DEIF SCM4.1 Output module parameters

DEIF SCM4.1 Output module parameters

Always install the sensors in accordance with the Sensor Operating Instructions. In general, sensors for lighter than air gasses should be located at a high level and sensors for heavier than air gasses should be located at a low level. Do not install the sensors: a. Where the normal air flow may be impeded. b. In corners of rooms where static air pockets may exist. c. Near sources of heat such as convector heaters. Do install the sensors: a. As close as possible to the potential source of gas to be detected in order to give the maximum possible warning. b. So that they are accessible for maintenance work.

Sensor Line Resistance

 Sensors should be located such that the line resistance of cable does not exceed the maximum permitted. The table gives a quick guide to the maximum cable lengths permitted for specific sensors, when connected by stranded copper conductor cables of various sizes to a System 57 running at the minimum dc input voltage. The figures in the table provide a useful reference guide to maximum cable lengths, however, in many circumstances longer cable runs can be used. eg. Where the dc input voltage is higher than the minimum. In these circumstances a more detailed analysis is required to determine maximum line resistance. The following sections outline how to calculate the maximum line resistance for catalytic sensors, loop powered sensors and transmitters powered from the System 57. See Section 11.3 for a guide on cable selection.

Catalytic Sensors

 The maximum line resistance of cabling for a catalytic sensor varies with the current and voltage requirements of the type of sensor installed. It is also subject to a maximum of 10V permitted across terminals S and NS at the Field Interface/Relay Card.The maximum line resistance of cabling for a 4 - 20mA loop powered sensor varies with the voltage drive requirements of the type of sensor installed. It is also subject to a 20V maximum loop drive voltage.The maximum line resistance of cabling for a 4 - 20mA transmitter powered from the System 57 varies with the voltage and current requirements of the transmitter. It is also subject to the minimum supply voltage available from the System 57.Making the above calculation using a Vr of 18V will accommodate the worst case low dc supply situation.

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