Early history of shunt diode safety barriers

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This reflected a Germanic preference for isolation which is still present and was possibly influenced by their skills with magnetic amplifiers. [Hartman Braun used them very successfully in the 1960's] This was a considerable setback at the time because even then the European market was significant from both economic and technical acceptance reasons. Later this attitude softened slightly and a PTB document in 1969 discussed the application of barriers but only allowed their application in Zone1 with well-defined earthing requirements.

Early prototypes and further evaluation 1963-1964

During this period large prototype open construction barriers as illustrated in Figure3 were used to promote discussion and generate interest.

Despite this construction not being suitable for large installations, one was installed in a carbon black plant in Scotland and was in use until the mid-1990's.


A number of people evaluated units. Tom Cook and Jop Beeftink of ICI were deeply involved and made numerous suggestions which were subsequently discussed with Noel Elliot who was head of the electrical section of the Factory Inspectorate. The outcome was that it was considered necessary to encapsulate the barrier so as to avoid problems caused by pollution and the possibility of unauthorised replacement of components. Encapsulation also improved the thermal dissipation. Discussions were held with Gresham Transformers Ltd, who had experience in the use of epoxy resin encapsulation of small transformers, which was a rare skill at the time. A practical design was produced by Doug Adams with Trevor Bolton acting as driving force. A significant event was the signing of a 'letter of no objection' by Noel Elliott on December 24th 1964 based on an ERA report. How far this was influenced by the Christmas spirit of goodwill and the fact that Noel was retiring on that day is open to speculation.


Freddie Arnaud a Senior Electrical Factory Inspector actively progressed the barrier project. His expertise on earthing and intense suspicion of auto-transformers had a major influence on the developing codes of practice. It was his concern to avoid the possible deluge of apparatus and system certificates which led to his proposal of the 'simple apparatus' concept. This was intended to avoid the need for certifying switches, thermocouples, resistance thermometers and other simple devices. The concept of 'simple apparatus' remains a significant factor in the design of intrinsically safe systems and this is almost entirely due to this early initiative.

Certification and wider acceptance

In order to justify the amount of effort being put in by the Factory Inspectorate it had to be made clear that the barrier concept should become 'public property ' and not limited to one organisation.


Figure 3 An early open-construction barrier Figure 3 An early open-construction barrier

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