We’ve Got It Down PAT: Appliance Electrical Safety Testing

Everywhere 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 perform the function of keeping us safe from the dangerous mains voltages delivered from our wall sockets.

Of course, we’ve all had appliances that have become damaged. How often 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 many readers are guiltily shuffling a particular piece of equipment out of the way at the moment.

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

In the United Kingdom,where this piece is being written, the law in question is the Electricity At Work Regulations 1989, which specifies the maintenance of electrical safety and that there should be evidence of regular maintenance of electrical appliances. It doesn’t specify how this should be done, but the way this is usually 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 usually referred to. Join me after the break for an overview 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 perform the electrical tests themselves, but for convenience and calibration they are invariably combined together in an application-specific self-contained unit, the PAT tester.

These instruments all perform 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 regulations is then usually satisfied by the application of a sticker with the test details. In most cases a green sticker for a pass, and a red one for a fail.

Astoundingly, there are installations in which this plug would work.
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. Inspect 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 instantly failed, or at least it must be repaired if that is possible before it can proceed to the electrical tests.

A final physical test for UK appliances concerns the fuse fitted in the plug. Our BS1363 plugs are unique in the world in having an inbuilt fuse, this is because our wiring topology means our sockets are capable of delivering more current than they are rated for. Thus our plugs have an inbuilt fuse whose purpose is simply to protect the appliance cable from fire in the event of a short circuit. The safety check is therefore twofold: that it has a correctly fitted fuse, and that the fuse current matches the rated current of the flex. Since 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 most 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 normally 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 typical PAT tester
A typical PAT tester

The earth bond test is for non double-insulated appliances, and ensures that the earth pin on the plug has a good quality connection to any exposed metal parts of the 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 aim of the test is to ensure that the earth connection has a suitably low resistance, and can take a high current without burning out.

The leakage test is designed to ensure 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 ensure that it is low enough to pass. Normally the pass figure is set in the order of one or two mA.

The insulation test applies a very high voltage, in the order of 500 V, to the appliance, and measures the insulation resistance, which should be in the very high M ohm range. This ensures that in the event of a high voltage transient spike there will be no breakdown and therefore no risk of shock to the user.

In Plain Sight

These tests are vital 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 tested 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 many power cords have rough lives and end up damaged, and so many appliances pick up knocks and scrapes that can expose the user to danger.  It’s thus important to understand that the visual inspection is the most important part, and not owning a PAT tester does not mean that you are unable to ensure your appliances are safe.

What has been described here is based upon the appliance testing regime of just one country, but it should 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 glad that you did.

21 thoughts on “We’ve Got It Down PAT: Appliance Electrical Safety Testing

  1. Wasn’t this sort of thing at least part of the reason why (the much larger part being internationalization) the IEC 60320 coupler specs became so widespread? Seems like localized mains plug-to-IEC cables would be the focus of most of the visual inspection called out here and just replacing the whole thing the path of least resistance (ha ha) when it fails.

  2. Not even yet started reading:
    “How often have you seen a plug held together with electrical tape, or a cord with some of its outer sheath missing? ”
    Sounds like the Qualified electrician here at work (On break now BTW).

    My bench has a very well kept Electric ecosystem that I wired together and the above person approved of without any exposed ends etc, always use 3-pin PSUs (I have experienced the rubbery 120hz of the “double isolated” figure-of-8 connected SMPS on a metal chassis, think infamous Macbook PSU with plastic GND pin!!!).

    I’m not a qualified electrician, however the law and regulations (Here in the UK) have provisions for “Instructed persons”, “experienced Technician” and “Qualified Technician/electrician. I fall under the, “Experienced technician” category. Whilst I can wire up my own mains stuff, I have to have approval of the Qualified electrician. However it is difficult to prove in court that you are an experienced technician and thus need to have a statement or more from qualified technicians/electricians. Some things ONLY a qualified electrician is allowed to do, like wiring up a main feed breaker (300A I mostly see) due to the high risks of working on an always live mains!

    Instructed persons: Do exactly as instructed, otherwise they are placed liable unless they can prove faulty instructions.
    experienced personnel: can legally do more than an instructed person with risk of losing court cases/life if worst happens.
    Qualified personnel: Can do most things within the realms of regulation + qualification, must stay updated with regulations!

    1. These are formal terms, both legally and technically. All EU (Cenelec) product safety standards are (supposed to be) harmonized the the IEV technical vocabulary(IEC60050), as follows:

      electrically skilled person – person with relevant education and experience to enable him or her to perceive risks and to avoid hazards which electricity can create.

      electrically instructed person – person adequately advised or supervised by electrically skilled persons to enable him or her to perceive risks and to avoid hazards which electricity can create.

      ordinary person – person who is neither a skilled person nor an instructed person.

      You do not get to ‘edit’ these definitions until your Brexit is complete. And you brits are quite welcome to become our 51st state upon finalization of brexit (said while donning flame-proof clothing).

