Transformer Xo bonding in concrete building

[dirk]

I have a client who owns a concrete high rise building. There is no means of bonding transformer xo terminals to ground. None of the transformers in the building now have connections to ground at the xo terminal. A new transformer will be installed on the 41st floor as part of this project. I am trying to explain in layman?s terms why the transformer needs to be bonded without just saying it is code required. Specifically if a grounding electrode is to be installed as part of this installation why it would be best to install an electrode system up through the floors so that all the other transformers can be effectively bonded.
Can anybody be of assistance?

 

[dereckbc]

Re: Transformer Xo bonding in concrete building

Dirk, the Xo connection purpose is the same as any service GEC connection. To provide a discharge path for lightning and static, stabilize system voltage in reference to earth, and provide a planned fault path in the event of accidental contact between the ungrounded conductors and building steel, water pipes, and any metallic objects in the imediate area.

 

[madmike]

Re: Transformer Xo bonding in concrete building

Dirk,

I would have to assume that all of the transformers were intended to be installed as "derived" voltage systems. In any case, with the XO terminal floating, a major safety hazard exists. An excellent article on building grounding can be read and downloaded from the guys at PowerCet, a Power Quality company. See http://www.pqtoday.com/downloads/pqtsum97.pdf
I don't believe it will be necessary to install an entire grounding system. The building steel which is encased in concrete should do fine. A possible approach would be to Cad-Weld a ground fitting to a building column I-beam adjacent to each transformer. Then bond both the transformer cabinet and the XO terminal to the building steel.
r,
Mike

 

[charlie]

Re: Transformer Xo bonding in concrete building

Mike, why would you bond both since these are all SDS?

 

[dirk]

Re: Transformer Xo bonding in concrete building

Mike,
The building is poured concreteand & rebar. No I-Beams. I do not think that chipping away the concrete at a column and attaching to rebar would be acceptable.

 

[websparky]

Re: Transformer Xo bonding in concrete building

OK......Let me see if I have all of the info.

Concrete multi-story building with electrical service.
There must be a "main" electrical service disconnect somewhere that is grounded somehow to something.

If I am understanding this correctly, there are transformers on each floor to supply electricity to the wiring system on each floor.
When this building was built, nobody installed a grounding conductor with the phase conductors that travel up the building to each floor.
When the transformers were installed, somebody ran the phase conductors to the transformers and did not run a grounding conductor with them.

Do I have this right so far?

 

[pierre]

Re: Transformer Xo bonding in concrete building

Dirk
I am with Dave here, something does not sound right. I do not see how these transformers can operate safely without the proper bonding. What is stabilizing the voltage of the separately derived system?
It would seem to me that the equipment grounding conductor run with the feeders ( at least I hope there is one), is acting like the grounding electrode conductor(?)

Pierre

 

[dirk]

Re: Transformer Xo bonding in concrete building

There are no grounding conductors. The EMT is being used as the ground. A bonding jumper is being installed from a bonding bushing to the Xo terminal.

 

[dereckbc]

Re: Transformer Xo bonding in concrete building

Dirk, if I understand you correctly, you have got a problem. I have worked a lot in high-rise environments, and there are a few solutions.

1. The best is building steel, but you have indicated it is not available.

2. Would be the water pipe, but being on the 41st floor I doubt it's integrity.

3. Run a riser cable from the electrical service up the service shaft to facilitate transformer connections.

Generally speaking the architect would run some sort of ground bus or cable up the service shaft and bond it to building steel on every floor, but not always.

The best thing I can thank of to do is run a 3/0 riser cable to at least your location to facilitate transformer Xo connection. Then I would bond to the water pipes in the immediate area. This is by no means the best solution as it will leave grounded objects at earth potential during a lightning strike, while the 41st floor would be several thousand volts higher since you cannot bond to building steel on the 41st floor.

Anyway you do have a code problem not to mention the rest of the transformers located in the building. Good luck!

