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Author Topic: Where to attach ground?  (Read 6044 times)

David Buckley

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Re: Where to attach ground?
« Reply #10 on: November 24, 2015, 09:32:22 pm »

Doesn't the transformer require it's own ground?  My understanding of Jamin's question is with venues where there isn't a legit ground available.

Nope, that's the beauty of it. The transformer isolates you - both ways - from the venue power, and provides (through a neutral/ground bond on the output) a local, "real" ground for everything plugged into it.  Same as using a generator with neutral/ground bond on it.

So from the perspective of the venue, the transformer is just a load between hot and neutral.  From the perspective of the load on the other side of the transformer, it looks and works just like a modern, fully code compliant feed.
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Jamin Lynch

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Re: Where to attach ground?
« Reply #11 on: November 24, 2015, 10:25:11 pm »

Some of these old dancehalls were built years ago and still do not have up to date electrical. They are way out in the country and I guess overlooked by inspectors.

Most of the sub panels had no ground bar at all.

I've seen some guys "attach" the ground wire to the hard conduit that comes into the sub panel.

We don't have basements anywhere in my area.  :)
« Last Edit: November 24, 2015, 10:46:48 pm by Jamin Lynch »
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Stephen Swaffer

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Re: Where to attach ground?
« Reply #12 on: November 24, 2015, 11:59:49 pm »

Nope, that's the beauty of it. The transformer isolates you - both ways - from the venue power, and provides (through a neutral/ground bond on the output) a local, "real" ground for everything plugged into it.  Same as using a generator with neutral/ground bond on it.

So from the perspective of the venue, the transformer is just a load between hot and neutral.  From the perspective of the load on the other side of the transformer, it looks and works just like a modern, fully code compliant feed.

Actually, the transformer provides a local, real neutral or grounded conductor.  Code still requires the transformer derived neutral to be bonded to ground.  The problem in a sub panel fed by only three wires is that typically load current is conducted by the neutral wire raising it to some voltage above "ground" which gives rise to audio noise.  This will still happen if you install a transformer and ground to the neutral in the panel.  Code specifically prohibits current other than fault current from flowing on an Equipment grounding conductor, so a new transformer will have to have a new wire run to meet code.  If you are going to run a new wire, why not just run a ground for the panel?

From a safety standpoint, you could run everything on GFCI's-code allows this. This may or may not be acceptable from an audio standpoint.

If every load on the panel is 240 volt, then the third wire could be used as ground.  In that case (perhaps a heating sub panel ) a transformer could work-but that would be an uncommon situation.
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Steve Swaffer

David Buckley

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Re: Where to attach ground?
« Reply #13 on: November 25, 2015, 05:46:06 am »

Code still requires the transformer derived neutral to be bonded to ground.
Yes, code does mandate (250.30) the derived neutral (ie the transformer secondary) to be bonded to the supply ground with a separately derived system as the NEC calls it.  But this is one of those occasions where the NEC is just plain wrong.  It gets even more dangerous when an isolating transformer is used in a marina or on a boat, where it gets really deadly.  The requirement for the bond defeats the very purpose of an isolating transformer.

This is a situation where there is a choice: safety or compliance.  I'll choose safety every time, thanks.

Here is the NEC way from ecmweb, note the solid green ground from the supply on the left all the way through, and also note that the ground fault current represented by the mostly left facing arrows on the solid green line end at the transformer secondary centre tap, and do not go all the way to the supply



This is the rest-of-world way of doing it; the fault current path is identical, there is no compromise on safety, and the isolated supply has nothing at all to do with the the primary supply.



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Stephen Swaffer

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Re: Where to attach ground?
« Reply #14 on: November 25, 2015, 08:25:22 am »

Yes, code does mandate (250.30) the derived neutral (ie the transformer secondary) to be bonded to the supply ground with a separately derived system as the NEC calls it.  But this is one of those occasions where the NEC is just plain wrong.



IMO, this is a dangerous mindset.  Even if you use an iso transformer, you need a ground. You claimed the transformer creates a local ground-which it cannot do.  Even the "rest of the world way" has a ground. Otherwise, you have a situation where someone lays a 58 on a stage and creates one "ground" connection unintentionally and another fault becomes dangerous. Metal objects that are not intended to be energized need to be grounded, and the right way to mitigate hazards is with GFCI protection. Code compliant, safe, defensible if something goes really bad. Y

Another issue with the isolated, no bond setup is what happens when (and sooner or later it will happen)' there is a fault to ground. With no neutral bond, no fuse or circuit breaker will blow.  There will not be any obvious indication of a fault until the second fault occurs. Whether you realize it or not you no longer have an isolated system.

In a sub panel with no neutral bar, I would look for a box bonding screw (I assume if you are in a panel, you are qualified and if you are qualified finding that or verifying a absence should be doable). If it exists, clamping to conduit is the same as attaching to the neutral bar.  If not, conduit is an acceptable EGC, if it is installed correctly for that purpose-which you probably can't really verify. At some point you have to hope the installer got it right.  From a practical standpoint, I would either go to conduit, or if bonding screw exists go to the neutral bar and be very vigilant in my testing and verification.  The main hazard would be the neutral voltage being raised by a load and that raising the "ground" voltage which becomes on issue if you are on a damp concrete floor or near building steel, etc.
« Last Edit: November 25, 2015, 09:08:59 am by Stephen Swaffer »
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Steve Swaffer

Stephen Kirby

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Re: Where to attach ground?
« Reply #15 on: November 25, 2015, 12:38:36 pm »

Thanks Stephen,  I've not dealt with iso transformers but I've had lots of step up/down transformers in industrial applications and they all had a ground rod at them with a neutral bond.

