Device Protection

RBergmann

Member
Join Date
Jun 2002
Location
California
Posts
258
Knowing full well that this is not an electrical forum but for PLCs, I have a question that applies to both.

We have a series of equipment that generally is field wired by the customer's electrician. One 24VDC PLC input, via terminal blocks, is for a "dry contact" (with our system providing the voltage source) signal to initiate and maintain a sequence.

For some reason, we are getting 240VAC applied (presumably by mistake) and it's taking out most, if not all, of our DC devices. This gets expensive as these can include power supplies, PLCs, HMIs as well as various other equipment.

By the time we are made aware of DC device failure, it's too late. Our equipment is located in various locations of the U.S.

Is there a circuit or device that can provide us a kind of front line defense against these mistakes? An MOV (wired in parallel) + fuse (wired in series with one leg of the dry contact circuit) was offered as a possible solution.
 
My first response to that is "if they can't read the wiring diagrams, they shouldn't be wiring it". If they've gone and sent 240VAC straight to a 24VDC digital input, they either haven't checked the wiring diagram and just made a wild guess at what to do, or they really don't understand the task. In either case; this is very clearly the customer's screwup, and your hands are clean.

But, that doesn't help you.

Perhaps you could try a different approach for getting this "run enable" signal. Instead of asking for a dry contact, tell the customer that they must provide a powered output, and ask them to specify the output voltage they intend to use. Install a relay with a coil of that voltage, connect your field terminals to the coil, and wire the contacts to your PLC's digital input.
In the first instance, it forces the customer to get someone who has an appropriate level of understanding involved, to answer your "what will your output voltage be?" question. And secondly, if Dumb@ss Darryl comes along and puts his 240VAC onto your relay anyway, at least you only blow the relay. Even Dumb@ss Darryl can replace a plug-in relay.
 
RBergmann,

If this is just one site, I would say a mistake.
if this is happening on multiple sites, I would do the following.

1. Go to the customers and ask them to show you they wired the system up. Takes notes and say nothing, TAKE NOTES! Then ask very detailed questions, could be that their answers will give you a clue what is happening.

2. Take the customer(s) prints and installation manuals.
3. Analyze those prints with a fine tooth comb. I suspect that the prints that you use to wire the machinery by is not the same as what the customer gets.

4. If you understand the prints and manuals, then they need to be redone, something is being lost in the reading of the instructions.

5. INSTALL an interface relay between your panel and the customer that the customer can power. Interface relays are cheap when compared to the entire plc system and other components. if they put 240 volts on a 24 volt coil, your out $25.

regards,
james
 
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I can't help with your question but wanted to make sure you had thought of all possibilities of preventing this mistake.

Presumably, this is happening at installation. If so, our guys seem to pay more attention if they have to remove a sticker or plastic cover that says "DO NOT APPLY VOLTAGE TO THESE TERMINALS! DAMAGE TO EQUIPMENT WILL OCCUR."

Attached is a picture of simple plastic covers. A sticker over the terminal screws will also work.

Good Luck.

$_1.JPG
 
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RBergmann,

If this is just one site, I would say a mistake.
if this is happening on multiple sites, I would do the following.

1. Go to the customers and ask them to show you they wired the system up. Takes notes and say nothing, TAKE NOTES! Then ask very detailed questions, could be that their answers will give you a clue what is happening.

2. Take the customer(s) prints and installation manuals.
3. Analyze those prints with a fine tooth comb. I suspect that the prints that you use to wire the machinery by is not the same as what the customer gets.

4. If you understand the prints and manuals, then they need to be redone, something is being lost in the reading of the instructions.

5. INSTALL an interface relay between your panel and the customer that the customer can power. Interface relays are cheap when compared to the entire plc system and other components. if they put 240 volts on a 24 volt coil, your out $25.

regards,
james
You're correct, James ...

I generated the schematics, so I know to what they're wiring. I don't think the documents are being used. A lot of feely, touchy stuff going on. These are not machines, but systems located in rural areas (as a matter of fact the three problem areas have been withing 25 miles of Nashville).

You and ASF have a good point. I may introduce relays for this signal, but does put a 'foreign' voltage in our cabinet. I also like the sticker idea that OdinIII mentioned. May include that also.
 
If you are worried about introducing a foreign voltage into the panel, have a small subpanel outside your main panel. Only 4 wires in there: their power wires and your contact wiring. We have done this several times.

Nashville, 1 hr. from Macon county.

regards,
james
 

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