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.
28. Digital TV can suffer more from EMI than analogue
Digital TV is more likely not to deliver a programme to the viewer than the analogue TV services it replaces. It appears that this newer technology is less robust, and that its users will on average suffer a higher loss of service than they may have become used to.
Broadcast digital TV, which can be picked up with existing TV antennae, has a sharper cut-off in performance as signal strength declines. R.S.Sandell, a Fellow of the IEE writing in IEE Review November 98, is concerned that: “Whereas analogue viewers can live reluctantly with a picture that has to be viewed through varying angles of ‘venetian blind’ and alternating densities of ‘boiling porridge’, they can still follow the programme plot for most of the time. This dubious advantage may not be available for some members of the digital generation, who will be confronted by a blank screen. In particularly unfortunate reception location this condition may come and go with time as field strengths vacillate”. Viewers using indoor aerials (the TV with the rabbit ear antenna in the kid’s bedroom?) may find they need to invest in new external aerials or aerial amplifiers and splitters.
Satellite-delivered digital TV is very susceptible to lightning storms, both at the uplink and downlink ends. This leads to the odd situation, when watching digital satellite TV in South Africa during very fine clear weather that thunderstorms near the uplink in Europe can cause all 100 channels (or however many there are) to disappear all together for periods of several minutes. This phenomenon was well understood by the satellite broadcasters, who broadcast a little presentation on this topic every now and again.
(From Keith Armstrong of Cherry Clough Consultants, www.cherryclough.com.)
29. Jam GPS over 200km range with 4W pocket-sized Russian unit
GPS is another example of an advanced technology that everyone wants to use, but which has important susceptibility problems. The signals from the GPS satellites are very weak, so the receivers have to be correspondingly sensitive, which means they are readily swamped by interference from industrial sites. Even though they are at microwave frequencies, interference with satellite communications caused by such commonplace things as poor quality power line connections has been observed several times. Added to this, the need to “see” several satellites at once means that GPS is unreliable in the urban canyons of cities.
I was intrigued to see two items on GPS in the New Scientist magazine dated 10th January 1998. The first was an article about the concern of the US military about a Russian GPS jammer. With only 4W of power (about the same as a hand-held CB or security guard walkie-talkie) this device is claimed to prevent GPS systems from working over a 200km radius (yes, 200 kilometres!). Apparently any competent electronic engineer could build such devices from readily available components. The second item in the same issue was an advertisement from BT for their MoBIC mobility system for blind people. This uses a computerised map, speech simulator, and GPS to guide blind people to their destination. Quote: “Getting around the shops is much easier since I started using the US Military’s Satellite Guidance System.”
Designers building GPS into their products, especially where these are used for critical purposes, might like to consider the lack of robustness and ease of jamming of this system. I have visions of hordes of planes, cars, and pedestrians all milling around a factory until a certain machine is switched off, because their satellite navigation systems are blocked by its microwave noise.
(From Keith Armstrong of Cherry Clough Consultants, www.cherryclough.com.)
30. Rodent repeller interferes with Amateur Radio
Interference in the Amateur Radio 144MHz band traced to an ultrasonic rodent repeller.
(Brad Thomson, Editor of Test and Measurement World, Feb 95, www.tmworld.com.)
31. Domestic appliance interferes with surround-sound processor
The operation of a domestic appliance used to reset a surround-sound processor, causing a ½ second gap in the audio
(From Neil Gardner, Plantronics, August 98.)
32. Taxicab radios interfere with control of Hi-Fi system
An advanced hi-fi system would change input selection due to taxicab radio transmitters when they called at a public house 100yards away.
(From a Technical Director of Lumonics, 1996.)
33. Cell phone interferes with digital watch
A Tissot Two-Timer digital/analogue wrist-watch went into time-travel mode (about x 60) whenever a particular Motorola Micro-Tac portable phone nearby had someone actually speaking into the mouthpiece.
(Chris Duprés, 7th July 1998. It was his watch!)
34. Several examples of interference
With over 18 years in EMC I could go on listing interference incidents for a long time. Some examples this year already: My computer (FCC Class B) interferes with my cordless phones, to some degree on all 10 channels. My fax machine (FCC Class B) interferes with my TV and some channels of my cordless phones. My garbage disposal unit interfered with everything! My small personal fan destroys my monitor picture.
(From Derek at LF Research, 6th July 98, www.lfresearch.com.)
35. Microwave cookers interfere with car security
Domestic microwave ovens can activate the microwave security sensors fitted to some vehicles.
(From Terry Beadman, Motor Industry Research Organisation (MIRA), 6th November 1998, www.mira.co.uk.)
36. EMI implicated in two capsizes
The Canadian Centre for Marine Communications claim there is evidence that EMI may have contributed to two boat capsizes, via autopilot malfunctions. One was the 16metre fishing vessel the “Dalewood Provider” on August 17 1989, the other was the 64 tons “Martin N” on April 25th 1987. In the latter case three lives were lost. In both cases the concern is that the on-board VHF radiotelephone system interfered with the autopilot sufficiently to turn the rudder hard over. Staff at the Centre report that erratic alterations in a boat’s course when autopilot is engaged and VHF radio used is commonplace, generally due to insufficient EMI suppression at the autopilot’s interface and control cables.
