9ENG8008 GFM436 User Manual
Appendix B – Understanding Instruments For Use In Flammable Atmospheres- 79
1.
2.
Methane Least easily ignited I
17.3 Design and construction of intrinsically safe instruments for use in
flammable atmospheres
The technique of intrinsically safe design and construction is defined in the standard BS EN 60079-11:2012. The
technique identifies potential sources of ignition (by spark or by heat) and specifies the design of safety circuits that
limit the energy of the spark and/or the component temperatures such that they cannot ignite the gases expected
within the hazardous area. There are two categories of intrinsically safe instruments:
‘ia’ The safety circuits used will prevent the generation of a spark or temperature capable of igniting the gas
even if the instrument develops TWO faults.
‘ib’ The safety circuits used will prevent the generation of a spark or temperature capable of igniting the gas
even if the instrument develops ONE fault.
By testing components in fault conditions (as required by ‘ib’) the worst-case temperature within the instrument is
determined. This is used to give the instrument a temperature classification. A guide to temperature classifications
is given below.
Temperature Class Maximum Temperature (⁰C) Maximum Temperature (⁰F)
T1 450 842
T2 300 572
T3 200 392
T4 135 275
T5 100 212
NOTE: This instrument is suitable for use in flammable atmospheres caused by the presence of gases in
Group I, Group IIa and Group IIb.
NOTE: The R Series is designed to meet the requirements of category ‘ib’.
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