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I have to wire a few smoke detectors to a PLC. The PLC has both analogue input and digital input cards.

After reading the type of smoke detectors available in the market, I am a little confused as to which card to wire them to, i.e. AI or DI cards and how to specify my detectors?

If I need line monitoring, do I just add a end-of -line resistor to the last detector and wire to a AI card? If I do not need line monitoring, do I just wire to a DI card? But, I don't see contacts (N.C or N.O.)mentioned int he smoke detectors available at the market? The detector catalogue just mentioned terminals L, L1, L2. Can anyone help out?

I think the third wire on the detector is referenced to the neutral connection, but I'm not sure if it goes to line voltage with smoke or perhaps something lower, why not just buy one and check to see.

I expect whatever the voltage is it must be universal to allow different brands to work together.

If the output is AC you will need a discrete input card.

Roy

A smoke detector should have a potential free contact or fixed contact that switches in the event of smoke being detected. This should be wired to a digital input card of the plc. You can save cabling and just wire all the smoke detectors in series through the NC contacts onto 1 input. This will also detect any wire break with your detectors

I am assuming you are using a general purpose PLC and not a controller specifically designed for interfacing to fire sensors.

>After reading the type of smoke
>detectors available in the market, I am
>a little confused as to which card to
>wire them to, i.e. AI or DI cards and
>how to specify my detectors? <

It depends on the type of output offered by the device - some offer volt free contacts, others (the majority in my experience) offer a solid state output. See below on line monitoring. Most detectors were originally made to interface to controllers specially designed for fire and gas detection service although times have changed now.

>If I need line monitoring, <

The need for line monitoring depends on where you are and the national legal or corporate requirements. I am pretty sure from memory both the American NFPA standards (NFPA 72) and the European standards both require open and short circuit line monitoring for input/detector circuits. Line monitoring may also be required on outputs depending on the type of device (e.g. sounder or trip output to HVAC). Make sure you comply.

>do I just
>add a end-of -line resistor to the last
>detector and wire to a AI card? If I do
>not need line monitoring, do I just
>wire to a DI card? But, I don't see
>contacts (N.C or N.O.)mentioned int he
>smoke detectors available at the
>market? <

Assuming the need for line monitoring. If you have VFCs you will need both a end of line (EOL) resistor and a resistor in series with the contact to detect both open and short circuit as well as alarm. You will most likely need an AI for this but there maybe a DI card available that allows you to do this.

If you have a solid state output from the device you will need to get an equivalent circuit from the device manufacturer (most will be willing and happy to supply this from their manual) and do some relatively simple DC circuit analysis (Ohms law, Kirchoffs, etc.) to make sure you can capture open & short circuit & alarm and wire it to an AI. You could also perhaps purchase a device on sale or return from a manufacturer and trial the device for compatibility.

Hope this helps.