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Author Topic: Isolation vs bonding  (Read 6167 times)

David Buckley

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Re: Isolation vs bonding
« Reply #10 on: September 27, 2016, 04:06:10 pm »

There's been a ton of experiments about how low current shocks (a few mA) will reduce milk production on dairy farms.

I'm kinda proud that this effect was discovered here in New Zealand, its reference 19 in your linked-to paper.  Phillips, D. S. M. 1962. Production of cows may be affected by small electrical shocks from milking plants. New Zealand Journal of Agriculture, 105:221

I read the paper some years ago, but have never been able to find an online copy.

This problem is very prevalent in the USA, because of the multi grounded neutral distribution system[1]  and the fact that an isolation transformer can't solve the problem because of 250.30(A).1, a solution that works in regions where the NEC does not apply.  Because of that "feature", the USA had to invent Neutral Isolators.


[1] See The Hazardous Multigrounded Neutral Distribution System (PDF), considered controversial and political, because it challenges the status quo of the electrical distribution system, and has a load of rubbish about fees and property rights, but engineeringly, a good read.



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David Buckley

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Re: Isolation vs bonding
« Reply #11 on: September 27, 2016, 05:21:18 pm »

This should theoretically be safe near water where the potential difference between two earth points would be low but could be a shock hazard in dry areas where the difference could potentially be quite high between two earth points .

Yep.

So in the original article the grounds would need to be bonded together unless the op was going to dig in a ground rod every single time he deploys the transformer and even then there could be a potential shock hazard should the two different ground come into contact if that is even up to code.

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 ????
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John Roberts {JR}

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Re: Isolation vs bonding
« Reply #12 on: September 27, 2016, 06:06:37 pm »

 


Today I learned this BBS software does not like several question marks on the run, like ????; left to its own devices, you get ????
They're called emoticons and nobody in their right mind uses 4 question marks in a row, right? 

JR
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David Buckley

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Re: Isolation vs bonding
« Reply #13 on: September 27, 2016, 06:19:19 pm »

But it sill requires everything on the boat dock to be bonded to that systems ground-which is bonded to the neutral for that system.  I am still trying to wrap my head around how this would work in a stage situation.

As I noted previously, imagine instead of a transformer, you had a genset.

Follow the advice here for a floating neutral generator.

It's from boatyville, but the following diagram shows the idea (source)



Where 220V is availble, then a 220 to 110 transformer would be even better.

Now in practice, the better approach to safe stage power would be a single box solution, with a (preferably non-metallic, like a C17) input socket (or cams, for the big boys!) for connection to venue (or generator!) power, and then a standard distro for the secondary side, with the N/G bond in the distro like a service entrance.  The iso transformer should ideally be in a non-metallic enclosure within the distro.

For many constructions of isolating transformer, there is an inter-winding shield, that requires to be connected to the supply ground.  This is the only thing in the distro the supply ground connects to.  Ideally, the isolating transformer should be of the construction that is inherently safe without the grounded inter-winding shield, and that then means there is no ground wire needed to the transformer at all, and thus you don't even need a supply connection to ground; no earth cam!  If its a 220 to 110 transformer, you don't need a neutral incoming cam either; just two hots.

An alternative construction is to have a transformer as a separate box, which has an input socket (or cams) and an output socket(s) or cams.  The enclosure is usually metallic.  The supply ground should be connected to the metallic enclosure.  The secondary N/G bond should be within the enclosure, creating a ground bar, like in a service entrance, and then there are output sockets/cams, protected by breakers and preferably GFCI/RCDs, which have their ground connected only to the isolated ground bar.

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David Buckley

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Re: Isolation vs bonding
« Reply #14 on: September 27, 2016, 06:20:38 pm »

...nobody in their right mind uses 4 question marks in a row, right? 

Obviously, I'm not in my right mind !

Glad we got that cleared up :)
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David Buckley

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Re: Isolation vs bonding
« Reply #15 on: September 27, 2016, 07:12:54 pm »

The IEEE article lists proposed code changes that enumerate 5 conditions that must be met in order for this to be an acceptable practice.

I assume that's this five, and in my comments:

(1) The isolation transformer shall have overcurrent protection on the supply side as required in 450.3.

Very sensible

(2) The isolation transformer shall be provided with a ground fault protection device on the load side.

Even more sensible.  I would myself allow multiple devices, as that makes sense in many situations, and RnR is such a situation. 

(3) Metal enclosure of the transformer shall be connected to the supply side neutral and grounding system as required by 250.4 (A).

Tick.

(4a) The load side neutral and equipment grounding conductors shall be connected together

Tick.

(4b) and grounded on the secondary side of the transformer as required by 250.20(B).

Hmmmm.... 250.20(B) is specific to premises; in the premises context, I'm in agreement this is a sound principle, but in a non-premise environment, not so sure.

