Early history of shunt diode safety barriers



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A verifiable record of the early days of shunt diode safety barriers

  1956 U.S. National Electrical Code published by the National Fire Protection Association [NFPA] recognised intrinsic safety by defining it
  1958 BS 1259 entitled 'Intrinsically safe electrical equipment and circuits for use in explosive atmospheres' published
February 1959 Redding patent 918534 using Zener diodes as rectifier bridge following transformer
July 1960 Kent Transdata isolator used Zener diodes as output rectifier
June 1961 Northampton College of Advanced Technology, Summer School. Redding discussed safety barriers using zener diodes
March 1962 Patent application for Redding barrier filed. No 977,913
April 1962 Redding IEE paper describing barrier and other applications of Zener diode limitation
June 1962 Drawing of Redding type barrier in terminal block created
January 1963 ERA note listing Dr Gehm's [PTB] objections to barriers. 'barriers will not be permitted in Germany'
November 1963 Kent report no 83 Open construction barrier with duplicated diodes discussed. Report was widely circulated
February 1964 Evershed 0-10 mA isolator drawings created
  1964 Cook and Beeftink of I.C.I. involved in testing early barrier design. Need for encapsulation evolved
September 1964 Meeting with Elliott Electrical Factory Inspector produced further design requirements. James of G.E. heavily involved at this stage
September 1964 Two channel 10V and three channel diode barrier [large design] drawings created
October 1964 Early meetings at Gresham Transformers Ltd on barrier manufacture. James of G.E. involved. Adams subsequently was largely responsible for the encapsulated design, Bolton was the driving force
  1964 Meeting of IEC in Warsaw discussed intrinsic safety
July 1964 Evershed Series 300 granted PTB certificate (Ex) s G5
  1964 Riddlestone ERA report on 10V 47Ω barrier
December 1964 Kent report on Type 1 [10v 47Ω]
December 1964 Letter of 'no objection' from H.M Electrical Inspector of Factories
May 1965 Kent sales literature on 10V 47Ω barrier published
April 1965 Early document on U.L, approach to IS certification
August 1965 KEMA [Arnhem] report which allowed installation at ICI Rosenberg
October 1965 Article in Kent technical magazine [circulated to customers] The Instrument Engineer on 10v 47Ω barrier
March 1966 Evershed Factory Inspectorate certificate for ER 300 Series issued
May 1966 ERA Report on Type2 5V 10Ω barrier issued
June 1966 ERA report on Foxboro barrier using a reed switch
July 1966 Kent internal memo discusses Evershed certificate IS 3302 covering output stage as galvanic isolator for 0-10 mA
  1966 T.J.Cook [I.C.I] paper on intrinsic safety suggests that barriers 'would overcome most of the disadvantages of the present system
  1966 Kent report [not dated] on Type2 5V 10Ω barrier published
July 1966 South African Bureau of Standards approved Type 1 and Type 2 barriers
December 1966 Redding + Towle IEE paper discusses 10V 47Ω and 5V 10Ω barriers
December 1966 Early 28V barrier design drawings
May 1967 NFPA tentative standard on intrinsic safety
March 1967 Comprehensive SMRE test report on 10V 47Ω barrier
August 1967 1966 Report of Factory Inspectorate contained details of barriers and some galvanic isolators
  1967 BASEEFA formed, Changeover smooth and slow
August 1967 Factory Inspectorate certificate IS 6020 for 10V 47Ω barrier issued
September 1967 Redding paper on barriers and intrinsic safety in Instrument Practice
January 1968 First draft of BIMCAM publication on barriers
February 1968 Taylor note on application of 28V 300Ω Type3 barrier
April 1968 Type 3 Taylor barrier Approved by Factory Inspectorate IS 3344 28V 240Ω
December 1968 4V 10Ω barrier approved. Certificate No IS 6026
July 1969 Redding filed patent on improvements to barriers. No 1 310 354
  1969 PTB Mitteilungen 2/69 on safety barriers published
July 1969 Kent publication on use of barriers with RTDs
May 1970 Taylor filed master barrier patent with active current and voltage limits

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originally printed in Measurement and Control, March 2014