[jsb]

i have a 12-2 uf cable running 1000' to a sprinkler pit and i am monitoring 3 tamper switches. if i disconnect both ends of the cable and test it to ground there is .4volts dc on each of the conductors. this is causing a ground fault on my FACP.

it reads over 5M ohms to ground on each conductor so i know it is not really a ground fault.

if i stick both leads in the ground about 5' apart i read over .01vdc

i know you cannot induce dc voltage but i have no idea what is going on here.

any suggestions or input

 

[MDShunk]

What kind of tamper switches get grounded?

 

[jsb]

none, why did u ask that? i disconnected them to eliminate that part, i have the uf cable disconnected from anything at this point when i am testing it.

 

[MisterCMK]

You probably still have a ground fault. Many times your regular DVOM can't read the ground fault because the the voltage that it uses to measure resistance isn't high enough. We have some custom build meters here at work that have a bunch of batteries wired into them to better detect ground faults that otherwise wouldn't show on a meter.

 

[ralph]

What kind of equipment are you working with ? I had the same problem with Gamewell once.

 

[MDShunk]

Makes me wonder if it's not an inductance issue from the long run. Something twisted might have solved the problem.

 

[RePhase277]

1000' of cable is just a huge antenna that picks up every radio station and every spark plug. It is also a big capacitor. If this is a DC signaling circuit, a diode in reverse (from normal circuit polarity) across each end may help.

If you can figure out the capacitance of the circuit, maybe you could add an inductor to cancel it out?

 

[jsb]

in the end i called a guy who was in vietmnam and did some stuff with radio frequencies etc. he hooked up to the wire and gave me an exact location on a ground fault. once we got that repaired it was all good after that. he is AMAZING!

that doesnt diminish the fact of the Vdc on the cable. that was very odd and no one can explain it. luckily it wasnt over the tolerances of our equipment, and didnt effect it in the end. everything is good to go now.

lesson learned: never let anyone bully you into doing what you dont want to do. i have never run FA (or anything elese for that matter) in direct burial. the customer did not want jboxes throughout the site every couple hundred feet so he talked me into doing direct burial. that will be the last time.
it would have probably never happened if it was in conduit. not to mention i rented a miniX to dig it up and fix it, and when i was returning it the rented trailer broke and caused $4000 damage to my van, and they say it is not their responsibility because i unloaded and reloaded the equipment.

 

[RePhase277]

TDR is pretty cool stuff. But the issue of the 0.4 VDC is just an artifact of the Earth and digital volt meters. There is a pretty high field gradient between the sky and the Earth, and this can be detected in long runs of cable.

At any rate, it was nice of that guy to come all the way from Vietnam to help you out.

 

[jsb]

yeah that doesnt look right does it sorry

 

[drsparky]

TDR is pretty cool stuff. But the issue of the 0.4 VDC is just an artifact of the Earth and digital volt meters. There is a pretty high field gradient between the sky and the Earth, and this can be detected in long runs of cable.

At any rate, it was nice of that guy to come all the way from Vietnam to help you out.

If any one has a Oscope with a gated output I can show you how to use it as a TDR.
Chuck

 

[MisterCMK]

Aren't ground faults fun to track down? :no:

 

[leland]

i have a 12-2 uf cable running 1000' to a sprinkler pit and i am monitoring 3 tamper switches. if i disconnect both ends of the cable and test it to ground there is .4volts dc on each of the conductors. this is causing a ground fault on my FACP.

it reads over 5M ohms to ground on each conductor so i know it is not really a ground fault.

if i stick both leads in the ground about 5' apart i read over .01vdc

i know you cannot induce dc voltage but i have no idea what is going on here.

any suggestions or input

Your cable is bad. change it out.

 

[slowforthecones]

that sucks your van got phuckered up with the rental equip. i always prefer to haul with my now lowboy.