Banana Skins – June 2022 (#383-384)

383. Walkie-talkies interfere with electronic door locks on aircraft cockpits

Here’s another good reason why the use of mobile phones on planes should remain banned: your call could lock the crew in the cockpit. The problem was first reported in December 2003, when a Northwest Airlines mechanic scrambled the electronic locks on the security doors of an Airbus A330 by using his walkie-talkie in the vicinity of the flight deck. 

By June 2004, Boeing had discovered that similar problems affected 1,700 of its aircraft. The solution has been a two-year, top-secret repair schedule. Boeing reports that all its jets were fixed by the end of September, while Airbus says it still has doors to mend. The faulty system has now been augmented by a technical innovation described as “a manually operated sliding bolt”.

(Taken from ‘News’ in the ‘Travel’ section of the Sunday Times, October 16 2005, page 19,

384. Interference and the European Rail Traffic Management System (ETRMS)

Emissions: Until relatively recently the only limits on emissions from electrical equipment on rail vehicles operating in the UK were those related to signalling interference. Problems of incompatibility between equipment within the train were dealt with on an ad-hoc basis. As a result most electronic equipment on older vehicles is relatively “hard” and does not suffer problems due to interference from adjacent electronic equipment.

However, some older electronic equipment has been found unexpectedly sensitive to emissions. Examples of this are the brake units fitted to HST power cars which were found to be affected by mobile telephones and NRN radios. One type of CSR radio unit which is often affected by conducted emissions from conventional control equipment has been found to be non-compliant with EN 50121 immunity requirements.

The conclusion from this is that compliance with the current standard set out in EN 50121-3-2, tables 4 to 6 will avoid introducing unreliability into existing train borne systems in the majority of cases. To cater for the small percentage of vehicles where problems will be encountered it is recommended that, when ERTMS systems are fitted to vehicles that have electronic systems without proven immunity, tests are carried out on such systems to ensure compatibility.

Immunity: For similar reasons to those stated above with regard to emissions, the situation with regard to immunity requirements for new electronic equipment is perhaps even less certain. There is a large amount of highly inductive electrical equipment on most rail vehicles and on older vehicles this has never been subject to any formal assessment of electromagnetic emissions. 

The author is personally aware of electrical fields in the passenger saloon of a 25kV electric multiple unit that were so strong whenever the main vacuum circuit breaker was operated that a 12mm arc would be generated between two 1m long conductors held in free air. This phenomenon was accentuated by a faulty connection to the main transformer secondary output but similar effects were also observed on other units.

The arc would be alarming to passengers but apparently had no effect on the electronic traction control equipment. Historically the most troublesome sources of interference have been less spectacular and associated with control gear at battery voltage. The required immunity limits in this area are well documented in RIA 12, EN 50121 and EN 50155. 

It has been suspected that older vehicles may generate interference levels outside these limits. In an attempt to provide some quantifiable measure of the typical conducted EMC environment, measurements were taken on a small sample of vehicles. These consisted of a diesel locomotive, an electric locomotive, a DMU and an EMU. The results of these tests are contained in report 02/T087/ENGE/014/TRT – ERTMS Engineering Interfaces – Supplies and EMI Tests. 

The test results suggest that the conducted electromagnetic interference on older rail vehicles is broadly in line with the present test limits in EN 50155 and EN 50121-3-2. Some electrical disturbances outside the limits were measured as follows: 

  • Repetitive high frequency waveforms were noted in several cases. The amplitude of these gave no cause for concern but the frequency was above the test limits in EN 50121 and RIA 12 and could corrupt data signals. 
  • Significant voltage differentials were found between negative and the vehicle frame in some cases. This may cause problems if care is not taken with equipotential bonding of ERTMS components.
  • On some vehicles there was a high level of ac ripple superimposed on the dc supply. This need be nothing more than an irritation to the designers provided it is considered at an early stage in the design.

It is expected that more extreme electrical disturbances will be found in service due to random combinations of circumstances that occur from time to time. It is recommended that the ERTMS specification should call up full compliance with EN 50155 (despite the evidence in this report that this will not be sufficient in some cases – Editor).

Power supplies – Voltages: The tests carried out on Class 155 DMUs indicated that, even under ideal conditions, the voltage would dip below the lower limits for several seconds during engine starting. Anecdotal evidence indicates that a 110V dc diesel locomotive battery voltage can dip to below 15 V dc during cold weather starting. It is recommended that this be brought to the attention of prospective suppliers of equipment via a requirements specification. It is essential that during such dips, the equipment continues to function within specification or shuts down to a safe condition.

There may be significant ripple on supplies on certain vehicles under different charging conditions. The current requirement according to EN 50155 is for a maximum or 15% ripple on the nominal dc voltage. In previous years, the limit according to RIA 13 was 30%. Tests carried out on vehicles indicate that even this expanded limit is sometimes exceeded (actually up to 50% – Editor).

It will be noted that there is a significant difference between the measured ripple for Class 43 at 0.4% and the worst case on Class 508 at about 50%. It is assumed that the difference is due to the characteristics of the load because the basic dc supply system is the same in all cases, comprising an ac source and simple rectifier. The conclusion that must be drawn is that significant ripple can occur in many types of vehicle when certain conditions exist. It is recommended that this possibility be drawn to the attention of prospective suppliers so they can take appropriate precautions.

(Extracted from the Rail Safety & Standards Board Report 02/T087/ENGE/002/TRT, Issue 3 08/03, sections 11.1, 11.2 and 11.3 on pages 16, 17 and 18. The report is available from, but easier to find with a Google search for 02/T087/ENGE/002/TRT.) 

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:, 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.

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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