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Author Topic: Hot and Neutral swapped.  (Read 12565 times)

Kevin Maxwell

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Hot and Neutral swapped.
« on: September 18, 2014, 11:08:57 am »

Hot and Neutral swapped.

Yesterday when installing a new mixer in a church gymatorium I was checking the outlets and when I checked one of the stage outlets I discovered that the hot and neutral were swapped. It is used to power instruments like the keyboards on stage. My question is how dangerous is that? I know it is bad I am just wondering how bad? I figured that with the right combination of errors someone could windup having a hot instrument on stage and then they would complete the circuit with their lips to a grounded mic. And then they would need to get another instrumentalist.   >:(

I figured I would rewire the outlet properly so I found the circuit breaker and turned it off and swapped the wires. I also found that the mis-wired neutral was also not tight at all. So after I rewired it I turned the breaker back on and it immediately popped. This circuit seemed to be feeding else ware also. I opened it back up again and removed the jumper wires (that were wire nutted to the incoming lines) completely from the box and put it back together disabled.

Before I had removed the old mixer I checked to see what was working and what wasnít. I discovered that a keyboard that was plugged into that mis-wired outlet was clearly receiving an AM radio station (1500) thatís transmitter is less then a mile away. When the keyboard was powered down the station got real loud. It was being feed into a cheap DI, when we put it into a slightly better DI the station got quieter but it was still there. I donít know if that is caused or made worse by the bad outlet or is a completely different problem.

I have put in a call to the facilities manager to fill them in on my findings. 
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Steve M Smith

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Re: Hot and Neutral swapped.
« Reply #1 on: September 18, 2014, 11:15:36 am »

Whilst it's not right, it's not really dangerous in practice.  As far as the equipment is concerned, live and neutral are electrically the same.

Sounds like you have some other problems as well though!


Steve.
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Mike Sokol

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Re: Hot and Neutral swapped.
« Reply #2 on: September 18, 2014, 11:32:50 am »

Hot and Neutral swapped.

Yesterday when installing a new mixer in a church gymatorium I was checking the outlets and when I checked one of the stage outlets I discovered that the hot and neutral were swapped. It is used to power instruments like the keyboards on stage. My question is how dangerous is that? I know it is bad I am just wondering how bad? I figured that with the right combination of errors someone could windup having a hot instrument on stage and then they would complete the circuit with their lips to a grounded mic. And then they would need to get another instrumentalist.   >:(

Look very carefully to see what the green/ground screw on the receptacle was connected to. If it was jumped to the white/neutral screw, then you have a classic and somewhat dangerous  "bootleg ground". If you have a bootleg ground PLUS reversed polarity, the you have an RPBG (Reverse Polarity Bootleg Ground), which is VERY dangerous. See http://ecmweb.com/contractor/failures-outlet-testing-exposed But neither a standard bootleg ground or RPBG can be found with a standard outlet test, so exactly how did you discover that the outlet had H-N swapped? Those 3-light cube testers will lead you astray in a hurry, so you need to do an actual metered voltage test against a known ground to be sure of what's happening.

From your description I would guess that the radio in the keyboard is caused by a floating (EGC) ground somehow. That in itself can be dangerous since any leakage current to the chassis in a back-line amp could electrify a guitar with potentially lethal results.

And there's no GFCI on this outlet, correct? So that trip was the current circuit breaker in the panel? If so, then you have something seriously wrong with the Ground-Neutral separation in that branch circuit.  I would suggest you get a licensed electrician in there to check this (and all other) outlets for correct wiring. Where you find one rat, you'll often find a bunch more....  >:(
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John Roberts {JR}

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Re: Hot and Neutral swapped.
« Reply #3 on: September 18, 2014, 11:42:27 am »

Hot and neutral reversed is generally not dangerous in modern 2-wire line cord gear. Inside such products the primary circuits are double insulated. 3-wire line cord products are less safe, since they are single insulated and rely upon the safety ground to shunt a primary fault and take out the breaker. A floating ground outlet, or cut off ground pin, could result in an energized chassis from a primary fault.

As has been widely stated the real killer is when a 3 wire outlet is wired to a 2 wire drop, and the ground pin is boot-legged to neutral, that in the reverse hot-neutral case is not neutral at all. This can and does kill people when they get across two chassis grounds with full mains potential between them.

JR
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Kevin Maxwell

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Re: Hot and Neutral swapped.
« Reply #4 on: September 18, 2014, 12:44:43 pm »

Look very carefully to see what the green/ground screw on the receptacle was connected to. If it was jumped to the white/neutral screw, then you have a classic and somewhat dangerous  "bootleg ground". If you have a bootleg ground PLUS reversed polarity, the you have an RPBG (Reverse Polarity Bootleg Ground), which is VERY dangerous. See http://ecmweb.com/contractor/failures-outlet-testing-exposed But neither a standard bootleg ground or RPBG can be found with a standard outlet test, so exactly how did you discover that the outlet had H-N swapped? Those 3-light cube testers will lead you astray in a hurry, so you need to do an actual metered voltage test against a known ground to be sure of what's happening.

From your description I would guess that the radio in the keyboard is caused by a floating (EGC) ground somehow. That in itself can be dangerous since any leakage current to the chassis in a back-line amp could electrify a guitar with potentially lethal results.

