Re: Freak Accident:
Hal I agree with you that in most NEC compliant electrical systems, there should be no problems, But there is one setup that will cause a problem every time and the electrical is NEC compliant,
It is when a large show is done in a building or place (like a park) where there is more than one service, And the audio equipment is split with the sound stage at the back fed from one service and the main stage is in the front fed from another service.
Since the neutral is bonded to the grounding at each of these services the common audio cable shields will also share the neutral current.
There is no reason the audio shield need to be connected to the power grounding, this was a left over from the old tube amps that used to power the main PA system. All new amps have power supply's that would isolate the audio section of the amp, and the shields connection to the chassis can be removed with out any problems. This connection is not needed and the amp will function with out it.
I think what you are asking is that audio equipment be made idiot proof
No I'm not This is something that is not very common knowledge with the people installing this equipment as we can see by the people getting hurt or killed by it, Look at the NEC for the most part it doesn't idiot proof electrical systems, It address common problems that many electricians didn't understand that was causing fires and shocks, If we didn't have a table to tell us how many amps a wire could safely handle many would be overloading them, Why did the AMA reduce the length of kitchen cords in 1969? It was because many parents didn't know about the dangers of kids grabbing the cord and pulling a hot appliance onto them. Would you call that parent an idiot? Or someone who didn't have the right knowledge?
I'll bet there are many electricians who will not know what cause the hum in a audio system, It's more of a lack of knowledge that the AMA or which ever org. covers how these amplifiers are built should make some changes to stop this un-necessary danger.
Remember when they came out with the XLR balanced audio system they removed the signal negative connection from the shields but left the shield connected to the chassis grounding. This was to extend the max length of runs and to help in reducing the hum and noise on the audio cables. Well it did help in removing some but since there was still a parallel path with the shielding it still cause the hum. I use to keep a bag of XLR isolation transformers on hand because we had that exact setup at a park here, the main stage was up front on the rear of the concession stand and where to sound men wanted to always set there sound both up there was a pedestal meter panel combo, I had told them in the beginning that they had to use these isolators between their sound booth and the main stage because of the two different services, well they didn't listen and had many problems, Now these was large venue bands, not barroom bands but like Charlie Daniels and others.
The worst was a rodie was shocked when taking apart an XLR connection that was the result of a voltage differential between the two service neutrals that was paralleled on this cables shielding. They thought I was nuts when I required the isolators, well after that the park department had us run dedicated feeders from the same service that the main stage area used and installed a 200amp 3? panel, so this wouldn't happen again.
Will it stop any problems in the future? Probably not since any mis-wired cord used by a band can create the same problem.
The only common denominator is to stop connection the audio shields to the power supply grounding and reconnect them to the input signal negative, this will isolate it from the power system keeping the shields/equipment safe to touch.
