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Loop Testing And LED Lamps: WHAT’S GOING ON?

loop testingHave you recently started to get weird results – or even no results at all – when you’re trying to carry out loop tests on lighting circuits? You’re not alone; it’s becoming a common problem. Peter Wade of Megger explains what’s going on and what to do about it.

You’re carrying out a three-wire non-trip loop test on a lighting circuit and your tester gives you a peculiar result that you were not expecting. No big deal; that happens sometimes. So you borrow another tester, which is a different make from the first and this time the result is suspiciously low – less than 0.1 Ω.

At the Megger telephone help desk in Dover, we’re starting to regularly hear stories just like this and of course, the callers want to know what to do so that they can complete the certification of the installation they’re working on. So let’s find out what’s happening.

The first clue is that these issues always relate to lighting circuits – and not just any lighting circuits. The problems only occur in lighting circuits that include dimmable LEDs. Surely then, the answer must be to disconnect the LED lamps before testing. In many cases this isn’t as easy as it sounds as LED lamps are often hard to disconnect and, especially in applications that include arrays with large numbers of LED lamps, disconnecting them may involve a lot of time-consuming work.

In any case, disconnecting the lamps before carrying out the loop test turns out to be pointless. With no lamps in circuit, some sort of loop test result may be obtained, but it will still be unreliable and potentially misleading.

So what exactly is going on? To answer this question, Megger carried out a series of tests in its application laboratory. The initial tests showed, as expected, that the problem didn’t occur in circuits where non-dimmable LED lamps were used. More surprisingly, perhaps, there were also no problems in circuits using dimmable LED lamps but WITHOUT dimmers. Further testing revealed that the problem only occurred in circuits equipped with modern “intelligent dimmers”, which are specifically designed for use with dimmable LED lamps.

Older types of dimmer, which have been used for decades, don’t apparently affect loop test results – but see later in this article. These older dimmers don’t, however, work with LEDs. The new intelligent dimmers control the brightness of the LEDs by modifying the waveform of the voltage applied to them, and it’s this waveform modification that upsets loop testing.

In a nutshell, it’s not the dimmable LED lamps that cause the problems; it’s the dimmers that are used with them. But if that’s the case, why does disconnecting the lamps dramatically alter the loop test results obtained? After all, the dimmers are still in circuit.

downlight fittingThe answer is that intelligent dimmers need a minimum load to function. With the lamps removed, the dimmers are quiescent and their influence on the loop test is greatly reduced. With lamps fitted, however, the dimmers see a load, their circuitry is activated and they start to modify the waveform, making it impossible to carry out reliable loop testing.

Loop impedance values are needed to certify an installation in line with the IET Wiring Regulations, so what needs to be done in installations that include intelligent dimmers? The answer is that ZS values should be established by calculation using R1+R2, or the dimmers must be removed from the circuit before loop testing is carried out.

This may seem like a costly and inconvenient chore, but the truth is that all dimmers – even the conventional types that have been used for many years – should always be removed before performing loop tests. Contrary to the common belief among electricians and electrical contractors, simply removing the lamps is not sufficient.

It is also important to remember that, unlike filament lamps, LED lamps are electronic devices, as are their associated dimmers. This should be considered when carrying out certain electrical tests such as insulation resistance and loop testing.

Best practice suggests that loop tests should always be performed on a circuit with no load connected. Any electronic dimmer that remains in circuit is in reality a load, and it can have a significant effect on the loop test result. This is because, when measuring at the lamp socket with the dimmer in circuit, it’s not the circuit loop impedance that will be measured but the internal impedance of the dimmer itself.

It’s worth stressing that all of the points made in this article apply irrespective of the manufacturer of the instrument used for loop testing, and also irrespective of the test method used.

The conclusion can be quickly presented: always disconnect dimmers before carrying out loop tests, and your unusual loop test results will hopefully be a thing of the past!

Two final comments: first, if you are in any doubt whatsoever, seek advice from the lamp and/or dimmer manufacturer and second, if you would like further information about replacing conventional lamps with LED types, a very useful resource is the Electrical Safety First document, “Best Practice Guide 9 – Safe Installation of Retrofit LED Lamps”, although this does not specifically deal with the loop test issues discussed in this article. The document is available as a free download from

Explanations of the different types of LED lighting dimmer and how they affect the characteristics of the circuit are given in the IET Code of Practice for the Application of LED Lighting Systems.


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