(Extracted from: “Need for EMI/EMC Standards and Regulations on Small Boats: a Canadian Perspective” by Byron R Dawe and Albert Senior of the Canadian Centre for Marine Communications, and Peter Ryan of Fisheries and Oceans Canada, EMC Technology magazine, Nov/Dec 1998, pages 17-19.)
37. Chlorine gas release caused by mains transients – prosecution
The HSE recently prosecuted the supplier of an item of equipment which led to a release of chlorine in a semiconductor plant. The equipment was not sufficiently immune to mains transients (and proven to be so by the HSE’s own labs). They were prosecuted under section 6 of the Health and Safety at Work Act because the supplier, though aware of the problem, did not inform the users of the equipment. The company pleaded guilty.
(From Simon Brown of the UK’s Health & Safety Executive (HSE), 13th January 1999.)
38. Cell phones can interfere with pacemakers
The Therapeutic Goods Administration of Australia (TGA) continues to review findings of clinical and laboratory research indicating a potential for temporary interaction or interference between mobile phones and the operation of pacemakers and implantable defibrillators. The findings have indicated that interference may be caused by holding the phone within about 150mm of the implanted device, or in direct contact between the phone antenna and the user’s skin. Interference can occur with the phone in standby mode, as well as in use. Some phones incorporate magnets, at least in their loudspeakers, and while held close to the implanted device these can cause them to go into their “magnet” mode, which for a pacemaker is a fixed pace.
Based on the most recent testing, simply moving the phone away from the implanted devices will return it to its correct state of operation. Recommendations for users of implanted pacemakers or defibrillators include: not keeping the phone in a pocket over the site of an implant; using the ear that is furthest away from the site of the implant when using the phone; and not allowing the phone antenna to touch any part of the body.
(From Compliance Engineering’s European edition Jan/Feb 1998,
www.ce-mag.com.)
39. Pinball machines interfere with emergency services’ radio comms
There is a story of how something was causing havoc with the emergency services’ two-way radio communications in Nevada, i.e. police, fire, and ambulances. An exhaustive investigation led to one or more really noisy pinball machines at a roadside pub (editors note: I thought they had bars in Nevada instead of pubs). The owner was ordered to get rid of them. He got rid of them and the problem went away. However, it soon reappeared, as another pub owner wound up with the same machines.
(From George Alspaugh of Lexmark International, 7th July 1998.)
40. Kitchen fan triggers security lamps
I have a security floodlamp system for my backyard, equipped with a thermal motion sensor. I have found that I have a reliable, though unintentional, remote control capability simply by flicking the kitchen range fan on and off a couple of times. I told my wife that it’s a special purpose, hard-wired, digital controller.
(From Ed Price of Cubic Defense Systems, San Diego, 8th July 1998, www.cubic.com.)
41. Eurostar north of London delayed by concerns over interference
Eurostar and Railtrack officials admitted this week the threat of EMI causing signal failures is delaying the introduction of European rail services north of London. EMI generated by overhead power lines can affect the trackside signals such that red lights are forced to green. A Eurostar spokesperson said: “In electrical terms, we have found with new trains, such as Eurostar, there tends to be a degree of stray electrical current. This can cause an interference with signalling and affect the integrity cause a signal to go from red to green.” Railtrack, responsible for the track and signalling systems, is refusing to allow the trains to run commercially until Eurostar can demonstrate their safety.
“We are working hand-in-hand with to solve this problem as quickly as possible,” Railtrack said. Eurostar engineers have designed an interference current monitoring unit. When it senses EMI, the motor is stopped and the train coasts to a stop. However, for the highest safety the unit must be set to maximum sensitivity. This could cause the train to stop every few miles.
(From Electronics Weekly October 23rd 1996) (Editors note: has anybody seen a Eurostar north of Watford Junction yet? How much has this cost our national economy, especially northern companies? I understand that all Eurostar trains have had TCFs done for them under the EMC Directive and that traditionally both British Rail and Railtrack always imposed stringent EMC immunity standards on their signalling equipment, using the RIA series of standards.)
Also see the Lords Hansard text for 14th July 1998 (180714-02) in which Baroness Hayman, Parliamentary Under-Secretary of State, Department of the Environment, Transport and the Regions stated:
“My Lords, the technical issues which prevent the operation of regional Eurostar rolling stock on parts of the existing network relate to electrical interference associated with track circuits. These matters have prevented the issue of safety clearances which are required before passenger services can be operated.”
(From online Parliament statement.)
In October 2003 the interference problems still did not seem to have been solved. The editor has since been informed that plans to run Eurostar trains north of London have now been shelved.
42. Railway trains banned over interference
There are a number of railway trains that have been unable to be taken into service because they interfere with signalling.
(From Ray Garner of Datel Defence Ltd, November 98, quoting an earlier article in a national newspaper.)