I am thinking that the old buildings Jamin is talking about probably have full metal conduit.  Although some of the joints may be a bit corroded by now.  And the neutral bar bolted to the box which is effectively the same as the conduit.  Probably have all iron piping as well.  Although someone may have spliced plastic in some above ground repair.  Would an iron pipe that goes into the concrete be more trustworthy?
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Stephen Swaffer

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Re: Where to attach ground?
« Reply #16 on: November 25, 2015, 01:56:47 pm »

Would an iron pipe that goes into the concrete be more trustworthy?

The forums favorite answer-it depends  :) Actually that would make a good ground in most cases-somewhat depending on how much of the pipe is in contact with the concrete.  However, the biggest issue here is really properly called bonding.  Bonding is intentionally creating a metallic conductive path-and is required by code for everything on a service that is likely to or can become energized.  Mike posted a link to a video not too long ago regarding the difference between bonding and grounding.  "Grounding" to the metal pipe would likely not ever trip an overcorrect protective device, if that pipe is bonded to the service an overcorrect protective device would trip.  If that pipe is a good enough ground to be considered a grounding electrode, code still requires all grounding electrodes on a premises to be bonded to each other.

In short, you really need a properly sized metallic path back to the ground neutral bond to really have a safe setup-and that path cannot intentionally carry current, which rules out using the neutral.
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Steve Swaffer

Mac Kerr

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Re: Where to attach ground?
« Reply #17 on: November 25, 2015, 02:18:35 pm »

Thanks Stephen,  I've not dealt with iso transformers but I've had lots of step up/down transformers in industrial applications and they all had a ground rod at them with a neutral bond.

I am thinking that the old buildings Jamin is talking about probably have full metal conduit.  Although some of the joints may be a bit corroded by now.  And the neutral bar bolted to the box which is effectively the same as the conduit.  Probably have all iron piping as well.  Although someone may have spliced plastic in some above ground repair.  Would an iron pipe that goes into the concrete be more trustworthy?

I can't believe I am saying this again... A "ground rod" stuck in the the dirt has nothing to do with electrical safety bonding. It is for lightning protection, not electrical safety. It will not trip a breaker in a fault condition. It will trip a GFCI because it will create a leakage current path, but it will not pass enough current to trip a breaker.

To trip a breaker you need a return path for fault current that returns to the source. If you have a separately derived system like a generator that is where the safety ground conductor needs to return to, and that is where the neutral/ground bond needs to happen. If the generator neutral is not bonded to the generator ground (chassis/ground lug) there is no return path to the source to trip the breaker.

Mac
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Mike Sokol

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Re: Where to attach ground?
« Reply #18 on: November 25, 2015, 05:21:17 pm »

In short, you really need a properly sized metallic path back to the ground neutral bond to really have a safe setup-and that path cannot intentionally carry current, which rules out using the neutral.

Yup, and I use a Ground Impedance Tester to verify that the bond has a low enough impedance to quickly trip any over-current device. All my GIT's have set threshold of 1 ohm. So any bond impedance greater than 1 ohm is a fail, while a bond impedance less than 1 ohm passes. Any bond that measures really low (less than 1/10 of an ohm, I think) is a suspect bootleg ground.

As Mac has stated, ground rods really aren't the same thing as your EGC "Bond" point. In fact, a ground rod can have an impedance of up to 25 ohms to earth and still be within code. A little quick math shows that 120 volts / 25 ohms = 4.8 amps, which isn't enough to trip any over-current device. That's why a ground rod isn't a substitute for a proper ground BOND to the Neutral conductor inside the service panel, and by extension any sub-panel. Ground rods are for lightning protection and keeping the local ground plane of your building close to earth potential. And they do provide enough leakage current to trip a GFCI.
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David Buckley

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Re: Where to attach ground?
« Reply #19 on: November 25, 2015, 05:51:42 pm »

IMO, this is a dangerous mindset.  Even if you use an iso transformer, you need a ground. You claimed the transformer creates a local ground-which it cannot do

No, I didn't claim anything of the sort.  What I claimed, correctly, was  "...provides (through a neutral/ground bond on the output) a local, "real" ground for everything plugged into it".

That's just fact. 

When I say "everything plugged into it", I'm making the assumption that the "everything" is your disto.

It may not have been clear in the explanation above, so let me make it really clear now.  The isolation transformer with a neutral/ground bond creates a gound point.  About that there is no doubt.  That ground point still needs to be connected to everything on the stage, which, assuming one uses normal three prong plugs with three core cable, will be the natural case assuming one wires one's stage right and uses normal techniques.

When a sound provider puts up a PA for a band, it is really important that every conductive element that the band can touch on the same are at the same potential, to prevent shock.  The band need to be in an equipotential zone.  Every ground wire from everything on the stage needs to end up at a single point that you can trust.  Since Buddy Holly's day, the most hazardous situation that occurs onstage is live guitar, grounded mic. 

The audio world is very familiar with distros, a device intended to distribute power, but which actually has the more important function of providing a common ground point for everything connected to it.  Step down a level, and there is the poor mans distro, a device that provides a common ground to everything plugged into it.

As long as everything is plugged into the distro, then everything has the same ground potential.  So if our guitarist touches the mic, both things are at the same potential, no shock.   

I really do know what I'm on about with this stuff, I'm not making it up as I go along.  If there is something you don't understand, just ask.



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Re: Where to attach ground?
« Reply #19 on: November 25, 2015, 05:51:42 pm »


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