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Author Topic: Help understand cause of Shock  (Read 6154 times)

frank kayser

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Re: Help understand cause of Shock
« Reply #10 on: November 21, 2014, 05:27:43 pm »

The other vector for shock is the bass amp... The mic could be providing a valid safety ground but if the bass amp energizes the guitar and player the current will flow to ground through the mic.

Does the bass amp have 3 wire plug with chassis bonded to safety ground.

JR
Is the cord intact? Has someone "lifted" the ground at the plug?  Chassis? Plug strip?
frank
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Jonathan Johnson

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Re: Help understand cause of Shock
« Reply #11 on: November 21, 2014, 10:47:44 pm »

The non-contact tester will help identify unusual voltages on ground.

If products were plugged into different power drops (outlets)  maybe measure voltage between grounds on the different power drops.

JR

This. When you are metering as a safety check, you have to measure against a "known, good" reference.

When you walk up to a receptacle the first time, it is an unknown. You cannot assume that the ground connection is at 0V potential.

Ideally, you would compare it to the building's grounding electrode system. To do this plug an extension cord in the receptacle to be tested, and measure between the ground contact and the metal of the service panel. If it is close to 0V, measure the resistance, too -- it should be no more than an ohm or two -- to ensure that there is good continuity in the ground connection.

If it's too far, you can test a closer receptacle and then use that as a reference.

Another possibility for Nitin's situation is that the ground conductor in the building wiring or power drop had failed. Sometime during the set, the bass amp or other equipment experienced a short-to-chassis failure. Because the EGC was broken, the circuit to ground was not completed so the OCPD didn't open. As a result, high voltage was applied to the signal lines, damaging the equipment.

Of course, that is just a theory that can only be verified by testing.

This highlights the importance of proper testing, and de-energizing faulty circuits until they can be repaired.

So why would the ground conductor have failed? One of the most common failures is a loose conduit connection, as can happen when someone bumps a conduit, dislodging the fittings. In some places it is permissible to use the conduit as the ground conductor; other places require a separate ground wire in the conduit. Another common failure is a broken or missing ground pin on a female plug.
« Last Edit: November 21, 2014, 10:49:45 pm by Jonathan Johnson »
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Mike Sokol

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Re: Help understand cause of Shock
« Reply #12 on: November 22, 2014, 07:00:05 am »

Another possibility for Nitin's situation is that the ground conductor in the building wiring or power drop had failed. Sometime during the set, the bass amp or other equipment experienced a short-to-chassis failure. Because the EGC was broken, the circuit to ground was not completed so the OCPD didn't open. As a result, high voltage was applied to the signal lines, damaging the equipment.
I've seen several instances where the branch circuit's EGC (safety grounds) were all connected together, but not terminated back at the service panel. As JJ described, this would allow the hot-to-chassis failure of one piece of backline gear (bass amp?) to energize the ground contacts in numerous receptacles on that branch circuit. While it's not common for a power transformer to develop a hot-to-chassis short, something like an extension cord with the insulation compromised by a split could easily allow a hot conductor to come in contact with a piece of gear, creating this type of condition. And as previously noted, if the EGC doesn't have a low-impedance path back to the service panel, then there's nothing to trip the OCP circuit breaker.

This hot-chassis condition is dangerous in two ways. If your musician get's between a piece of energized gear (mic, guitar strings, ect...) and anything grounded (damp concrete floor, grounded railing, grounded mic, etc...) then they can sustain a dangerous and potentially deadly shock. But this same hot-chassis is often connected to other gear which could be correctly grounded. In that case there the chance of a lot of current flow in wires never designed for that kind of amperage. The result is cables melting down, input and output electronics fried, and circuit boards being fried. Because just about everything nowadays has a CPU (computer) in it, this is an expensive repair.

I believe that standard setup should include a quick test for hot grounds. While the gold standard involves testing between each receptacles ground and a wire run back to the service panel, that's probably only going to be done for serious troubleshooting or after somebody dies from electrocution (and maybe not even then). However, while not perfect, a NCVT such as a Fluke VoltAlert will beep and blink when it encounters anything large with 40 volts or more of AC. That allows you to do a very quick test of outlets for an RPBG (the entire front of the outlet appears to be energized) as well as hot chassis (simply touch it to the microphones, guitar amps, mixing console, ect...) and if it beeps/blinks then you need to do further investigation before something bad happens. And a NCVT will find a floating ground from a broken off ground pin in a power cord since most gear plugged into a power line has some internal leakage and will try to float to half of the line voltage. The EGC's job is to keep that bias voltage close to zero by draining the current back to the service panel's G-N-E bonding point. But without a proper EGC connection, then none of this works properly and shocks or meltdowns can occur. 
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Frank DeWitt

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Re: Help understand cause of Shock
« Reply #13 on: November 22, 2014, 12:32:41 pm »


However, while not perfect, a NCVT such as a Fluke VoltAlert will beep and blink when it encounters anything large with 40 volts or more of AC. That allows you to do a very quick test of outlets for an RPBG (the entire front of the outlet appears to be energized) as well as hot chassis (simply touch it to the microphones, guitar amps, mixing console, ect...) and if it beeps/blinks then you need to do further investigation before something bad happens. And a NCVT will find a floating ground from a broken off ground pin in a power cord since most gear plugged into a power line has some internal leakage and will try to float to half of the line voltage. The EGC's job is to keep that bias voltage close to zero by draining the current back to the service panel's G-N-E bonding point. But without a proper EGC connection, then none of this works properly and shocks or meltdowns can occur.