(4c) To provide adequate isolation, the installed grounding electrode shall be located at least 6' from the nearest grounding electrode of the supply side and shall be connected to the transformer by an insulated grounding conductor.

If you accept (4b) applies to the context and thus a ground rod (or two) is required, then this is eminently sensible.

(5) The location of the isolation transformer shall be on the load side of the service disconnecting means

Yes, assuming there is a "service disconnecting means", which implies there is a "service".  Without a service, it's all a bit moot.  What I think it is prohibiting is an isolating transformer ahead of the service entrance, which is not unreasonable.

(5 cont) and shall not be below the electrical datum plane.

Had to look up what an "electrical datum plane" was.  Article 682 definitions.  In an environment that 682 refers to, this is entirely sensible, though outside of a 682 environment, the point would be moot.

Now if one looks at these stipulations outside of the 682 environment of pools and marinas, and removes the 682 specific stuff, its still a sensible set of rules for an isolating transformer in any context, be it in an installation with a service entrance, or in a field with a tent over it.

These code changes also apply specifically to boat dock conditions. 
Yes, but they don't need to be.  Its easy to see why this should have been done to marinas and boats decades ago, as people die from this every year, but the wheels of regulation turn slowly.

I possibly see some value in this-but it also requires a non metallic sheathed wiring method to keep the 2 systems isolated

Outa time, need to think about this some more.



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Stephen Swaffer

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Re: Isolation vs bonding
« Reply #16 on: September 27, 2016, 10:55:33 pm »


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?????

Regardless of the seperately derived source-generator or transformer-the NEC requires all grounding electrodes that exist on a premesis to be connected.  I assume that they are viewing the boat dock as a separate premesis in the proposed rule.  The same would be true of a stage in the middle of a field-a separate premesis.

A stage in a venue with a concrete floor has the possibility of different potentials-on and on we could go.  In a theroearetical world then yes it makes sense.  Even the neutral isolator you showed has a warning against using it in situations where life safety is involved.  It certainly has its place-and is an engineering solution to be considered.  In the US, the NEC has addressed concerns by requiring equipotential bonding grids and not allowing aluminum to be used for grounding conductors in livestock operations.  That is no doubt a more expensive solution-so one could debate if the expense was worth the added safety or not.

I guess I still fall on the side of not fixing "dodgy" grounds by relying on a transformer that is (hopefully) isolated.  I say hopefully, because the second you have a fault, isolation is gone-and you are back to relying on the GFCI for protection.  Why not rely on the GFCI-with maybe a stinger cap ground like JR has advocated?  That's lighter and less expensive than a transformer anyway?  (Of course the obligatory disclaimer-the GFCI/stinger cap ground is not recognized or approved by UL or the NEC).  Seems like both methods are in the same boat-work able if precautions are followed but unapproved.
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Steve Swaffer

David Buckley

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Re: Isolation vs bonding
« Reply #17 on: September 27, 2016, 11:08:06 pm »

Why not rely on the GFCI-with maybe a stinger cap ground like JR has advocated?

Because (and I keep saying it!) a GFCI does not protect against ground-to-ground shocks.  A correctly configured isolation transformer, along with GFCIs, cover all the bases.  OK, it means the distro is heavier, but if the distro is a rack on wheels then that isn't that big of a deal.
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David Buckley

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Re: Isolation vs bonding
« Reply #18 on: September 27, 2016, 11:13:01 pm »

One was to sense the current flowing in the ground lead and disconnect power and ground if the ground current exceeds several mA.

Old people, like me, will remember a time before GFCI/RCDs, when that is exactly what one did, using ELCBs.  They had these in the school labs, and one could trip them with a carefully crafted mains plug, much to Sir's annoyance.
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David Buckley

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Re: Isolation vs bonding
« Reply #19 on: September 28, 2016, 07:08:14 am »

Having re-read the thread here, lets not lose track of what this is about: its about Article 250.30(A).1 negating the most important advantages of isolation transformers.  Its good that there is action to remove some of that stupidity in certain circumstances, and, perhaps, over time, the NEC will come into line with the rest-of-the-world thinking and comprehension.  Or maybe not.

None of this is saying you cant, in the RnR context, in most circumstances, achieve an entirely safe setup by doing nothing more than plugging a distro into a socket on thew wall, and having cabling that is correctly wired.  And if there are GFCIs in the mix, then another whole class of problem is mitigated.  And if one uses wireless, as JR notes, yet another class of problem goes away too.

An isolating transformer done right can be used to deliver a safe setup in situations where other solutions don't deliver.  The necessity to do this should be rare, in the lives of most folks. 


« Last Edit: September 28, 2016, 07:12:54 am by David Buckley »
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Re: Isolation vs bonding
« Reply #19 on: September 28, 2016, 07:08:14 am »


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