Isolated ground broken

[Jnewell]

So this is a bit confusing and I can clarify anything, if asked.

I was called out to troubleshoot an outlet for a UPS system for cash registers at a grocery store that keeps turning off. Upon inspection of the panel it is a single phase 240v panel with an isolated ground, with multi wire branch circuits wired of a 3 phase panel. So instead of a two hot shared neutral for single phase, it’s wired with a 3 hot and shared neutral, which was the first issue I saw. All of the branch circuits go into one pipe and daisy chain through to where they land, there is 8 registers total. When I plugged in my plug tester it showed a missing ground, and somehow the conduit had been broken so we were called out to replace the flex and run new wire from the junction box.

When I shut the power off and checked continuity between the dirty ground and neutral it was at all 8 registers. But when I checked it between isolated ground and neutral/dirty ground it was reading continuity betweenat the 5th register but not the 6th, which tells me that it is disconnected/broken somehow. I then turned the power on and checked with my plug tester after removing the broken flex and wire and it read normal not missing a ground. I was also getting a very high resistance reading between isolated ground and neutral/dirty geound(which I assume was caused by induction). When I checked voltage there was not any voltage between isolated ground and neutral/dirty ground at register 5 but after that I was getting about 7v, but when I plugged in the UPS systems I was getting as high as 71v depending on which breakers were on, but typically about 32v

I didn’t check which phase the other breakers were on that was giving me the highest voltage reading(Didn’t think of it til after). I have some theories but not sure if there valid. I know what needs to be don’t to fix it, but could some explain the science behind it to me? Thank you

 

[LarryFine]

A high-impedance voltmeter will read high voltages on an unterminated wire run with energized wires. A low-impedance tester, like a solenoid tester or even your voltmeter with a low-wattage bulb in parallel with the test leads would show almost no voltage.

 

[hillbilly1]

The biggest issue I find with isolated ground systems is electricians don’t understand what an isolated ground is. If you go back to the panel, you will probably find the ground bar is truly isolated, no ground at all connected other than the branch circuits. Get that with a lot of step down transformers too, they “Think” isolated ground, and don’t bond XO. I’ve seen systems work for years like this until a component actually shorts out, and makes the isolated grounds “Hot”. A piece of equipment plugged in to the IG receptacle may be sitting on something else that actually is grounded, giving the appearance that it is grounded. When you unplug that piece of equipment, the ground goes open, which may cause the condition you describe.

 

[Jnewell]

It was bonded correctly in the switchgear, and the only thing on the isolated ground bar was the cash register daisy chain loop.

 

[hbiss]

Didn't you already ask this question here https://xenforo.mikeholt.com/threads/ups-system-issue.2555084/
Seems like you are getting the same advice.

-Hal

 

[Jnewell]

Yes I did but I’m still confused why it was reading continuity when on, even though it is broken/disconnected and only one of the UPS systems was experiencing problems when the IG is disconnected to 3 of them.

 

[kwired]

Does modern point of sale devices even need IG anymore?

I though part of problem years ago was involving shields on communications cables, anymore they are using catV or VI (no shield) or even wireless communication.

 

[hillbilly1]

Isolated grounds supposedly came about due to grounding was done by the conduit, which introduced noise and other problems due to loose locknuts and/or improperly tighten couplings and connectors ( mainly EMT) .

 

[hbiss]

I'm having a problem understanding your description and problem.

You have 8 receptacles daisy chained with pipe from a single phase panel. The receptacles are wired with a MWBC as if the panel was 3 phase instead of single, so you have 3 hots and a neutral on two breakers or three? Not good, but shouldn't have anything to do with anything.

You also have an IG conductor daisy chained from the panel ground bus through each receptacle. All receptacles are orange IG receptacles? Is the IG conductor only connected to the IG receptacle ground screws?

There is apparently some flex conduit is involved and a junction box. Apparently the flex was damaged and you replaced it as well as the IG conductor from a junction box to I don't know where. Is the junction box one of the receptacle boxes in the daisy chain or a splice box in the run? Did you replace the IG conductor going from one receptacle to another? Because you lost me by talking about continuity tests, turning power back on, testing with your receptacle tester THEN removing the broken flex and IG conductor then wondering about the results.

Yes I did but I’m still confused why it was reading continuity when on, even though it is broken/disconnected and only one of the UPS systems was experiencing problems when the IG is disconnected to 3 of them.

Taking apart and inspecting each receptacle and wiring would have told you why.

Replace the broken flex and IG conductor, inspect all receptacles to see that they are wired properly and of the proper type, turn the power back on, check all receptacles to ensure there is voltage and ground then have a nice day.

As has been mentioned, IG receptacles went the way of the Dodo bird so you might just want to replace all those receptacles with regular receptacles and make the IG conductor into a normal EGC by bonding it to each box with a pigtail to the receptacle. If you have flex more than 6 feet in length you need an EGC anyway.

-Hal

 

[hbiss]

Isolated grounds supposedly came about due to grounding was done by the conduit, which introduced noise and other problems due to loose locknuts and/or improperly tighten couplings and connectors ( mainly EMT) .

