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[Kevin Maxwell]

Please correct me where I am wrong which may be everything I write here. This may be a stupid question but if the cause of this kind of incident were actually a properly grounded mic completing the circuit of an improperly wired guitar amp, what would it take to put some kind of protection on the ground for the mic. I assume that a mic cable with a telescoping ground, pin one only connected at the mixer but not at the mic itself, would have not completed the circuit causing the deadly shock. Is it possible to fuse the ground right at the mic connector with something small enough to fit in the connector that would open in a situation like this but not open in normal use? Do we need special mic cables to keep guitarists with bad gear from killing themselves? If the pin one not connected at the mic would work, do we need to carry special guitarist/vocalist cables like that for them, specially marked to not get mixed in with normal mic cables?

Even if we are doing everything properly is there anything extra we can do to prevent tragedy because of other peoples mistakes.

[Mike Sokol]

Please correct me where I am wrong which may be everything I write here. This may be a stupid question but if the cause of this kind of incident were actually a properly grounded mic completing the circuit of an improperly wired guitar amp, what would it take to put some kind of protection on the ground for the mic. I assume that a mic cable with a telescoping ground, pin one only connected at the mixer but not at the mic itself, would have not completed the circuit causing the deadly shock. Is it possible to fuse the ground right at the mic connector with something small enough to fit in the connector that would open in a situation like this but not open in normal use? Do we need special mic cables to keep guitarists with bad gear from killing themselves? If the pin one not connected at the mic would work, do we need to carry special guitarist/vocalist cables like that for them, specially marked to not get mixed in with normal mic cables?

Even if we are doing everything properly is there anything extra we can do to prevent tragedy because of other peoples mistakes.

It's difficult (read expensive) to fuse a mic body so it would blow at 10 mA or so. But there are guitar string grounding systems with in-line fuses. However, that small of a fuse can be blown by even a static shock, so it's not a universal fix.

However, a GFCI in the right place can be a real lifesaver. I believe they belong right where the backline amplifiers plug into the power distro, ideally in the distro itself. I just found a quickie GFCI ShockBuster that might be a good idea for backline stage power, but I've not tested one myself. I'll pick one up tomorrow if I can find it in the store and evaluate it for build and operation.

http://www.lowes.com/pd_145275-33536-30339011_0__?productId=1135923&Ntt=gfci+receptacles&pl=1&currentURL=%3FNtt%3Dgfci%2Breceptacles&facetInfo=

 

[Steve Bradbury]

So here's an interesting side question. If a receptacle based GFCI has its load and line terminals accidentally reversed, I'm pretty sure it won't trip to protect you from an actual hot-to-ground fault. But will it  pass its own "self test" button even if it's wired backwards? If so, that's a false sense of security. I don't have a spare GFCI laying around to try tonight, but I'll try to pick one up tomorrow for a quick check.

I’m not sure if a receptacle based GFCI works the same as our RCDs, but as it is the imbalance between the supply and return that causes the trip, theoretically it shouldn’t make any difference which terminals are the supply or load. Protection only occurs on the side being used as the load. The only downside is that the internal circuitry will still be energised after the trip, and if the trip resistor is under specified it would burn out if held down.

If you cross wire (just the phase in/out reversed) the unit should trip with no fault.

Some of the RCD devices that mount in UK distribution boards don’t seem to be labelled with regards which terminals are the supply, others do. One reason for labelling in and out, that I have been told, is that the load rating is greater if the supply is on the fixed contact rather than the moving contact. Whether this is true I don’t know.

Having suffered a serious shock when I was 17 (one hand at 240V the other earthed) and had a gig where the bass player earthed himself through the microphone (fortunately he survived) I’m always conscious of the potential for serious injury.

[John Roberts {JR}]

Please correct me where I am wrong which may be everything I write here. This may be a stupid question but if the cause of this kind of incident were actually a properly grounded mic completing the circuit of an improperly wired guitar amp, what would it take to put some kind of protection on the ground for the mic.

I have actually spent a few decades thinking about this and regarding the debate about whether isolation or a robust safety ground is safer the robust ground usually wins. However for a guitar without a power supply inside it, the need for a safety ground is less. 

The only mic that is completely safe is a wireless mic, and if I was wading in a baptismal pool with a live mic, I'd prefer battery and RF isolation.

