There has been much debating over the years about the most appropriate test current to check the integrity of the protective earthing conductor on electrical appliances.
Up until recently a higher test current of between 10A and 25A was most usual as it was thought that it was most likely to detect any damaged conductors.
However, it has been reported recently that with modern electronics this is not necessary any more - now the industry generally uses lower test currents of 100mA to 200mA.
But both the higher and lower test currents have their benefits and the IET Code of Practice recommends both 25A and 200mA.
The earth bond test is performed to check that there is a good connection between the mains and the earthed metal parts. A protective earthing conductor is there to prevent electric shock through allowing current to pass under fault conditions. Class I appliances need to have a protective earthing conductor resistance of a low enough value to prevent voltage on external metal parts becoming a level where shock is likely and to be certain that the fault current is large enough to cause the fuse to operate quickly and clear the fault.
To test this it is usual to plug the electrical equipment into the PAT tester and clip the test lead to an earth point on the appliance. The earth continuity test passes a test current along the earth cable and the tester measures the resistance. Any damage to the earth connection - the earth resistance reading will increase.
However, measurement of the protective earthing conductor is made from different components of electrical resistance which could affect the results of tests – ‘bulk’ resistance will be affected by temperature and physical pressure under certain circumstances.
Contact resistance is variable and depends on the interface between the two surfaces in contact. In reality, the contact area surface is smaller than it is thought to be, this results in the possibility of increased resistance because the current is channelled through small areas of contact. Any dirt or oxide on the surface of the material can increase the resistance at the contact.
Any of these factors can affect the results of an earth continuity test. It is important to be careful so that accurate results are achieved.
It is a common belief that higher test currents of 10A to 25A are capable of overcoming contact resistance, however large currents in the earth conductor can increase the risk of surge voltages which could damage sensitive electronic components. This is one of the reasons the 25A test is not used on IT equipment.
On the other side of this argument is that the 100mA to 200mA test may be softer and be safer for these sensitive components, but it could still have inconsistencies because of its ability to achieve a good contact point where contact resistance and contamination may result in a fail test.
This means that some test results for Class I earth continuity can produce a variable or a false fail result.
Seaward are one of the manufacturers aiming to overcome this issue by developing new technology which solves the problem. Seaward say they have incorporated their new zap test circuit technology into all their tester series models.
So, what’s your experience, do you find this a problem, is the new technology from Seaward needed?
Written by Sara Thomson