Early history of shunt diode safety barriers

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
Feb	1959	Redding patent 918534 using Zener diodes as rectifier bridge following transformer
July	1960	Kent Transdata isolator used Zener diodes as output rectifier
Jun	1961	Northampton College of Advanced Technology, Summer School. Redding discussed safety barriers using zener diodes
Mar	1962	Patent application for Redding barrier filed. No 977,913
Apr	1962	Redding IEE paper describing barrier and other applications of Zener diode limitation
Jun	1962	Drawing of Redding type barrier in terminal block created
Jan	1963	ERA note listing Dr Gehm's [PTB] objections to barriers. 'barriers will not be permitted in Germany'
Nov	1963	Kent report no 83 Open construction barrier with duplicated diodes discussed. Report was widely circulated
Feb	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
Sep	1964	Meeting with Elliott Electrical Factory Inspector produced further design requirements. James of G.E. heavily involved at this stage
Sep	1964	Two channel 10V and three channel diode barrier [large design] drawings created
Oct	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
Jul	1964	Evershed Series 300 granted PTB certificate (Ex) s G5
	1964	Riddlestone ERA report on 10V 47Ω barrier
Dec	1964	Kent report on Type 1 [10v 47Ω]
Dec	1964	Letter of 'no objection' from H.M Electrical Inspector of Factories
May	1965	Kent sales literature on 10V 47Ω barrier published
Apr	1965	Early document on U.L, approach to IS certification
Aug	1965	KEMA [Arnhem] report which allowed installation at ICI Rosenberg
Oct	1965	Article in Kent technical magazine [circulated to customers] The Instrument Engineer on 10v 47Ω barrier
Mar	1966	Evershed Factory Inspectorate certificate for ER 300 Series issued
May	1966	ERA Report on Type2 5V 10Ω barrier issued
Jun	1966	ERA report on Foxboro barrier using a reed switch
Jul	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
Jul	1966	South African Bureau of Standards approved Type 1 and Type 2 barriers
Dec	1966	Redding + Towle IEE paper discusses 10V 47Ω and 5V 10Ω barriers
Dec	1966	Early 28V barrier design drawings
May	1967	NFPA tentative standard on intrinsic safety
Mar	1967	Comprehensive SMRE test report on 10V 47Ω barrier
Aug	1967	1966 Report of Factory Inspectorate contained details of barriers and some galvanic isolators
	1967	BASEEFA formed, Changeover smooth and slow
Aug	1967	Factory Inspectorate certificate IS 6020 for 10V 47Ω barrier issued
Sep	1967	Redding paper on barriers and intrinsic safety in Instrument Practice
Jan	1968	First draft of BIMCAM publication on barriers
Feb	1968	Taylor note on application of 28V 300Ω Type3 barrier
Apr	1968	Type 3 Taylor barrier Approved by Factory Inspectorate IS 3344 28V 240Ω
Dec	1968	4V 10Ω barrier approved. Certificate No IS 6026
Jul	1969	Redding filed patent on improvements to barriers. No 1 310 354
	1969	PTB Mitteilungen 2/69 on safety barriers published
Jul	1969	Kent publication on use of barriers with RTDs
May	1970	Taylor filed master barrier patent with active current and voltage limits