chasing a ground problem

[laz]

Have a multi complex apt. building with the main disconnects and meter pacs in a single mechanical room. They leave the room running down into a slab on grade first floor. All conduits leaving the room are 1 1/4 GRC. They then go underground to the apt they feed. three story building so some go thru the walls and flooring up from the main floor. Some runs are in excess of 200 feet.......52 unit apt. building built in 1968-69.

I am having a grounding issue and am wondering if some of the buried ridged conduits have somehow come apart and not making a good ground anymore..
Is there ideas on how to check if this is the case? Can a megger be hooked up to the conduit at one end and then the other to see if the readings are the same. Or will the voltage from the megger just go straight to ground without running the length of the conduit to the apt. ? Which is what i envisioning happening, but i have only used meggers on testing insulation values.

Or is there any instrument/s out there that will do the above............or any other ideas on how to prove the conduit has and is getting a good ground......other than just continuity.

thanks for any help you can contribute....

 

[mopowr steve]

Is it possible to get a roll of some #12 THHN and connect it to the service neutral and up to said apartments to see what ohm reading you get from EGC at apartment panel and temporary ground wire. I guess I would really question things if above 10 ohms unless you can figure total ohm’s per foot of # 12 and ohms per foot of GRC.

 

[ptonsparky]

Are you looking for an open Equipment Ground, not an open neutral?
What made you look for a poor return path of an EG via the GRC?

 

[don_resqcapt19]

The traditional use of the term "megger" is for an insulation resistance tester. I don't think that is what you want to do.
If you are trying to verify the conduit as the EGC, you can measure the resistance between the neutral at the panel and the panel feeder conduit with feeder circuit off. It should be pretty low. However that does not mean that the path is sufficient for fault clearing.

What type of "grounding" problem are you trying to resolve?

Is it possible to get a roll of some #12 THHN and connect it to the service neutral and up to said apartments to see what ohm reading you get from EGC at apartment panel and temporary ground wire. I guess I would really question things if above 10 ohms unless you can figure total ohm’s per foot of # 12 and ohms per foot of GRC.

!2 AWG copper is about 5 ohms per 1000 feet, so with his longest run being 200 feet, that would be 1 ohm. I would expect that the conduit would have a lower resistance than the copper, if it is in good condition, and so I would want to see a reading of less than 2 ohms.

 

[Cow]

Buried rigid and the PNW, do they know what happens to rigid in wet climates?

Technically, I'm in the PNW, but it's eastern Oregon and it's a desert here with little rainfall. Rigid still rusts but it takes many years to get there. With your install being 51 years old, that's not going to be a good thing.

Are they using the rigid as the ground? Only 3 wires in the conduits?

 

[laz]

We are getting anywhere from 50--60 volts to ground thru the coax cable. 8 demarc boxes around the building feeding 6 units each and a 9th that feeds the rest. When this first came up there was one that read 122 volts to ground and bite a cable guy. They called the power company and they checked their neutral and did find some deterioration and actually pulled another neutral into the underground vault that feeds the complex. They also put a tester on one of the open meter slots and ran back over 800 volts from each phase ( three phase service) back to the neutral with no problems.

The one that read 122 volts was cleared from a 15 amp circuit with a loose wirenut on the neutrals. But here is the kicker............when the unit that read this...when that units power was turned off ...it still read the 122 volts....this is a 3 story building and the unit’s power that, when turned off, was two storied above the one where we were reading the 122 volts.....the coax in all units i have checked go from the demac box to a splitter on the outside wall and a run goes into the bedroom on the same wall about 3-4” away and the second run goes past that drop a couple more feet and goes into the living room......two only...

These wires read voltage when removed from the splitter in the demarc box so the only place the other end goes is in each individual apt. So how’s it getting voltage from the units power 2 stories above it...?????

