Text Box: Patol Analogue EOLs comprise a 22M resistor and meet the “Simple Apparatus” criteria; their parameters being effectively inconsequential in relation to LHD cable characteristics. Patol Digital LHDC monitor circuits employ resistive EOLs with similar “conforming” characteristics. Digital monitoring circuits using “capacitive” EOLs are not recommended, and other techniques such as EOL zener diodes require special consideration . Barrier examples which can be supplied by Patol are MTL778 & MTL7778 units which are both dual channel 28V 600 ohm A.C. shunt diode safety barriers. © Measurement Technologies Ltd. The maximum permitted hazardous area cable parameters for these particular units with the channels combined in the hazardous area are:-  The characteristics of Patol LHDC are :-  It can be seen from the above that in the case of a 28V MTL 7778 Barrier employed for Analogue LHDC, without any interposing cable, the limiting parameter for the Group IIB gas area is LHD cable capacitance, which is directly related to length. In this example, the maximum permissible length would be:- 0.083uF = 650000 = 1857 metres 350pF 350 From an Intrinsically Safe point of view 1857 metres may be acceptable, but with Analogue LHDC this zone length would most likely be further limited by the temperature length relationship for the installation. Ref. LHDC Data Sheet. With higher voltage barriers the predominant factor is capacitance and as Digital LHDC has generally lower C/m, longer lengths may be installed than with Analogue LHDC.

Hazardous Area Fire Protection by

Linear Heat Detectors - Installing LHDC using Intrinsically Safe Barriers - Page 2

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