Early history of shunt diode safety barriers



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Fortunately at that time BIMCAM [later became part of GAMBICA] had an active Flammable Atmospheres Group [FLAG] with representation from all the major UK instrument companies under the chairmanship of Ken Brown of Foxboro.

NOTE: BIMCAM was The British Industrial Measuring & Control Apparatus Manufacturers' Association. The initials GAMBICA do not translate into a name which adequately covers the combination of instrumentation orientated organisations which form its membership; hence these letters are just a name.

At that time Foxboro, Taylor, Honeywell, Kent and General Electric were all members of FLAG and lunch at the St Ermin's hotel, Westminster was a considerable attraction. FLAG plus Bob Redding became the core of the co-operative action on barriers. Possibly the group's most useful action was the publication of the BIMCAM code of practice on barriers which served industry well for a number of years before being replaced by the BASEEFA code of practice SFA 3004. The publication of this document was due to the persistence of Ken Brown who overcame the initial resistance of the BIMCAM council. The relationship between the various participants was stretched at times but problems were usually resolved because in general the problems did not involve what was perceived as high commercial stakes and the participants were engineers. A typical problem was that barriers had to carry a manufacture's name because of the certification requirements. The 28V 240Ω barrier was developed by a combination of Kent and Taylor activity. Ron Sunderland and Olec Binski of Taylor Instruments Ltd were deeply involved and the question of whose name should be used inevitably arose. The problem disappeared when a senior director at Kents refused to pay the

 
Figure 5 Encapsulated barrier Figure 5 Encapsulated barrier

certification fee [a few hundred pounds] and Taylor with a better sense for future potential willingly stepped in. In the late 1960's the co-operation weakened, and Foxboro, Honeywell and Taylor all introduced variants on the theme, but the major usage remained with the Kent/Taylor versions. At about this time other UK manufacturers, notably Robertson and Davy United developed barriers for specific applications.

During the period from 1964 to 1967, the early discussions were mostly with 'Widge' Widginton [testing being done by 'Bish ' Bishop ] with inputs from other members of SMRE and ERA and all other relevant and some irrelevant organisations. Topics included were, whether to use the UK cadmium curves or the PTB version, the characteristics of fuses compared to the surge rating of diodes and the reliability of earth connections. The use of cable parameters and other aspects of system design were all under discussion at this time and contributed to the general confusion. The possibility of applying and certifying barriers initiated many of the questions relating to intrinsically safe system design and this made a major contribution to the subsequent development of the 'entity concept' and system standard.

 

The British Approvals Service for Electrical Equipment [BASEEFA] was created in 1967. Many of the SMRE staff transferred to BASEEFA and continued to deal with barrier certification and consequently the changeover had little effect on the progress of certification. However BASEEFA was, and in its modified form still is, a major influence in the development of barriers and intrinsic safety as a whole.

The Factory Inspectorate certificates emerged in 1967-8. A comprehensive test report on the 10V 47Ω barrier was compiled in March 1967 [signed by J.Cartwright].The certificate followed in August and stipulated the use of the instructions card illustrated in Figure 6.

The first significant use of the encapsulated barriers was at an ICI Rosenberg [Amsterdam] site in1965. At that time UK certification was pending and only partially acceptable in the Netherlands and hence approval by the appropriate authority KEMA in Arnhem was sought. There were fears that the plant start-up would be delayed for lack of acceptable approval. Vivid memories of a long fast drive across Holland on the last day to avoid a delay in start-up, bearing the KEMA documentation in the company of Jop Beeftink and Gordon Kingham [two ICI plastics stalwarts] remain as part of the barrier history. From that time the use of barriers began to increase and gained momentum as the three types of barrier [ 3V 10Ω, 10V 47Ω and 28V 300Ω] were fully certified by the end of 1968. Usage was given a boost as the use of computer control became the norm. One of the early large installations was at a Glaxo plant in Northumberland which used an early Kent K70 computer and led to a clarification of many of the perceived problems on earthing for operational and safety reasons.


cont..

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