Help with electrical idea for wastewater system

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Old 10-22-16, 10:08 AM
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Help with electrical idea for wastewater system

Hello, folks. I have a specialized wastewater system that is used when the ground on a lot doesn't perc, and boy is it a pain for me. The short version of the story is that the Siemens PLC in my system (in the upper left of the picture below) has blown its 24V DC supply. That supply came from the PLC and powered the inputs to the components of my system (solenoids for water flushing, floats for depth checking, etc). This happened once before 2 years ago and as a test, I bought a little 24V DC power supply wall wart, stripped the wires out of it, and ran an extension cord out to the side of the house and attached that to the inputs and everything worked as expected. Once I knew that was the problem, I replaced the PLC (not cheap).

So, I don't really want to put a new PLC in for various reasons and my question is more of a safety/regulation one. I know I can get the thing to work by supplying the 24V DC again, but I obviously can't have an extension cord running outside to get power to the wall wart. So my plan is to double tap (via a pig tail) one of the breakers that are in there (in the upper right hand corner of the box) and just connect up a little outlet right there in the box in one of those plastic junction boxes. My question is primarily, is it an obviously crazy plan that is likely to burn the house down? What are my potential code violations in this plan (if they are not innumerable)? Does doing something like this to a closed system necessarily make me negligent?

I am in Virginia. Thank you so much for any advice.

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  #2  
Old 10-22-16, 10:49 AM
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I'd be more inclined to put in an open frame industrial grade 24VDC power supply with it's own breaker or fuse, both on primary and secondary.

Something like this, depending on how much current you need:

PMT-24V100W1AA Delta Electronics | Power Supplies - External/Internal (Off-Board) | DigiKey
 
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Old 10-22-16, 11:44 AM
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Thanks, Paul. Would that be because of moisture concerns? Does anyone think this seems like an unsafe approach in general? I am mostly worried about fire and electrocution, obviously.
 
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Old 10-22-16, 12:05 PM
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Mostly, I think that kind of supply will be more reliable than a wall wart, but yes, most wall warts are not intended for outdoor use.

I don't see anything fundamentally wrong about your plan. There's already AC in the cabinet. You're not increasing the overall power dissipation or load, so I don't see a major problem.

One concern would be how you intend to disconnect the failed supply from the things it is powering so you are not back feeding the failed supply with the new one because that could cause problems.
 
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Old 10-22-16, 02:55 PM
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Gotcha. What kind of problems (from the backfeeding)? I'm not too worried about frying the PLC anymore because it's already going to have to be replaced anyway eventually.
 
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Old 10-22-16, 08:21 PM
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Really hard to say without knowing details of the PLC power supply. You would be connecting two power supplies together. It could be fine, or the failed supply could load down the good supply and cause it to not work or to fail, or the PLC could be further damaged. It could work for a while and then die. As I said, really hard to say. I wouldn't recommend going down this route if you can't disconnect the failed supply somehow.

Since this is the second time that PLC supply has failed, one must wonder if the external loads are too much for it and it's rating is being exceeded.
 
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Old 10-22-16, 11:50 PM
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That's a control panel from American Mfg. company. Do you have the wiring manual for it ?
 
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Old 10-23-16, 05:45 AM
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Hi Pete, yes it is! Mine is the 4-zone panel (although only 2 zones are hooked up) found on page 5 of
http://americanonsite.com/american/pdf/01DRIPRES15.pdf

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Does that help? If not, I can request a wiring diagram from them.
 
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