      1. Personally, in my opinion:
        I’d prefer if we (The UK) can become an extra state of India.
        The UK had a turn of ruling over India back in the Empire days, it is only fair to let India have its fun ruling the UK/England.

        That, or bring back exile laws between participating countries.
        It is too cold here, the police are annoying at best, bribes get called “fines” and the lawmakers have no idea what they are doing!

  3. We too have these tests in Oz. I’m considering setting up to become a tester as I have the expertise and most of the equipment needed. It may be easier to just buy a tester but they are pretty costly. One organization I belong to has so much stuff that needs to be tested it does sound like a good idea to do it myself and save them money. I just have to get the official paperwork correct.

  4. The problem with British ‘workplace’ equipment regulations is that they are poorly harmonized with general electric code (BS7671), and particular product safety standards scoped for your electrical equipment (EN60950-1,61010-1,60065, 62368-1, etc) such that No. 635 can be contradictory. One of the few things we did correctly on this side of the cod pond was to ensure that OSHA workplace regulations (29CFR1910) points to the scoped ANSI standards. One of the things yanks got very wrong was no national requirement for recurring tests of fielded equipment outside of a few special categories.

    The article mentions a common problem with these regulations – leakage current limits can vary several orders of magnitude for various electrical equipment, so an “order of one or two mA” is actually hundreds of uA to tens of mA, depending on the equipment class and the scoped safety standard and the mains wiring and how much Guinness was had for lunch (medical equipment has much less touch and ground current).

    But the one important thing that we got correct in Estados Unidos de America(a.k.a, las antiguas colonias britanicas, or land of the free??), was that an engineering lab can do about anything to equipment so desired as long as the dead bodies are not allowed to pile up and decay.

  5. Safety tagging is a joke here in Australia. Just about everything that draws less than 7.5 Amps (1800 Watts) uses a 3 pin 7.5 Amp IEC lead (computer cord) that is removable from the appliance. In a business environment where equipment is often taken for repair or shifted around, the wall plugs are too hard to get to so the power cords remain when the equipment is replaced.

    The net effect is that we have 90% certified safe power cords and nobody has any clue about the safety of the actual equipment.

    1. That sounds like a problem with the implementation rather than the regulation. The standards are quite good here in Australia (AS3760) and are a legislative requirement for all workplaces. What you describe is not atypical, the solution is that you test both the cables and what they are connected to, as per the requirements for test.

      1. “the standards are quite good here in Australia”…

        To do PAT testing in the UK you have to be an electrician.

        I Australia, I know of a big tax dodging cleaning company that puts a select few of their female staff through a days “training”, give them a PAT tester and they perform testing, whilst presumably cleaning…at an army base in Queensland.

        Standards my arse.

        1. Agreed. Also the test tag is put on the cord which is most often an IEC cord that can be unplugged from the tested equipment and then plugged into untested equipment. Defeats the whole purpose really!

  6. I worked for an NRTL earlier in my career.

    The scope of any UL standard is public. This article would be more helpful if it included the scope of UL60336, 60950, 60063, 60601 and UL61508.

    These all have international requirements pulled into them. Their table of contents indicate “Performance Tests” so that you can see which class of appliances receive which class of testing.

    I don’t understand the requirement for follow up maintenance on equipment that is sold to an end customer. Who gets the stick if the equipment is not used or maintained or installed properly in the UK? What happens if you don’t comply?

  7. Is the reason you’ve given for a fuse in the plug real? I would think that *any* mains outlet almost anywhere in the world is capable of supplying more current than the plug/socket/cord can handle.

    In Australia, GPOs (wall sockets) are rated at 10 amps*, most plugs 10 amps (but lots are less), and the cable connected to that usually even less again. However, the main breaker box “fuses” (breakers) are 16 amps. I can pull 16 amps through my GPO forever without tripping the circuit (at least until something catches fire, burns and shorts the active/neutral).

    * Yes, you can get 15, 20 and 25 amp versions, but these are uncommon, use thicker in-wall wiring, and have a higher fuse rating (in the breaker box).

    1. In this case, not more, a lot more. UK wiring often uses a Ring Circuit, so the main breaker box fuses need to be rated to the total load and usually have 30 – 32 Amp fuses or breakers.

    1. Applicable to most things, yeah!

      Electricity, even with all the laws, loopholes, etc… I’ve seen, heard and I also experienced shocks myself, albeit most of my shocks were when I was still young and freshly kidnapped into a so-called “care-system”, father already told me about the dangers of electricity before the legal-kidnapping (Also I learnt to safely wire a plug at the age of 4), yet a lack of self worth at the time and curiosity almost killed the cat so to speak.

      Whilst a cheap ripoff PSU took out my Managing Directors laptops’ Scottkey diodes (part of the PSU/Battery management), I warned him of the dangers of fakes and gave him a genuine spare. Also found the shorted diode looks like it saved the entire laptop. So he was doubly lucky not to be both electrocuted and out of pocket!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s