Julian Grant, Managing Director of Chauvin Arnoux, continues his educational series of articles by looking at the impact that harmonic currents can have on earth leakage.
In two previous articles that have been published in recent issues of PE, I’ve introduced the topic of harmonics, explaining what they are, where they come from and why they matter, while also highlighting the need to ensure test instruments have enough “bandwidth” so that they are capable of including them in measurements.
Having shown how large the harmonic currents produced by modern day non-linear loads can be in comparison to the fundamental 50Hz current, I’d like to discuss the impact and considerations this also has on earth leakage, or residual currents. It stands to reason, after all, that if the current being drawn by a piece of equipment contains large amounts of harmonics, then the current that is leaking to earth will also contain them.
Not only that, but it is the case that higher frequency currents are able to travel more easily through insulation and across air gaps than those at lower frequencies, sometimes resulting in disproportionally higher levels of harmonic currents being leaked than those at 50Hz. This is due to the capacitive properties of insulation, and the fact that the impedance of a capacitor is inversely proportional to frequency. In other words, the higher the frequency the lower the insulation resistance to ac currents.
Two obvious considerations
The presence of harmonics in earth leakage currents in an installation raises two obvious considerations with regards to ensuring compliance with the latest version of the IET wiring regulations, along with other potentially more complex issues in some extreme circumstances.
Regulation 531.3.2 states in section (ii): ‘in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation of such currents downstream of the RCD shall be not more than 30 % of the rated residual operating current.’ In order to be able to accurately measure such currents an earth leakage clamp with a measurement bandwidth in excess of 1.25kHz will be required if it is to cover the first 25 harmonics, and 2.5kHz if it is to see the first 50. Sadly, as is the case with conventional current clamps, many of the products in use and available to buy on the market today max out at 400Hz or 1000Hz.
The bottom line here is that if your earth leakage clamp is not including harmonic currents in the measurements then you’re not measuring all of the leakage current in the circuit. More specifically, the readings obtained will all be erroneously low.
The second consideration concerns the selection of the protective device itself. Regulation 531.3.3 states: ‘Different types of RCD exist, depending on their behaviour in the presence of DC components and frequencies. The appropriate RCD shall be selected……..’ In this instance the frequencies referred to include harmonics, and an earth leakage clamp with too narrow or low bandwidth will not see them or include them in displayed results.
Earth leakage clamp capability
To ensure measurements include all of the harmonics generally considered today, a clamp with a bandwidth exceeding 50 x 50Hz (2.5kHz) would be required. A quick call to manufacturers’ tech support lines, or a search of their website for the data sheet and specifications, will reveal if your earth leakage clamp has such capability.
An additional handy feature to have would be the ability for the earth leakage clamp to include and exclude the harmonic currents from the readings. This would enable measurement of the level of fundamental (50Hz) current and the level of harmonic currents – vital information to assist in the selection of an appropriate RCD for the circuit. Without getting into the finer detail of RCD selection, published research on high frequency behaviour of residual current devices has shown that type A and AC RCDs are unlikely to see harmonics above 200Hz, although the presence of third harmonic in the residual current often increases the RCD sensitivity to 50Hz currents. The larger the 150Hz third harmonic current, the lower the 50Hz tripping current required to operate the device.
Type F and Type B RCDs include detection of high frequency currents up to 1kHz, although there is debate as to the suitability of this upper limit in light of modern-day non-linear loads with 1kHz considered by many as too low. Based on this, Type B+ RCDs now exist with a 20kHz upper limit.
As a cautionary note, having discovered the presence of harmonic earth leakage currents in an installation, changing a Type A or AC RCD to Type F, B or B+ may result in nuisance tripping that hadn’t previously occurred. Having now installed RCDs capable of reacting to the harmonic earth leakage currents present, it’s a good idea to be prepared to have to go on and resolve those issues.
Accuracy is vital
The take away here is that it is vitally important to make sure that the earth leakage clamp you’re using is designed to give accurate results in the presence of harmonics.
The F65 from Chauvin Arnoux will measure leakage currents down to a resolution of 10 mA with frequencies up to 3kHz – better than the 2.5kHz required to see the first fifty harmonics. Unusually for a leakage clamp, it also has a selection of other functions including measurement of voltage, frequency, resistance and continuity.
Today, where almost everything is controlled electronically, problems with harmonics are here to stay. Wise are the contractors who familiarise themselves with these problems and ensure that they’re well-equipped to deal with them.
To view the Chauvin Arnoux F65 Earth Leakage Clamp technical data sheet click here