Editor’s Note: We regularly receive requests from readers to publish stories about real EMI/EMC problems faced by real engineers. We are pleased to bring you Banana Skins, a new monthly column in In Compliance, and a 20-year tradition that began in the pages of the EMC Journal. We hope you enjoy the column and look forward to continuing the tradition of sharing these valuable stories.
20. Licensed TV transmissions interfere with intensive care, kills babies
While taking classes in the early 80’s, my professor got involved with a terrible incident down in New Jersey. Seems a hospital had a high incidence of infant deaths in the intensive care section of the maternity ward. Late at night, the alarms on the babies’ monitors would go off for no apparent reason. Annoyed, the nurses would turn them off and do the rounds on foot.
After some preliminary investigations, my Prof found out that a nearby TV transmitter was allowed by their FCC license to increase their output wattage by some enormous amount after say midnight but had to reduce it prior to 6am, or some such arrangement. The cable interconnecting the nurses station to the various baby monitors sang like a lark with these frequencies and set off alarms with the induced voltages.
Not sure now of all the specifics except what I have related above nor the name of the hospital, but they lost something like 6 kids before fixing it.
(From Doug Mckean, via email@example.com, 29th July 1998)
21. ‘Impossible’ 50% brown-out occurs for 8 hours in the UK
Undervoltage AC supplies (brown-outs) are common in underdeveloped countries, or where the AC supply network is incorrectly configured. Parts of Spain are known to experience around 150Vac for lengthy periods during the day, apparently due to network loading, despite an officially-specified mains supply of 230Vac ±6%. I had never experienced a brown-out in the UK, except maybe for a second or so prior to a complete supply failure during a thunderstorm, and I used to think that it must not be possible because of the way the supply network is operated here.
On Sunday 26/7/98 around 5pm in Denshaw village, Lancashire, U.K., the supply dropped to around 140Vac RMS (40% below nominal), and stayed at that level for about three hours before shutting down completely as the engineers arrived to fix the problem. We switched off our fridge and other motor-driven appliances, mainly because they were making very strange noises. Our computers kept running, but the CRT screens blanked, making us concerned about what was happening to our data, so we switched them all off as well.
I am aware of electronic control equipment that can misoperate when operated considerably outside its specified AC supply range, and also understand that undervoltages can damage coils and motors. Apparently the motors can stall due to the low voltage, so they don’t generate back-emf, so they draw excessive currents and overheat, damaging their insulation and suffering premature failure (if not electric shocks and fires).
Manufacturers of products for the developed world, and their safety test laboratories, usually do not test at supply voltages outside ±10% (sometimes ±15%). Until Sunday 26th July I had not thought this important. So far we have not discovered any damage to appliances or to data.
(Submitted by Keith Armstrong, Cherry Clough Consultants, www.cherryclough.com.)
22. Ball lightning during in-flight refuelling
There was film footage on TV some years ago of a British in-flight re-fuelling exercise where the tanker aircraft was hit by lightning, but there was no on-going discharge downwards, the implication being that the plane was left charged to 100kV – 1MV or whatever.
A few seconds later great balls of glowing gas came off the back of the fuselage and wing tips (where the fuel hose was) into the slip stream, presumably taking away a whole load of surplus electrons, or holes – whatever it was – as “ball lightning”.
(From Chris Dupres via emc-pstc)
(The Editor comments: New Scientist magazine recently reported that a theoretical basis for ball lightning may have been found, www.newscientist.com.)
23. Electrostatic charging of helicopters
Helicopter blades and bodies tribocharge as they swish through the air, and they don’t have a convenient green/yellow wire handy. There are reports of an oil rig computer system crashing whenever a helicopter landed, due to its sudden electrical discharge into the metal decking, and there is a film which shows a crew-member getting an awful shock when he reached up to touch the skids of a hovering helicopter.
