Jim,

I have been tasked with doing a design to change out all 480 to 120/208 VAC and 480 to 240/120VAC transformers in a very large and old building (1945). The transformers are fed from two substations (one new with GF protection and one old without). My management doesn't trust the existing steel bonding and grounding so wants to put in a separate EARTH GROUND system for these new, replaced transformers and panels. My question is this. Wont a separate Earth Ground (different from where the power is fed from) split the possible return path for a Bolted Ground fault and possibly causing problems because circuit breakers may or may not trip? Management will not talk to me about it and will not discuss my concerns without an argument. Is this concern worth questioning? What are the NEC and IEEE requirements? Is there a difference in this being an upgrade and not a newly built building? The building has 7 floors and Subs are on the 1st floor. Transformers being replaced are on 2nd and 3rd floors.

thanks

Put in the new earth ground grid, but bring pigtails to the existing grid, and bond them all together.

I would not change them unless a specific unit has an issue. On the other hand, new dry type transformers 15 kVA and larger are extremely energy efficient (DOE standard) when compared to transformers as little as 15 years old,

We have to be careful in this discussion the terminology can be a problem, most of the following comes from 250.30:

1. Each transformer requires (NEC) an equipment grounding conductor originating at the panel that houses the overcurrent protection for the transformer. This can also be a metallic conduit.

A, Keep in mind, that transformers have inrush currents that are not associated with the load, therefore, the fuse or breaker must be sized correctly. Once you size the fuse or breaker, you then select a wire size that equals or exceeds the OCD rating.

B. If you install a new transformer, do not forget the secondary protection. There are some NEC rules that allow you to omit the secondary protection, however, with three phase transformers they are hard to apply. I spend a lot of time in different plants and I would say the lack of secondary protection is one of the most common NEC code violations I see, The addition of secondary protection also dramatically drops the incident energy on the secondary.


2. Each transformer must have a grounding electrode conductor. The following connection are show in the NEC Handbook, page 184. This connection can be made in the transformer or no further away than the first the enclosure housing the overcurrent devices (OCD) for the load side.

A. There are options, however, in your case it will be building steel and we do not care about perceived continuity/bonding issues. You do not want to establish an earth connection at the transformer by driving a rod. For simple dry type transformer installations in the interior of a building, this practice in many cases can create issues.

There is also a method called a common electrode conductor for multiple transformers this can be found in 250.30(A)(6)

B. The NEC also requires a connection to the closest metal water piping system (250.104). This would be sized the same as the electrode conductor.

Thanks for the responses!

1. The conductivity of steel conduit vastly exceeds EGC's.
2. Depends on how you do ground fault protection. If you use the vector sum of the 3 phase CT's or if you have one big CT where all phase conductors pass through it (variously called a ground fault, BYZ, sensitive Earth fault, or core balanced CT) to measure it directly, basically "in = out". If there is any return path other than through the phase conductors, the residual current is a ground fault regardless of the path it takes and it can be easily detected. In practice the vector sum approach is cheaper because there is no additional hardware but because you are dealing with the sum of the errors of the CT's, the 4th ground fault CT approach is much more accurate. Calculated ground fault currents tend to be fairly noisy.
3. Consider high resistance grounding while you are at it. You can limit ground fault currents to say 10-15 A, enough that it won't destroy anything and the size of the grounding conductor becomes a nonissue but you can easily detect and isolate faulted equipment. At 480 V the equipment to do this is very inexpensive.

To PaulEngr:

Thanks for you input. The High resistance grounding is something I need to BoneUp on!

Out of curiosity, do you have a professional engineer doing the design or providing you with expert advice? If not, I'd suggest you consult the experts. That probably means paying for their time, but that's part of the deal. If the company is spending the funds to upgrade the electrical system, that also means getting the authorities having jurisdiction to approve what you plan, meaning a building permit or facility general permit followed by inspections and approval as part of the process. Failing to get the AHJ to review, inspect and approve modifications is asking for trouble. Should someone be injured in an electrical accident or if property was damaged or destroyed, you can be certain any insurance company and other public agencies will be looking to insure that everything was done properly.

to wilHendrix
Thanks for the good technical advice.

Thanks everybody. This helps.

The permit process is for protection of the public, not private business.



Not really. Once construction is complete, they issue an occupancy permit. They cannot force an owner to pull permits when they undertake work themselves, whether it's a private homeowner or a business. And frankly when I have pulled permits (I think 3 times in 25 years) for an industrial facility, the inspectors always left with a learning experience rather than the other way around.



Businesses are routinely inspected by all manner of federal and state agencies, AND if they carry business loss insurance which is what you are talking about, they will be inspected by private insurance inspectors. I know that Hartford and FM in particular have some pretty tough inspectors until you figure out where they publish their standards and simply follow those. If it's commercial though and all they carry is property and general liability insurance then there is little to no inspection done for the most part until something happens. Then you get a claims adjuster sniffing around making sure everything was done to Code. Or if it's an injury, you get an OSHA inspector sniffing around too.

And finally at least for industrial plants, think of the practical side of things. They have their own maintenance staff or contract one. Almost no matter how small they are, they are constantly making modifications and capital improvements almost 12 months out of the year. It's rare to find a plant that isn't under almost constant construction. They have access or have their own engineering staff, too. Some states require licensing for electricians though it's usually just a revenue grab by the state. They know what quality work looks like. So unlike commercial/residential industrial plants are inspected far more often than a local Code inspector could ever dream of, and have the staffing to where they vastly exceed the capabilities of the local inspector when it comes to self policing.