Jim Wallace, research and technology manager at Seaward Electronic, looks at the merits of using 25A and 200mA currents to test protective earthing conductors in electrical and electronic appliances.

Jim Wallace, research and technology manager at Seaward Electronic, looks at the merits of using 25A and 200mA currents to test protective earthing conductors in electrical and electronic appliances. Debate in the appliance industry on the most appropriate test current for checking the integrity of the protective earthing conductor has been around for many years. Historically a higher test current of 25A has often been favoured on the premise that it will best detect any damaged conductors present.

In addition, when analogue instruments were widely used for low resistance measurement, it was often necessary to use high test currents to produce sufficient voltage drop across the sample to generate the necessary needle deflection.

With modern electronics this is no longer necessary and more recently, given the growth in popularity of portable hand held test instruments, others have come to prefer a lower test current of 200mA as a means of eliminating any risk of damage to the equipment under test.

In reality, the different test currents both have their merits and the IEE Code of Practice for In-service Testing and Inspection of Electrical Equipment recommends both 25A and 200mA.

However, for routine testing and testing after repair of appliances and testing of fixed installations, the majority of European standards now specify a test current of 200mA.

Protective earthing conductors are designed to prevent electric shock by allowing the passage of electric 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.

A variety of national and international standards define a maximum acceptable level of resistance of a protective earthing conductor.

These standards not only specify the maximum resistance values but also define the test current, the open circuit voltage and the duration of that test.

With any item of electrical equipment it is likely that the protective earthing conductor will comprise various lengths of flexible cable linking the equipment to the point of electrical supply.

It is also possible that various types of switching mechanism may exist including relays and electrical switches.

Any measurement of a protective earthing conductor will therefore encounter both bulk and contact forms of electrical resistance.

Both these types of resistance can have implications on the use of different test methods with varying currents, voltages and time durations.

Bulk resistance is the material along the conductors' path.

This will tend to be constant although it will be affected by temperature and in certain cases by physical pressure.

Contact resistance, however, is a variable resistance that occurs at the interface between two conducting surfaces.

Contact resistance is made up of constriction resistance and film resistance and will be dependent on the contact force between the two surfaces in contact.

Careful inspection of the contact interface between two conducting materials will show that surfaces that may appear flat and uniform to the naked eye will invariably comprise a series of rough peaks and valleys when viewed under a microscope.

In reality, the two mating surfaces will therefore only make contact with each other where the surface peaks (asperites) meet and the actual surface area of this real contact area is typically much smaller than may be apparent.

In these circumstances constriction resistance occurs as the electrical current is channelled through small point contacts that occur at these peak points or interfaces.

Layers of oxide and dirt that are formed on the material's surface also create film resistance.

These oxides have higher resistance than the conducting material on either side of the junction.

The impact of these different types of resistance can therefore have significant impact on the results obtained from varying levels of test current.

It follows, therefore that irrespective of the test current, contact resistance between the test probe and the appliance under test can give a variation in measurement performance.

It is therefore important to ensure a secure connection to the equipment under test.

The perceived benefit of the relatively high 25A test current is that it will be capable of overcoming the implications of film resistance.

However, and conversely, excessively high levels of test current will cause temperature rise throughout the protective earthing conductor path.

If applied long enough will have a significant impact on the resistance value measures.

In the event of a damaged protective earthing conductor, where several strands are broken, a high current test may also detect the damage by 'fusing' the cable.

Fusing occurs due to the heating effect of the test current - the current flows, generating heat and the wire melts apart resulting in an open circuit.

The fusing action is produced by a temperature rise in the cable and it therefore takes a finite time for the cable to fuse.

The temperature rise and hence the ability to fuse a damaged cable depends upon the test current and the test duration.

In protective fuses this is referred to as the I2t rating.

The higher the current or the longer the test duration the higher the probability of fusing the damaged cable.

The probability of the test fusing a cable with broken strands will therefore depend on: a) how many strands are broken b) the magnitude of the test current c) the duration of the test The purpose of the earth continuity test is to ensure that accessible conductive parts, which rely upon protective earthing as a means of protection against electric shock, are connected to the protective earth of the supply.

There may also be accessible conductive parts which are protected through other means such as double insulation or protective impedance but which are connected to protective earth for functional reasons such as signal screening.

These earth paths may not be designed to carry high currents and passing a high test current through them may therefore result in damage to the equipment under test.

A 200mA test current is rapidly becoming the European standard for in-service testing and testing after repair.

In particular, those test instruments that comply with the requirements of EN61557-4 are capable of making accurate resistance measurements using a 200mA test current.

The use of a lower test current such as 200mA also reduces or eliminates the risk of damage to the EUT caused by passing high test currents through paths to ground that are not intended to provide protective earthing.

One of the reasons often provided for the use of a higher test current is that the resistance values being measured are in the order of 0.1 ohms and, in principle, a higher test current will aid the measurement process.

However, this particular argument loses some of its merits with the development of modern test technology that enables very accurate resistance measurements to be made using low test currents.

This work has been pioneered by Seaward Electronic in the form of a new-patented high intensity pulse or spike test that overcomes the previous contact resistance problems that inhibited the wider application of protective earth testing using 1A or 200mA test currents.

As a result the new concept successfully conquers variations in measurement that can be caused by weak contact resistance between the test probe and the appliance under test, 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 test technology introduced by Seaward enables valid 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 new test feature is now incorporated across all instruments in the company's new PrimeTest range of hand held portable appliance testers.

In summary, both 25A and 200mA are recommended internationally as a valid test current for the in-service inspection of electrical equipment and both are of value to electricians and test engineers.

However, a high test current doesn't necessarily detect a damaged PE path and does not always give better accuracy.

In addition, modern electronic technology means that low current testing can now be applied more effectively than may have been the case in the past.

Whatever the test current, contact resistance is an ever present variable.

However, a short duration high current spike prior to a 200mA test can overcome such problems.