1747-L20A inputs

[todster]

I have a 1747-L-20A. The inputs have the common labeled as common and are tied to gether internally. The inputs themselves have 2 diodes opposed. Can the line voltage be connected to the commons and the switches be used as a sink of sorts if you will. It would be much easier to run a common wire to the switches on the inputs than run 120v to them. It would also mean that if one shorted to ground the only result would be that the input would turn on which in my application would be ok. I'm using this on a wood furnace and would prefer not having to have any 120V near the flue and would prefer that the wires if they did short out simply turn the input on. By default the inputs here would only indicate overheat and shut the furnace down as well as turn the room air blower on.Also after so many minutes of the blower on and over heat in stack and plenum an alarm will come on.

 

[bernie_carlton]

While switching the ungrounded line is the industry standard you have obviously thought out the consequences (failed safe)of a short to common.

I would assume that AC COM is not connected to AC NEUT (there wouldn't be much isolation if that were true) but check it out with an ohmmeter anyway. If they are isolated then I don't see any problem in the wiring method you propose.

If for a commercial application you may have to check with industry requirements (UL) though.

Realize that while the sensing switch is open line voltage will be present on one side of the switch. So you haven't really avoided that problem. Its current capability may be limited by the input circuitry but it's not a fail-safe limitation. You may possibly want to design a GFCI into the circuitry if shorting is a possibility.

 

[todster]

According to the specs the max current is like 4ma or something in that area. Even if the wire does short out before the switch it will not fuse all the wires into one solid mass and stink up the house. I was using a keyence on it with 24vdc but want to play with hmi and the keyence kv40 isn't really set up for that with out going through a lot of headaches. That and used A/B i/o cards are dirt cheap for future expansion. The plc will have a gfci at the wall outlet. The common and neutral seem to be isolated but ohming the inputs to the common is also such a high resistance that my meter can't read it. I guess it only cost me $20 on Ebay so I'll hook it up and if she smokes i'll have to invest another $20 or upgrade to an slc500/3 and a rack. Just have to tell the wife furnace won't work till I get that new rack and let her smell the old one as proof.

 

[todster]

It will work with the common used as L1 and the inputs as common. The current draw is only 45uA.