Basic Pat Mistakes

8/6/2019 Basic Pat Mistakes

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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

don’t understand complex IT or electronic equipment power supplies and filters,

beware just going ahead with the usual standard tests, when in fact - don’t test!

You will encounter equipment where you won’t be sure what values or scales you

should test at and furthermore won’t understand the results.– don’t pass as fit what

you don’t understand and don’t 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 don’t 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 don’t need to use a probe for the insulation test).

If the earth path isn’t tested and proven good, you can’t 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 theappliance should be at 500V. The tester detects (via a probe or the earth wire) any

voltage escaping to the appliance housing. But if you haven’t switched it on the

voltage can’t 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 don’t 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. (That’s 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 withseparate earthed paths, each one needs to be separately measured.

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6. If you have multiple earth paths

Don’t take the average reading – it’s the worst case that matters.

7. Only doing one insulation testFor 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 don’t 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 doesn’t 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 andearth. 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 manufacturer’s 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 it’s not always a good idea testing at 500vdc!

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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 anddangerous.

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)

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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 stillsafe for service and will make his judgement based on whether the protective

conductor current increased from the previously recorded value or from the

manufacturer’s 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 don’t pass as fit what you don’t

understand and don’t test complex equipment that you might ruin.

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Basic Pat Mistakes