Carrying out an Earth Continuity Test

This test is carried out on all Class I appliances. The purpose of the test is to check that there is a good connection between the Earth pin on the plug and the case of the appliance. A good connection is defined as having a resistance of less than 0.1 ohms (or 100 milliohms).

(This blog post is a chapter extract from the "Manual of Portable Appliance Testing")

The Earth Continuity test is sometime referred to as the Earth Bond test or the Earth Resistance test. In effect they all measure that there is a good connection between the mains plug and the Earth point.

The conventional way to carry out this test is to plug the appliance into a PAT Tester and clip the Test lead to a suitable Earth point. The figure below shows the Earth Continuity test being carried out on a kettle. When the test is done, if the resistance measured is less than 0.1 ohms, then there is a good Earth connection and the appliance is considered safe.

When carrying out this test, there are a number of practical things to be taken into consideration.

Earth point on appliance

In order to carry out this test, it is important of course to clip the test lead to a suitable Earth point. If this is not done, then there is no return path for the current. This will result in the appliance failing the test.

Finding a suitable Earth point will come with experience. Alongside are some examples: (Fig13.1 to 13.8)

It is essential to get a good connection to the Earth point. Rusty metal or scaling on kettle elements may prevent a good low resistance connection. If this is the case then rotate the connector and use the teeth of the crocodile clip to scrape away the rust or the scaling to get a good connection. It is also acceptable to clip onto a screwdriver and use this to connect to an Earth point.

The Golden Rule is: When testing Earth Continuity, if you do get a fail initially, do not immediately fail the appliance. Make sure you have a good connection and try again. If necessary try a different Earth point as not all metal parts might be connected to Earth.

Class I Plastic appliances

On some Class I appliances it may not be possible to access an external Earth point if the appliance is totally enclosed in plastic. For example there are many electric fans in use which fall into this category. This presents us with a problem because if we cannot connect the test lead to an Earth point, there is no circuit made and the Earth Continuity test will fail.

On these appliances, it is important to recognise that their construction makes them safer than conventional Class I metal appliances. As well as having an Earth connection, they essentially are enclosed in plastic giving them an extra layer of safety.

The way to test these appliances is to still carry out the Insulation Resistance Test. If this passes, then record this pass on the Equipment Test Record and make a comment to the effect that that the Earth Continuity test could not be carried out due to the plastic enclosure.

Test Current

The person carrying out the testing can use one of the two options below.

Option 1: Use a test current of 1.5 times the fuse fitted in the plug up to a maximum of 25A. For example, when testing a kettle with a fuse of 13A, a test current of 25A should be used. When testing a lamp fitted with a fuse of 3A, a test current of around 4.5A is used. The current is normally applied for a period of between 5 and 20 seconds.

Note: This test current should NOT be used on IT or other equipment where there are electronic components in use. The large currents flowing in the Earth conductor has been known to induce surge voltages that have damaged electronic components.

Option 2: Use a test current of between 20 and 200 mA. Whilst this test is being carried out the cable should be flexed to bring out any obvious faults.

This is very safe test and can be used on ALL Class I appliances including IT equipment.

Test Limits

As mentioned above, to pass this test, the Earth Continuity resistance has to be less than 0.1 ohm. As this is from the pin on the plug to the Earth point, an allowance can be made for the Mains lead that is used on the appliance.

For example, a Class I lamp has a 5m mains lead (current capacity of 3A) attached to it. The total resistance of this lead is 0.195 ohms. To this one would add the 0.1 ohms for the appliance itself to give a limit of 0.295 ohms. When testing this lamp for Earth Continuity, if the resistance was say 0.22 ohms, then one would pass the appliance.

Say a PC has a 5m cable of current capacity 13A. The total resistance of this is 0.078 ohms. The pass limit for this PC would be 0.1 + 0.078 which is 0.178 ohms.

If the appliance has a short mains lead, then this will not contribute a lot to the Earth resistance. For example a 1m cable for an electric fire would have a resistance of only 0.015 ohms (Appendix 3 on cable types outlines how to calculate resistance of different types of cables).

When using a PAT Tester with a display, the user will be able to work out whether the appliance passes or fails whilst taking the length of the mains lead into account.

On Pass/Fail PAT Testers, there is an automatic allowance made for the mains lead. Typically the limit is set to around 0.25 ohms to allow for up to 5m of mains cable. On some Pass/Fail PAT Testers, allowance can also be made for longer cables, say when testing 10m or 20m extension leads.

The following points must be considered when carrying out the Earth Continuity test:

• The pass limit is less than 0.1 ohm.

• When testing Class I metal appliances, getting a good Earth connection can be difficult and it is important to persevere.

• One cannot get an Earth connection on Class I plastic appliances.

• Take care not to use a test current greater than 200 mA when testing IT or appliances with sensitive electronics.

• Allowance can be made for longer power cords by increasing the pass limit.