3.3 Installation in a Hazardous Areas
Where gas detector heads are to be mounted within a potentially explosive atmosphere then
electrical safety barriers must be fitted in order to limit the electrical energy that is supplied into
the hazardous area. This prevents an incendive spark occurring under an electrical fault
condition.
There are two methods by which safety barriers can be provided:-
a). using an optional Status Scientific Controls barrier type FGDIO fitted integral to each
output channel of the MCU unit.
b). using externally mounted proprietary safety barriers of the correct type in series with
each remote detector head.
When using barriers to create an I.S. supply, certain restrictions are imposed on the parameters
of the interconnecting cables used. These parameters are defined by the manufacturer of the
barrier and limit the maximum capacitance, inductance and inductance to resistance ratio of the
cable. The installation is only intrinsically safe when the combination of the barrier and connecting
cables comply with the manufacturer’s specification.
As with a non-I.S. installation, the cable length is restricted by the cable loop resistance. With the
introduction of a barrier, the cable loop resistance is reduced because of the internal resistance of
the barrier. The end to end resistance of the barrier must therefore be subtracted from the overall
cable loop resistance when calculating cable lengths.
Barriers must be selected to restrict the parameters of the I.S. supply to the gas detectors within
the following limits:
When considering the capacitance and inductance allowable across the barrier output terminals,
note:
There is zero capacitance and zero inductance between terminals 0V and SIG on any
model of FGD Detector head.
There is an equivalent of 1.4F capacitance and zero inductance between terminals 0V
and PWR on the FGD2 and FGD3 flammable gas detectors.
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