What happened to the contest.  I was hoping to win one of these so I could live.  (Grin)
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Jonathan Johnson

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Re: Help understand cause of Shock
« Reply #14 on: November 22, 2014, 02:41:07 pm »

I've seen several instances where the branch circuit's EGC (safety grounds) were all connected together, but not terminated back at the service panel. As JJ described, this would allow the hot-to-chassis failure of one piece of backline gear (bass amp?) to energize the ground contacts in numerous receptacles on that branch circuit. While it's not common for a power transformer to develop a hot-to-chassis short, something like an extension cord with the insulation compromised by a split could easily allow a hot conductor to come in contact with a piece of gear, creating this type of condition. And as previously noted, if the EGC doesn't have a low-impedance path back to the service panel, then there's nothing to trip the OCP circuit breaker.

I think that whatever happened in this case, there must have been TWO failures. First, a failure of the safety ground system, then secondly an equipment failure of some sort which allowed the safety ground system to become energized. Had the safety ground not failed, the second failure would likely have caused the circuit protective device to trip. Had the equipment not failed, you'd never know the safety ground had failed unless you tested it.

Troubleshooting when there are two failures is difficult because we tend to look for single points of failure. We resolve what appears to be a problem but the symptom is still present so we tell ourselves "that must not have been the problem," when indeed it was but ONE of multiple problems contributing to the symptoms.

Whenever there is a shock, injury, electrocution, or property damage due to an electrical fault, there almost always are multiple failures contributing to the incident; one of those failures is almost always a problem with the safety ground.

Failures of the safety ground are almost never obvious until there is an unfortunate incident. Regular testing would probably be a Good ThingTM.
« Last Edit: November 22, 2014, 02:50:02 pm by Jonathan Johnson »
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Stephen Swaffer

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Re: Help understand cause of Shock
« Reply #15 on: November 22, 2014, 10:16:40 pm »

I've seen several instances where the branch circuit's EGC (safety grounds) were all connected together, but not terminated back at the service panel. As JJ described, this would allow the hot-to-chassis failure of one piece of backline gear (bass amp?) to energize the ground contacts in numerous receptacles on that branch circuit. While it's not common for a power transformer to develop a hot-to-chassis short, something like an extension cord with the insulation compromised by a split could easily allow a hot conductor to come in contact with a piece of gear, creating this type of condition. And as previously noted, if the EGC doesn't have a low-impedance path back to the service panel, then there's nothing to trip the OCP circuit breaker.
 

One install problem I come across far too often is a missing bonding screw in the panel.  Inspectors in my area are requiring a separate ground bar even in a service panel-and many panels now come with the bars installed-but if the bonding screw is not installed then there is no ground neutral bond.  IMO, it would make more sense to ship panels with bonding screws installed-I feel like it would be more likely for a a subpanel to be installed by someone who knows enough to remove the screw; whereas, it seems many service panels get installed by DIY that have no clue what that green screw is for or why they should expend the effort to install it.  In any case, a missing bonding screw could create the scenario Mike describes.
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Nitin Sidhu

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Re: Help understand cause of Shock
« Reply #16 on: November 23, 2014, 01:12:14 pm »

Gentlemen thank you all for your time.

Im back in town and making a visit to the venue tomorrow to see if I can get some answers.

is there anyway that the TC voice processor could be in play ? There are two tangents to this, the TC live power adapter is only 2 pin with no grounding, and the the receptacle where it was connected had a live ground.

Regards.
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Nitin Sidhu

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Re: Help understand cause of Shock
« Reply #17 on: November 23, 2014, 01:23:26 pm »

That's the nasty part of this RPBG wiring mistake that is so insidious - the metering you went through (congrats on that, BTW) will produce perfect results -just as you saw.
frank

A RPBG, if i understand correctly, will not meter correct. I would presume that we would now have a voltage measurement between Neutral and ground, instead of Live and ground (as per pin positions).

It does seem possible however, that the 2 pin Voice live adapter is plugged in upside down, and that is done all the time.
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John Roberts {JR}

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Re: Help understand cause of Shock
« Reply #18 on: November 23, 2014, 02:43:54 pm »

A RPBG, if i understand correctly, will not meter correct. I would presume that we would now have a voltage measurement between Neutral and ground, instead of Live and ground (as per pin positions).
A reverse polarity bootleg ground will measure good with a local meter. Neutral and ground will measure 0V difference and hot will measure full voltage to either neutral or ground, BUT the neutral and ground will actually be hot and, hot will be neutral. 
Quote
It does seem possible however, that the 2 pin Voice live adapter is plugged in upside down, and that is done all the time.

Ungrounded gear is always suspect. Units with 2 wire line cords should be double insulated. If the mic he was getting a shock from was connected to an ungrounded product, that seems like a good place to start looking.  While that product could also telescope a chassis ground (or energized chassis) from a cable shield if XLR output connected to another powered product.

It will be easier to parse out the rouge hot with a non contact tester.

JR
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Re: Help understand cause of Shock
« Reply #19 on: November 23, 2014, 03:42:17 pm »



It will be easier to parse out the rouge hot with a non contact tester.

JR

It took me a few seconds to parse THAT.  My brain was wondering why you'd say "rouge hot" rather than "red hot"...you rogue, you.
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Re: Help understand cause of Shock
« Reply #19 on: November 23, 2014, 03:42:17 pm »


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