Several Isolation transformers & Isolation Grounding.

[EEAOC]

One of the engineers I work with wants to provide several transformers to establish ISOLATED GROUNDING in a regular office building setup. Is this a standard practice? I was under the understanding that the isolated grounding conductor has to be taken directly all the way to the building’s service transformer neutral or a separately derived service such as building generator. I have had several isolation transformers in hospital setup where each operating rooms has to be on separate isolation transformer via an isolation panel.
I am not sure that several isolation transformers are needed in a standard office building. Can anyone clarify this whether this is done in regular office building? Thank you.

 

[dpc]

The isolated ground must still be bonded to the building ground.

Use of an isolated grounding sysem is not a normal practice in a typical office building. The NEC permits it, but does not require it. The NEC allows a grounding conductor on a branch circuit, such as to a receptacle to be insulated from the metallic boxes and conduits. But eventually, everything must be bonded together.

Does this engineer have a problem he thinks this will solve, or he just thinks it's a good idea?

 

[alehman]

Adding transformers solely for the purpose of isolation for general office equipment is probably not necessary unless there are specific problems to be solved. If you do install transformers, the secondary winding neutrals must be grounded to the building structure and/or service grounding electrode. Transformers create separately derived systems. If you install isolated grounding conductors, the N-G bond of each transformer must be the origin of the isolated ground for branch circuits served by the transformer.

 

[ScottyUK]

This practice isn't uncommon in electrically noisy environments such as power plants and the like, where it is used to prevent mains disturbances typically caused by switching operations on large drives and similar loads. The neutral point of the secondary should be bonded to the main earthing terminal of the building. The transformers would normally have an earthed interwinding screen to reduce capacitive coupling of noise from the dirty side of the transformer.

This sounds overkill for an office building unless the office happens to be fed from the same service as some very heavy industrial loads.



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[mc5w]

The original purposes of isolated grounding were that the coaxial cable Ethernets interconnected the equipment grounds with the result that a ground fault could partially flow through the coaxial cables. Large mainframe computers also had a similar problem because TTL logic would run through ribbon cables from one device to another. By having a single point where the appliance equipment ground was connected to the equipment ground for conduits, cable sheaths and so forth you would get rid of accidental loop antennas and so forth and groung faults were less likely to blow up your computer.

Today the main use of isolated grounding is for sensitive electronic equipments such as nuclear magnetic resonance imaging. In this case the entire room is an isolated grounded appliance and is insulated from building steel and metal conduits except for a single isolated grounding wire. Drives electrical inspectors crazy. I have not read enough of 2005 National Electrical Code to find out if they put an exception into the grounding rules.

Mike Cole, mc5w@earthlink.net

 

[busbar]

 
Another caveat is that most “stock” drytype transformers run blistering hot at nameplate loading, with a rated temperature rise of 150°C. There are cooler-operating {longer delivery} models that have 115° or 80°C rise, but they are significantly more expensive than plain-vanilla “commodity grade” stocked at most supply houses.

 

[rbulsara]

as dpc (and may be others) said..eventually all grounds need to be connceted to building ground (building steel is most common)..

Refer to IEEE/ANSI std 1100 "recommeded practice for grounding of sensitive electronic equipment"

If in NEC is applicable in your area, you can not violate NEC and NEC is right..

To answer your question, most data centers (in the usa) do use multiple stepdwon transformers which also act as isolation transformers..But primarily they are there to step down 480V to 208/120V (or applicable voltages). These tranformers are part of what is commonly known (in the usa) as PDUs (power distribution units) which is a glorfied packaging of a stepdown (and isolation) transormer and branch circuit panelboards.

 

[mc5w]

You also need to have multiple stepdown transformers with various phase shifts to neutralize harmonic currents. SquareD has started making some that can be used in pairs to neutralize the 5th and 7th harmonics.

K-rated transformers no longer work because modern fluorescent ballasts no longer generate enough 5th and 7th harmonic current on the primary side to cancel 5th and 7th harmonic current from computers on the secondary.

Mirus International makes some single output transformers like SquareD does and also makes dual secondary and quad secondary the will allow you to neutralize harmonics up to the 21st if the nonlinear load is reasonably balanced between the secondaries. Their 112.5 KVA Harmony-4 transformer has four 100-amp secondaries that can be hooked up to four 200-amp 3 phase load centers that each use a 100-amp back fed main circuit breaker which is a lot cheaper than a 400 amp panelboard that has an 800 amp neutral.

There are a lot of harmonic mitigation transformers that are by far better than a plain K-rated transformer.

Mike Cole, mc5w@earthlink.net

 

[alehman]

Harmonic cancellation devices and K-rated transformers serve different purposes. K-rated transformers are not intended to reduce harmonics. They will of course short out triplen harmonics, as will any transformer, if there is a delta connected winding. The K rating only indicates ability to withstand the harmonic currents without overheating.

 

[glenn02]

Putting an isolation transformer is a matter of design perspective depending on the expected presence of disturbances in the system. Due to the practice of multi-point grounding in a commercial facility, the presence of normal or common mode noises, impulses ( line to ground, neutral to ground) cannot be controlled. If the deisgn engineer thinks that such disturbances can cause problems in your sensitive electronic equipments, then isolating the sensitive equipment or equipments is an effective alternative to block such disturbances.

For more information about this, I can refer you to the following reading materials:

1) IEEE 1100 Recommended Practice for Powering and Grounding Electronic Equipment
2) TM 5-689 ADP/Computer Electrical Installation and Inspection
3) MIL-HDBK-419A Grounding, Bonding and Shielding for Electronic Equipments and Facilities
4) FIPS 94 Guideline for Electrical Power on ADP Installation
5) EC&M Practical Guide to Quality Power for Sensitive Electronic Equipment