43. Induction furnace interferes with high street store’s computers
A 1.5 MW induction furnace controlled in on/off time-proportioning mode (using large contactors to switch the current) interfered with the computers in a Marks and Spencer’s store ½ mile away.
(From Laidler Associates Consulting Services, June 1998, www.laidler.co.uk.)
44. Turbogenerator support distorted by heating from incorrect cable installation
A new large turbogenerator in a UK power station was designed to have its 20kA three-phase output busbars split either side of one of its support pillars, because of a lack of space. The support pillars were steel, part of a steel framework, and created a single-shorted turn around one of the busbars. In operation, the pillar (made of 2 inch thick steel members) got hot enough to blister its paint, and increased in height by 5mm, putting a bearing out of alignment and causing a terrific noise which caused the station workers to run for their lives. (Editor’s note: a large turbogenerator up to speed and adrift from its bearings is a fearsome object!) The cure was another shorted turn, this time around the pillar and made of ½ thick aluminium.
(Conversation at Mersey and District Club Européen, 28th January 1999.)
45. Further information on Banana Skin No. 37: Chlorine release
The case referred to was heard at Swindon Magistrates Court on November 25th 1998. The defendant entered a plea of guilty to a charge brought under S6 (1) (d) of the Health & Safety at work etc. Act, 1974. The magistrates imposed a fine of £5,000 and made an order for the defendants to contribute £7,000 towards HSE’s costs of £9,482. The case concerned a microprocessor based valve control panel used to control the flows of chlorine and nitrogen in a semiconductor plant. There had been a release of chlorine resulting from all of the valves in the control cabinet being set to an open position.
Investigation by the HSE found that the unit was susceptible to conducted transients on the mains supply. There were no precautions against electrical interference in the power supply and the microprocessor watchdog was not effective in ensuring a safe state following detection of a fault. The HSE inspector who dealt with this case was Eifion Davies in our Cardiff office.
(From Simon Brown of the HSE, 3rd March 1999.)
46. Monitor image wobble caused by magnetic fields
Scenario: Large open-plan office in a publishing company. Lots of eager beavers with 21 inch displays on their MACs, doing all sorts of clever graphics things for page make-up and other arcane processes.
Problem: The displays on only some of the monitors oscillate sideways about 0.5 mm at most, at about 1 Hz.
Diagnosis (partial): The combined magnetic fields of mains cables under the floor and a power transformer on the floor below are sufficient to cause this very small effect. Unfortunately, once you notice it, it keeps catching your eye and it eventually drives you mad! The 1 Hz is due to a beat between the third harmonic of 50 Hz and the second harmonic of the 75 Hz frame rate of the displays.
Solution: Move the transformer. Replace the large feeder cable to it by individual lower-current feeds to the loads served from it, spread out across the void below the office floor.
Continuing problem: Now that the sideways movement has been eliminated, an even more subtle “vertical” movement of the displays is discovered. Again, it’s difficult to see, but once you see it, you can’t ignore it. This effect is not continuous: it occurs for a few minutes and then disappears for about ten minutes or more.
Diagnosis: An air-conditioning unit is found to have an intermittent fault to earth, resulting in some 3 A flowing in the armour of the cable feeding it. This current is not balanced by currents flowing in the conductors of the cable, and creates a 50 Hz magnetic field with a horizontal component sufficient to cause the effect.
Solution: By turning a monitor through a right-angle, so that the strong horizontal component of the field is parallel to the electron gun axis, the movement disappears. However, it is obviously necessary to correct the potentially hazardous fault in the air-conditioner.
The main point here is that the tolerable amount of display movement is “very small indeed” when people are working on complex artworks on large-screen displays.
(From John Woodgate, 8th March 1999.)
47. Mains spikes blow fuses in poorly-designed control panel
The relaxed attitudes of those times did not always pay off, however. Slightly later in my career, I moved to a company where engineering standards were, lets put it politely, a little lacking. For example, interlocking between contactors in reversing pairs was considered an unnecessary expense, and no one would ever consider such niceties as interrupting capacity when selecting a fuse. If the current rating was right, the fuse was good enough for the job.
The error of these ways wasn’t long in revealing itself, however. In one mechanical handling job, we had around a dozen reversing starters, all protected with totally inadequate fuses. Even worse, the contactors were controlled by some very dodgy solid-state switches which had been “designed” in-house. Now, in those days, EMC hadn’t even been invented. The result was that even the slightest spike on the supply made these solid-state switches turn on – just for an instant – but long enough for all of the contactors to jitter. Frequently, both contactors in a reversing pair would close for an instant, placing a short-circuit across the supply. This meant a mighty bang as the inadequate fuses shattered and spilled their silica contents all over the floor of the enclosure.
After a lot of time on site, during which much wiring was re-arranged and many capacitors were added to the system, we managed to get the equipment working after a fashion but, ever since, I’ve been suspicious of control panels with a layer of silica sand in the bottom!
(Taken from “When I was a lad…” reminiscences by Keith Wilson, Panel Building Magazine, February 1999, page 17.)
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.