 

[don_resqcapt19]

Re: Transformer Xo bonding in concrete building

Putting the actual code rule aside, what technical benefit is there to providing a grounding electrode conductor sized per Table 250.66 for this type of installation? The primary equipment grounding conductor will provide all of the grounding that is required for the "earth connection" of the transformer secondary. What additional safety would be provided by installing this GEC? Of course any "local" piping or steel would have to be bonded if likely to be come energized, but it appears that these items do not exist in this installation.
Don

 

[dereckbc]

Re: Transformer Xo bonding in concrete building

Don, the only problem I can see is in a high-rise enviroment where lightning strikes the building.

Lets assume you either have a GEC or primary EGC originating in the basement, and is kept isolated as it rises through the building to the upteenth floor and terminated to the Xo of a transformer. This would put you equipment at earth potential. If the building is struck, a voltage divider would form along the length of the building from top to bottom.

So the problem I see is the floor would be at a very high potential (lets say 1000-to-20,000 volts) during a lightning strike while the electrical equipment would be at earth potential (0 volts).

Now to solve that problem I require the Xo to be bonded to the building steel on the floor it is located on and any local piping.

Other than that I agree with your point of view

 

[dirk]

Re: Transformer Xo bonding in concrete building

Gentelmen,
I can assume that we are all pretty much in agreement on this situation. I feel the best way to fix this problem is to install a GEC riser up through the building from the required, "within 5'section" of water piping to the top floor. All the existing Xo terminals and any future transformers, can now attach to this GEC at another time. This is IMHO the best theoretically and code compliant solution. Would we all agree?

 

[brian john]

Re: Transformer Xo bonding in concrete building

One item that was mentioned, but not answered, is the water pipe on the floors with the transformers bonded to the transformer XO. Would recommend this in addition to the 3/0 GEC.

Lighting most likly would kiss the 3/0 GEC good by due to excessive lenght, bends in the conductor.

 

[dirk]

Re: Transformer Xo bonding in concrete building

Brian John,
The water pipe is not grounded to the Xo terminals on any of the floors that do have transformers. Although not sized right now, we most likely will size this GEC at 350kcmil or a 500kcmil. The cost difference should not be to dramatically different.

 

[pierre]

Re: Transformer Xo bonding in concrete building

Dirk
Something just struck me as I was reading further in these posts. You say that this is a concrete building, and at least 41 stories tall. I did not know that a building could be built that tall without steel framing. Are you sure there is no steel framing for this building?

Pierre

 

[dirk]

Re: Transformer Xo bonding in concrete building

pierre,
I was unaware that this could potentially be the case. I am trying to track down a set of structural drawings for the building now. If this is in fact the case then as someone indicated earlier we could chip away the concrete at a column weld on a lug and install a GEC at every XFMR location.

 

[dereckbc]

Re: Transformer Xo bonding in concrete building

Dirk, building steel is the best electrode you can possible use in a high-rise. If possible use it.

 

[bennie]

Re: Transformer Xo bonding in concrete building

I worked on a 15 story apartment building in Los Angeles. This building was constructed from pre cast concrete modules. The rebar, in the module walls was welded to each other. The floors were poured in place.

There was no effectively grounded steel columns.
I do not remember if the rebar was bonded or not. I don't believe it was.

The service to the building was bus duct 2400/4160 running beside the elevator shaft. Taps to transformers were made with plug in switches.
grounding was also by the bus duct.

 

[don_resqcapt19]

Re: Transformer Xo bonding in concrete building

There are reinforced concrete high rise buildings that do not have a steel structure. The only steel in the structure of these types of buildings is the rebar. Look here.

 

[pierre]

Re: Transformer Xo bonding in concrete building

Don
I looked at that site, and still I am not convinced that tall buildings can be structured without steel beams. The site mentions that the building used concrete encased steel beams (for fire proofing) and steel cable - I believe the cable is in the flooring to assist in the swaying of such a tall structure. BTW - the building has 37,000 tons of steel in its construction.

I think you would be hard pressed to find a 40 or more storied building built in the US without steel beams. JMO

My suggestion is to search a little further to determine if in fact there is a steel superstructure in the building.
I would be interested in the result.

Pierre