All Class I appliances are subject to this particular test, which ensures that there is a satisfactory connection between the Earth pin in a plug and the case of the appliance.
The earthing of an appliance means it is deemed safer to use because the chance of electrical shock is vastly reduced, and combining these together with standard insulative properties enhances the safety of an appliance even further.
This particular test is also regularly referred to as an Earth Bond or Earth Resistance test, but they all measure the same thing.
Performing the Test
In order to perform an earth continuity test, the appliance’s mains plug must be inserted into your PAT testing device, and a test lead must be connected to a suitable earth point on the appliance.
If you take, for example, a kettle, then this test would be performed by plugging the kettle directly into the PAT tester, then clipping a test lead to the internal metal inside the kettle.
In order to pass the test, the reading you gain from the PAT tester has to show that the resistance that has been measured is less than 0.1Ω + the resistance of the supply cable.
Earth Points on an appliance
Of course, one of the things you need to work out when doing this test is where a suitable earth point is located on the appliance.
On many appliances there are metal points on the external side of the appliance that are suitable for connecting to earth. However, not every metal point on an appliance is a direct connection to earth; it is important to repeat the test on another potential earth point if you get a fail reading, just to be sure the connection isn’t incorrect.
Rusty metal can also play a factor in the testing – where possible try to connect to clean metal, or you can alternatively use the crocodile clip’s teeth to scrape away some of the rust.
When testing ‘Flat bottomed kettles” where the heating element itself is not exposed you will generally always fail the test. Instead test the earth pin generally on the side of the detachable base. You should be able to obtain an earth connection there.
If testing a PC try and get the earth connection as near to the power supply which is generally in the top right hand corner as possible. This is where the IEC lead enters the appliance.
Class 1 “No Earth”
These appliances are generally limited to shredding machines, Laptop / Monitor power supplies and Desktop fans. There is simply nowhere to attach the clip of your pat testing machine to.
The way to approach these appliances is generally to omit the earth bond test. After all if no-one including yourself can touch metalwork then neither can a user. They are therefore not likely to receive a shock by touching exposed metalwork that has become live as a result of a breakdown in the insulation.
Earth Bond Test Currents
There are generally two types of earth bond test current used.
Low current which is generally in the range of 100 to 200 milliamps and high test currents in the range of 10amps or 25 amps.
The earth bond test passes a test current along the earth cable from the pin of the plug to the bodywork of the appliance. The appliance tester then measures the resistance of that connection.
If the connection is rusty, non-existent or corroded then the earth resistance reading will increase.
In layman’s terms it is a little like testing a piece of rope with the earth wire being the rope. By adding the weight (current) to the rope you are testing that in the event of a fault such as a breakdown in the insulation of the appliance that the earth wire will be able to withstand the 25amp test current that is applied to it.
Many of the PAT testers available today are battery powered and only carry out the 100 or 200 milliamp test which is fine according to the code of practice.
The mains powered PAT testing machines carry out a 10 amp or a 25 amp test. This is like testing the rope or cable at full strength.
IT equipment should have the test carried out at 100 or 200 milliamps whereas heaters, washing machines and other high powered equipment should ideally have the test carried out at 25 amps if your tester has the ability to do this.
Written by Barry Atkins
