120 VAC input safety

Trying to figure out the best way to take accept a PLC input from an existing 120VAC lighting circuit in order to detect when the lights are on. I see 2 possibilities since my PLC is 24V AC/DC:

1. Use a relay
2. Use an optoisolator

What I want to know is how should I go about protecting either of these? In this case, the voltage is not being fed by the panel's control transformer, and therefore it has a much higher available fault current - the lighting is fed from a 20 amp breaker.

Is it safe to use the little terminal block style fuse blocks that take glass fuses, or do I need to upgrade to midget fuses? Time delay or fast acting? What rating? Maybe I don't need to bother with fuses at all?

Is the PLC in the area of the lights?
Be lazy and use a photocell.

seth350 said:

Is the PLC in the area of the lights?
Be lazy and use a photocell.

Click to expand...

lol... not really in the same area. Maybe I'll just use a RIB relay instead of bringing the voltage right into the control panel. It will be a bit safer that way, but if the relay stopped working it would be harder to troubleshoot and fix.

I would cuss and stomp if I had to go get a scissor lift to take a light fixture apart and replace an RIB because something else on the floor ain’t working. lol

I would bring the 120V into my panel then through the main disconnect, to a fuse rated for the load it will need. As long as the fuse is rated less than the MCB and you use the same size wire from the fixture to your fuse.

seth350 said:

I would cuss and stomp if I had to go get a scissor lift to take a light fixture apart and replace an RIB because something else on the floor ain’t working. lol

I would bring the 120V into my panel then through the main disconnect, to a fuse rated for the load it will need. As long as the fuse is rated less than the MCB and you use the same size wire from the fixture to your fuse.

Click to expand...

Yep, me too - definitely trying to avoid that. Since the load will just be an interposing relay or opto, I guess I would just use the smallest fuse that I can find like 1/4 amp or something. But going through the main disconnect doesn't work so well when I have several of these circuits to monitor in the same way.

Then I'll be sure to carefully label the multiple energy sources.

seth350 said:

I would cuss and stomp if I had to go get a scissor lift to take a light fixture apart and replace an RIB because something else on the floor ain’t working. lol

I would bring the 120V into my panel then through the main disconnect, to a fuse rated for the load it will need. As long as the fuse is rated less than the MCB and you use the same size wire from the fixture to your fuse.

Click to expand...

I would cuss and stomp if I found someone bringing a 120V lighting circuit into my PLC control panel just because they didn't want to interface it properly

Personally, I'd put a small j-box nearby with a small fuse/CB and an interposing relay in it. 120V cable up to the light circuit, 24VDC cable into your control panel. You can put it down human level near the light switch if you want, then neither Seth nor I will need to do any cussing and stomping

Could you use a current switch?

Say maybe 24v up to a lighting j-box big enough to mount the switch?

Or the opposite; drop leg of the lighting circuit down to a j-box near the control panel?

I wouldn't have the 120 enter the control panel without a way to isolate it locally.

Just stick a relay in the lighting circuit, and 24v to the contacts it is generally good practice to put this in a small box with a warning label indicating where the supplies are from and what voltage. It is also common to use a striking colour for the wiring to the terminals for the signal to the (in your case PLC) for example orange which often denotes supply from other sources (will depend on country I suppose). You could also use a local isolator to isolate in/out voltages.

I'm pretty space-constrained on this project, but I'm liking the idea of having a separate j-box or trough located above the control panel which would contain the relays and any other current sources that aren't fed from the panel disconnect. I'll see if there's a way I can fit that in.

I use current monitoring relays. After measuring current in normal state with light switched on, I put 10% lower value as threshold. It worked flawlessly many times.

ASF said:

I would cuss and stomp if I found someone bringing a 120V lighting circuit into my PLC control panel just because they didn't want to interface it properly

Personally, I'd put a small j-box nearby with a small fuse/CB and an interposing relay in it. 120V cable up to the light circuit, 24VDC cable into your control panel. You can put it down human level near the light switch if you want, then neither Seth nor I will need to do any cussing and stomping

Click to expand...

I agree, but that's what most guys use the RIB's for... Isolation. Most I see are in HVAC (commercial) work, which is about as close to Lighting as I know, so what's the issue? The RIB has an LED to indicate the signal, and it isolates it from the panel's circuits.