This test is carried out on all Class I appliances during PAT Testing. 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 milli-ohms).
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
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 during PAT testing, there are a number of practical things to be taken
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 during PAT testing. This will result in the appliance failing the test.
Finding a suitable earth point will come with experience. 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
PAT testing again. If necessary try a different Earth point as not all metal parts
might be connected to earth.
PAT testing - 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 carry out PAT testing on 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 the earth Continuity test could not be carried
out due to the plastic enclosure.
PAT Testing - 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. This test 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.
PAT Testing - Test Limits
As mentioned above, to pass this test, the Earth Continuity resistance has to be
less than 0.1 ohm when PAT testing is carried out. 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. 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.