back feeding ML 1200?

Does anybody know if the ML1200 allows having a voltage (120VAC) on an output while the output is off? This is for the 120VAC model and not the 24VDC if it matters.

Thanks

What type of outputs are they? Triac, relay?

relay outputs.

As always more information on what your trying to do would be helpful. If they're relay outputs, you would need to put some external voltage on them. The PLC doesn't supply any voltage on them. But I believe some of the relay outputs have common sources.

I guess I don't understand the question. By definition a relay output switches voltage supplied by a device external to the PLC. Therefore you would have to have voltage at one terminal of the output channel in order for the output to do anything. A PLC relay output is just like a regular external relay, except that the coil is inside the PLC and operated by the PLC logic. The way a regular relay is operated is having an external voltage operate the relay coil.

I have a plc output that energizes a coil which closes a NO contact causing a VFD to run. For safety purposes another device has the ability to energize the same coil (fire alarm) causing vfd to run. I believe it is ok to wire the plc as stated but I thought I would get a second opinion.
thanks

Let me see if I understand the situation:

You have a ML1200 with relay outputs. One of these relay outputs drives another device (relay?) with a 120-volt coil, which startss a VFD. You want to also have a second 120-volt source from a fire alarm that also needs to start the same VFD. Is all this correct so far?

If so, then the only problem I see is that you should probably make sure that your 120 volts is "common", coming from the same transformer and the same phase. Otherwise, if they ever both get connected at the same time, you could have a short circuit. This short will be downstream of the PLC relay output, so should not bother it, but it could blow a fuse or burn some wires. Therefore I would make sure that I use a common supply for both 120 volt sources.

If that is not possible, or you don't want to take the chance that someone sometime will swap one of the two voltage sources, then you can do what many of use have had to do in the past, use another double-pole double-throw relay downstream of the PLC (call it R1). When the PLC outut is ON, it energizes a NO contact on R1 which starts the VFD. The fire alarm output can be connected to one of the NC contacts on R1, and then to the same VFD Start circuit. When R1 is not energized by the PLC, then the NC contact will allow the fire alarm to start the VFD, with no chance of a short circuit.

As long as the two 120VAC sources are the same, it won't matter. No different than having two different switches or push buttons power the same coil.

You should use a relay to isolate the two power sources. The fire protection circuit should be before the disconnecting means of the power to your plc. You should not use the same circuit to feed the for this reason

The fire alarm output can be connected to one of the NC contacts on R1, and then to the same VFD Start circuit. When R1 is not energized by the PLC, then the NC contact will allow the fire alarm to start the VFD, with no chance of a short circuit.

Click to expand...

It would be better to use the relay to switch power from the fire curcit line and have the switch legs from the relay and fire pump switch parreled to the coil that way your not adding another contact that could fail