The zap test approach to reliable earth continuity testing
26th March 2014
Amy Lyons, Applications Engineer in Seaward’s Test & Measurement Division, explains how specialist technology overcomes incorrect earth continuity test results.
Historically, there has been considerable debate on the most appropriate test current to be used to check the integrity of the protective earthing conductor of electrical appliances.
Until a few years ago, a higher test current of between 10A and 25A was often favoured on the premise that it would best detect any damaged conductors or overcome any contact resistance present.
With modern electronics this is no longer necessary and more recently, given the growth in popularity of battery powered and portable hand held test instruments, the industry has generally preferred a lower test current of 100mA to 200mA.
In reality, the different test currents both have their merits and the IET Code of Practice for In-service Testing and Inspection of Electrical Equipment recommends both 25A and 200mA.
Protective earthing explained
The earth continuity test, or the earth bond test, should ensure that there is a good connection between the mains plug and any earthed metal parts.
Protective earthing conductors are designed to prevent electric shock by allowing the passage of current under fault conditions. In Class I electrical equipment the protective earthing conductor resistance needs to be of sufficiently low value to prevent the voltage on external metal parts rising to a level where the shock potential presents a hazard to life and to ensure that the resultant fault current is sufficiently large to cause the fuse to operate quickly and clear the fault.
The traditional method applied to this test is to plug the electrical appliance into a PAT tester and to clip the test lead to an accessible earth point on the appliance. The earth continuity test passes a test current along the earth cable from the pin of the plug to the contact point on the appliance. The appliance tester then measures the resistance of that connection.
If the earth connection is damaged, non-existent or corroded then the earth resistance reading will increase. For the equipment to ‘pass’ the measured resistance is usually less than 0.1Ω plus the resistance of the protective conductor in the supply cable.
In reality, any measurement of a protective earthing conductor is made up from different components of electrical resistance which may have implications for the results of any testing.
For example, so called ‘bulk’ resistance, which is linked to the conductor material and which will tend to be constant, will be affected by temperature and physical pressure in certain cases.
Contact resistance, however, is a variable resistance and will be dependent on the quality of the contact interface needed to complete the test circuit – i.e. between the earth clip and the contact point on the appliance.
Despite appearances, the actual surface of any contact area may be much smaller than is apparent. As a result, increased resistance may occur as the electrical current is channelled through relatively small areas of contact. Layers of oxide and dirt that are formed on the material’s surface can also increase the resistance at the contact interface.
The impact of these different types of resistance can therefore have a significant impact on the earth continuity test results and care needs to be taken to achieve reliable readings.
A new concept
One of the perceived benefits of the relatively high 10A to 25A test current is that it will be capable of overcoming any contact resistance. However, large currents flowing in the earth conductor can also induce surge or transient voltages that may damage sensitive electronic components. For this reason the 25A test is generally not applied to IT equipment, for example.
On the other hand, the softer 100mA or 200mA test current reduces or the risk of damage to the equipment under test. However, the low current test may still be subject to inconsistencies caused by the inability to achieve a satisfactory contact point where contamination or contact resistance might cause a fail.
As a result, when testing earth continuity on Class 1 appliances, some battery powered testers can give variable readings or an incorrect fail result.
A new patent pending test concept pioneered by Seaward in the form of special ‘zap’ test circuit technology overcomes this problem.
This proprietary method successfully combines a low current test with a high intensity, very short ‘pulse’ that overcomes any contact resistance problems but does not pose any risk to the integrity of the appliance being tested.
As a result the new concept successfully conquers variations in resistance measurement that can be caused by a poor earth test clip contact, for example, when measuring continuity of tarnished or corroded parts such as a kettle element or in detachable IEC power cables.
Importantly, the unique low current zap test technology introduced by Seaward enables valid and reliable earth continuity tests to be carried out using battery powered testers, significantly increasing the portability and versatility of hand held testers and speeding up the testing process.
This specialist feature is now incorporated as standard across the Seaward handheld PAT range, including the new Apollo testers as well as the PrimeTest series.