Yes it is a three wire system feeding each unit. At the meter base location i have “redundantly” grounded the meter pacs and related equipment there. The 1 1/4 conduit leaving the meter pacs do not have either a grounded bushing nor a grounding loc nut. They are double lock nutted one on each side of the encloser..... And I do not think the GRC’s integrity is good enough for fault clearing.......

I am sure that trying to pull out the three feeders and reinstalling a 4 wire run would not be possible. Maybe some of the runs would come out but i’d bet most would just break the wire in mid run in the conduit...

So what’s the fix???

Only thing at this point i can think of is to just install a bonding jumper at each panel and label all the equipment that it has been done that way.........i know, i know, not code but seems to me to be a whole lot safer than no return for the ground at all.....and yes i would have to clear it with the state before i did this........if i do.....

Thoughts????? and thanks the help.............

 

[ATSman]

Have a multi complex apt. building with the main disconnects and meter pacs in a single mechanical room. They leave the room running down into a slab on grade first floor. All conduits leaving the room are 1 1/4 GRC. They then go underground to the apt they feed. three story building so some go thru the walls and flooring up from the main floor. Some runs are in excess of 200 feet.......52 unit apt. building built in 1968-69.

I am having a grounding issue and am wondering if some of the buried ridged conduits have somehow come apart and not making a good ground anymore..
Is there ideas on how to check if this is the case? Can a megger be hooked up to the conduit at one end and then the other to see if the readings are the same. Or will the voltage from the megger just go straight to ground without running the length of the conduit to the apt. ? Which is what i envisioning happening, but i have only used meggers on testing insulation values.

Or is there any instrument/s out there that will do the above............or any other ideas on how to prove the conduit has and is getting a good ground......other than just continuity.

thanks for any help you can contribute....

Like you said, a megger is used to test the integrity of the insulation system for wires, cables, motors, xformers, etc.
The instrument you want to use is a Ductor (low resistance ohmmeter.) The one we use is made by Biddle and named
Digital Low Resistance Ohmmeter (DLRO) It will measure resistance down to 1 micro-ohm (1/1,000,000 of an ohm.) see attached pic
We use it mainly to measure the main contact resistance on ATS's and circuit breakers. There are many other brands and models available.
They will verify the continuity of grounding systems as in your case. Also can be used to check the torque on the bus bars in switchgear and
bus ducts & bussways. It is also stated in the NETA specs in measuring the winding resistance on large power transformers (shorted winding test.)

 

[electrofelon]

The problem is not a bad or insufficient EGC.

 

[ptonsparky]

Do you think the conduit is energized?

Double locknuts are an adequate bond.

 

[texie]

The problem is not a bad or insufficient EGC.

 

[AKElectrician]

Test your bond by taking a denergized hot to ground temporarily and check to see you got continuity on the other end.

 

[Fred B]

Like you said, a megger is used to test the integrity of the insulation system for wires, cables, motors, xformers, etc.
The instrument you want to use is a Ductor (low resistance ohmmeter.) The one we use is .........Also can be used to check the torque on the bus bars in switchgear and bus ducts & bussways. It is also stated in the NETA specs in measuring the winding resistance on large power transformers (shorted winding test.)

I know this ? will be off from OP but, how are you using an ohmmeter to check torque? Torque is a mechanical measurement.

 

[ptonsparky]

I know this ? will be off from OP but, how are you using an ohmmeter to check torque? Torque is a mechanical measurement.

i could certainly put a piece of plastic between the conducting materials and torque it to 250 in lbs.

SEWAG, there would be some measurable conductance between clean, properly torqued connections.

 

[Fred B]

i could certainly put a piece of plastic between the conducting materials and torque it to 250 in lbs.

SEWAG, there would be some measurable conductance between clean, properly torqued connections.

What sort of difference in R value would you get between say 250inLbs and 150 to tell you that you've achieved listed torque value?

 

[ptonsparky]

What sort of difference in R value would you get between say 250inLbs and 150 to tell you that you've achieved listed torque value?

