaired or taken out of service

In the United Kingdom,where this piece is being written, the law in question is the electrical power At work policies 1989, which specifies the maintenance of electrical safety and that there ought to be evidence of regular maintenance of electrical appliances. It doesn’t specify how this ought to be done, but the way this is typically achieved is by a set of electrical tests whose official name: “In-service Inspection & testing of Electrical Equipment”, isn’t very catchy. thus “Portable Appliance Testing”, or PAT, is how the process is typically referred to. join me after the break for an summary of the PAT system.

Consider Both Physical and Electrical

The PAT test has several parts, of which some are physical and some are electrical. A competent electrical engineer with a well-stocked bench could probably create a set-up to carry out the electrical tests themselves, but for benefit and calibration they are invariably combined together in an application-specific self-contained unit, the PAT tester.

These instruments all carry out substantially the same tests, but are available all the way from basic manual devices up to computerised machines with asset tagging and label printing. once a test has been performed, the evidential part of the policies is then typically satisfied by the application of a sticker with the test details. In many cases a green sticker for a pass, and a red one for a fail.

Astoundingly, there are installations in which this plug would work. wrong polarity, earth instead of neutral, blown fuse replaced with foil. All testers will have seen plugs like this one.
The first part of a PAT test is always a visual one. examine the appliance, look for physical damage. Is there any point at which the protection between user and mains voltage has been compromised? has the case been cracked or broken, or is there any way that something could be inserted to touch mains voltage? then the power cord, has it been damaged, and is the plug in one piece? has its strain relief been compromised? open the plug up, and has it been wired incorrectly? If the answer is “yes” to any of those questions, then the appliance is quickly failed, or at least it should be repaired if that is possible before it can proceed to the electrical tests.

A final physical test for UK appliances issues the fuse fitted in the plug. Our BS1363 plugs are distinct in the world in having an inbuilt fuse, this is because our wiring topology implies our sockets are capable of delivering much more current than they are rated for. thus our plugs have an inbuilt fuse whose purpose is simply to safeguard the appliance cable television from fire in the event of a short circuit. The safety check is for that reason twofold: that it has a correctly fitted fuse, and that the fuse current matches the rated current of the flex. because our sockets are rated at 13A a lot of British plugs are supplied with a 13A fuse by default, so a significant part of a British PAT tester’s job involves levering out 13A fuses and replacing them with 3A, 5A, or 7A equivalents.

Arguments will no doubt rage in the comments about the merits of different world mains connector standards, but meanwhile many people involved in PAT testing here will have a significant surplus of 13A fuses. Do you have any project ideas for this windfall?

A Battery Of Electrical Tests

Once the physical tests have been passed, it is time to turn to the PAT tester for the electrical tests. The computerised models will typically have a set of pre-programmed tests for different types of appliances rather than the physically selectable tests of the physical models, but the basic tests themselves are the same. There will be an earth bond test, an earth leakage test, and an insulation test.

A common PAT tester
The earth bond test is for non double-insulated appliances, and guarantees that the earth pin on the plug has a good quality connection to any exposed metal parts ofthe appliance. An earth bond lead from the tester is clipped to external metalwork, and a high DC current of about 40 A is passed between it and the earth pin on the socket. The objective of the test is to guarantee that the earth connection has a suitably low resistance, and can take a high current without burning out.

The leakage test is developed to guarantee that any leakage from the live supply to the earth wire or to the exposed metal parts of the appliance is at acceptably low levels. A low voltage AC is passed to the appliance, and the current returning down the appliance earth or the earth bond wire is measured to guarantee that it is low enough to pass. typically the pass figure is set in the purchase of one or two mA.

The insulation test applies a very high voltage, in the purchase of 500 V, to the appliance, and steps the insulation resistance, which ought to be in the very high M ohm range. This guarantees that in the event of a high voltage transient spike there will be no breakdown and for that reason no risk of shock to the user.

In plain Sight

These tests are important to ensuring the unseen aspects of the appliance’s safety are in place and it does not pose a risk to its users. but it is worth remembering that they alone are only a part of the full battery of tests. When you have PAT evaluated a large number of appliances you will find that it is very rare to fail an appliance on the electrical tests, but very common to fail one on the visual inspection. So numerous power cords have rough lives and end up damaged, and so numerous appliances pick up knocks and scrapes that can expose the user to danger.  It’s thus essential to understand that the visual inspection is the most essential part, and not owning a PAT tester does not imply that you are unable to guarantee your appliances are safe.

What has been described here is based upon the appliance testing program of just one country, but it ought to have universal application wherever in the world high voltage mains power is used. The prospect of electric shock or fire due to poor electrical wiring is something we can all face, so even if your country does not require any of these tests we’d still urge you to consider enacting something like them. When your inspections have found a sizeable heap of appliances in a lethal condition, you’ll probably be pleased that you did.

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