As I have posted before it is possible to put a capacitor in series with the ground of a guitar sized adequately to limit the current from an AC mains fault to less than lethal levels while still shielding noise. This is cheaper than an exotic (10mA is not normal) fuse and probably more predictable, plus the cap will not blow.

I suspect but have never tried to float the handle and screen of a mic through a similar coupling cap. The circuit ground needs to be low impedance, but the mic handle and screen can probably be effectively shielded through a cap.

Both of these hypothetical fixes are not in the amp or inside the console but inside the peripherals.     

I assume that a mic cable with a telescoping ground, pin one only connected at the mixer but not at the mic itself, would have not completed the circuit causing the deadly shock.

And it wouldn't work if phantom power was being used.

A dynamic mic does not need the pin one connection to pass audio, but shielding to reduce noise pickup is still useful.   

Is it possible to fuse the ground right at the mic connector with something small enough to fit in the connector that would open in a situation like this but not open in normal use? Do we need special mic cables to keep guitarists with bad gear from killing themselves? If the pin one not connected at the mic would work, do we need to carry special guitarist/vocalist cables like that for them, specially marked to not get mixed in with normal mic cables?

Why protect a consoles 20+ inputs when you can protect the one guitar, or better yet use a GFCI outlet strop on the back line. (But I've said this before.)

Even if we are doing everything properly is there anything extra we can do to prevent tragedy because of other peoples mistakes.

If you meter all outlets with NCVT including the mics and guitars, such events should be rare. Many musicians experienced with playing in questionable venues have been shocked enough times that they do not just casually grab a mic for the first time without testing first for sparks. Some will touch the mic to the guitar strings without touching both themselves, to see what happens.   

JR

PS: GFCI and RCD are AC devices so polarity should not matter.

[Mike Sokol]

If you meter all outlets with NCVT including the mics and guitars, such events should be rare.
JR

A standard sensitivity (90 to 1000 volt) Non Contact Voltage Tester (NCVT) such as a Fluke VoltAlert will easily find a guitar or microphone with 40 volts or more AC on its chassis. If there's a line-to-chassis short and a missing EGC (safety ground) which puts 120-volts on the chassis, then a NCVT will beep and light up from 6 inches to a foot away from the energized surface. If you keep a NCVT in your pocket it only takes a few seconds to point it at each of the stage amps, electric guitars a wired mic to make sure there's no hot-chassis voltage.

Here's a pic from my No~Shock~Zone Clinic on electrical safety for musicians.

[Kevin Maxwell]

So it sounds like it would make sense to only supply GFCI back line power. Isn’t there a different type of safety circuit out there now that’s supposed to be even better? What are the advantages and disadvantages of doing this? Are they more likely to pop when they shouldn’t? A lot of the shows that I have been involved with lately we are running the back line power also. So unless there is a good reason not to I am going to recommend we make this change.

[Stephen Swaffer]

Modern GFCIs are designed so that will not function if they are wired incorrectly-I believe that is why they have to ebe "reset" on install.  Of course, this may not be true of bargain basement GFCIs-but the difference in cost between a quality name brand GFCI and bargain basement is negligible when you consider it is a safety device.

IMO, fusing a ground is always a bad idea.  While it might be nice for it to blow when a person gets in the circuit, any other time I want sparks to fly and the OCPD supplying the power that is trying to get to ground to trip so I know there is a problem that needs to be addressed.  I have dealt with other situations where fusing a ground seemed to be the safer alternative, but I just could not find a way to justify it-there are other better ways.  One way I considered was sensing current in the ground wire and disabling the device when current above a set threshold was detected-with say a shunt trip type of circuit breaker.  While that may seem extreme-if current is flowing in a ground wire the right course of action is to stop using the gear and fix the problem.

I understand JRs concern about wired mics in baptismal pools-the issue I see is that way too much focus in the mics.  If the pool is energized ANY ground is dangerous-mics happen to me the most common and perhaps more dangerous because they are likely to be gripped firmly with a wet hand-still I think the focus needs to be on safely supplying energy to the needed devices-be they lights, pumps, heaters, or PA systems.  I think making grounded objects the "bad" guy is a mistake-it is simply too difficult to safely isolate objects and handrails, and even concrete floors in the vicinity can be a source of grounding a person.

