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That little A/C receptacle tester....

#21
cavemanbass said:
Many thanks to OP for sharing, have to admit I have a tester like that and never use it. Plan to change that!

Question about the original incident... if OP had used a simple passive DI instead of the DI in his amp, would he still have fried his amp? Just the DI box? Both / Neither? I had never thought about this, I have a passive DI on my pedal board just for convenience... but if something was going to blow I'd prefer it was a $200 box rather then an $800 box...
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Passive DI wouldn't have necessarily prevented the issue either, the fault it through the ground of pin 1 on the DI to transfer to the ground of the amp. The input ground of the amp usually routes back to the star ground via PCB traces, so this would still be vulnerable. If the pin 1 ground lift was switched open, you wouldn't have had the fault current but you could be exposed to shock instead.
 
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#22
The situation with a piece of equipment plugged into a faulty circuit on one side of the DI, and a piece of equipment connected to a correctly wired circuit on the other side, assuming the ground was lifted, depends on which side the equipment you are operating is connected to. If your amp is on the good side, and the ground connection to the other equipment is open, you would be safe, and not suffer any damage. If you are touching equipment connected directly to the faulty circuit, you are at risk of electrocution.

Once you flip the ground lift switch to connect the two ground circuits you and your equipment are at risk. Some point in the circuit that is the weakest link due to conductor cross section area will most likely burn up before the 15 or 20 amp circuit breaker trips, but it might take enough time for you to receive a shock, and other circuit elements to get fried. If the fault opens a circuit and does not trip the circuit breaker, and if you are touching equipment on the faulty circuit after the smoke clears, you are still at risk of electrocution.

Maybe a low amperage fuse (or two, depending on the construction) in a DI box would offer added protection to equipment subject to these fault conditions, but it doesn't take very much current (through the right path) in a human body to cause death.
 
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#25
Hoochie Coochie Man said:
The situation with a piece of equipment plugged into a faulty circuit on one side and of the DI, and a piece of equipment connected to a correctly wired circuit on the other side, assuming the ground was lifted, depends on which side the equipment you are operating is connected to. If your amp is on the good side, and the ground connection to the other equipment is open, you would be safe, and not suffer any damage. If you are touching equipment connected directly to the faulty circuit, you are at risk of electrocution.

Once you flip the ground lift switch to connect the two ground circuits you and your equipment are at risk. Some point in the circuit that is the weakest link due to conductor cross section area will most likely burn up before the 15 or 20 amp circuit breaker trips, but it might take enough time for you to receive a shock, and other circuit element to get fried. If the fault opens a circuit and does not trip the circuit breaker, and if you are touching equipment on the faulty circuit after the smoke clears, you are still at risk of electrocution.

Maybe a low amperage fuse (or two, depending on the construction) in a DI box would offer added protection to equipment subject to these fault conditions, but it doesn't take very much current (through the right path) in a human body to cause death.
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If the console was connected to the incorrectly wired power (hot/gnd reversed), there would still be 120V between the mics (handle/grille) which would be st 120V and the bass strings which would be at ground.

It's simply a reversed perspective, but the danger remains.
 
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#26
agedhorse said:
If the console was connected to the incorrectly wired power (hot/gnd reversed), there would still be 120V between the mics (handle/grille) which would be st 120V and the bass strings which would be at ground.

It's simply a reversed perspective, but the danger remains.
Click to expand...

I've gotten one to the lips touching a mic back in the days before three wire power cords and such.

One more reason I don't sing.;)
 
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#29
Hoochie Coochie Man said:
I've gotten one to the lips touching a mic back in the days before three wire power cords and such.

One more reason I don't sing.;)
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#30
It would be interesting to know the actual cause.
Faulty premises wiring?
Where did the remote broadcast equipment get power? Did they use a generator?
Was there a fault inside a piece of equipment connected to the premises wiring?

I could see if it was at an old venue that had undergone work by unqualified people, but an event hall at a theme park is likely to employ a qualified electrical maintenance worker, and not have been built before equipment grounding was required.

If there had been an injury and an insurance claim involved, or an ambulance call with a police report, an investigation would most likely have been done to determine what caused it.
 
