Text Box: Hazardous Area Fire Protection by Linear Heat Detectors Installing LHDC using Intrinsically Safe Barriers Both Analogue and Digital Linear Heat Detection Cable (LHDC) may be installed in a “Hazardous Area” by the use of an appropriately certified Intrinsically Safe Shunt Diode Barrier. The Barrier must be installed between the LHDC and the Zone monitor (Interface circuit). Both Barrier and LHD Zone Monitor must be located in a designated “Safe Area”. The Linear Heat Detection Cable and End of Line Terminator (EOL) can constitute “Simple Apparatus” in both of the Analogue and Digital installation configurations. The approval certificate for any particular barrier will require that certain conditions are met, and it is important that the maximum permitted parameters associated with field mounted equipment are observed. These parameters are dependant on both the Gas Group classification within the hazardous area and whether the barrier circuits (channels) are interconnected in the hazardous area. . For Analogue LHDC, Patol recommends that 22V to 28V zener barrier units are employed in combination with the range of Patol analogue LHDC zone monitoring units. These 22/28V types are also often the most appropriate when Digital LHD is operated directly in conjunction with 24Vdc fire system trigger circuits.  Figures 1 and 2 show a typical Analogue LHDC connection scheme. The arrangement uses dual channel barriers which means that whist the supply 0V must remain within 2V of earth there is no direct circuit connection to the IS earth at the barrier.

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