Text Box: The LHDC can be arranged to operate via a single channel barrier by employing a ‘return to earth’ as per Fig.5. Full isolation between earth and Fire System is realised by using the LDM-519-DDL volt free relay contacts.  The maximum permitted hazardous area cable parameters for the MTL7765 for isolated (discrete) channels (no interconnection in hazardous area) are:-  Example : For the above configuration with Group IIC gas and no interposing cable. Patol Digital LHDC Standard LHDC : 120pF/m 0.95uH/m 5.5uH/Ω. The L/R ratio is (5.5uH/Ω) less than 66uH/Ω. Available capacitance : 0.58uF : 0.58uF/120pF = 4833m Available inductance : 1.45mH : 1.45mH/0.95uH = 1526m Inductance is the limiting factor with the maximum permissible length of LHDC = 1526m Power Supply Earth Referencing Aside from the intrinsic safety aspects it is important that the 24 Vdc power supply to the zone monitor, address loop interface, etc. is correctly referenced with respect to the Intrinsically Safe Earth bus in order that the LHDC signal is not affected by zener barrier leakage currents. Patol produces Analogue zone monitor types with the LHDC signal screen common to both positive and negative supply poles. The LHDC screen should stay within 2V of IS Earth. LDM-519-LP Has negative screen - Supply negative must be referenced to IS Earth. LDM-519-SEN-N Has positive screen - Supply positive must be referenced to IS Earth. For other Zone Monitors refer to appropriate data sheets or contact Patol Limited. Isolators – Galvanic Barriers IS isolators/galvanic barriers can be used with Patol Digital LHDC. Measurement Technology Ltd. Models MTL3043 & MTL4061 are recommended by MTL for ’Fire Detector’ applications, however the simple ‘volt free’ nature of the LHDC may make other 4000 series switch monitoring type isolators more suitable in certain applications. Some isolators require a separate power supply.

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Linear Heat Detectors - Installing LHDC using Intrinsically Safe Barriers - Page 5