Earth loop impedance testing is essential if installations are to meet the Wiring Regulations and Part P. But unless you’ve the patience of a saint and the skills of a juggler, it isn’t always easy.

Ask users what they most want from a loop tester and the answer is always the same. They want a tester which is simple to operate, requires only two connections and doesn’t trip RCDs. And, of course, it must provide accurate, dependable results. That may not sound too much to ask, but manufacturers have been trying for years to develop a product that meets all these requirements simultaneously.

However, after 18 months of research and development expenditure totalling hundreds of thousands of pounds, Megger claims to have become the first company to succeed.

With a patent application currently in progress, the exact details remain a little sketchy, but essentially, the new LTW300 testers use a low current test signal that is guaranteed not to trip 30 mA RCDs. The tricky part is achieving high accuracy with such a small test current, and this is where the company’s research effort has been directed. Mark Hadley, product manager for Megger explains: “The solution is provided by sophisticated digital signal processing techniques that allow the test signal to be reliably separated from the fundamental frequency of the mains supply.”

The instruments also analyse a large number of measurements over a period of ten seconds, providing accurate results in spite of the noise on the mains supply. If particularly high levels of noise are detected, the test period is automatically extended to ensure the accuracy of the results is not compromised.

Standard models can measure loop impedance down to 0.01 ohm, but the range also includes the LTW425 model which can measure down to 0.001 ohm. This is particularly useful when it is necessary to make measurements close to the source of supply. Past problems have included instruments unable to take measurements due to high levels of electrical noise, those that were unable to cope with high PSC values and others which gave erratic readings. The LTW425 addresses all of these issues.

Nevertheless, there are limitations when testing close to the source of supply. Hadley says: “Megger is pleased to provide advice on this issue, but note that the results always err on the side of safety.”

He adds: “The Cat IV rating means they are safe to use at any point in an installation, even before the consumer unit or main distribution board. Few other loop testers of any type can make this claim.”

The new testers have been field tested in demanding applications, including underground railways. “They have fully lived up to their promise,” says Hadley. “This means that users have access to the holy grail – two-wire instruments that can be used anywhere and which are guaranteed not to trip RCDs.”

You would expect this kind of new technology to come at a price. But you’d be wrong. The new testers cost exactly the same as previous models.

A trip down memory lane – why traditional loop testing is set to become a thing of the past

The most basic loop testers work by connecting a 10 ohm resistor between the live conductor and earth for a very short time. By measuring the supply voltage and the current, the instrument can calculate the total impedance of the circuit. After deducting the value of the resistor, whatever is left is the earth loop impedance.

This method is simple, accurate and dependable – provided the circuit under test isn’t protected by an RCD. If it is, the RCD is guaranteed to trip, which is not only inconvenient but also means that the test result is unreliable.

A workaround is to link out the RCDs during loop testing, but that is equally awkward, and there’s always the risk that a link will accidentally be left in place, leaving the affected circuit without earth leakage protection.

To avoid these problems, three-wire loop testers were developed. These need connections not only to the live conductor and earth, but also to the neutral conductor. They work in much the same way as a simple tester, but start by measuring the impedance from live to neutral instead of live to earth.

Then they measure the impedance from neutral to earth and use these two values to calculate the earth loop impedance. At no stage are there any current flows which would trip an RCD, so all is well…

Or is it?

There are many cases, such as centre tapped site supplies or even ordinary light switches, where no neutral is available, and others where connecting to the neutral is difficult. Also, it’s fairly easy to work with two test leads, one in each hand, but unless you’re an avid circus performer in your spare time, juggling three leads is another matter.

So perhaps reverting back to two-wire testing and finding a way to stop the RCD from tripping is the best idea? One technique depends on the RCD having a small internal transformer to detect earth leakage. By injecting DC during the loop test, it is possible to saturate the transformer’s core, and fool the RCD into ignoring the test current. This works so long as the RCDs are mechanical. Unfortunately, many modern electronic types trip whether or not DC is applied.

So why not simply go back to the original two-wire test method and, instead of a 10 ohm resistor, use much higher value so the current is insufficient to trip the RCD?

That works too, but now the earth loop impedance is so small compared with the value of the resistor that measuring its effect on the current flow accurately
is difficult. Things are made worse by the noise and fluctuations that are present on every supply system.

There are testers that use this technique, but the lowest value of earth loop impedance they can measure is about 1 ohm. Anything below this is displayed as < 1 ohm. When the test result is used to calculate fault currents close the source of supply, a much more accurate result is needed.

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Electrical and Mechanical Contractor

Postscript

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