Lost my 4-20 ma loop

cirtcele

Member
Join Date
Apr 2007
Location
Illinois
Posts
64
I lost my 4-20 loop a couple days ago it appears to be an open since i can induce a signal in the building that has the plc that connects to the main scada plc and get a flow displayed on scada. I've tried to use an Amprobe wire tracer to track the wire but end up getting a lot of hits on the terminal strip where many 4-20 loops are connected, i even get strong hits on the shield drain wires on some loops. I connected the signal generator to one of the loop wires + the other to ground and leave the - loop wire connected. The shield drain is in the flow meter vault still connected to ground. Thats the way i noramally connect to trace non process wires. Anybody know of a better way to find this loop. We are a wastewater treatment facility and this loop travels underground to different buildings and j boxes but nothing is labeled. Been trying for two days to track this down.
 
1)Try a product called Fox & Hound. Don't know if this will help or not.
2)For dead wires, ground one end. Go to other side and use ohmeter to find which wire is grounded.
 
If you have the full Amprobe kit you should have sufficient tools to find break in cable ASSUMING it is not buried 20'.

Disconnect cable at one end short all together and ground them. Go to other end to find shorted grounded cable. Then two guys with radios find which if any conductor is broken.

Food for thought
You ground the shield at the sendor. How many others including me ground at the PLC?? Are you creating ground loops in the shield??

Dan Bentler
 
I'll try shorting the signal wires to the shield then to ground. We have the full Amprobe kit maybe i'm not using it correctly. What about shorting the signal wires together then connecting the Amprobe signal gen to them. This loop contains at least one other device in the loop don't really know how many or where because of poor documentation.
 
I'll try shorting the signal wires to the shield then to ground. We have the full Amprobe kit maybe i'm not using it correctly. What about shorting the signal wires together then connecting the Amprobe signal gen to them. This loop contains at least one other device in the loop don't really know how many or where because of poor documentation.

I am not sure about hooking the Amprobe to all 3 shorted. Would have to read directions.

I am not sure about fox n hound ie tone generator like the phone co guys use. Looks like that may work also. May be good for tracing the cable unless in conduit (?).

The thing I can see may be a problem is fox n hound or Amprobe will put a voltage on the cable - will that damage instruments?

Otherwise it is trace cable and or conduit and find the next box. Always a ball of fun. That is what happens when they dont take the time to make drawings or save them.

Dan Bentler
 
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...this loop travels underground to different buildings and j boxes but nothing is labeled.
It seems like a case of poor installation: not labelling the wires. A drawing won't help much if the wires are not marked. They all look alike in a J-box.

Cirtcele, I know that as soon as you find which wire is which, you are immediately going in there with some wire markers and label all of those different places.....
 
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