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Author Topic: Isolation vs bonding  (Read 6485 times)

Stephen Swaffer

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Re: Isolation vs bonding
« Reply #20 on: September 29, 2016, 01:09:04 pm »

Something else to consider is that 250.30(A).1 does not preclude the use if isolating transformers that are truly isolated.  It only applies to AC systems that are grounded-and AC systems that are required to be grounded are listed in 250.20-a key word in that being "premises" wiring.

That being the case, I wonder if Scott's device actually has grounded "neutrals".  It is not designed to supply premises wiring.  It would be regulated by UL requirements-which as JR has showed us, are not necessarily as transparent as the NEC.

The reason I started the discussion was the implication that 250.30(A).1 was just stupid when in fact, in the vast majority of cases, it makes the entire system safer.  When certain carefully controlled conditions are met, I agree that it could be a useful tool in eliminating some issues caused by connected grounds.  In most case, those same issues can be resolved in, what I would argue is a safer manner, with other methods.

Unless you are the event organizer, it would be difficult to control all of the variables that would potentially trip up the usefulness of the isolation transformer.

I do not know all of the historical details, but I do know that various grounding schemes including ungrounded systems have been tried and used for various reasons-most being rejected or losing favor becasue of hazards associated with them.

One challenge I am currently trying to resolve in my day job is an electric heating element that intermittently blows fuses due to a ground short.  I find it interesting that electric furnaces are allowed to use ungrounded systems-no doubt because of these types of issues.  My application doesn't fit the definition, so I am going to have to try to correct it another way.  I don't understand the reasons my application doesn't fit-maybe becasue it is too ssmall a niche to be argued for-or maybe becasue there is a real issue with doing so.
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Steve Swaffer

John Roberts {JR}

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Re: Isolation vs bonding
« Reply #21 on: September 29, 2016, 01:40:44 pm »



One challenge I am currently trying to resolve in my day job is an electric heating element that intermittently blows fuses due to a ground short.  I find it interesting that electric furnaces are allowed to use ungrounded systems-no doubt because of these types of issues.  My application doesn't fit the definition, so I am going to have to try to correct it another way.  I don't understand the reasons my application doesn't fit-maybe becasue it is too ssmall a niche to be argued for-or maybe becasue there is a real issue with doing so.
I'm not sure I understand the problem. If the energized element shorts to ground and takes out the fuse that is the proper result. If the power was floating (isolated transformer winding) a short to ground would just shift the floating power relative to that short (until there is another short  :o ).

It seems a more productive solution is to improve the insulation associated with the heating element since an energized chassis is usually unacceptable, and floating power relatively expensive.

A fuse/breaker or GFCI/RCD should both protect against that fault hurting anybody. 

JR
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Mike Sokol

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Re: Isolation vs bonding
« Reply #22 on: September 29, 2016, 02:30:39 pm »

I do not know all of the historical details, but I do know that various grounding schemes including ungrounded systems have been tried and used for various reasons-most being rejected or losing favor because of hazards associated with them.
When I built precision resistors and missile guidance controls for the military back in the 80's, we had an entire section of test gear powered by isolation transformers that were connected to a separate ground rod. It was right in the middle of the building in a little "pit" cut into the concrete floor. We had to dump water and electrolytes in the "pit" every so often and certify the actual ground resistance on a monthly basis IIRC, just as we had to calibrate all of our test gear annually and trace everything back to NBS standards. Any stray ground currents could mess up resistance measurements in the ppm (parts per million) range, and we had to certify many components down to 5ppm or so. But that's the only time I've worked on an AC power distro system with a truly isolated ground.
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Lyle Williams

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Re: Isolation vs bonding
« Reply #23 on: September 29, 2016, 08:03:40 pm »

The equipment we provide not only needs to be safe in its own right, but needs to exist safely in an ecosystem of other equipment.