In response to Kevin's "Are they more likely to pop when they shouldn't", I would refer to the thread on the surge protector being incompatible with GFCIs.  I don't think the issue is with GFCIs tripping when they shouldn't as much as with gear that leaks when it shouldn't.  If GFCIs need to trip at 6 mA to protect a person, then manufacturers of gear-including surge protectors-need to step up to the plate and figure out how to build gear that doesn't shunt current to ground like a human body when plugged in.

IMO, the only time isolation would be preferred to a robust ground would be a in the very specialized case of high voltage "bare hand" work-but that is a rare type of work most of us will never deal with and involves too many other safe guards for day to day work.

[John Roberts {JR}]

So it sounds like it would make sense to only supply GFCI back line power. Isn’t there a different type of safety circuit out there now that’s supposed to be even better? What are the advantages and disadvantages of doing this? Are they more likely to pop when they shouldn’t? A lot of the shows that I have been involved with lately we are running the back line power also. So unless there is a good reason not to I am going to recommend we make this change.

I haven't looked them up lately but several people sell inexpensive outlet strips with GFCI breakers built in... one or two for back line could be useful. A smart guitar player should own his own... but I'm not holding my breath for that.

JR

[Mike Sokol]

IMO, fusing a ground is always a bad idea.  While it might be nice for it to blow when a person gets in the circuit, any other time I want sparks to fly and the OCPD supplying the power that is trying to get to ground to trip so I know there is a problem that needs to be addressed. 

+100%

I, for one, want anything connected to AC power or potentially energized by power to have a low-impedance Equipment Grounding Conductor (EGC) back to the service panel's Ground-Neutral-Earth (G-N-E) bonding point. The obvious suspects involve back-line stage amps, PA systems, mixing consoles, generators, and anything else music related. Portable generators need to create their own G-N-E bonding point with a ground rod, just like a service panel.

But this also includes non-obvious peripheral objects such as (but not limited to) metal stages, hand rails, chain link fences, metal roofs, baptismal pools, and lighting trusses. Plus in your home it includes plumbing systems, furnace ducting, electric gates, outside outlets powering block heaters, farm pumps and water heaters, etc., etc....

Of course, GFCIs are required by code in many of these powering situations, but as note by others, that's really a secondary line of defense. A solid ground is your best friend when you're standing on a stage surrounded by AC powered sound and lighting gear.

[John Roberts {JR}]

IMO, the only time isolation would be preferred to a robust ground would be a in the very specialized case of high voltage "bare hand" work-but that is a rare type of work most of us will never deal with and involves too many other safe guards for day to day work.

I can't imagine any safe high voltage bare hand work except perhaps for science fair Van de Graf demos...

An important distinction about using a fuse in series with ground (BTW I do not advocate using fuses), is where is the current coming from and where is it going?  Passing through a human is always bad.

The securely grounded metal chassis of a powered product offers protection to shunt any inadvertent failure of powered circuits "inside" that chassis to a safety ground, tripping mains fuses or circuit breakers in the process.

A marginal case is something like a passive DI that does not have any power supply inside, but can connect between two powered products.  In my judgement the input and output grounds need to be well bonded so a fault upstream will still trip the mains protection devices and remove the hazard. Of course many DI provide ground lifts, and I have seen reports of DI with wimpy internal ground traces that have been vaporized by mains power faults. That ground trace acting exactly like a fuse DID NOT PROTECT THE USER by stopping the shock hazard condition like it could have.

Finally the only case were an argument for isolation holds some attraction is hand held instruments like guitars or mics. Generally these devices need some amount of grounding for signal integrity to prevent hum and noise pickup, but with no power inside there are no local power faults to contain and shunt to ground.

I am repeating myself but I prefer the capacitor vs fuse to provide a shield ground, and might like the fuse solution better if such tiny fuses were more common, cheaper, and certain. Fuses operate by thermal overload and the stress from a few mA even at 100V is not a lot of heat power. An electronic breaker could be very sensitive and even resettable, but expensive and solid state devices have a bad tendency to fail as a short circuit. So I prefer the low tech capacitor to current limit the AC fault

Caps can fail too, but a small film cap failing as a short is rare, and they make caps specifically for use around main voltage.   

JR

PS: Some chatter from Argentina, but no real new information yet.

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