Q: The rear-screen heater in my BMW 3-Series causes so much interference when switched on that it’s impossible to listen to the car radio. Our local BMW dealer suggested replacing the entire rear screen as at cost of more than £600. This seems drastic. — KR from Hertfordshire.
A: This is a known problem within the trade. It stems from the fact that the rear screen includes both the heating elements and the radio aerial. The high level of electrical current required by the heating elements is being picked up by the aerial. Fortunately, it is usually possible to fit one or more electrical suppressors into the heated rear window wiring, as close as possible to the window itself. These reduce the electromagnetic interference from the screen to a level where it shouldn’t interfere with the aerial. This will cost much less than a new screen.
(Taken from ‘Car Clinic’, Sunday Times, March 13 2005, page 26, http://www.sunday-times.co.uk.)
Just in case anyone in the EMC community was away from the media for the last few days, electromagnetic interference is one of the suspected culprits in a fuel sensor malfunction that resulted in the cancellation of the first planned space shuttle launch earlier this month. Four hydrogen fuel sensors read either wet or dry, and a dry reading from all four sensors triggers engine cutoff and an aborted launch. After the originally scheduled flight was called off on July 13, literally hundreds of engineers tried to recreate the electromagnetic environment in which one sensor failed intermittently. Reportedly grounding was improved, and the entire craft re-examined for possible sources of EMI.
Still, the exact cause of the intermittent failure was not identified, and NASA rules were modified so that lift-off could take place with three operational sensors. Fortunately, the intermittent fuel gauge glitch did not reoccur during Tuesday’s lift-off. (Sadly, as we prepare our final copy, word of foam problems and cancellation of future flights has just been released to the media.) Clearly, NASA personnel face some daunting challenges in the months ahead. Moreover, every modification to the extremely complex craft alters the EM environment in which delicate instrumentation must function.
(From Interference Technology E-News, 29 July 05.)
I’ve always been suspicious of admonitions to turn off mobile phones on planes, in hospitals and so on, believing them to be yet more examples of the culture of bossiness that pervades modern life. It turns out that the bossy-boots are right.
You know how it is – one minute the car is working perfectly; the next – literally – it has conked. So it was last weekend when I was trying to transport four extremely heavy lead planters, overlooked by the removers, from the old flat to the new house. Various people emerged to have a look and offer advice, including the mechanic who lives opposite (who said Londoners aren’t neighbourly?), but the nature of the conking-out remained a mystery. “It sounds as if it is trying to start, but isn’t.” said one man, helpfully (who said men know all about cars?).
Then the AA man arrived – all yellow van and flashing lights, just like in the adverts – and solved the problem immediately. Had I, he asked, kept the car key anywhere near my mobile phone? Well, of course I had. Like most women I lug around a miniature version of my life in my handbag.
This was my mistake. The mobile phone signal had corrupted the chip in the key, disrupting the central locking system and knocking out the ignition. The thing was that none of us knew about this – not me, the neighbours, friends I have told about it, not even the mechanic. I pass on the information so that no one else finds herself stuck on a yellow line in the middle of London on a searingly hot day with a car that is going nowhere.
(From Rachel Simhon’s column in the ‘Diary’ section of the Daily Telegraph, Saturday, July 16 2005, page 23.)
Being involved in providing audio induction loops for hearing aid users, I am interested in cases of audio magnetic interference. The modern electromagnetic environment has an increasing number of these.
While installing loop systems in a building in Wolverhampton Science Park I checked for possible interference. A coil of red-coated pyrotenax cable in the ceiling was interesting. That being part of the Fire Alarm installation, I used the monitor receiver to listen to the “Break-Glass” alarm point. Again, I heard the same digital noise right across the audio range. This kind of interference is a continuous background buzz for hearing aid users wanting to use an induction loop. This compares with listening to a car radio, or to a CD with a noisy fan in the room. In practice, the magnetic field from the Break-Glass would not be a problem. But the field from the cable routing and the coil of surplus cable just above someone’s head could be.
We found a similar interference in a new building in Edgbaston High School. Most of the hall is clean, but there is one corner with significant digital noise. Again, it happens to be next to a storeroom containing the displays for the fire alarms. Otherwise, in that room, the fire alarm is cleaner than at Wolverhampton.
A case which may not be an audio magnetic field (I did not have the right detector available) was at my mother-in-law’s. We heard that the Hi-Fi had developed a fault, and there was a buzz. I found this buzz on the cassette deck, though the radio was clean. Moving the BT DECT cordless phone cured the problem. The wiring for this, from the power supply as well as the phone line, seems to be the main radiator. Since it is plugged in to the same mains point as the hi-fi, separating the items is difficult. It is not clear whether or not this is magnetic interference. But cassette tape play heads are susceptible to magnetic fields. For example, with separate items, placing a tape player on top of an amplifier usually causes a loud hum from magnetic coupling to the amplifier power supply.
Audio induction loops are a common “aid for the disabled” using the audio magnetic spectrum coupling to a pick-up coil in the hearing aid. The target magnetic field is 100mA/m to match normal hearing aid microphone levels. Installers, and public buildings, use monitor receivers to check loop systems. One such monitor is the Ampetronic ILR2 which allows people with normal hearing to hear what a hearing aid user would hear.
I happened to have a loop monitor with me when I was shown around the brand-new library building in Bournemouth. They have the, now ubiquitous, standard screening arrangement to detect books being smuggled past the check-out. This generates such a strong magnetic field that the nearest induction loop on the counter has to be about 4 metres away. Any closer and the noise, a constant whistle at about 1kHz or so, is intolerable for hearing aid users.
