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[Lyle Williams]

PS: opening the safety ground is a foreign concept for UL et al. but who can argue with opening all three?

I think the issue is that the chassis ground of Device A needs to be able to trip a breaker when a fault in Device B makes it live.

[John Roberts {JR}]

I think the issue is that the chassis ground of Device A needs to be able to trip a breaker when a fault in Device B makes it live.

While I am repeating myself, I have spent decades wrestling with this conundrum. Yes, ground bonding the chassis of single insulated, powered gear is the standard for human safety. That said it is precisely this scenario (a musician finding himself between two different mains power drops) where properly ground bonded chassis can actually be the hazard from mains wiring faults that kills the musician (it has happened). The human mains power shunt will not trip the circuit breaker.

In general I do not embrace floating the chassis in a single insulated design, especially for a vacuum tube amp that has it's own dangerous internal voltages. BUT a GFCI in series with the guitar amp should protect against at least any primary power supply fault. I am not confident that a GFCI on the FOH power would protect against a RPBG.

The only fully protective mod is cap coupling inside the guitar, and guitar makers would be smart to do that (IMO). While we do not have numerous dead musicians driving this, something as cheap as a cap seems reasonable. 

I have not investigated this but it seems a UL approved stinger cap design "should" (?) require double insulated primary wiring to reduce that hazard.
 
JR

Note: they do sell GFCI plugs that can be attached to gear while not as cheaply as I would like. A guitar amp manufacturer could incorporate GFCI less expensively but that is a price competitive market so unlikely to happen.

[Stephen Swaffer]

I think the issue is that the chassis ground of Device A needs to be able to trip a breaker when a fault in Device B makes it live.

But this needs to be addressed at the supply side of device B-there are just too many potential paths to ground many of which are not connected through any gear/device/receptacle.

[Mark Dawson]

As a bit of a sideways swerve, on the topic of protecting consoles with multiple mics into them, how do we factor in the use of a UPS between the Gfci and the console/stage box or similar?    At a first thought I'd consider a UPS like an isolating transformer, but I don't think that's correct for many designs..

[John Roberts {JR}]

As a bit of a sideways swerve, on the topic of protecting consoles with multiple mics into them, how do we factor in the use of a UPS between the Gfci and the console/stage box or similar?    At a first thought I'd consider a UPS like an isolating transformer, but I don't think that's correct for many designs..

I would be apprehensive that a UPS might conceal a ground fault occurring post the UPS. I'd prefer a GFCI outlet  on the UPS.

That said a RPBG may make all of that attempted protection moot, assuming the now hot safety ground passes through the UPS.

Danger Will Robinson...

JR 

[Stephen Swaffer]

The ultimate solution might be a ups incorporating a GFCI output, with EGC current monitoring to disable the output.  The problem is going to be getting a UL listing as the cost of doing so is going to be spread out over a limited number of units.  As much as I hate code mandating good practices, forcing the use of a protective device like this may be the only way to drive demand up high enough to justify the cost.

It would help if there were another industry that the technology would make a lot of sense on-RVs Mike?  It would be a short step from an ice cube relay to a 50 o even 100 amp definite purpose contactor that would energize the entire RV upon a fault?

I assume that once a concept design is listed that variants of that design require less exhaustive testing?

[Mike Sokol]

I would be apprehensive that a UPS might conceal a ground fault occurring post the UPS. I'd prefer a GFCI outlet  on the UPS.

That said a RPBG may make all of that attempted protection moot, assuming the now hot safety ground passes through the UPS.

Danger Will Robinson...

JR

Yes, the RPBG (Reverse Polarity Bootleg Ground) condition will energize the EGC safety-ground wire and everything with a grounded power cord plugged into it. Since the NEC/UL doesn't allow the ground wire to have a disconnect contact, then it follows that no currently approved voltage monitoring or protection device will disconnect you from a hot ground caused by one. Seems crazy, but that's how it works.

Of course, an RPBG outlet should NEVER happen in the first place, especially with newer wiring (post 1970). However, I've discovered several RPBG outlets in churches just in the last year where a DIY volunteer has upgraded the old 2-banger outlets on stage to grounded ones. So that's now my warning sign. If I'm in an old church and see new outlets on the back wall behind the band, my spider sense starting tingling and I get out my NCVT for quick test even if I'm not using them for my seminars. I also double-check the chassis of my mixing console after plugging it in to make sure it doesn't have a hot-chassis condition.

Just know that any old bar stage with new outlets on the wall behind the band is suspicious as are new grounded outlets in old garages. In short, anyplace with older wiring (especially K&T) that has new "grounded" outlets needs a double-check for RPBG mis-wiring. You also need to know that a 3-light cube tester won't discover an RPBG mis-wiring condition. It will show the outlet as being wired correctly, when the ground and neutral are at 120-volts and the hot is at 0 volts.

[Mike Sokol]

It would help if there were another industry that the technology would make a lot of sense on-RVs Mike?  It would be a short step from an ice cube relay to a 50 o even 100 amp definite purpose contactor that would energize the entire RV upon a fault?

I have one of the EMS (Electrical Management System) manufacturers considering a sensor that would at least warn you that an RV hot-skin condition (common RV slang) has occurred, even if they're not allowed to disconnect the RV from the hot-ground wire at this time. However, that's exactly the sort of exception that the NEC might make. And you're correct, that one exception just might make it possible for an EMS type of device to monitor and disconnect the sound gear ground if a hot-chassis condition occurs.

[John Roberts {JR}]

As long as we are considering relays, why not just open up hot, neutral and ground with a 3 pole relay if 6 + mA current is detected on the EGC?  It would obviously need to be a manual reset-but IMO, if you are going to open a ground you need to open the mains with another set of contacts.

I am thinking the circuitry is already designed-the exact same circuit that detects a 6 mA difference  between the hot and neutral in a GFCI would detect a 6 mA current in a single wire-so if you had a GFCI with a second trip circuit detecting current on the EGC that would shut down the circuit opening all three wires it should provide very thorough personnel protection.

Prototyping one might make for an interesting winter project.  It would stand to reason that using circuits that are already UL listed would help with development costs-but maybe that is not a reasonable process?

As for a cheap inline GFCI, I can buy a quality GFCI receptacle for $11 and have seen GFCI's designed to be hardwired in without a receptacle for $15.  Granted the inline is likely plug and play and listed for such use-but if we are making modifications that listing goes out the window anyway.

I am liking this approach. Two different current transformers, one differential transformer for the hot and neutral, a second current transformer for just the ground lead. If hot and neutral are mismatched by 6 mA, or if the ground lead carries 6 mA, a 3 pole relay or latching switch opens all three conductors. (UL might insist on breaking the hot-neutral before breaking the EGC which sounds more expensive.)

This is a foreign concept to UL who do not think in terms of opening EGC paths, but in my judgement is the only way to protect a guitar player from both a faulty amp or bad electrical service, his or FOH.

Thanks Stephen, et al.

JR

PS: I am going to try to bounce this idea off a guy with a couple GFCI patents and he may have some ideas about a cost effective approach. 

[Stephen Swaffer]

The one place NEC and UL do condone disconnecting the ground is with a plug and cord connected device.  If you discovered a RPBG the first thing you should/would do would be to unplug any connected gear.  I would sell this to them as an "automatic unplugger".  Build the device itself in an enclosure to double insulated standards so it does not itself require an EGC.

I also considered they may want to see a "force guided" contact relay.  (I think that is the term that was used.)  So these quite often in safety circuits-the idea being that all the contacts had to make/break together.  It is possible for a contact in a standard ice cube relay to stick while the others operate. (Not common but possible.)