454. Video projector interferes with audio induction loop
Installers of various equipments may not be aware of audio magnetic induction loops, which are an aid for the disabled. These produce magnetic fields across the whole audio range but primarily from 100Hz to 5kHz with a nominal field strength of 100mA/m in the listening area. Achieving this may mean fields of at least 10A/m a few centimetres from the loop cable.
BS7594, Guidelines for Induction Loops, includes advice to minimise susceptibility to magnetic interference. Basically, these give ways of avoiding a large area earth loop.
A Church where we had installed an induction loop bought, a few years later, a new video projector and computer system. We were called because the installer of the projector was complaining that our loop did not meet specifications, and was causing interference. The picture had “hum bars” moving up and down the screen.
I cured this with nothing more than a 5 metre mains lead. I plugged the projector into the same mains socket as the computer, thus minimising the earth loop. As a further test, I measured loop current and magnetic field, and could show that the installation was close to a text-book example of the theory in BS7594. The video projector supplier had plugged this into a mains supply installed for other equipment.
This was a text-book example of the earth loop which BS7594 warns against. During this dispute, the video supplier had arranged a demonstration of an induction loop which he hoped the Church would purchase. Hearing aid users were not impressed with the loop quality compared with the existing installation. Installing the loop cable on the steel reinforced concrete floor reduced the interference to the video projector. But it also reduced the useful signal to hearing aids, and was not acceptable.
(Sent in by Robert Higginson of AREAC, 2nd August 2007.)
455. Switch-mode power supply emissions vary strongly with mains voltage
It’s very tempting to believe the CE mark and Declaration of Conformity of a bought-in power supply, but… A power supply with active mitigation (PFC) was tested at 220V and behaved perfectly reasonably. But at the UK version of the harmonized European 230 V, actually 244 V, it drew current from only one half-cycle of the mains supply, and emitted very significant amounts of even harmonic currents and DC. This behaviour persisted down to 227 V, then the current in the ‘other’ half cycle gradually increased, so that at 222V the waveform was reasonably symmetrical and the second harmonic emission was very low.
This has been reported anecdotally before, but the effect is not widely known. It can be even more marked if the output current is well below the rated value. Half-wave operation is, of course, a big ‘No-No’ according to the standard, except under very special circumstances.
Ideally, all users of OEM power supplies should check the emissions with a power analyser, which can be much less costly than the full-specification
IEC/EN 61000-4-7 instrument and gives reasonable results on fluctuating loads. But at least an observation of the input current waveform, using a current probe or low-value resistor and an isolating transformer, will detect half-waving and other misdemeanours, such as cycle-skipping, that may affect the equipment being powered, or even other associated equipment.
(Copied entire from: “John Woodgate’s Column”, by John Woodgate, The EMC Journal, January 2007, pp 13-14.)
456. EM emissions from hybrid vehicles
The results from the emission testing carried out on the seven alternative powertrain vehicles have, with one exception, been found to exceed the emissions limits as specified by 95/54/EC, CIPSR 12 and 97/24/EC. The majority of the excessive emissions correspond to vertical polarisation for broadband, and the maximum frequencies for these excessive emissions were 127MHz for broadband and 144MHz for narrowband.
Since the vehicles were tested under dynamic conditions where practicable, the vehicles may well have met the requirements of 95/54/EC when tested in the normal ‘static’ mode, whilst producing emissions in excess of the legislative limits when the power electronics and electrical machines are activated.
Nonetheless, the results from one of the hybrid electric vehicles demonstrate that a well-engineered hybrid electric vehicle need not present any more of a threat than conventional IC (Internal combustion) engined vehicles.
(Extracted from “Investigation of electromagnetic emissions from alternative powertrain road vehicles,” Alastair Ruddle, Executive Summary, MIRA report No: 01-845060, 28 May 2002.)
457. Computer interferes with amateur radio
One investigation revealed a computer that was putting out a strong fifteenth harmonic on the two-meter amateur bands (144-148MHz). It was energetic enough that a mobile operator found that the resistivity of the coolant did not follow Ohm’s law. The liquid resistivity showed an increase when the electric field was increased. This phenomenon was also observed for hydrocarbons by Kinkenberg and Van der Minne.
A more recent event occurred in January 1996 with the integration and testing of the Spacelab Multi-Purpose Experiment Support Structure (SL MPESS) carrier for the United States Microgravity Payload-3 (USMP-3) on the ST-75 shuttle mission. During the Freon flow balancing of the USMP-3 Freon system, a leak was discovered in one of the Freon flex lines. ESD was suspected and later confirmed by the Kennedy Space Center (KSC) material science division and this author.
