Text Box: Lower Voltage Zener Barriers Lower voltage barriers may be employed for Digital LHDC, however in many cases it will be found that leakage currents through the Barrier zener will cause mal-operation of circuit monitoring with 24V fire system equipment. It should be noted that with lower voltage zener barriers inductance usually becomes the predominant factor in determining LHDC zone lengths. There can be a length advantage with lower voltages, especially if single barrier channel configurations are employed. Patol produces an interface module LDM-519-DDL which in addition to permitting the use of a 15V barrier provides a digital display of the distance to the ‘fire’. See LDM-515-DDL Data Sheet D1137. The recommended Barrier is MTL7765 which when employed in combined dual channel mode as per Figure 4 maintains earth isolation at the barrier.  The maximum permitted hazardous area cable parameters for the MTL7765 with the channels combined in the hazardous area are:-  Example : For the above configuration with Group IIC gas and 50m of hazardous area interposing cable specified at : 71pF/m 0.39uH/m 17 uH/Ω. Patol Digital LHDC Standard LHDC : 120pF/m 0.95uH/m 5.5uH/Ω. Both L/R ratios are less than 22uH/Ω. Available capacitance : 0.58uF – (71pF x 50m) = 0.5796uF : 0.5796uF/120pF = 4830m Available inductance : 0.32mH – (0.39uH x 50m) = 0.3005mH : 0.3005mH/0.95uH = 316m Inductance is the limiting factor with the maximum permissible length of LHDC = 316m

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Linear Heat Detectors - Installing LHDC using Intrinsically Safe Barriers - Page 4