Usefullness of Isolated Ground Circuits 2

I have no references for you, but FWIW, I've never seen isolated ground circuits used in an office building. Sometimes the owner insisted on their use in a dedicated computer room. I'm sure you're aware that these isolated ground circuits must still be connected to the building ground at some point.

The switching power supplies in the computers are creating most the noise in most offices anyway.

I've only ever seen isolated ground receptacles / circuits in hospitals and bowling alleys...

Best to you,

Goober Dave

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Sheesh, is anyone still requiring isolated grounds? I thought this ridiculous practice had died out in recent years.

OK, I gotta ask: why bowling alleys? I've seen them spec-ed and used for entertainment sound equipment to supposedly eliminate ground loops and 60Hz hum injection into mic lines. It works, sorta, sometimes, kinda.


SceneryDriver

DRWeig, fwiw isolated grounds are no longer allowed in hospitals starting with NEC 2011.

Yay on the retraction of IGs in hospitals thing! They're expensive and also a pain.

SceneryDriver, in the not-too-distant past I had to design electrical for a couple of bowling alleys. There seems to be a major static discharge concern back in the pinsetter area. Think rubber balls sliding down a plastic-coated lane into plastic-coated pins. The auto scorekeeping manufacturer's engineer advised me that the power ground got mighty noisy from discharging all that static, and he didn't want it showing up on the signal cable shields or the chassis grounds. They pulled the IG directly from a ground bus on the service entrance bond with big fat wires, and apparently it did away with most all of the static discharge spikes.

So we had IG receptacles inside the scoring table, up above the approach area, at the cash registers, and in the video arcade. Supposedly it helped a lot... I hope it wasn't just a waste of money.

Best to you,

Goober Dave

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A quick Google search didn't reveal too much. I can't say for certain that any recent studies have been done regarding IG receptacles in office environments, but I tend to agree with your suspicion. The only times we incorporate them into our designs are when the owner or the IT/technology consultants insist on them. Their use is usually confined to IT/IDF/MDF rooms. Most of the other EEs that I've discussed this topic with feel that the use of IG receptacles is an outdated practice that serves little purpose in modern facilities.

When grounding of branch circuit equipment was first required, grounding and bonding methods left a lot to be desired. BX (armoured) cable did not have a grounding strip. The armour was used as the ground conductor. After a few years of service, with either aluminum or steel armour, surface corrosion would make the turn to turn resistance from turn to turn higher than the resistance around the loop of armour. The armour became an induction coil with the added resistance of the strip of armour stretched out flat.
EMT or thinwall conduit used indent type couplings. These were a straight sleeve type coupling that was indented into the conduit in four places by a special tool. It just took a little bending for the connections to become a little loose. Mechanically the couplings would hold even when loose but electrically, after a few years and a little surface corrosion, they were a disaster for electronic signals.
Most small commercial buildings housing supermarkets were wired with a combination of thinwall conduit and armoured cable.
Over the years grounding and bonding methods were improved to the point that they were the equal of isolated ground circuits.
The first problems that I was aware of came with the advent of Point Of Sale devices (Cash registers) with a data link to a computer.
The designers assumed that a ground connection was automatically a secure connection to the ground mass of the earth. Despite a clause in the North American codes of the time that no equipment may depend on a ground conductor for its operation except for grounding devices, data designers saved a wire by using the equipment ground as a signal return path.
Through code committees the data people lobbied for changes in the Codes to allow isolated grounds.
Concurrent with this code grounding methods were being upgraded. Although many data installations in older buildings needed upgrades to the grounding and bonding systems to function reliably, bonding methods in new construction was generally suitable for dependable data transfer.
I suspect that many data problems were blamed on grounding when the actual problem was elsewhere. When the problem was eventually located no-one jumped up and said;
"Hey! I made a mistake! The grounding was good all the time!!"
After a long career in construction I have a few anecdotes to support these assertions but no studies.
A good study or two may have saved us a lot of wasted time and money on isolated grounds.
I will be the first to admit that there are a very few exceptions where isolated grounds are a good thing, such as in operating theaters.
There have been studies years ago when explosive anesthetics were used when the combination of an explosive gas and a high frequency scalpel caused a patients lungs to explode on the operating table with concurrent injury and sometimes death of the operating team.
This was surely a case for small transformers with grounded inter-winding shields and isolated grounds.
For most data systems a well installed code ground has been the functional equal to an isolated ground for many years. (like 20 or 30 years).

Bill
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"Why not the best?"
Jimmy Carter