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[Stephen Swaffer]

What I don't understand is how this is any safer than using the neutral bar in the non compliant panel as your single bonding point for your equipotential plane? Neither case meets code, and as long as you are using the neutral, a bad neutral creates problems either way.  Yes, it could work IF you had complete control-until someone plugs a fan in and sets it on stage or a late musician (won't ever happen I know) plugs his amp into the nearest receptacle instead of the distro, or whatever. To really be safe you still have to be bonded back to the service. Most of the guys on the NEC code committees are pretty smart, too.

[Jonathan Johnson]

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.

Academic point: if you use the neutral busbar as a common bonding point between the distro neutral and ground, AND the chassis of the subpanel is also bonded to this same point, AND you also drive a ground rod into the dirt and connect it to that SAME bonding point, you have achieved the SAME scenario as the service entrance that is fed from the utility transformer without a separate ground wire from the transformer to the service entrance. (Yes, it's common for utilities to supply a service entrance with a neutral conductor but not EGC.) If current on the neutral that's feeding that subpanel causes a voltage rise at the bonding point, the ground rod helps to minimize that voltage rise by attempting to bring the EGC in the distro to the same voltage potential as the dirt.

If there is a hot-to-grounded-chassis fault you may not trip a breaker UPSTREAM of the subpanel, but you should still trip the breaker of the branch circuit the faulty device is connected to. If there is a hot-to-EGC fault in the cord supplying the distro, it should trip the breaker feeding that cord (you did tie into a breaker in the subpanel, didn't you?).

If there is a hot-to-dirt fault, you might not trip a breaker. But even if everything was installed exactly according to code, and there is a hot-to-dirt fault, you might not trip a breaker.

EDIT: If the neutral busbar is NOT bonded to the chassis of the panelboard, and ALL of the existing circuits are in conduit, then you should NOT tie the EGC of your distro to the neutral busbar. (There may be later additions that have installed a branch circuit EGC and connected that to the neutral busbar. This is incorrect practice.) In that case, even though there is no grounding busbar, the conduit system acts as the EGC (Equipment Grounding Conductor). If you attach the EGC of your distro securely to something metal that is bonded to the panel chassis, that would suffice. For example, you might have an electrician bolt a lug to a hole drilled in the cabinet. You might want to remove paint or install a star washer between the lug and the panel. Even then, it's a good idea to do a ground impedance test, as a loose fitting on the conduit supplying the subpanel could render the ground useless.

The best option is to have an electrician install a grounding busbar. This should be bonded to the cabinet. The busbar should also have a jumper to a grounding bushing installed on the conduit supplying the panel. If possible, a separate EGC should be added to the conduit from the main panel to the subpanel.

[David Buckley]

What I don't understand is how this is any safer than using the neutral bar in the non compliant panel as your single bonding point for your equipotential plane?

It's safer because using a random neutral as ground is just that - random, one has no idea of the integrity of the neutral.  If the neutral fails then you have the "ground" wire raised to line potential, and there may well be other things that penetrate the equipotential zone that are grounded to a different potential causing a genuine shock hazard.  if the premise has missing or inadequate ground wiring, then it may well have missing or inadequate bonding too.

With an isolating transformer and its N/G providing a local ground, then you are certain of the integrity of that ground at the point of supply.  If the building neutral fails, then you lose the show, but there is no shock potential.

... until someone plugs a fan in and sets it on stage or a late musician (won't ever happen I know) plugs his amp into the nearest receptacle instead of the distro, or whatever.

OK, lets take that case where the PA and backline is supplied through an isolating transformer, with all this kit running off the distro thus bonded to the locally created ground.  We'll further assume there is a metal encased fan on stage plugged into some local outlet, and we don't know anything about the ground pin of that outlet.  Lets further assume that the vocalist, SM58 in hand touches that fan.  The SM58 is connected to our local ground.