A US Coastguard Chief related to me that he had seen “arcs as bright as a welding stick” when an emergency pump was delivered to the deck of a freighter one night. He also told me that the Coast Guards’ standards practice calls for NEVER lowering a flotation ring or sling to a person in the water; the person will automatically reach for the line! Instead, they dip the line into the water and drag it to the person.
(The above are from Chris Dupres, via firstname.lastname@example.org.)
24. Interference with a travelling crane
A new CNC machine being installed in a factory had a spindle controller which was a small inverter drive in a plastic case. When the spindle was first operated emissions from the inverter caused a overhead travelling crane to start up and drag its chains down the length of the factory. Luckily, the 18 ton casting the chains had been attached to had just that minute been released.
(Submitted by Phil Hampton.)
25. Two more examples of interference with cranes
There was the famous case reported by the DTI in the early days of their EMC Awareness Campaign of the guy who was standing under his crane’s load using his radio-control pendant when interference caused it to release its load, crushing him to death. Many recent crane incidents are due to the use of radio control, especially where crane radio-control systems share the same frequency bands as amateur radio and/or car radio-keyfobs. The soon-to-be-introduced TETRA system also shares some of these bands, and use 25W transmitters – so expect more wild cranes.
Just to prove that modern technology can’t teach old technology any interference lessons, I once worked for a company that I was told had made the controls and drives for the first large-scale hovercraft testing tank in the late 1960’s. It was in effect a sophisticated travelling overhead crane, which ran a gantry along overhead rails and towed a hovercraft shape along a large pool of water in an even larger building. In those days they used resistor-transistor logic which ran on a 40V rail to provide noise immunity. During commissioning the machine suddenly started up by itself and proceeded towards the far end of the pool – it had been set off by “some sort of mains transient”. All the personnel on the site were standing by the access ladder to its gantry, but the only emergency stop button was on the gantry – but it and its ladder were moving just faster than running speed and they couldn’t get to it. Since it was not operating according to its (hard-wired) programming, the crane ignored its limit switches and crashed clear through the end wall of the building. Luckily nobody was hurt. The next version had E-stops all around the building.
(Submitted by Keith Armstrong, Cherry Clough Consultants, www.cherryclough.com.)
26. Foetal heart monitor picks up cell phone conversations
A foetal heart monitor in a clinic in the UK in June 1998 picked up a cellphone conversation from elsewhere on the premises quite clearly. The visual output of the monitor was unaffected, but the staff tend to use the audio output, and the cellphone conversation was so loud that it swamped the heart signal they were listening for.
It must have been an analogue cellphone, and it must have been getting in via the audio stages, or else the visual output would have been distorted. Even slow opamps will demodulate 900MHz signals (as hearing aid wearers are only too aware!). In common with many healthcare promises, the use of cellphones on the premises was banned, but you can’t rely on people to read or follow signs.
(Submitted by Ian Ball.)
27. RF welder fields set fire to bed 60 yards away
A 40kW RF welder (a dielectric welder for plastic materials) in use in a factory caused a mattress in a bed manufacturer’s factory 60 yards away to catch alight. The bed springs must have just been the right length to make an efficient antenna at the frequency the welder was using.
(Anonymous submission, date of event not specified.)
The regular “Banana Skins” column was published in the EMC Journal, starting in January 1998. Alan E. Hutley, a prominent member of the electronics community, distinguished publisher of the EMC Journal, founder of the EMCIA EMC Industry Association and the EMCUK Exhibition & Conference, has graciously given his permission for In Compliance to republish this reader-favorite column.
The Banana Skin columns were compiled by Keith Armstrong, of Cherry Clough Consultants Ltd, from items he found in various publications, and anecdotes and links sent in by the many fans of the column. All of the EMC Journal columns are available at https://www.emcstandards.co.uk/emi-stories, indexed both by application and type of EM disturbance, and new ones have recently begun being added. Keith has also given his permission for these stories to be shared through In Compliance as a service to the worldwide EMC community.
We are proud to carry on the tradition of sharing Banana Skins for the purpose of promoting education for EMI/EMC engineers.