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Last Post 02/16/2015 5:28 PM by  Jim McKelvey
CR4395 For Open Heater Application
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Jim McKelvey
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02/16/2015 5:28 PM
    I am interested in using the CR4395 for determining failure of a heating element in a railroad signaling environment. I have read through the relevant application document and still have a question about the operation of the output function (I am currently basing my design on use of a relay output).

    In my design, a PLC (installed in an instrument house in the field) receives a call for heat from a central office and in response energizes an output which turns on a heater control case (located some distance from the instrument house). The idea is to have a contact from your current sense relay wired in series with an auxiliary contact that closes when the high voltage heat contactor closes. The series combination of these two contacts will be wired back to the same PLC in the instrument house with the intent of providing an alarm for either of the following two conditions:

    1. Current sense relay detects a loss of heater load
    2. High voltage contactor fails to close

    In addition to this, we also want the system to default to the alarm state if the current sense relay fails.

    Given the fact that both dry contacts associated with the current sense relay and contactor aux contact are wired in series, this single feedback circuit to the PLC will need to turn on the appropriate input when the system is activated an no alarm is present (loss of this input will represent an alarm condition but will only result in the PLC generating an alarm when the logic knows that there is a call for heat). Assuming that I use connection scheme "B," this implies that the current sense relay contact must change from an open to a closed state when the heater circuit is turned on and change to an open circuit if the heater element fails. If the relay contact does not change state when control power is applied, I could use the closed contact however if the current sense relay fails, I assume that it will not be able to change state and the alarm circuit would not reflect this failure (which is what I want).

    Can the relay contact be made to change state when power is applied and then change to the other state if it senses a low-current condition on the supply feed to the heater?

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