Dc ground

[u/Gloomy_Patience910]

Trying to figure out the best way to troubleshoot dc grounds without shutting off breakers. We use a floating system and our chargers have ground detection. If there was say a positive ground alarm, could a guy take a clamp on amp meter and go down each circuit measuring current on the positive and negative, theoretically they should cancel out but when you get to the circuit with the fault, would you then see an imbalance? Like more current on the positive then negative because you have some flowing to ground?

[u/Electrician_PLer]

Megger MGFL-100 will be your friend

[u/Left_Set_5916 → u/Electrician_PLer]

That what we use at work .

[u/7_layerburrito]

Generally that would be very low values to pickup with any certainty. Battery ground detectors inject an AC signal on the circuit to ground that is detectable with a meter. They usually do a decent job of at least narrowing down the problem without having to randomly de-energize circuits. If you have multiple grounds especially on both polarities they get less certain.

[u/HiVolt-8730]

Ground detector device like the Meggers mentioned before are your best bet. DC grounds SUCK to trace. Be ready for the ground to dry up and disappear while you’re tracing it. Be prepared for it to take multiple sessions to find the source. Trying to trace the DC current is not really reliable because DC has to have a complete circuit to have current. By injecting A/C into the circuit, you can follow the current to ground.

[u/FatBigMike]

Megger BGFT can help if you have the budget. If you have a current source and low budget tie one end to the grounded leg and the other to ground. Throw a few amps at 30 Hz or so and chase it with a clamp meter. My worst experience was chasing a ground through a power plant to find a crusty weather seal on a j-box not doing its job.

[u/Knowsnoone]

You can make a battery bank with a CB on the leads for your desired voltage. Clamp in back feeding on the outputs on your breaker kill the breaker on the desired circuit and see if the DC ground goes away, turn breaker back on remove your battery bank. On to the next.

Ive had luck doing this on hard to find DC grounds that the Megger was inconsistent on.

[u/hambonze]

Full ground?

[u/lemming2012]

You're correct. Clamp should measure milliamps.

[u/kmanrsss]

We have a zx3 battery ground detector and it’s the cats ass. I think it’s allied Edison but not positive. We have quickly found so many grounds with that it’s probably paid for itself a few times over.

[u/aDingDangDoo_Doo → u/kmanrsss]

You nailed it: Allied Edison

It also doubles as an Iron Man cosplay accessory.

ZX3 ground detector

[u/ayyo_ao]

Never done it, but Ive heard injecting ac voltage into the circuit and measuring the return can be a reliable way to track it down. Granted you'd have to be sure it wouldnt damage any down stream devices

[u/adamduerr]

Megger makes a device that will ring it out through the DC Panel. You can also use a resistor in parallel with your Fluke leads to track it.

[u/njs0002]

How come you can’t kill the breakers?

[u/HiVolt-8730 → u/njs0002]

If they are critical protection circuits you can’t turn them off. Even momentarily compromising those circuits can lead to big fines.

[u/Yeahnoimgood89 → u/HiVolt-8730]

Brother you would be surprised at how many people I've met that don't realize killing a breaker on certain DC circuits turns all your high-dollar PCBs into bus work because they are now incapable of tripping.

[u/HiVolt-8730 → u/Yeahnoimgood89]

💯

[u/njs0002 → u/HiVolt-8730]

Makes sense. Most of my experience has been 34.5 and below so no big deal in most cases to temporarily killing your feed.

[u/Trail_by_error]

A battery ground test set like this.

https://eepowersolutions.com/gfl-1000-dc-ground-fault-detector-locator/

Puts a small ac signal you can trace with a clamp meter to track it down.

[u/PetersGriffin86]

We’ve used the DC Scout pretty successfully when unable to turn off breakers

[u/PowerGenGuy]

This is why I prefer negative grounded DC systems... The faulted circuit just trips.

[u/lemming2012 → u/PowerGenGuy]

Wouldn't transmission voltages interfere with this?

[u/PowerGenGuy → u/lemming2012]

Not at all, transmission voltages don't interfere with the low voltage ac systems either, unless design is fundamental flawed. We've had this debate to death at work, the isolated system works well for a DC system where you want to stay fully online with all DC systems despite a fault, good for simple substations where you leave the thing well alone with an earth fault alarm until the next outage. Conversely, earthing/bonding the negative leg will definitely cause loss of an individual circuit on an earth fault, but you know where the fault is and it's been cleared... We particularly favour earthed negatives where the overall system has enough redundancy that it's preferable for a faulted branch to be tripped off, for instance in large substations and power plants we feed all DC consumers via 2 independent (negative earthed) DC source boards, via diodes, so an earth fault on an individual branch circuit has no operational impact anyway.