Point IO burned up.

[dftmaint]

We have a system with several Point IO panels. Today had one that burned from the Field Potential Distributor Module. The 120vac output card beside the FPD looked like it had a lot of burn marks but since it is the next card to the right its hard to tell where the problem originated from. Most of the damage was around the terminal block where the FPD receives 120vac power. The other Point IO panels get 120vac from the same source.

Reading about the FPD and its function i think i can rule out the DC side being part of the problem. The 120v output supplies three solenoid valves to control liquid. The valves all test ok using a meter.

There is no suppression devices on these valves. They are not even fused. What do you guys think about adding simple fused terminal blocks to these? Has anyone had one of these fail like this? Any thoughts on the cause? Thanks

[Ken Roach]

I've certainly seen 120V AC circuits burn up when they get contaminated, overloaded, shorted to ground, etc.

The fact that it was in the context of a POINT I/O assembly and a 1734-FPD Field Potential Distributor just suggests to me that the isolation did its job and kept the damage to the associated circuits, not the network, the adapter, or the backplane.

If everything's charred there's not much chance of finding contamination, or loose screw terminations, that could be the cause.

I agree that it would be prudent to protect the field wiring for those solenoids with a fuse on the "hot" leg where it enters the 1734-FPD, sized to accommodate all of the solenoid loads while protecting the wiring.

What is the wattage of the solenoid coils ?

In general, solid-state AC outputs don't suffer from the same kind of flyback / spike risk and damage as dry relay contacts do. What exactly was the 1734 I/O module ? Was it an "OA" (triac) or an "OX" (relay) ?

[dftmaint]

Originally Posted by Ken Roach

I've certainly seen 120V AC circuits burn up when they get contaminated, overloaded, shorted to ground, etc.

The fact that it was in the context of a POINT I/O assembly and a 1734-FPD Field Potential Distributor just suggests to me that the isolation did its job and kept the damage to the associated circuits, not the network, the adapter, or the backplane.

If everything's charred there's not much chance of finding contamination, or loose screw terminations, that could be the cause.

I agree that it would be prudent to protect the field wiring for those solenoids with a fuse on the "hot" leg where it enters the 1734-FPD, sized to accommodate all of the solenoid loads while protecting the wiring.

What is the wattage of the solenoid coils ?

In general, solid-state AC outputs don't suffer from the same kind of flyback / spike risk and damage as dry relay contacts do. What exactly was the 1734 I/O module ? Was it an "OA" (triac) or an "OX" (relay) ?

It was charred real bad. I couldn't see the wattage but each were reading around 180 ohms which would be pushing the rated .75 amps each output is rated. It was an OA4.

[Ken Roach]

Thanks for those details !

You are correct, of course, that the 1734-OA4 is rated for 750 mA per output, and 2A total per module.

Sanity check: V/R = I, so 120/180 = 0.660 Amperes.

While POINT isn't as rugged as ancient 1771 stuff, the ratings are still fairly conservative and the max current is for continuous operation at 113F ambient.

Maybe there was a floating or loose ground and you ended up rattling those solenoids on and off and burned the module up with surge current.

If you wanted to be even more conservative, put in another 1734-OA4 and move one of the solenoids to it.

But in general there's nothing mysterious or complicated about an OA4 triac output. Fuse the output to protect the wiring, and check all the wiring all the way through for loose terminals on the neutral side.

[dftmaint]

The plc is a compactlogix and we don't have Rslogix 5000 to make changes. I have some, but not a lot of RS5000 experience. Really just the few programs i did in tech school and a trainer i completed. Would i would need to change an address in the program if i added another terminal block and module to the point IO? Most of our stuff is slc 500, micrologix, or automation direct. May just add interposing relays to get isolation. Thank you for all the help.

[Ken Roach]

Yes, you would need to change the I/O configuration in RSLogix 5000, and maybe make a change to the POINT rack size through its web browser too.

Generally I/O modules don't just burn up spontaneously on overload if they've been specified correctly, especially if you have similar circuits that don't overload.

Inspect all the wiring to those valves carefully. A loose neutral could certainly cause a malfunction like what you described, typically along with "valve chatter".