ground loops

[ohmhead]

Question I have two 4000amp main switchboards each at 480 volts 3 phase .
I have a tie breaker common to both and each is fed from separate utility transformers . When installing the grounding electrode conductor is two separate grounding electrode conductors required one to each service or can one be run to just one switchboard if the ground bars are common to each switchboard and tie breaker .

When grounding electrode conductor is attached to one or both ?

Is it attached to main breaker section or any section close to bonding jumper ground bar or in any location in switchboards ?

 

[don_resqcapt19]

Take a look at 250.24(A)(3).

 

[ohmhead]

So looking at 250.24 A (3) I read dual fed double ended switchboards with a tie breaker a grounding electrode conductor must be applied to both . Because it states from each power source but the first part states a single grounding electrode conductor is miss leading . So from my grounding system ground rods should I run one GEC to the tie breaker or run one to each main sections .

 

[JoeStillman]

Some ground-fault protection schemes for double-ended switchgear rely on a common bonding jumper for both transformers. You need to be sure you understand your ground fault scheme before you decide how to ground your neutrals.

 

[ohmhead]

Some ground-fault protection schemes for double-ended switchgear rely on a common bonding jumper for both transformers. You need to be sure you understand your ground fault scheme before you decide how to ground your neutrals.

OK can you explain this in more detail what GFI scheme would be needed for installing a grounding electrode conductor in the last section of say one end . Now if it was not install in the main section say the branch section would that be a problem if all the ground bars were connected to all sections . Would this be a violation of code and would it effect the trip of the GFI in others down stream .

 

[Smart $]

OK can you explain this in more detail what GFI scheme would be needed for installing a grounding electrode conductor in the last section of say one end . Now if it was not install in the main section say the branch section would that be a problem if all the ground bars were connected to all sections . Would this be a violation of code and would it effect the trip of the GFI in others down stream .

This is getting into a lot of equipment design specifics which we can't answer unless you give us all the necessary details. If you have to ask these questions, those details will have to include essentially all manufacturer and engineering documentation (or links thereto).

Assuming the equipment was already pre-engineered and ordered per spec' for your installation... the easiest way to put it is tie one GEC in the tie section/enclosure. It can't be that much more wire that you would push the limit to possibly getting flagged on inspection.

 

[ohmhead]

This is getting into a lot of equipment design specifics which we can't answer unless you give us all the necessary details. If you have to ask these questions, those details will have to include essentially all manufacturer and engineering documentation (or links thereto).

Assuming the equipment was already pre-engineered and ordered per spec' for your installation... the easiest way to put it is tie one GEC in the tie section/enclosure. It can't be that much more wire that you would push the limit to possibly getting flagged on inspection.

Thanks for the input just a simple question if you had a double fed switchboard the grounding electrode conductor on any project would be connected to the first disconnect .

Details on project on a forum would not be wise .

Just a general simple question would in any case the GFI be effected on trip if the GEC was not close to the main section or disconnect .

 

[Smart $]

Thanks for the input just a simple question if you had a double fed switchboard the grounding electrode conductor on any project would be connected to the first disconnect .

Details on project on a forum would not be wise .

Just a general simple question would in any case the GFI be effected on trip if the GEC was not close to the main section or disconnect .

Other than connecting a GEC in the Tie Section, you can connect a GEC to any accessible point on the service entrance grounded conductor. For separately derived systems, in either the source or the disconnecting means enclosure (if in separate enclosures, and in whichever enclosure the SBJ is located). Preference is to do it the same for both supplies... but not required by Code.

GFI shouldn't be affected by where the GEC(s) are connected. Without project specifics, I'll neither confirm nor deny I said that.

 

[ohmhead]

Question double ended switchboards which end to EGC

Question double ended switchboards which end to EGC

Other than connecting a GEC in the Tie Section, you can connect a GEC to any accessible point on the service entrance grounded conductor. For separately derived systems, in either the source or the disconnecting means enclosure (if in separate enclosures, and in whichever enclosure the SBJ is located). Preference is to do it the same for both supplies... but not required by Code.

GFI shouldn't be affected by where the GEC(s) are connected. Without project specifics, I'll neither confirm nor deny I said that.

Ok so if lets say we had a 7 section double ended gear each separate and fed by two different utility transformers.
center a tie breaker installed .
we have 3 sections gear 1# and 3 sections gear 2# tie breaker in the middle of both . Now the GEC its was connected to the load section of the most outer section in section 1# .

Now the bonding jumpers are made up in each main breaker section of each service and the neutral and ground bus are connected to both ends via the tie breaker section which connects all sections 1# & 2# together from one end to the opposite end .

What if you had a ground fault in one end of the double ended gear furthest away from GEC end which was installed in the furthest load section .

One single GEC connected to the opposite load side end section with out any GEC connected in its main service disconnect section.
Would this effect the fault or GFI tripping or be a issue of any kind beside a code violation on connection to first disconnect means .

Would then code require both main brk sections to have a GEC or just one as to say one single GEC a loop over to the other service disconnect .

