Banana Skins – January 2021 (#307-315)

307. Poor power quality makes cooker switch itself on

Typically, regular voltage quality spot checks are made throughout a local distribution system with additional measurements taken when a customer asks; is there a voltage problem? Indeed, this was the initial question that prompted an investigation into the cause of a modern electric cooker switching itself on.

Initial discussions between the local Distribution Network Operator (DNO) and customer revealed that this phenomenon first took place at Easter, then July and was experienced repeatedly throughout the summer months. However, the problem did not then re-occur until late September.

Standard voltage quality analysis – to BS EN 50160 ‘Characteristics of voltage supply in public electric power supply networks’ – simply showed an increased level of flicker, only slightly above normal levels. However, investigations did reveal that a cable-car was located some distance away from the residential area. This cable-car comprised a double lift installation, having a 65kW lift with slipring motor compensated with 20kVAr capacitor power, as well as a 75kW lift with a B6-circuit rectifier drive. Although electrically distant from the local distribution substation, the cable-car was supplied from the substation by a two-core 2´95mm2 copper cable, approximately 400m long.

Further investigations indicated that the oven malfunction seemed to coincide with periods of increased cable car use, normally at peak periods in the summer. A half-day network analysis at the lift equipment connection point in the local distribution system was carried out. Oscillograms were captured when the envelope trigger or transient level trigger varied by 20% of the voltage fundamental peak, or at least 65V between the cable and earth. Results showed that these limits were exceeded by a factor of two, by commutation spikes caused by the operation of the B-6 rectifier drive of the 75kW lift when it was the only lift in use.

Going back to the cooker manufacturer revealed that the cooker’s electronic oven controls initiated switching commands via pulses. It was possible that the commutation spikes – with their steep slopes and zero-crossings – were being mistaken as switch-on commands.

(Adapted from “Power Detectives”, by Stephanie Horton, Engineering Manager at LEM UK Ltd, in the IEE Power Engineer Journal, October/November 2004 Issues, pages 40-41, http://www.theiet.org.)

(Editor’s note: Keith Armstrong reports that in private communications with an officer enforcing product safety laws in a mainland European country, the officer said that they had experienced several instances of household appliances turning themselves on by mistake. This includes appliances such as saunas, and fire safety was a significant concern. Interference was regarded as the likely culprit.)

308.  Darth Vader toy switched on by low-level interference

What is electromagnetic noise and why is it proclaimed dangerous and unwanted? This extract from Ministry of Commerce’s Field Offices newsletter is a graphic example of EM noise and interference (EMI) it may cause. “… A not so long time ago in a distract not so far away, a certain Technical Officer’s son received a Darth Vader remote control toy for Christmas. The parents noted that this toy displayed the renowned C-Tick mark for EM Compatibility (EMC)! The children played Star Wars games until the sun set.

And that’s when the real story began. You see, once the children were in bed at night, the parents could hear the occasional synthesised sound of “you underestimate the power of the dark side!”. A quick check revealed all children asleep and the remote control untouched. Once a number of these occurrences had been heard, an “interference investigation” was launched.

The Technical Officer called himself back on duty and quickly found that the darned toy operated at 27.120 MHz and responded to electrical noise! This EMI could be generated from such simple things as light switches being turned off, and washing machine pumps switching during normal wash cycles…”

(Taken from “The Back Page” of the EMC Society of Australia’s Newsletter, September 2004 Issue Number 27, http://www.emcsa.org.au.)

309.  Computer company learns that EMC compliance pays

In its formative years, a major US PC manufacturer felt that FCC certification was not a barrier to marketing. Standard operating procedure was to sell while the authorisation process was in process. Then the FCC arrived to shut down the factory. The VP of Engineering met with the FCC in Washington at the last minute and worked out an agreement that kept the factory running. After that point, FCC certification and other agency approvals became a requirement before shipment was authorised. Today, that company has a world class compliance operation and I am proud to have taken part in that process.

(Richard Woods, in a correspondence on the IEEE’s emc-pstc list server, 15 July 1998.)

310.  Interference before World War I

The US Military first encountered Radio Frequency Interference (RFI) some time prior to World War I when a radio was first installed on a vehicle.

