Several Isolation transformers & Isolation Grounding.
Several Isolation transformers & Isolation Grounding.
(OP)
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.
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.






RE: Several Isolation transformers & Isolation Grounding.
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?
RE: Several Isolation transformers & Isolation Grounding.
RE: Several Isolation transformers & Isolation Grounding.
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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RE: Several Isolation transformers & Isolation Grounding.
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
RE: Several Isolation transformers & Isolation Grounding.
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.
RE: Several Isolation transformers & Isolation Grounding.
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.
RE: Several Isolation transformers & Isolation Grounding.
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
RE: Several Isolation transformers & Isolation Grounding.
RE: Several Isolation transformers & Isolation Grounding.
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