No, not true. Isolated grounds were to provide a dedicated ground conductor back to the main panelboard or service entrance ground to eliminate noise on the regular ground caused by other equipment that might share it.

IG conductors are not supposed to be daisy chained from receptacle to receptacle either. Each IG receptacle must have a dedicated IG conductor running back to the panelboard or service entrance ground bus.

-Hal

 

[hillbilly1]

No, not true. Isolated grounds were to provide a dedicated ground conductor back to the main panelboard or service entrance ground to eliminate noise on the regular ground caused by other equipment that might share it.

IG conductors are not supposed to be daisy chained from receptacle to receptacle either. Each IG receptacle must have a dedicated IG conductor running back to the panelboard or service entrance ground bus.

-Hal

Apparently the article in EC&M magazine was wrong then......

 

[hbiss]

Apparently the article in EC&M magazine was wrong then......

Likely whoever wrote it wasn't even alive back when data was RS232...

-Hal

 

[Jnewell]

There’s multiple issues and single phase 240v system MWBC with 3 single pole breakers sharing one neutral and no breaker ties.

we were originally called out due to the floor guys bumping the register and breaking the flex under it that was ran from out of the underground j box to the receptacle, when you put in a plug tester on the IG receptacles that were orange with a green triangle with the IG tied to them it showed it was missing the IG. When I tested continuity with a multimeter between IG and neutral/dirty ground with the power off it would end at register 5, so 6, 7, and 8 did not have a connected IG, it was somehow lost. The register that was bump was register 7.

After I fixed the flex and was turning it back on my helper noticed the plug in tester kind of dance and dim as we were turning breakers back on, which eventually showed it was right, so I started checking voltage which led to continuity tests because I was getting voltage between neutral and IG cause the IG was open.So it showing grounded only when the power is on and plugged in is what threw me for a loop.

I hope that clarifies a little better

 

[hbiss]

I hope that clarifies a little better

Umm, no. At that point I would have put away my test equipment and opened everything up and looked for a problem. DVMs will give false readings because they have high impedance inputs and there is no telling what those little plug-in receptacle testers will show if there actually is voltage on the wrong conductors. So you use them with caution. I actually like to use a 25W light bulb in a pig-tail socket for stuff like this. Never lies.

-Hal

 

[kwired]

Isolated grounds supposedly came about due to grounding was done by the conduit, which introduced noise and other problems due to loose locknuts and/or improperly tighten couplings and connectors ( mainly EMT) .

Might be reason the IT guys wanted them, NEC never really allowed those loose locknuts or improperly tightened fittings, unless you knew it would be a bad connection for good reason and intentionally bonded around it.

 

[kwired]

No, not true. Isolated grounds were to provide a dedicated ground conductor back to the main panelboard or service entrance ground to eliminate noise on the regular ground caused by other equipment that might share it.

IG conductors are not supposed to be daisy chained from receptacle to receptacle either. Each IG receptacle must have a dedicated IG conductor running back to the panelboard or service entrance ground bus.

-Hal

Pretty much always been my understanding. And a lot of issues went away when they stopped using shielded cable types for data transfer but many still think IG is needed for receptacle intended for IT equipment. Now most data is transmitted either UTP Cat V or VI, optical fiber or wireless. Any shielded cables still in use are generally a short length of 6 feet or less to a printer or similar accessory connected to another device nearby, but even those are becoming more and more rare.

 

[hillbilly1]

Might be reason the IT guys wanted them, NEC never really allowed those loose locknuts or improperly tightened fittings, unless you knew it would be a bad connection for good reason and intentionally bonded around it.

Not allowed and what is actually installed is two totally different things! LOL!

 

[hbiss]

Any shielded cables still in use are generally a short length of 6 feet or less to a printer or similar accessory connected to another device nearby...

Forgot to mention ground loops. Consider a computer and a printer with grounded line cords in two separate parts of the building. Maybe on two separate panels. They are interconnected with a shielded cable with the shield grounded to the chassis of the computer at one end and the printer at the other. The shield becomes a parallel path from the EGC at the computer location to the EGC at the printer location conducting noise and current and coupling it to the data conductors which it is supposed to protect.

So, if the receptacle serving the computer and the receptacle serving the printer each had a separate isolated ground conductor going back to the same exact location in the service panel, that problem is eliminated.

This is the purpose of an IG receptacle.

-Hal

 

[jim dungar]

... same exact location in the service panel, that problem is eliminated.

IG conductors need to be run to a same location, like a common point or bus at their source, not necessarily the service entrance.

480V service entrance -> step down transformer-> 120V receptacels - the IG just goes to the transformer.
208V service entrance ->UPS->120V panel - the IG just goes to the UPS/Panel.

There have been many improper IG, as well as dedicated ground, systems installed by people trying to get back to the service entrance in a multi-story facility covering several acres.

I think the only people still really pushing IG are the device manufacturers and consultants that design by boilerplate.

 

[hbiss]

IG conductors need to be run to a same location, like a common point or bus at their source, not necessarily the service entrance.

Yes. I only used that as a simple example.

I think the only people still really pushing IG are the device manufacturers and consultants that design by boilerplate.

They still can be useful in audio where shielded cables are still used. But even audio is going IP and fiber.

-Hal