Early history of shunt diode safety barriers

Chris Towle


The history of the development of shunt diode safety barriers in the United Kingdom from 1961-1971 is used to illustrate the time taken and the many factors affecting the development of light current equipment for use in hazardous areas. 

This document attempts to record the early development of shunt diode safety barriers within the United Kingdom.[1961-1971], with a brief reference to the subsequent expansion. It demonstrates that to succeed it was necessary for many people from different sections of the process instrument industry, to recognise the need for barriers and to become involved in the development and acceptance process. The significant contribution of many of these people is generally not recognised.

During this period the author was employed at Kent Instruments Ltd in the Research Department, which at that time allowed individuals the freedom to explore their particular individual interests. Support for the barrier project was limited to a few individuals and it was opposed by an even smaller [but influential] number. The majority of the people who might have been expected to have an interest in the technique because of the possible effect on process instrumentation were totally indifferent.

Inevitably some people who made significant contributions will have been omitted from this account or possibly had their names miss-spelled and profuse apologies are offered in anticipation of this error. Any account of the past is always influenced by the author's recollections and prejudices. If these are too evident in this particular case a further apology is offered.
Developments in the field of intrinsic safety always take a long time. The development of the initial concept and the need to consult others who are involved in certification or the application of instruments and can bring a different viewpoint are factors which contribute to the time taken. The tortuous path through the certification labyrinth, which usually includes the creation of new standards or the modification of existing standards and the creation of an acceptable Code of Practice also contribute to the elapsed time. All these factors are illustrated by this document. This long time scale and the indeterminate costs involved serve as a deterrent to the majority of financially controlled manufacturers entering this field but the long term rewards can be significant. Shunt diode safety barriers are still manufactured in significant numbers and several million have been manufactured and installed since the concept was introduced.

The following history is based on some recollections but reinforced by the information listed in the appendix which is supported by existing documentation. This document only discusses the development within the UK. Parallel development in other countries did take place towards the end of this period particularly in Germany but detailed accurate information of this development is not readily available to the author and hence is not included.

The concept years 1961-1963
It is recognised that the 1960's were very different from the current situation. Then BS 1259 was the current standard for intrinsic safety, and CENELEC and IEC standards did not exist. The current equivalent of BS 1259 [18 A5 pages] is a combination of the general requirements BS EN 60079-0 [88pages],apparatus standard BS EN 60079-11[141pages] and the system standard BS EN 60079-25 [74 pages]. The grand total of 303 A4 pages as compared with 18 A5 pages represents the advance in technology or the submerging in bureaucracy depending on your viewpoint. Probably it is a little of both.

At this time, certification of intrinsically safe equipment for surface industries in the UK was the responsibility of the Factory Inspectorate, based on testing by the Safety-in-Mines Research Establishment [SMRE] in Sheffield. In 1961 intrinsic safety as a technique for instrumentation in surface industry in the UK was beginning to be an acceptable technique in the petrochemical and pharmaceutical industries but was not widely accepted in refineries. Germany was further advanced in accepting the technique and the USA was firmly wedded to the plumbing of the explosion proof technique.