Look back...Semi Educated Wild Ass Guess. IDK what the value would be. It would depend on the size and material of the buss bar. Another SEWAG.

We know that improperly torqued connections have a higher resistance value. We see it in heat damaged connections all the time.

 

[laz]

RE............. The problem is not a bad or insufficient EGC.

what "is" the problem then sir........??????


RE.................
Do you think the conduit is energized?

Double locknuts are an adequate bond.

No, i don't think the conduit is energized. Except by the stray voltage found in the coax.

Yes, i agree that the lock nuts are more than adequate.....

How many 50 year old galvanized water pipes are still in existence and working today?????
I think a good majority of the conduits are rusted through in multiply places.....
Having said that how can it be fixed....i doubt that the wire would ever be able to be pulled out in one piece....let alone ever be able to pull new feeders in......so what's the fix.....these are under a slab on grade.... and under a three story structure....

 

[RichB]

RE............. The problem is not a bad or insufficient EGC.

what "is" the problem then sir........??????


RE.................
Do you think the conduit is energized?

Double locknuts are an adequate bond.

No, i don't think the conduit is energized. Except by the stray voltage found in the coax.

Yes, i agree that the lock nuts are more than adequate.....

How many 50 year old galvanized water pipes are still in existence and working today?????
I think a good majority of the conduits are rusted through in multiply places.....
Having said that how can it be fixed....i doubt that the wire would ever be able to be pulled out in one piece....let alone ever be able to pull new feeders in......so what's the fix.....these are under a slab on grade.... and under a three story structure....

In Seattle area here---WSDOT requires a ground/bond wire in ALL conduits due to the buried RGS deteriorating after being buried--I for one will not trust that a circuit still has a usable EGC if it is using the conduit as an EGC--seen way too many that are rusted out and/or the threads are corroded

 

[ptonsparky]

RE............. The problem is not a bad or insufficient EGC.

what "is" the problem then sir........??????


RE.................
Do you think the conduit is energized?

Double locknuts are an adequate bond.

No, i don't think the conduit is energized. Except by the stray voltage found in the coax.

Yes, i agree that the lock nuts are more than adequate.....

How many 50 year old galvanized water pipes are still in existence and working today?????
I think a good majority of the conduits are rusted through in multiply places.....
Having said that how can it be fixed....i doubt that the wire would ever be able to be pulled out in one piece....let alone ever be able to pull new feeders in......so what's the fix.....these are under a slab on grade.... and under a three story structure....

The conduit is not energized by the voltage in the coax. It is your ground or earth reference.

 

[Fred B]

Look back...Semi Educated Wild Ass Guess. IDK what the value would be. It would depend on the size and material of the buss bar. Another SEWAG.

We know that improperly torqued connections have a higher resistance value. We see it in heat damaged connections all the time.

Sorry didn't know that acronym, thought that you actually had a definitive means of measuring accurate torque with a megger, and was wondering what that was. Agree with resistance/heat damage statement.

 

[laz]

RE>> In Seattle area here---WSDOT requires a ground/bond wire in ALL conduits due to the buried RGS deteriorating after being buried--I for one will not trust that a circuit still has a usable EGC if it is using the conduit as an EGC--seen way too many that are rusted out and/or the threads are corroded .............

Yes, agreed...........and that's a valid requirement....to bad it's not in the NEC or at least the WAC rules........I highly doubt that it was a requirement in 1968.

I also have seen way to many questionable indoor systems that rely on the conduit for ground just for the fact of lazy people not tightening set screw couplings and or connectors.....way to many....

i am ~80 miles north by north west of seattle..

......so what is the fix....

i am assuming all here know that it could be fixed by putting a bonding jumper in all the panels.....is there any presidence to getting special permission from Olympia to do this......that you are aware of?? the mechanical room is under lock and key with no access to unauthorized personal...

i would imagine it would cost more than the whole building cost new to run 52 new conduits to each unit....