The Devil is in the Detail

As I pen this article by a log fire, with outside covered by natures seasonal nocturnal white blanket, it occurs to me that Electromagnetic Compatibility and related matters can be influenced by The British Obsession: the weather. Over a period of time certain situations have arisen that show that EMC is just as much making things work as debating the finer points of EN61000-X-X and, does have direct influence on safety.

Our business is obscure to most, the design and manufacture of Scientific Equipment for sampling dust from chimneys and flues. It is a multi-disciplinary compelling subject which for most will demand a brief explanation. The picture shows the pioneer researchers over forty years ago collecting dust samples to determine the particulate effluent from the stack they are testing. They are inserting a nozzle into a stack and drawing out gases to collect the entrained dust particles, it also clearly indicates that protective clothing in those days was somewhat more stylish than today. Plainly this activity is not for the faint-hearted or of a sedentary disposition and putting a foot wrong can have terminal consequences.

So that a representative sample can be collected, indicating the quantity of dust expelled from a stack: the gases drawn into the nozzle have to be at the same velocity as those travelling up the stack, that is, isokinetic. Achieving this represents a considerable challenge and it is very easy to get it wrong and have to repeat a test. Unlike those in the picture, today we have handheld computers and other electronics to guide us through tests and take measurements.

We received a telephone call from a fire chief who was trying to rescue a member of a sampling team who had collapsed up a stack. He wanted to know certain aspects about our equipment that the other team members couldn't answer. It transpired that he was hypoglycaemia, that is blood glucose levels too low. This puzzled us because the individual in question is clear thinking, an ex-pit deputy and manager with many years experience of dust sampling both in mines and chimneys. The conclusion by all concerned was that it was "one of those things". This incident persuaded us to set up a dedicated help line.

A letter from the health and safety executive told us that an individual had almost fallen from a stack; a height that would have certainly been fatal, due to an electric shock. The implication was that our equipment was responsible. Our equipment runs from 110V centre tapped, that is, two phase, any voltage over 130V it switches off and disconnects itself from the supply via relays on both phases, it is able to withstand inputs up to 415V. The cables are flexible armoured using BS4343 connectors and built with metal boxes at least 4.5mm thick in short; belt, braces, garters and string.

Upon investigation the accident occurred with the equipment disconnected as the team were packing up and lowering equipment down the stack. The individual had been diagnosed as having suffered an epileptic fit which to us seemed strange for a young, fit and healthy individual. This was a double blow to the person concerned as it not only put into question him dust sampling but his private pilots licence as well. The weather at the time was zero wind speed and temperatures well below freezing. At this stage officialdom lost interest as there was no simple reason for the incident and we too where completely puzzled.

Within a few days of this, the author was using a well know brand of suction cleaner and there was a most impressive and unexpected pyrotechnic display from the aluminium pipe. Upon inspection, where one was flared so the other could couple into it, there was a plastic liner; some experimentation could generate this effect at will. Generally it doesn't happen as the tube is held and the resistance of the user effectively stops the build up of static electricity.

Enquiry indicated that at the time of the incident equipment was being lowered by rope and pulley, although we were assured that this was metal we still borrowed it and had a look. Very well made with rolling bearings, the rollers stainless steel and the rest of the bearings plastic. The capacitance between the pulley and the body was hundreds of picofarads and with 25kV across the bearing as much as we can muster, gave an excellent imitation of an open circuit. Combined with the polypropylene rope, it formed a crude and surprisingly effective Van de Graaf generator. From the way the incident was described, it was most likely the individual affected discharged the rope and pulley through his head! At a conservative estimate there was half a joule of energy stored which is sufficient to upset the heart, and most likely the function of the brain too. It is not recommended any one tries this to find out. The pulley and block was easily modified to stop this recurring and the individual is still dust sampling and flying, but not at the same time, as far as we know.