The important thing here is that there is no common reference between the local ground of the SM58, and the supply feeding the fan.  If the socket feeding the fan is as it should be, then there may be a slight voltage difference with a very small current drive capability between the SM58 and the fan, from leakage,  Less than you would see if the the socket was wired with reversed N/G (bootleg), which is a small voltage with significant current drive capability. 

If the fan socket has no ground, then it is also possible that the fan may be faulty, in which case its metalwork is at mains hot potential.  Our vocalist might well get a bit of a tingle when he touches it, due to leakage between our local ground and the supply ground. 

Contrast that with the case of what would happen without the isolating transformer.  The vocalist would get the full mains across him with high current drive capability, and would be seriously shocked.

The bottom line is this: if there is a properly grounded supply available, then one can build a distributed supply that is adequately safe for entertainment purposes by just using the right kit and plugging it together.  When such a supply is not available, then the question then becomes how can one safely put on a show.  Getting a set of mole grips out and attaching a cable to a bit of conduit and calling it ground, or using a neutral bar as a bootleg ground may well get you through the night, but these are not approaches without risk.  Using an isolating transformer with a local ground is a strategy that will always work and deliver a safe outcome.

Most of the guys on the NEC code committees are pretty smart, too.

Yeah, they are.  But they are not perfect.  They fail to see the advantages of isolating transformers.   Every year, Americans die because of electrical incidents around water and marinas, and these folks could be protected were isolating transformers used to break the ground circuit.  GFCI/RCDs are fabulous, one of the best inventions in the world of electricity, ever, and an essential part of any distro, but a GFCI/RCD cannot protect against ground to ground shocks.  No protective device you can (still) buy provides ground to ground shock protection.  This is why its really important to get grounding right.

I think the code making guys are influenced by long standing practices in power distribution in the USA.  For an interesting (but somewhat political!) summary of some of it, have a read of he Hazardous Multigrounded Neutral Distribution System (PDF, 256KB).

[Stephen Swaffer]

Electrically there is no difference between your isolating transformer and the scenario that occurs if someone wires a home (or outbuilding) with a 3 wire service from the POCO and does not install a grounding electrode.  The POCO's transformer is essentially an isolating transformer-yes it has a grounding electrode, but the bottom line is that with your "local ground" your connection to earth-and anything else on the premises is an unknown quantity.  Set an equipment rack or lay a mic cord on a concrete floor and spill a drink and you just earthed the "local ground".  It doesn't take much of a connection to carry 6 mA.

Your "equipotential" zone is exactly what the NEC demands of EVERYTHING on the premises.

What happens if there is an insulation breakdown in your isolating transformer? Undersize the transformer to save money and weight and this becomes a real concern.

The line between "feeling a tingle" and a dead musician is an awful fine one to walk.

Then the elephant in the room.  If something bad happens and you wind up in court it will be your word against the wording of the NEC.  If those on the panel at the NEC cannot understand the advantages of an isolating transformer, do you really expect to successfully explain that to a judge or jury? 

Probably the best plan for the OP's scenario would be to carry enough #6 or #4 wire to run a ground wire back to the main panel, unless a true EGC can be ascertained at the panel in question. In an steel framed building the building frame should be part of the grounding electrode system so it would provide a ground connection point-but those are the only connections I would really be comfortable with. Even better, invest in the equipment necessary to verify the integrity of the connection to the service ground/neutral bond.

Obviously, the problem in the OP's scenario is business.  If you create a stink, there is probably someone else that will just hook up and go.  In my business as an electrical contractor, I finally figured out that I don't need every job out there-if the customer doesn't want to do things right and safe often it is better to just walk away-the customers that are willing to do things right are usually more successful and ultimately a more profitable customer anyway.  It should be worth at least a service call to get an experts opinion-and if there is no ground bar but there is conduit, installing a properly bonded ground bar as shouldn't break the bank.

[ProSoundWeb Community]