278. Interference issues on a research and recovery vessel
The experiences of the crew of the research vessel (R/V) Deep Scan, a privately owned research and recovery ship, offer some insight into the complexities of integrating commercial off-the-shelf (COTS) computing equipment into a shipboard electromagnetic environment.
R/V Deep Scan is constructed as a commercial vessel with many of the electrical characteristics of military mine-clearing ships. Its hull and deck structures are constructed from wood, closed cell foam and fibreglass, and it shares EMI/EMC problems common to non-metallic ships.
Computing equipment on board is said to be compliant with FCC Part 15 for radiated emissions. A commercial workstation processes sonar and navigation track data from multiple transducers. A 386 PC processes both electromagnetic survey data from multiple detection transducers and data for navigation. Navigation data is provided by COTS GPS (Global Positioning System) and LORAN-C receiver systems. Depth information is provided by COTS depth sounding equipment. Heading data is provided by a COTS fluxgate compass.
Operating the marine VHF transmitter at more than 1W begins to corrupt collected data, and any use of HF SSB transmission causes the COTS computing equipment used for magnetic data collection and navigation to enter states that challenge rational explanation.
FCC rules limit the levels of unintentional electromagnetic radiation, but the close proximity of COTS computing equipment (the vessel is under 60 feet long) to the antennas used for data collection and communications is largely responsible for disruption of operations due to the EMI the COTS equipment generates.
EMI generated by the switching power supplies in the COTS equipment slightly degrades the LORAN-C signal-to-noise ratio through radiated coupling. COTS computing equipment generates sufficient radiated interference on the HF bands to render HF communications impractical. Broadband interference and harmonics from COTS computing equipment interfere with communications reception on selected VHF channels, in some cases enough to prevent useful communications.
Daily operations on board R/V Deep Scan are influenced by the EMI and susceptibility problems associated with the use of COTS computing equipment. Responding to a call on the VHF radio presently requires the crew to wait for a logical break in survey operations, or requires termination of survey operations. During survey operations, monitoring some VHF channels is not possible, HF transmission is impossible and HF reception is seriously degraded.
(Taken from the Radiocommunication Agency’s “EMC Awareness” website, now at: http://www.ofcom.org.uk/static/archive/ra/topics/research/RAwebPages/Radiocomms/index.htm.)
279. Inverter drive interferes with ultrasonic level sensor at water treatment plant
Problems associated with electrical noise have been solved at Northumbrian Water Broken Scar water treatment works in Darlington, County Durham. Such problems are an inevitable element of major pump installations, especially where large inverter drives are involved. This is sometimes a major problem for instrumentation, which generally involves cables carrying a smaller signal between a sensor and a control and analysis unit.
At Broken Scar, a large inverter drive meant that there was very high electrical noise. The ultrasonic level measurement unit that was originally installed, which used a coaxial cable to carry a signal, was swamped and unable provide a reliable measurement. The solution was a Pulsar ultrasonic level measurement system …. performing an initial (digital) conversion on the signal at the transducer head, communicating digital information to the signal analysis. Despite the noise still being present, the system can discriminate between noise and the “true” signal to give a reliable measurement.
(Adapted from an advertisement for Pulsar Process Measurement Ltd, in Plant and Control Engineering Magazine, Oct/Nov 2003, page 11.)
280. Mains supply dips getting worse
We have seen an increase in AC mains supply dips from typically seven per year to eighteen, which is causing increased losses in production.
(A comment by a representative from a major steel manufacturing company at the IEE’s Wales South-East & Wales South-West Power Specialist section’s “Power Quality Seminar,” held at the University of Wales Swansea on Wednesday 12th November 2003.)
281. Increasing interference with Public Safety Communications in USA
Signals from wireless communications transmitters are continuing to create significant interference issues for public safety officials, according to a recent report in the Washington Post. The interference problems reportedly stem from the close proximity of spectrum allocations for public safety communications and some older style wireless telecommunications technologies, which operate in the 800 Megahertz band. As we’ve previously reported (see Conformity, August 2002), more than 70 government agencies in 27 states have reported interference problems with wireless communications services used by public safety officials.
According to the Post article, communications problems most often arise when a public safety official (such as a police officer) is far from a transmitter that carries emergency radio signals but close to a transmitter for a wireless system carrier. In these cases, the signal from the wireless system overwhelms the weaker emergency signals, effectively blocking emergency communications.
The communications system operated by Nextel Communications appears to remain the principal source of most of the interference complaints. The Nextel network was originally cobbled together in the 1980s from underutilized portions of spectrum allocated for limited specialty uses. However, as the Nextel network has grown, its use of spectrum has begun to overlap with the frequencies used by public safety agencies.
State and municipal public safety authorities are responding in various ways to the continuing problem. Some jurisdictions are attempting to upgrade the communications infrastructure to provide stronger signals to radios operating on public safety bands. Still others are attempting to pass ordinances that require wireless carriers to certify that their signals do not interfere with public safety communications. Meanwhile, some police officers have reportedly found a simpler solution to interference with public safety communications bands, They carry their own cell phones!
