Rule #1: Don't piss off the sound guy!
Is a portable entertainment PA Company power distro considered a sub panel when
considering whether or not to tie the ground and neutral together inside the distro?
Should the neutral bus bar be isolated from the grounds and the distro case?
This distro is a single phase type with Cam connections - 2 hots , neutral and ground.
When I built my first distro, I had a licensed electrician advise me to tie the neutral and ground together inside my panel.
To date I have had ZERO issues with ground hums, and ZERO shock issues.
I would suggest consulting the NEC and also a good source of information would be the Building Electrical Inspector
for your particular area. Or for the sake of liability issues, purchase premade distribution modules that are UL Listed and meet proper codes as of the date of their manufacture. Brands such as Whirlwind, or Motion Labs come to mind.
Currently, bonding neutral to ground any place but the "service entrance" is a violation of the NEC. Don't do it. The service entrance is usually defined as the primary point of electrical entry past the secondary transformer winding of the service or generator output lugs. So for regular buildings, that would be at the main panel the power lines connect to. It can also be any additional transformer past the primary service entrance as additional transformers downstream are considered a separately derived system. Any sub panel or distro tried to or past a main service is not to have the neutral and ground bus bonded together.
No distro should have the neutral and ground bonded. And I second Bill's suggestion of using a known brand of distro. I'm all Motion Labs main and rack distros which are ETL certified and solid.
But didn't Bill hook his neutral and ground together under the advice of a licensed electrician.
He is in violation of the codes as far as I can tell and what people are telling me here.
My Distro in question has Motion Lab edison and twist 30 recept panels but its put together with a home circuit breaker panel all inside of a rack.
The panel has the neutral and ground buss hooked together and I think the original builder just left it that way.
I just noticed it and I am going to fix it when I can find a neutral buss isolation kit which lifts the neutral bar off the panel with plastic insulators.
A local electrician here has told me the neutral and ground bars are not be bonded together anywhere except at the original service entrance panel.
The transformer service in my local Theater that I take my cams off of, is still in question as to whether it is tied together there.
I've run into a fair number of licensed electricians who are not up to date on the latest NEC. It changes all the time. I believe there was a time when bonding neutral to ground in sub panels was considered ok. But that was some time ago.
I rebuilt the Distro yesterday, Elevated the neutral bar, separated the main ground and neutral, and ran separate neutrals back to it from every circuit, as it had been built with as many as 6 - 30 amp twists parelled onto one neutral wire.
Yeah, the shared neutral wire is something you see in econo house wiring and it's not a great thing. It certainly has no place in high current live sound power distribution. Kudos for making the necessary repairs.
LEX products make some great distro systems. All UL Listed. We use mostly 3 phase and these systems are covered in rubber , watertight swichpanels. top notch gear.
My advice is simply this.
ALWAYS keep the ground and neutral separate. Bonding at the service entry is the right way.
I use UL approved devices for one very simple reason. I do not want novice electrical inspectors erring on the side of safety because it is something that was built in the shop somewhere. Motionlabs,Leviton, Coleman Cable X-TREME, systems are safe, and when the inspector sees them, they simply move on to other things.
My understanding of the ground vs neutral has to do with allowing for an alternate path to ground. In the case of a ground fault, the ground will provide the proper path to ground therefore, you would want them to be isolated. This is my speculation and not based on anything but my knowledge on the subject.
Sort of the same reason why ground lifts are not good to use on Bass players, it can potentially be an electrical shock hazard in the case of a ground fault.
Here's a good explanation given by an electrician/instructor:
"Frequently Asked Question: Why do the grounds and neutrals need to be separated in a subpanel? What happens if they aren't?
Answer: Though the neutral doesn't have significant voltage, it does carry current. Remember, it's current that kills, not voltage. In a 2-wire circuit, the neutral carries just as much current as the hot conductor. If the neutral and ground are connected in a subpanel, that current will travel on other paths, such as bare ground wires, equipment enclosures, and metal piping systems, on its way back to the service panel. One problem created by this condition is possible shock hazards, the severity of which depends on the locations of the equipment and the person touching the enclosure or piping system. Another problem is magnetic fields that do not cancel themselves out. Since the return current has multiple paths, the current remaining in the neutral will not counterbalance the current in the hot wire. The resulting imbalance creates a magnetic field that can interfere with sensitive electronic equipment. In a metal conduit system, the imbalance will induce current into the conduit, which could cause the conduit to overheat.
Q: Given that this is a fairly common condition, why don't we hear about problems, and why do some electricians not understand the problem?
A: For the feeder from one panel to another, the neutral only carries the imbalanced load between the two hot legs. Most of the time, the amount of current on the neutral is very low. However, in a situation where a single 120-volt appliance is in use, and there are few other loads in operation, the current on the equipment grounding conductors and other paths could be quite high, resulting in the problems noted above. Other conditions could cause the load imbalance to be quite high, if for example, all the lights in use at a given time just happened to be originating from the same phase conductor.
The National Electrical Code (NEC) has prohibited re-grounding the neutral after the service since the 1923 edition. Exceptions have been made for dryers and ranges that use the neutral as a grounding means, and for separate buildings. However, those loopholes have been effectively closed in the 1996 and 1999 editions of the NEC. The references in the 1999 edition of the NEC are found in 250-24(a)(5), 250-142(b), and 384-20."
Now I would ask a question.. I fully understand not wanting stray ground pathways and inducing magnetic fields into a conduit system. Both very undesirable. Now for the reality that most local sound guys live in.
Most small clubs do not have 220VAC or a cam setup available and if they do, the wiring in MOST of the clubs in this area is highly suspect. There has been little to no building code enforcement for as long as I can remember. Most venues have been remodeled numerous times with no electrical permits being issued and no inspections. If there is to be a show, the only alternative is to bare tail right on to the main wires coming in from the meter base at the top of the panel. There is no conduit to energize, no alternate pathways. At that point (here's the question) Is my distro a sub panel or a primary panel since I have bypassed all building electrical conduit and wiring??? If you are connected straight to the 3 wire drop (post electrical meter) I would consider that to be a primary panel.
Before everyone starts jumping up and down screaming about hitting a live service, YES I know its dangerous (don't try this at home kiddies), but it is also the reality of providing sound and lights in this market.
Check the picture below. This is a prime example of what I'm faced with constantly. This panel was in a operating bar in Pennsylvania that volunteered to provide power for a show being sponsored by the City Chamber of Commerce. We told them what the power requirements were. We told them they needed to have a licensed electrician on site to do the hook up and disconnect. When we arrived, there was a range plug hanging loose outside a window hooked to 10/3 which ran back to a fused disconnect in the kitchen which also had a deep fryer hooked to it. I was left on my own to make the show happen.