 

[Smart $]

Ok so if lets say we had a 7 section double ended gear each separate and fed by two different utility transformers.
center a tie breaker installed .
we have 3 sections gear 1# and 3 sections gear 2# tie breaker in the middle of both . Now the GEC its was connected to the load section of the most outer section in section 1# .

Now the bonding jumpers are made up in each main breaker section of each service and the neutral and ground bus are connected to both ends via the tie breaker section which connects all sections 1# & 2# together from one end to the opposite end .

The issue here is mostly literal interpretation versus one of physics (electrical). While the grounded service entrance conductors and both the grounded and grounding bus through the main bonding jumpers, and the GEC are all connected together electrically, under the NEC they are each distinguished by literal definition as a different conductor.

GECs are required to be connected to the service entrance grounded conductor directly, not through other conductors (in the literal sense). The NEC makes exception to that rule for dual fed services to connect a single GEC to the grounded conductor in the tie section/enclosure.

What if you had a ground fault in one end of the double ended gear furthest away from GEC end which was installed in the furthest load section .

The GEC/GES is not intended to be a path for for fault current. It does carry some current, both normal and fault, simply because it is connected. The majority of ground fault current should pass through the main bonding jumpers.

One single GEC connected to the opposite load side end section with out any GEC connected in its main service disconnect section.
Would this effect the fault or GFI tripping or be a issue of any kind beside a code violation on connection to first disconnect means .

Shouldn't effect OCP fault tripping, but it may affect GFP trip... depends on the GFP detection scheme... and we're getting back into specifics here.

Would then code require both main brk sections to have a GEC or just one as to say one single GEC a loop over to the other service disconnect .

You can have a single GEC, and make GEC taps to each main [250.64(D)]. You can't land a GEC in one main, on one lug, then from another lug, or the same 2-conductor lug, run a jumper to the other main. The only other option is to run individual GECs. Also note that when running a common GEC, it must be sized for the combined circular mil area of both services.

 

[ohmhead]

The issue here is mostly literal interpretation versus one of physics (electrical). While the grounded service entrance conductors and both the grounded and grounding bus through the main bonding jumpers, and the GEC are all connected together electrically, under the NEC they are each distinguished by literal definition as a different conductor.

GECs are required to be connected to the service entrance grounded conductor directly, not through other conductors (in the literal sense). The NEC makes exception to that rule for dual fed services to connect a single GEC to the grounded conductor in the tie section/enclosure.


The GEC/GES is not intended to be a path for for fault current. It does carry some current, both normal and fault, simply because it is connected. The majority of ground fault current should pass through the main bonding jumpers.


Shouldn't effect OCP fault tripping, but it may affect GFP trip... depends on the GFP detection scheme... and we're getting back into specifics here.


You can have a single GEC, and make GEC taps to each main [250.64(D)]. You can't land a GEC in one main, on one lug, then from another lug, or the same 2-conductor lug, run a jumper to the other main. The only other option is to run individual GECs. Also note that when running a common GEC, it must be sized for the combined circular mil area of both services.

Thanks Smart $ That's what I was looking for I had a debate with my PM over what was correct and what was not.
Someone tied the GEC to only one of the dual sides . It was tied first to a ground bar in room and then a mechanical lug was used to connect to the load section in one side of the double ended gear .

It was wrong imo first it was not continuous second it was a NEC code violation because that's not the first means of disconnect then they didn't tie the other double ended gear main with that GEC they used the common ground bar internal they never had a wire or GEC connection on the other main first means of disconnect per code .

Now I have current loop issues also on the ground main bar 3 amps to 5 amps flowing on other grounds which go to transformers and other ground bars in this building I know you can have milliamps which is common but when a clamp on ground rod tester shows 5 amps or more it shows a fault .

Also I don't like this but I maybe wrong have you ever used Lightning protection ground rods on a service main ground system they tied the lightning down rods to the main service . Meaning no service ground rods were installed for the service I would think this is wrong . Doesn't the NEC require a ground rod independent system you would think a Lightning strike would go directly into the gear .

This a job that I didn't start but must fix the amps flowing on the ground bars are odd and I don't have any open neutrals .

Whats you thoughts ?

I know its common practice to tie a grounding system together with the counterpoise LP but I never used there ground rods I always install my own rods in a delta system and then tie it to there LP system .

 

[Smart $]

I'm not familiar with LP systems and GE interconnection pros and cons... but IMO, Code is specific that all present GE's must be connected to the electrical GES, and I know of no exception for GE's of LP systems. The main thing to watch out for is the GES is not connected to the grounded conductor at two separate points of the same system. That will create voltage gradients across the equipment grounding system. I don't see any problem with the LP GE's being tied into the GES as long as the single point connection to the grounded conductor is maintained.

As to the the grounding current issues, I'd correct the GES connection(s) to Code compliant, then go from there. But I do understand you have to deal with boss(es)...

