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.
48. Screened leads that weren’t
I was testing an item of IT based instrumentation the other day that failed conducted emissions. We replaced its 3 metre long screened 25-way D-type lead, which had been purchased as a “fully screened cable” from a well-known distributor, with my own home-made 15 metre long 25-way D-type lead, which simply used a single braid cable and metallised plastic backshells. The conducted emissions problem (on the mains lead) went away. My customer is now trying to source cables which really are screened. So caveat emptor, even when buying from large distributors.
(From Ian Ball of A. D. Compliance Services Ltd, which used to be Dedicated Micros EMC Test Centre.)
Items 49-52 are four real-life case-histories of industrial projects that failed in a big way, due a failure to correctly appreciate EMC. The names and details that might allow identification of the companies concerned have been suppressed for confidentiality
These examples have been extracted from the paper “The Real Engineering Need for EMC” by John Whaley, General Manager of SGS International Electrical Approvals (UK), presented at the IEE event “Electromagnetic Compatibility in Heavy Power Installations”, Teesside, UK, 23rd February 1999, www.theiet.org.
The other papers from this event will also be of value to anyone involved with industrial products and installations (not just heavy power applications). Contact IEE Sales and ask them to send you digest reference 99/066. These cost £20 each for delivery in the UK, and they normally require a cheque for the full amount before posting. An extra postage charge may be made for overseas customers. Phone +44 1438 313 311, fax +44 1438 313 465, or e-mail: sales@iee.org.uk.
49. Failure to correctly specify EMC performance
A large manufacturer of industrial fasteners, negotiating with a major customer, agreed to install a packaging cell containing an automatic weighing machine that filled plastic packets with fasteners and an RF welding machine to seal the packets. For cost reasons the two machines were purchased separately. No assessment of the electromagnetic environment took place, and the machine contract specifications included no EMC requirements other than “shall meet all legal requirements“.
Both machines were supplied, installed, and tested successfully. Unfortunately when both were operated together the weighing machine suffered >25 % errors due to interference from the RF used by the welder (not an uncommon problem). In an 8 hour shift the cell should have packaged £20,000 of fasteners, but could have given away up to £4,000 of product in incorrect weights.
There was no comeback on the machine suppliers, whose products met specification. Both suppliers appeared willing to help, but when pressed blamed each other. Expert technical assistance was brought in and solved the problem. The fastening manufacturer lost 6 weeks production, suffered additional costs, and lost credibility with their major customer.
50. Over-specification of EMC
A machinery manufacturer needed a special inverter drive for a new range of machines, and out them out to tender. A manufacturer of AC inverters won the contract for this large project against stiff competition, but didn’t notice that the specification required meeting military EMC standards. Their normal inverter designs failed the EMC tests, and their customer refused to accept them. Not having experience in military EMC, time and effort was wasted only to find that the redesigned inverters would not meet functional specifications.
As a direct result, the inverter manufacturer went out of business. Their customer’s machine introduction was consequently delayed, leading to loss of revenue and loss of market credibility. The machinery manufacturer should have correctly assessed the electromagnetic environment of his product, and realised that military EMC standards far exceeded what he really needed.
51. A cost reduction exercise that didn’t
A manufacturer produced high-quality industrial equipment sold throughout the world. New management thought that poor financial performance was because their products cost too much to make, so began a cost reduction exercise that included employing a production engineer to make design changes.
The designers had been using historically-generated design rules to give their products their famous reliability. These included EMC protection developed over many years of reacting to interference problems in the field. The design departments had no real understanding of EMC, did not realise what protection was lost by the changes, and were unable to suggest cost-effective alternatives.
A number of machines were built to the new design, and with a new price structure sold well in the UK and particularly well in the USA. Unfortunately the product was unreliable due to poor immunity to real-life electromagnetic environments. The consequences included one customer rejecting a product, and the basing of a commissioning engineer in the USA for over one year, as well as loss of product reputation.
Reducing company profitability by employing cost-reduction techniques is not uncommon. Cost-effectiveness techniques should be used instead, taking account of all the consequences of change. In this case the history of the product should have made it clear that EMC expertise was required.
52. Mistakes with a cabling installation
A major manufacturer of automotive parts commissioned a series of robot controlled paint booths with a total project cost of over £2 million, and correctly specified their EMC performance. The successful supplier agreed to meet these EMC requirements, and accepted financial penalties in case of non-delivery. To save costs, it was agreed that the supplier would install his paint booths but the user would arrange for their cabling to be installed by local contractors.
When installed, the paint booths suffered apparently unconnected (and sometimes dangerous) faults and the user would not accept them. Investigations by both the user’s and supplier’s staff could not identify the problems. The user had problems meeting his production deadlines and had to employ extra painters, while the supplier started to incur financial penalties for late delivery. An independent consultancy quickly identified that the screens of all the interconnecting cables had been terminated in a daisy chain to a local earth (which was not the equipment earth), allowing interference with the control electronics.
The supplier normally used its own trained installation staff to install its products, and had no written instructions on the correct termination of the screened cables. Unfortunately there was no easy answer and 80% of the cables had to be replaced (using the correct screen terminations). The supplier picked up the bill.
| Costs to the Customer | Costs to the Manufacturer |
| Loss of production | Financial penalties under the contract |
| Extra painting staff costs | Additional costs of investigation (staff) |
| Additional costs of investigation (own staff plus independent) | Additional re-wiring costs |
| Loss of customer’s confidence |
The legal arguments about who was at fault continued for some time, but the lack of cable installation instructions from the paint booth supplier was the determining factor. Arguments that his staff normally installed his equipment were discounted, as he had agreed this would not happen on this contract.
53. Power dip problems solved using superconducting energy storage
The paper mill at Stanger (South Africa) has a modern electronic variable speed drive system rated at 1MVA. A thyristor-controlled rectifier controls the common DC bus voltage of the individual drives. The motors are independently driven, speed-synchronised units transporting the continuous paper web at high speed. Voltage dips of more than 20%, lasting
in the order of 40 ms, are enough to upset the sensitive controls and shut down the drives. This tears the paper web and results in several hours of downtime for cleaning and re-threading.
The paper mill used to experience at least one or two such voltage dips a week in its power supply, but since the installation of a superconducting magnetic energy storage system in April 1997, configured as a voltage dip protector, not one shutdown has been caused by voltage dips on the supply from the feeding grid.
(Adapted from an article by R Schöttler and R G Coney in the IEE Power Engineering Journal June 1999 special feature on electrical energy storage, www.theiet.org.)
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.