During investigation of the Spacelab fluid line failure, another ESD failure was found on a space shuttle ground support equipment fluid line. The fluid line carried N2O4 at the fuel storage facility. The KSC materials laboratory recommended conducting fluid lines in this application.
It is hoped that this article will encourage greater awareness and that special care will be taken in the design and routing of fluid line systems in the future.
(Extracted from: “ESD in Fluid Lines: Theory and Application in the Petroleum and Aerospace Industries,” Robert A Green and Robert S Axley, ITEM 1997, pp 108-139.)
458. ESD in fluid lines
From 1953 to 1971 over 35 accidents involving fire and explosions in aircraft during or after fuelling were attributed to electrostatic discharge (ESD). Most of these accidents involved JP-4 hydrocarbon fuel flowing through nonconductive fuel lines with TEFLON® as the hose liner. These fluid lines consisted of an extruded Teflon tube reinforced with a braided stainless steel outer jacket. Many of the ESD failures produced leaks in the hose. Fluid line leaks were also observed during the testing of the Pratt & Whitney J57P55, the Westinghouse J34WE46 and the General Electric CJ805 engines.
One failure that occurred in the aerospace industry during the late 1960’s on a spacecraft launch vehicle had an interesting effect on the spacecraft. The guidance computer commanded the propulsion to shut down early, preventing the vehicle from reaching its design altitude. After extensive review of the telemetry and the systems design, it was concluded that ESD had caused the guidance computer to malfunction. The source of the ESD was researched extensively and found to be the nonconducting Freon lines which ran internal to the computer. The ESD arced through the Teflon and into the computer causing the malfunction to occur.
Laboratory experiments on this configuration were performed to verify that ESD could occur. It was discovered that it took around 20,000 volts to arc through this Teflon thickness. Also, it was found that the resistivity of the coolant did not follow Ohm’s law. The liquid resistivity showed an increase when the electric field was increased. This phenomenon was also observed for hydrocarbons by Kinkenberg and Van der Minne.
A more recent event occurred in January 1996 with the integration and testing of the Spacelab Multi-Purpose Experiment Support Structure (SL MPESS) carrier for the United States Microgravity Payload-3 (USMP-3) on the ST-75 shuttle mission. During the Freon flow balancing of the USMP-3 Freon system, a leak was discovered in one of the Freon flex lines. ESD was suspected and later confirmed by the Kennedy Space Center (KSC) material science division and this author.
During investigation of the Spacelab fluid line failure, another ESD failure was found on a space shuttle ground support equipment fluid line. The fluid line carried N2O4 at the fuel storage facility. The KSC materials laboratory recommended conducting fluid lines in this application.
It is hoped that this article will encourage greater awareness and that special care will be taken in the design and routing of fluid line systems in the future.
(Extracted from: “ESD in Fluid Lines: Theory and Application in the Petroleum and Aerospace Industries”, Robert A Green and Robert S Axley, ITEM 1997, pp 108-139.)
459. Hi-tech ambulance gear goes to waste
Millions of dollars worth of communications equipment installed in ambulances nationwide has been sitting idle for six months because it interferes with radio reception.
New Zealand’s 550 ambulances were fitted with mobile data terminals and automatic vehicle locators worth $3 million in November and December as part of a $16 million call-centre upgrade, which saw eight communications centres merged into three.
“Six months on, we just have these useless things sitting on the dashboards and it feels like we’re operating blind half the time,” said an ambulance officer who did not wish to be named. Staff were told that the units – which were meant to relay patient information – would be in place before the merger early this year, but that had not happened, the officer said.
Since the closure of communication centres in New Plymouth, Hamilton, Palmerston North and Masterton in February, all ambulance calls have been routed through Wellington, Auckland and Christchurch.
At the time of the closures, St John communication centre coordinator Tony Blaber said local knowledge on the ground would not be lost thanks to the cutting-edge technology, which would allow dispatchers to pinpoint the location of calls and direct the nearest ambulance or rescue helicopter to incidents.
Mr Blaber told The Dominion Post that during trials the screens, which accounted for $500,000 worth of the project, had been found to interfere with the quality of radio reception at the “extremities” of the signal. “For most places in the country where the radio signal is strong, there would be no problem, but for isolated pockets where it’s marginal, it had a slightly degrading effect.”
Trials were under way around New Zealand and upgraded screens should be in place by the end of July. Though it was disappointing that the system was not completely operational as yet, the new centres had been doing an excellent job, he said.
(Copied entirely from “Hi-tech ambulance gear goes to waste” by Ruth Hill, The Dominion Post, Monday, 11 June 2007.)
