### Earth Continuity Limit

The limit for the Earth Continuity Test is (0.1 + R)Ω. Where the ‘R’ value is the resistance of the protective conductor (earth wire) within the supply cable. In practice when PAT testing, measuring the actual resistance of the earth wire within the cable is impractical, therefore the ‘R’ value is usually calculated using Table V1.1 in the IET Code of Practice.

Table V1.1 in the appendix of the IET Code of Practice, page 134, gives the nominal resistance values for a variety of cable sizes and lengths. The resistance values are in mΩ (milliohms), so need converting to Ω before using the (0.1 + R)Ω formula. Our simplified version of the table (Fig.2) gives the resistance in Ω per meter.

The cable size, cross sectional area (CSA), is usually marked on the outer sheath. The example in Fig.3 shows a three core cable with a CSA of 1.0mm2 per conductor. Therefore the maximum earth continuity limit for an appliance fitted with 3m of this cable would be: (0.1 + (3 X 0.0195))Ω = 0.16Ω.

Earth continuity readings higher than the (0.1 + R)Ω limit are often due to problems with the test procedure, rather than the appliance itself.

#### Contact Resistance

The contact resistance between the test lead and appliance case, or the appliance plug and test instrument, can significantly increase the measured resistance. The low current earth test (soft test) is particularly susceptible to the affects of contact resistance because it has less power to break though any oxidation layer on the plug or case. Cleaning the earth pin of the plug with an abrasive paper and making sure the test probe is making a good contact with the metal surface will often reduce the earth continuity reading. Pay particular attention to where the test probe or clip is placed. For example when testing a kettle, putting the test clip on a heating element without breaking through the lime scale will increase the resistance significantly. If possible use the high current continuity test (hard test) to reduce problems with contact resistance.