(From: “Wireless Interference with Public Safety Communications Growing,” Conformity, November 2003.)
282. Interference issues within UK railway networks
Historically, EMC issues on railways have been dominated by the possibility of interference from high power electric traction supplies, particularly DC, affecting safety-critical low power train detection equipment. The introduction of switched traction controllers raised further concerns and early variable frequency chopper drives have caused incidents, leading to the adoption of fixed frequency choppers for DC traction drives. By the time traction inverter drives for AC traction became commercially attractive, due to the introduction of GTO thyristors and subsequently IGBTs, the potential risk was well understood and care has been taken to design traction drives to be compatible with train detection systems, either by design or high-integrity monitoring.
Now, probably the greatest threat to train detection is to older track circuits operating on the same frequency as the AC utility used to produce the DC traction supply and older trains without power electronic traction controllers. In a recent incident on the UK network, a failure on the rectifier of a DC traction supply was detected by a number of new power electronic trains being brought to a halt by their interference current monitoring units even though each train was perfectly healthy.
(From: “GM/RT8015 and Safety” by Jeff Allen and David Bulgin of the Rail Safety and Standards Board, presented at the IEE Seminar “EMC Assurance in a Railway Environment,” 9th September 2003, http://www.theiet.org.)
283. New Automotive EMC Directive includes transient immunity tests
As part of the development of this directive (Automotive EMC, 2004/104/EC – Editor), it has become evident that many aftermarket devices can be found to be susceptible to the many switching events on a vehicle. This can be simply audio clicks or, worst case, result in hardware failure of the component resulting in damage to the vehicle itself.
For this reason, and following lobbying by the vehicle manufacturers, the committees responsible for the directive have included in this latest draft requirements for all ESAs (electronic sub-assemblies – Editor) to be immune from a series of transient events.
(From: “Transient Test Requirements for “e”-Marking – Necessity or Bureaucracy?” by James Gordon-Colebrook and Alex Mackay of 3C Test Ltd, presented at the Automotive EMC 2003 Conference, 6th November 2003.)
284. EMC Problems with Speed Detection Cameras cost Victoria Government AUS$30 Million
The state government of Victoria, Australia, has commissioned a special investigation into the Fixed Digital Speed Detection Cameras. This follows from the report of the independent testing of the fixed digital speed detection cameras, commissioned by the Department of Justice after concerns were raised about erroneous readings and incorrect infringement notices. On-site testing, engineering investigations and EMC testing showed the occurrence of both ‘over-readings’ and ‘under-readings’. The faulty readings were due to electromagnetic interference, poor installation and maintenance and degradation of the in-road sensors.
The faulty speed readings resulted in the issuing of fines and cancellation of driving licences, and this has called into question the reliability of electronic equipment used throughout the state’s traffic control measures and for other law enforcement activities. Many motorists who have had their licences cancelled are threatening to sue the state for damages and consequential losses. The State government has allocated about AUS$30 million to reimburse the fines imposed on motorists and to compensate those with claims for losses resulting from licence suspension due to penalties from the flawed cameras. The government will also have to meet the cost of replacing or improving the 41 fixed cameras throughout the city’s road grid.
(From Chris Zombolas, EMC Technologies Pty Ltd., Melbourne, Australia, http://www.emctech.com.au. Also see the media release from the Premier of Victoria, Friday, 14 May 2004: “Government acts on fixed speed cameras” and Melbourne Age, 15 May 2004: “$26 million Speed Payout”.)
285. TETRA interference with TV reception
The new emergency services radio system, called Airwave, has been blamed for interfering with television reception, but where the problems occur the fault lies with the filters on domestic aerial amplifiers. Trade and Industry Minister Steven Timms, in a Parliamentary written answer, said: “OFCOM is aware of instances of interference to domestic installations from Airwave radio base stations. In all the instances so far investigated the consumer’s own masthead aerial amplifier, used to boost weak signals, has had a pass-band wide enough to boost the television signal and, inadvertently, the unwanted radiocommunications signal (from Airwave – Ed.).”
Airwave is being rolled out across Great Britain for police and public safety communications, with completion due by 2005, when existing frequencies will be withdrawn. It is a digital system based on the ETSI-approved TETRA (Terrestrial Trunked Radio) standard.
Mr Timms went on: “Testing has shown that the TETRA transmitters were operating correctly and within their designated licence parameters. In most cases a suitable filter fitted between the masthead amplifier and the TV aerial will resolve the interference, and affected residents have been advised to have such filters fitted. As a goodwill gesture Airwave has arranged for filters to be fitted to the affected television installations in certain circumstances.”
(“Aerial amplifiers cause Tetra TV interference” from the IEE’s EMC Professional Network’s “EMC Industry News 2004-01-15”, 18th January 2004, http://www.theiet.org. TETRA has also caused significant problems for radio activated vehicle security systems, see Banana Skin No. 54. Other Banana Skins that mention TETRA are: 121, 122, 124, 252, 255 and 277.)
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.
