PAT testing

This is a basic "survival guide" to PT testing.

Summary

Portable Appliance Testing (PAT) is an essential procedure both before and after attempting to repair any mains operated electrical equipment, ensuring basic safety.

A PAT test comprises two parts: firstly a visual inspection (which in fact reveals the majority of faults), and an automated test using a PAT tester.

It's important to understand that the automated test still requires that you correctly identify the class of equipment, since if you get it wrong you could pass a faulty appliance as ok with potentially dangerous consequences.

Visual Inspection

Statistics show that 90% of faults are detected by visual inspections, which can reveal faults that an automated electrical test may not. A visual inspection should therefore should never be skipped.

You should perform the following checks:

Badly wired plug

Cracked plug

• Check for fraying of the mains lead, or cracking of the insulation, or any such deterioration which has simply been taped over. In the event of any cracking of the inner insulation exposing the copper conductors, the owner should be advised that the appliance is positively dangerous, and must not be used until the mains lead has been replaced as it is likely to constitute a fire or electric shock hazard.

• Check for a cracked or broken plug, or any signs of overheating. This will need to be replaced before the appliance can be declared safe.

• Check that the cable clamp on the plug (and on the appliance itself, if there is one) is firmly gripping the cable and allowing no movement. Often, this can be rectified by refitting the plug.

• Open the plug and check that it's correctly wired, the screw terminals are tight, and that a suitable fuse is fitted. Even if you don't open the plug, check the screws are tight. (In the case of a moulded-on plug there will be no screws and you won't be able to open it.)

Appliance Classes

Before you can perform the Automated Tests, it's necessary to understand the different Classes of appliance, otherwise these tests cannot be performed reliably. But as you'll see, you only need to worry about Class 1 and Class 2.

Class 1

The mains lead of a Class 1 appliance has 3 wires. In addition to the Live and Neutral (which carry the electric current to the appliance, and return it to the mains outlet), there is an Earth wire, connected to the metal frame. In the event of a fault, this provides a much easier return path for the current than through your body, so greatly reducing the risk of electric shock.

• The Live is brown. (That's the colour your trousers go if you touch it!)
• The Neutral is blue - a pretty neutral and benign colour.
• The Earth is green, with a yellow stripe. (Green for earth should be easy for earthlings to remember.)

Note that a Class 1 appliance may or may not have any exposed metal parts, but internally will normally have a metal frame.

Class 1 is divided into:

Class 1 Metal

This has exposed metal parts which are connected to the earth wire.

Class 1 Plastic

Internal metal parts such as a frame or a motor casing are connected to earth, but the outer casing is entirely plastic.

Class 2

Class 2 appliances just have 2 wires. (That's pretty easy to remember, isn't it?) These are the Live and Neutral. Class 2 appliances have no exposed metal parts which could possibly become live and create an electric shock hazard in the event of a fault. Nevertheless, they may still have exposed metal parts which are doubly insulated from the mains supply.

Class 3

Class 3 appliances are low voltage devices running off batteries or a separate mains adapter (which itself might be Class 1 or Class 2). Since low voltages don't constitute an electric shock hazard, no testing is required.

Class Identification Summary

To determine whether an appliance is Class 1 or Class 2, run down the following checklist until you have the answer.

• Does the appliance have a double-square "Double-Insulated" sign? If so, it's Class 2.
• If it has a detachable mains lead, how many holes does the appliance end of the lead have (or how many pins does the socket on the appliance have)? If 2, it's Class 2, or if 3, it's Class 1.
• Is the earth pin of the plug made of plastic, not metal? If so, it's Class 2. (The earth pin is the one which is bigger than the other two.)
• Remove the lid from the mains plug if you can. How many wires are there connected in the plug? If only 2 (brown and blue) then it's Class 2, but if 3 (brown, blue and green/yellow) then it's Class 1.
• If you can remove the cover from the appliance, try and see how many wires emerge into it from the mains lead. If only 2, it's Class 2, but if 3, then it's Class 1.
• Does the mains lead have an obviously oval shape, indicating just 2 wires inside, side by side? If so, it's probably Class 2. (But a round mains lead may equally be Class 1 or Class 2.)
• Does the appliance have a metal case? Then it's probably Class 1, unless it's audio/visual equipment which may be Class 1 or Class 2.
• Does the appliance have a case made entirely of plastic, with no exposed metal parts? Then it's probably Class 2. (Note that the element of a kettle counts as "exposed" as you could get your hand in there and you could certainly touch the water. But you couldn't get your hand inside a hair drier to touch the element.)

Safety

As previously stated, it is essential to correctly identify the class of an appliance otherwise a faulty appliance could be passed as OK with potentially dangerous consequences.

All mains powered appliances should be PAT tested both before and after a repair is attempted.

A faulty appliance must have its mains plug removed and the owner must be informed that it is unsafe unless the fault can be immediately rectified (e.g. by replacing a cracked mains plug).

External links

• External links as bullet points