#31
All equipment was plugged into the receptacles in the building. I was on the stage and the radio station had plugged onto a wall receptacle to the side of the stage. The wall receptacle was wired properly. The A/C on the stage was brought to me on an extension that was plugged in somewhere on stage. It was dark and wires were running everywhere so it was almost impossible to see where it actually came from. I had enough extensions with me that I was able to plug into the same receptacle the radio station equipment was plugged into...after I found that the A/C coming to me on stage was wired wrong.

In any case, the lesson is clear. I will check EVERY outlet before connecting. I will even check those I have used before just in case something has changed.
 
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#32
Reversed neutral and ground won't cause a problem unless there is a fault within the amplifier (or other equipment), safety standards insure that both sides of the primary are isolated from the chassis under all conditions.
 
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#33
This also happened to me at a rehearsal! I plugged my amp into a different outlet than normal and plugged an xlr into my amp d.i., then into the mixer channel. BOOM! blew the circuit breaker, fried both the mixer channel and my amp.

I wonder if the radio station's mixer was damaged like ours was.

I grabbed my circuit tester (which I use for work in machine repair) and this one outlet was wired backwards. I'd been rehearsing there for years, but no one had ever used that outlet for anything but a lamp.
 
#37
Checking my understand of the thread so far:

So the cause was most likely a miswired receptacle, with the hot and neutral wires reversed? And with one piece of gear plugged into the reversed outlet and another into a different (correctly wired) outlet, with a link between them via the DI line, there'd be a large potential between the two chassis?
 
#38
agedhorse said:
If the console was connected to the incorrectly wired power (hot/gnd reversed), there would still be 120V between the mics (handle/grille) which would be st 120V and the bass strings which would be at ground.

It's simply a reversed perspective, but the danger remains.
Click to expand...

Talking of such things, I am old enough to remember the sad fate of guitarist Les Harvey from Stone The Crows, back in the days when wiring faults were much more common...
 
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#39
Chuck M said:
Played a live radio show last night. The event is a volunteer event where gifts for underprivileged folks are wrapped. I have played it for many years and this year we were in a new venue....and that is where the trouble starts.

I arrived and the sound man for the radio station asked what I needed to get hooked up. I told him I just needed power and that he could plug in to the D.I. on my Aguilar AG 700. So he brings me power from an extension cord and I power up my amp and it is working fine until he plugs into the D.I.! Sparks...smoke..and a blown AG 700 and a blown Mackie Mixer (the radio station's mixer). Great.

I went to my car and retrieved my back up head..Genz Benz Shuttle 9.2. Plug it in but then thought about that little receptacle tester in my bag...plugged in the tester and it showed the power was wired wrong!

Got extension cords and ran power from a correctly wired receptacle and thought all was right. My amp powered up fine and sounded good. Then the sound guy went to plug in to the D.I. and again I see sparks fly! The Shuttle was still working but I had fears about the D.I. being damaged. Played the gig and came home and checked the Shuttle and, sure enough, the D.I. is damaged.

The radio station is going to pay for the repairs but, man, I sure wish I had used that little receptacle tester before I turned on my Aguilar.

This is a link to the tester.

https://www.amazon.com/Electrical-Receptacle-Outlet-Ground-Tester/dp/B0012DHVQ0

Get one and use it EVERY time you go to a new venue.
Click to expand...

That sucks. It bothers me when some people are just cavalier about electrical safety. A few years ago I was working with an acoustic guitar player who had a three-prong to two-prong adapter on the plug of his amp. Both his amp and mine had DI feeds to the mixer. I told him if he uses that adapter I was going to pack up and go home because I was not going to have my cable, DI, and amp be his path to safety ground if anything went wrong in his amp. He protested that without that adapter he gets buzz but it always goes away when he lifts the safety ground. I stood my ground. I pulled the adapter off and plugged his amp in. There was no buzz.

Ironically, those adapters are actually intended for making devices safer by allowing you to plug a three-prong plug into a two-prong outlet and connect the safety ground tab or lead to the center screw.
 
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#40
Ewo said:
Checking my understand of the thread so far:

So the cause was most likely a miswired receptacle, with the hot and neutral wires reversed? And with one piece of gear plugged into the reversed outlet and another into a different (correctly wired) outlet, with a link between them via the DI line, there'd be a large potential between the two chassis?
Click to expand...

It has to be hot and ground reversed. That's much worse than hot and neutral reversed.

The chassis should not be connected to neutral.
 
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