(Warren Kesselman and Herbert Mertel, writing in the “EMC Standards Activities” section of the IEEE EMC Society Newsletter, Summer 2000 Issue, http://www.ewh.ieee.org/soc/emcs.)

311.  New vacuum cleaner crashes car manufacturer’s computer

A cleaner in the offices of a major UK car manufacturer started to use a new vacuum cleaner, plugging it into the sockets in a corridor outside the room where their stock control computer lived. Unfortunately, the mains sockets in the corridor were connected to the same branch of the power distribution as the computer, and the conducted noise from the vacuum cleaner crashed the computer. This happened ever day for some time, costing the company a great deal of money, until someone realised the vacuum cleaner was the cause of the problem.

(Anonymous, private conversation August 1994.)

312.  Mains transients cause switch-on of toaster, burns gas station down

Transients in the mains supply of a gas station in the USA (called a petrol station in the UK) caused the spurious switch-on a microprocessor-controlled toaster one night after the staff had all gone home. Since the microprocessor wasn’t in its normal programme, it didn’t switch the toaster off.

The manufacturer of the toaster had omitted to include a thermal fuse, so the gas station caught on fire and burnt down. The PCB had been designed by a UK company, and its designers were later questioned intensely by a team of US lawyers for several hours.

(Anonymous, private conversation, August 1994.)

313.  Radar interference anecdotes

Our purchasing manager has a penchant for (expensive) cars. He had a ’92 Peugeot 605, and whenever he drove past the military airbase at Lyneham its air bag indicator would light. This was attributed to the site’s radar interfering with the car’s front wheel sensors. In addition the semi-automatic gearbox would drop into sports mode… The ’93 model he now has appears to be immune.

I myself suffered TV interference from ground radar when living 10 miles from Gatwick airport – bars would roll down the screen as the sweep went through.

(From Chris James, private communication, 7th July 1998.)

Banana Skins numbered 314-315 describe interference events that we might not be too surprised to hear about in or after 2015.

314.  2015: CE does not stand for ‘China Export’

A major electronics manufacturer has been ordered to suspend all sales in the EU while it fixes EMI problems with its products. Enforcement officials impounded products in warehouses throughout the EU. The average time to fix a product’s EMI problem is expected to be one month, but they have so many products that they expect it will be two years before they finish. They had argued that they thought the CE mark stood for ‘China Export’, said no-one had actually told them they had to comply with the EMC Directive, and that they were only doing what many of their competitors were doing anyway. The enforcement agents found these arguments unpersuasive.

(Possible electronic industry trade journal news item in 2015, or in fact in any year.)

315.  2015: Plasma beam weapon interferes with COTS

Western military forces have come to rely (unofficially) on the widespread use of consumer (‘COTS’) electronics such as GPS navigation, cellphones, and palmtop computers with wireless datacomms. Every soldier, sailor or pilot seems to own at least one of each, and they take them everywhere with them, including military exercises and operations. Some enterprising junior officers have even created their own ‘command and control’ nets, some of which seem to be much more effective than official ones.

But during a recent NATO exercise based around the new SHIVA particle-beam anti-missile tactical battlefield man-pack systems, a large proportion of this COTS equipment failed to work and the unofficial methods that had grown up around them fell apart, causing great confusion. It had not been realised by how much these facilities had come to be relied upon. As a result, the ‘attacking’ forces easily won the exercise, despite being on foot, armed only with weapons of Afghan war vintage, and communicating by shouting loudly.

(Possible article in Jane’s Defence Weekly in 2015, http://www.janes.com.) 

About The Author

Keith Armstrong

After working as an electronic designer, then project manager and design department manager, Keith started Cherry Clough Consultants in 1990 to help companies reduce financial risks and project timescales through the use of proven good EMC engineering practices. Over the last 20 years, Keith has presented many papers, demonstrations, and training courses on good EMC engineering techniques and on EMC for Functional Safety, worldwide, and also written very many articles on these topics. He chairs the IET’s Working Group on EMC for Functional Safety, and is the UK Government’s appointed expert to the IEC committees working on 61000-1-2 (EMC & Functional Safety), 60601-1-2 (EMC for Medical Devices), and 61000-6-7 (Generic standard on EMC & Functional Safety).

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