Similar anti-theft screens, the familiar pair of (usually) grey loops you walk between entering or leaving larger shops are very common. Because check-outs are near the doors, this is likely to be a problem where hearing aid loop systems are fitted. I happened to visit a major car accessory shop in Birmingham, again with a loop monitor to hand. The whistle from the security screen could be heard out in the car park.
(Sent in by Robert Higginson, 14 July 2005)
Audio frequency induction loops are an aid for hearing aid users which generate an audio frequency magnetic field. Often the siting of these is very restricted because of the way a building is built. While there is a specification for the field strength, this applies to normal listening position and there can be very high fields close to the loop cable. These fields can couple into other systems and equipment. Because EMC specifications are geared up for radio frequencies or mains power, high levels of audio magnetic fields are often forgotten.
We installed a desk loop system at an enquiries desk. These are supplied as kits, with a pre-formed loop coil usually located just underneath the desk, and driven hard so that the hearing aid picks up the spill field rather than the main field inside the coil. Having set the field to give the required “head height” signal, we found that the computer was responding to the magnetic field. The keyboard had a magnetic card reader used for staff to log-on. This was interpreting the audio signal as an erroneous user-name and password. Options were limited as there was only one tidy and vandal resistant place to put the loop. The solution was to move the keyboard away from the loop installation. In its normal place, the keyboard would be exposed to magnetic fields of several amps per metre.
Several years ago we installed a sound amplification system including induction loop in a church building. They also had a video projector, used with computers and video players with no problem. Recently they bought a new lap-top computer and the supplier offered a special package deal including a new projector. The new equipment picked up “hum bars” from the loop. The computer firm, apart from comments about “new regulations for loop installations” proposed the loop amplifier being switched off. We tried a range of other projectors and there was no problem. Only the Hitachi projector supplied packaged with the computer suffered hum bars. As an isolated equipment, a projector may pass susceptibility tests. But such equipment is never used in isolation, only as part of a system.
(Also sent in by Robert Higginson, 14 July 2005. Also see Robert’s article “Are You Hearing Me? An aid for the disabled lacking EMC protection” in the UK EMC Journal, June 1998, pp 14-16, available from the archives.)
(The Editor notes that the immunity standard for professional audio, video and lighting equipment and systems, EN 55103-2, includes requirements for immunity testing with audio-frequency magnetic fields. But system integrators often use ‘domestic’ equipment that has not been made compliant with EN 55103‑2, in their systems, and ignore their responsibilities under the EMC Directive by assuming that simply using CE marked items of equipment will result in a compliant system – the so-called CE + CE = CE approach, which does not work and furthermore has no legal or technical justification.)
A new hospital, commissioned in 2002, had large plasma display screens in their waiting rooms, showing fish swimming. But ultrasound diagnosis equipment in the rooms on the other side of the walls that the plasma displays were hung on suffered from interference. Close-field probing with a spectrum analyser showed significant levels of emissions leaking through the wall from the waiting room plasma displays, so they were assumed to be the culprits.
Moving the ultrasound equipment to the far side of the room from the wall shared with the waiting room reduced the level of the interference. When the interference levels are too high for a particular test, the ultrasound operators have permissions to switch the plasma displays off.
(Sent in by Clive Griffiths, 15th June 2005)
USA delegate opened a discussion on the new work being promulgated in the US by ANSI to address immunity concerns from wireless devices when used in close proximity to office equipment. (This had originally been raised by Goldman-Sachs who found this problem in their offices). The primary concern appears to be interference to telephone devices, although the draft ANSI standard is not limited to them alone. The standard being developed appears to be a product standard, not a basic standard. There is no chance of this being legislative but likely to remain a contractual issue.
This is a cause for concern to WG10 because of the risk of alternative test methods and attendant problems of multiple test regimes, inter-correlation, etc. It appears that this work has come about because of the lack of any immunity requirements in the US (for household, commercial and industrial products – Editor). Similar environments in Europe where CE marking is required have not evidenced such susceptibility.
(A report from a delegate to the WG10 Meeting 25-29 April 2005, Beijing, China, “Maintenance of 61000-4-3”).
Haag cite one experience where an intermittent failure was caused when the pulse driven motor of a crane went into resonance with the compensating capacitors of a nearby lighting system. It was interesting to note that because those involved were not familiar with the possibility of such mains linked resonances, all manner of costly time-wasting investigations were carried out and sources of blame sought, before the real culprit was found.
(From “Does Your System Have 50Hz Impedance Myopia,” Ron Neale, EMC Engineering Europe, March 1998, page 5.)
The point of the voltage interruptions tests in EN 61000-4-11, and the generic and product immunity standards that call them up, is that equipment should recover from such events as if nothing had happened, or at least as if they had just been switched on for the first time. Even for older equipment that pre-dated the above immunity standards, interruptions in the mains power is a rather obvious fact of life and designers could be expected to have designed accordingly. But recently I had to service a modified (and expensive) CD player that had some special system for background music.
There had been a 10-second-long power cut in Northampton, where it was installed, and the thing would not come back to life afterwards. Actually, it was not working when I collected it, but started working the next day (presumably after some capacitor had sufficiently discharged).
That 10-second power cut also put the Northampton traffic light system out of operation for a long time – increasing safety risks at road junctions. While working in the All Saints area of West Bromwich, we noticed several brief power cuts, the longest being perhaps only one second, and these appeared to put the traffic lights out of action for a lengthy period of time.
(Sent in by Robert Higginson, 13 September 05.)