A phone call from a sampling team about a technical query brought out the fact that one of their number had hurt his foot, by dropping a weighty piece of our equipment; being painful but not serious this was treated by the team as a bit of a joke. I talked to the individual and discovered that he uses a hearing aid and whilst up a stack he was startled by a very loud noise, to the extent that he dropped what he was carrying. The barometric pressure was unusually high, a still day and glorious sunshine. However, the time, day, weather and location was the same as the first incident. The hearing aid had been checked by the manufacturers and the individual assured me mobile 'phones didn't cause problems with it.

So what had happened? Our curiosity aroused, we started investigating on probable causes, the most likely culprit would be a high power RF source. There was no such equipment on the site and so our minds were cast further a field. Having exhausted usual RF radiators, we had a look at air navigation maps and went for a tour round. Some considerable distance away was a relatively new long range radar installation that tracked aircraft for hundreds of miles. These are pulsed radar working in the UHF spectrum with pulse repetition rates in the audio range. We then persuaded the individual with a sore foot to come down to the airfield and to see what happened, as soon as the radar was in use the hearing aid had to be switched off. Discussion with pilots showed that it also gave problems on their voice radio communications and, they had just learned to get used to it. At the time the incident occurred the weather was showing temperature inversion and under these conditions these frequencies can be refracted. During a subsequent similar weather pattern measurements up the stack showed field strengths of hundreds of volts a metre. The first incident was due to this source interfering with an insulin pump, who's manufacturers were very surprised to hear of the level of signal encountered and showed on their own tests that the pump was susceptible at the high field strengths and frequency. This shows how even diligent designers cannot foresee all scenarios.

The same piece of equipment kept being returned for repair, the same devices replaced each time. Discussing this with the customer who used it on just one site seemed to show that it occurred when it was being used on a new part of the installation, the users didn't seem to have any idea what was happening. A visit showed a new stack, when on the sampling gantry, to one side, was level with a 400kV power lines and down wind to the prevailing wind, so the overhead cables moved towards us during high winds. The sampling needed a 3.5 metre probe, at these voltages creepage and clearance takes on a whole different meaning. There was some difficulty in getting them to understand the risks until I got their safety officer to carry a fluorescent tube up the stack and when he was stood on the gantry with it glowing quite brightly, he turned a funny colour. They now sample from a different point.

Another 'phone call from the Health and Safety Executive explained that someone had received repeated electric shocks, luckily not serious from our equipment whilst sampling. Why companies don't contact us when they have problems of this sort defeats us. A visit where this had happened, showed a very modern and spotless installation, the only thing missing from the sampling point indoors were armchairs. Unfortunately, close inspection showed that the sampling was being done close to an electrostatic precipitator with high gas temperatures, which conduct electricity quite well. The shocks were coming from the precipitator, and until it was demonstrated with our equipment disconnected from its power supply it wasn't believed; interesting.

An interesting situation occurred to the author whilst sampling from an electric arc furnace at night. Our lamps on hard hats attracted insects, which is bad enough. The insects attracted the local bats who hung off our boiler suits as a convenient launch point for catching them. These charming, protected mammals have fine dentures and can carry hydro-encephalytus or rabies. The solution was to use ruby red lamps that didn't attract the insects. Is this an EMC problem? When radiating it is wise to do it at the correct frequency, or is this an obscure form of susceptibility?

My work as an expert witness is showing that companies and officialdom are increasingly going to litigation as the first choice. The foregoing situations plainly shows how with engineering the devil is always in the detail. However, we are looking at them, with all the wisdom of twenty-twenty hindsight. The key it could be argued is full and thorough risk assessment; is it reasonable to expect someone proposing to climb a stack to be on the lookout for radar systems over the horizon, or problems with bats at night? The lawyers dismiss the definition of "due diligence" as having to be determined by the courts, which makes me wonder whether this approach does really create a safer working environment. For when the sleet is coming at you horizontal the finer points of div, curl and grad are not upper most in peoples minds. For the lawyers; remember, arguing with engineers is like mud wrestling pigs: they enjoy it.

Bibliography Measurement of Solids in Flue Gases, by: P. G.W. Hawksley, S. Badzioch, J. H. Blakett Peter Williams is CEO of Tornado Technologies Ltd., and can be contacted by E-mail on: papawhisky@zetnet.co.uk