 

[ohmhead]

I'm not familiar with LP systems and GE interconnection pros and cons... but IMO, Code is specific that all present GE's must be connected to the electrical GES, and I know of no exception for GE's of LP systems. The main thing to watch out for is the GES is not connected to the grounded conductor at two separate points of the same system. That will create voltage gradients across the equipment grounding system. I don't see any problem with the LP GE's being tied into the GES as long as the single point connection to the grounded conductor is maintained.

As to the the grounding current issues, I'd correct the GES connection(s) to Code compliant, then go from there. But I do understand you have to deal with boss(es)...

Thanks Smart $ I guess the biggest thing now is 3 or 4 amps shown on the ground bar connections which run from one electrical room to the next . funny part is if you test for voltage theres is no voltage on these loops just current which is shown on a clamp on amp meter .

 

[GoldDigger]

If you have a wire with .1 milliohms resistance and only 3 or 4 amps, you will have a voltage if only 3 or 4 millivolts. Sounds consistent to me.
Basically return current from a load fed by either electrical room or from somewhere else entirely is flowing in the GES instead of the neutral.
Also measure the GEC current on either side to get more information.
There may not be a practical way to decrease it further.

 

[Smart $]

Thanks Smart $ I guess the biggest thing now is 3 or 4 amps shown on the ground bar connections which run from one electrical room to the next . funny part is if you test for voltage theres is no voltage on these loops just current which is shown on a clamp on amp meter .

No voltage (or rather an insignificant magnitude) is as GoldDigger pointed out.

Is this current affecting GFP?

Without knowing the exact arrangement of supplies and grounding connections, it's hard to say whether this is objectionable or inadvertent current.

When you say ground bar connections from one electrical room to the next, do these other rooms also contain service equipment with grounded conductors. With Code grounding requirements as they are, inadvertent grounded conductor current will be evident on grounding conductors between service equipment.

If these other electrical rooms are supplied from the OP equipment, you may actually have a ground fault somewhere... perhaps an improper grounded to grounding bond, or an actual grounded to grounding fault. Could also be an inadvertent grounding path paralleled with grounded conductors. For example, communications service that is bonded to both building and utility MGN. Grounded conductor current goes through MBJ, building GES to communications bond, through bonded communications service conductor to utility MGN, then back to grounded conductor at transformer.

 

[ohmhead]

Yes were in the millivolt range not the same measurement each time my thoughts were this is some return going from one room to the next room each has a ground bar . So basically there is a grounding riser in building which starts at main electrical room and connects to each other room to a ground bar in each elec rm . This ground bar is used to tap or ground each transformer down stream a connection to the transformer case and in the start the main electrical room this is attached to building steel cold water ground rods lightning protection counterpoise uffer ground. There is no spark when we lift these off ground bar no high voltage just milli volts but the clamp shows 3 or 4 amps on a few connections .

 

[Smart $]

Being on the load side of the MBJ's indicates it is objectionable current. Tracking down the cause can sometimes prove difficult and/or very time consuming.

 

[ohmhead]

Being on the load side of the MBJ's indicates it is objectionable current. Tracking down the cause can sometimes prove difficult and/or very time consuming.

Well whats interesting is the building is 100 % led lighting which have drivers on all branch lighting .
There are all VFD,s on 100 % of AHU,s and motors in building. Another issue is the typical single phase loads on each phase are so out of balance due to the engineered plans on the panel schedules. Since its a new project we go by contract drawings we don't deviate from what is contract . Heres the kicker Smart on one distribution switchboard emergency A phase is 1012 amps B phase is 435 amps C phase is 389 amps so the neutral is loaded and the balance is off .
This is do to the engineer plans AHU,s on emergency load . We didn't know each unit has a single phase electric strip heater built internal in each three phase AHU unit which when used it loads up the neutral. So its a tuff balance issue to fix.
In addition this is not the only unbalanced switchboard theres a few and its being looked at by engineer now as a big problem .

Could unbalanced phases cause the grounding connection to see small current leakage to system grounds and also with double fed switchboards with independent service transformers some how share this return ?

 

[Smart $]

...

Could unbalanced phases cause the grounding connection to see small current leakage to system grounds and also with double fed switchboards with independent service transformers some how share this return ?

Higher neutral current generally correlates with higher inadvertent current on grounding conductors.

Discerning between objectionable and inadvertent current on grounding conductors is, or can be a mind-boggling task.

 

[ohmhead]

Higher neutral current generally correlates with higher inadvertent current on grounding conductors.

Discerning between objectionable and inadvertent current on grounding conductors is, or can be a mind-boggling task.

Yes it is mind boggling I tested with another amp probe today 7.7 amps 3.45 amps over a period of time as the loads changed so did the ground bar grounds in amperage . I tested for voltage but removing the highest amp read ground and testing from ground bar to ground it was 13 millivolts at 7.7 amps I wish I could post a photo to show this .

I just don't see 7.7 amps at 13 millivolts no spark nothing . I don't see this as real current flow how could I touch this wire with my hand to bar and not feel anything in fact the millivolts went up to 24 millivolts from finger tip via meter test leads.

Now I have a Extech DVA 30 current sensor and it lit up bright on the grounds this is proof there is current flow but there is not enough voltage to make a spark .