Your equipment needs to be grounded so that it can be "a good electrical citizen" and clear faults in other people's systems.
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Stephen Swaffer

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Re: Isolation vs bonding
« Reply #24 on: September 29, 2016, 10:43:18 pm »

I'm not sure I understand the problem. If the energized element shorts to ground and takes out the fuse that is the proper result. If the power was floating (isolated transformer winding) a short to ground would just shift the floating power relative to that short (until there is another short  :o ).

It seems a more productive solution is to improve the insulation associated with the heating element since an energized chassis is usually unacceptable, and floating power relatively expensive.

A fuse/breaker or GFCI/RCD should both protect against that fault hurting anybody. 

JR

I agree-full disclosure and hopefully not hijacking my thread.  A replacement heating element is $1800-and there are 2 in the zone.  The manufacturer is less than helpful in giiving me what the insulation should test/meg at.  We are running on 480 single phase-277 V to ground.  And we blow a $30 fuse every 2-5 weeks.  Hence the head scratching before I just cut a check in hopes of resolving the issue. I won't modify the machine to float the supply ( that would be cost prohibitive anyway-I would need a handful of 90 kW transformers given all the heat used in this application)-but it was intersting to see that resistance heating furnaces are an exception to the grounded power supply requirements.
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Steve Swaffer

John Roberts {JR}

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Re: Isolation vs bonding
« Reply #25 on: September 29, 2016, 11:47:58 pm »

I agree-full disclosure and hopefully not hijacking my thread.  A replacement heating element is $1800-and there are 2 in the zone.  The manufacturer is less than helpful in giiving me what the insulation should test/meg at.  We are running on 480 single phase-277 V to ground.  And we blow a $30 fuse every 2-5 weeks.  Hence the head scratching before I just cut a check in hopes of resolving the issue. I won't modify the machine to float the supply ( that would be cost prohibitive anyway-I would need a handful of 90 kW transformers given all the heat used in this application)-but it was intersting to see that resistance heating furnaces are an exception to the grounded power supply requirements.
Got it... One strategy is the old penny in the fuse box and then monitor for what catches fire... Of course with a big dog heater it might not be that obvious.

Sometimes fuses are faulty.

I've been there before with vendors who can't explain failures that shouldn't happen.

JR   
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Jean-Pierre Coetzee

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Re: Isolation vs bonding
« Reply #26 on: September 30, 2016, 04:27:34 am »

Lets for a moment imagine that that isolating transformer wasn't actually a transformer, but a generator, another valid SDS.  Do we always dig a rod for a ground rod for a generator? 

Even if we do dig in a ground rod, we accept that for a generator, the presence of the ground rod is not to return fault current, as the impedance is far too high to clear a breaker of any reasonable size*.  We achieve safety by bonding all metalwork likely to become energized to the ground conductor,. which is itself bonded to the neutral of the generator.  So, assuming the distribution and stringers are all done right, the guitarists strings are bonded to the SM58, and also to the metalwork of the stage because the parcans have a ground wire and its all bondded back to the generator earth.  That's how we achieve safety.

So.......  if we replace the generator with an isolation transformer, why do we suddenly have to connect our SDS ground to some other ground?????

*] Though the small fault current that does flow is enough to trip a GFCI/RCD, and even without a ground rod, there is usually enough leakage to allow the few mA necessary to cause the trip to flow.  These devices really are lifesavers.

Today I learned this BBS software does not like several question marks on the run, like ????; left to its own devices, you get ????

I think the difference here would be being in the same venue versus being in a separate venue tapping power. Generally if you need to setup a gennie there is no other power available and therefore no chance that two different grounding systems can come into contact with each other through a person, if both grounds are bonded together then they will theoretically be at the same potential although there are things that can affect that as well, if they are not bonded together there can be a quite a lot of potential difference between the two and can pose a shock hazard.

The only time it would be safe is if you can guarantee that the two systems grounds will never be in contact with each other which I feel(and I'm sure the NEC as well) is not very easy to do unless there is quite some distance separating the two.
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Re: Isolation vs bonding
« Reply #26 on: September 30, 2016, 04:27:34 am »


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