All quiet on the EMC front (and other topics)

By J. M. Woodgate B.Sc.(Eng.) C.Eng.  MIET SMIEEE FAES Hon FInstSCE MIOA

All quiet on the EMC front

Since January, the top-level BSI EMC committee (GEL210) has seen only 28 documents so far plus a few 'liaison' documents, and the other committee (GEL210-11) has seen only about 80.  There have been times when those were monthly figures.

Not only are there few documents, there are no really hot topics to explain and comment on. This isn't a permanent state; new technologies and new frequency ranges will undoubtedly generate a need for more or modified standards in the coming year or three.

 Power quality

This is a subject that has generated many EMC standards, for conducted emissions and immunity therefrom, and, to some extent, the CISPR 18 series on the potential influences of power distribution systems on other infrastructure. It also has its very own standard in Europe, EN 50160. This came about because about 25 years ago, the European Commission decided that electric power is a 'commodity' (something that is bought and sold), which was not a surprise to people who had always been paying for it.

Like water and (fuel) gas, commodities have quality standards that are usually established in national legislation, but, perhaps because electricity cresses state borders more, or in a different way, than water or gas, it was decided that an EN would be produced. It was very controversial; the electricity suppliers (probably correctly) feared the imposition of unfeasible requirements, but agreement was reached and the first edition of EN 50160 appeared in 1994 (1995 as a BS EN). It applied to medium voltage supplies as well as low voltage. This first edition lasted until 2007.

It was updated to a new and much larger edition in 2010 and amended again in 2015. High voltage supplies are covered as well as medium and low voltage. It refers to IEC/EN 61000-4-30 for methods of measurement. The characteristics that are specified in EN 50160 are frequency, (voltage) magnitude, waveform and (voltage) symmetry of multi-phase supplies.

Electricity suppliers are clearly exempted from responsibility for failing to meet the requirements of the standard due to circumstances beyond their control.

Most people are concerned with the requirements for low-voltage supplies, so the following applies to them. For many purposes, the exact supply frequency is not important, but it is required to be within 1 % of 50 Hz for 99.5 % of the time. Much more important is the supply voltage, which is required to remain within 10 % of the nominal or 'declared' voltage. The latter is the voltage agreed between supplier and user, and may differ from the nominal voltage, which is 230  V. These voltages are 10 min mean RMS voltages. 

There are limits on rapid voltage changes and flicker, relating to IEC/EN 61000-3-3 and -11. Voltage swells are limited as well, but these are not covered in IEC/EN 61000-3-3 and -11. This is actually quite a complicated subject, so you need to read the standard to get the full picture.

Voltage unbalance in 3-phase systems is limited to 2 % for 95 % of any week. Again, this is a 10 min mean RMS value. Mains signalling voltages are limited to 9 % of the fundamental from 100 Hz to 500 Hz, sloping down to 5 % from 1 kHz to 10 kHz and then sloping down to 1.4 V at 100 kHz.

In the 1950s, the mains waveform looked like a sine-wave on an oscilloscope, even if it wasn't totally harmonic-free. This is in spite of all the TV sets with half-wave rectifiers, because they had 2-core mains leads and 2-pin plugs, so half were poled one way and half the other, neatly cancelling the harmonics (well, almost). Sometime in the late 1960s, the wider use of the 13 A three-pin plug prompted the electricity suppliers to ask the TV manufacturers to please get rid of the DC and even harmonics that were causing substations to go BANG, usually during the hours of darkness, and burn up. This swapped the harmonics from odd to even, so the sine waves became flat-topped, and they still are, although efficiency improvements in equipment power supply technology have caused the flat-topping to decrease a bit in recent years.

Harmonics are individually limited up to the 25th, and the total harmonic distortion up to the 40th harmonic, referred to the fundamental not, as in audio engineering, to the total RMS value, is limited to 8 %.

In May 2019, a new research project will start in Europe, to investigate the levels of voltages and currents at frequencies from 2 kHz to 150 kHz and to determine whether there are any ill-effects from them. Of course, such voltages are bound to be present; harmonic spectra don't stop suddenly at 2 kHz, and there are radio transmitters from 9 kHz upwards, eager to induce currents in the overhead power cables. Load equipment that uses switching techniques (a high proportion, these days) also produces high-frequency signals on the power lines

You may hear more of this in this modest column, because I have developed a very simple measurement technique. There is no real opportunity for precision measurements, because the levels of these signals vary a lot, both over short time intervals and at different points not very far apart. Whether this will be accepted by the people who strive to measure the EMI jelly with a digital micrometer remains to be seen.

There are two fairly long (as annexes go) Annexes in the standard; the first is about the special nature of electricity, while the second is about voltage variations and interruptions. Both are worth reading.