And there's no GFCI on this outlet, correct? So that trip was the current circuit breaker in the panel? If so, then you have something seriously wrong with the Ground-Neutral separation in that branch circuit.  I would suggest you get a licensed electrician in there to check this (and all other) outlets for correct wiring. Where you find one rat, you'll often find a bunch more....  >:(

I used a 3 light cube tester (Sperry Instruments) and it indicated that the hot and neutral were reversed. So I pulled the outlet apart and then metered the wires. The ground was thru the box itself it looked like it was metal clad wiring. And the black wire metered 120v to the box or to the white wire and the box to white wire metered nothing. 

No GFI it was the breaker in the panel that tripped.

When the property manager calls me back I will tell them to get an electrician in there to do it right. Now that I think of it I found another wiring problem in one of their other buildings, the one where their sanctuary is. And I think that was also a hot and neutral reversed. Isnít one of the problems with that when the power switch on a piece of equipment only switches the hot leg (as I assume from what I have seen is the standard way they do it) and it has a ground then you still have power in the device? And could have a hot chassis?   
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John Roberts {JR}

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Re: Hot and Neutral swapped.
« Reply #5 on: September 18, 2014, 12:57:15 pm »

Isnít one of the problems with that when the power switch on a piece of equipment only switches the hot leg (as I assume from what I have seen is the standard way they do it) and it has a ground then you still have power in the device? And could have a hot chassis?
No... Both hot and neutral are always at least single-insulated from the chassis irrespective of the power switch.

Note: some old guitar amps used a 2-wire line cord and a stinger cap from one leg to chassis, presumably to reduce hum by cap coupling the chassis to neutral. However if a cap is too conductive you can get a slight shock from those. The caps were originally sized to not be a human safety problem, but they stopped doing it. And if somebody replaced the stinger cap, who knows what happens.   
 
JR
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Mike Sokol

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Re: Hot and Neutral swapped.
« Reply #6 on: September 18, 2014, 12:58:49 pm »

And I think that was also a hot and neutral reversed. Isnít one of the problems with that when the power switch on a piece of equipment only switches the hot leg (as I assume from what I have seen is the standard way they do it) and it has a ground then you still have power in the device? And could have a hot chassis?

By itself, if only the hot and neutral are reversed, then any PROPERLY wired gear will operate normally. It really doesn't care if the hot or the neutral side is swapped since for sound gear it's always powering a transformer of some sorts. It won't draw power when the switch is off (as sometimes reported) and it won't create voltage on the chassis (as also erroneously reported).

However, a swapped H-N does become very dangerous when you're troubleshooting inside of energized gear like a guitar amp since you can easily get your hands on a "white" wire which is really the "black" wire. And turning off the gear's power switch won't denergize the transformer windings which leaves all sorts of things inside the chassis hot and potentially shocking for the technician doing testing. But on the outside of the "box" that should not matter at all. It should operate normally.

However, any (and especially multiple) hot-neutral swapped outlets in a building hints at very sloppy contractor wiring, with perhaps a lot more bad things going on. My main concern is that it should NOT have tripped the circuit breaker when you swapped the hot and neutral wires on the outlet itself. So something very fishy is going on here. I think your AC power system needs a very close look by someone who understands AC power for audio.

BTW: A swapped Neutral and Ground is even harder to find, but does all sorts of crazy things with ground loop hum. Guess I'll have to include how that works in my Ground Loop Sticky.
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Kevin Maxwell

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Re: Hot and Neutral swapped.
« Reply #7 on: September 18, 2014, 01:10:08 pm »

BTW: A swapped Neutral and Ground is even harder to find, but does all sorts of crazy things with ground loop hum. Guess I'll have to include how that works in my Ground Loop Sticky.

I thought everyone knew by now that the real reason that a sound system hums is because it doesnít know the words.   ;D ;D ;D ;D ;D
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Stephen Swaffer

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Re: Hot and Neutral swapped.
« Reply #8 on: September 18, 2014, 01:59:37 pm »


However, any (and especially multiple) hot-neutral swapped outlets in a building hints at very sloppy contractor wiring, with perhaps a lot more bad things going on. My main concern is that it should NOT have tripped the circuit breaker when you swapped the hot and neutral wires on the outlet itself. So something very fishy is going on here. I think your AC power system needs a very close look by someone who understands AC power for audio.

+1

I spent the morning sorting out a 3 way switch that had a few extra wires thrown into each box for good measure-and no EGC in the old romex.  My fluke DMM was reading anywhere from 6 to 145 volts on wires that were disconnected on both ends.  Kevin mentioned to gound was to the box-what was the box grounded to?  DMM readings can be misleading when grounding is questionable or non existent.
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Steve Swaffer

Mike Sokol

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Re: Hot and Neutral swapped.
« Reply #9 on: September 18, 2014, 06:02:23 pm »

+1

I spent the morning sorting out a 3 way switch that had a few extra wires thrown into each box for good measure-and no EGC in the old romex.  My fluke DMM was reading anywhere from 6 to 145 volts on wires that were disconnected on both ends.  Kevin mentioned to ground was to the box-what was the box grounded to?  DMM readings can be misleading when grounding is questionable or non existent.

I used to keep a drop light with a 60-watt incandescent bulb with big insulated alligator clips just for loading circuits like that. Open wires in parallel with live wires can develop so-called "ghost voltages" that the high impedance of a DMM won't meter properly. You need to make sure any voltage can sustain a real current, so a 60 to 100 watt bulb acts as an ideal "load" resistor.
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Mike Sokol
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Re: Hot and Neutral swapped.
¬ę Reply #9 on: September 18, 2014, 06:02:23 pm ¬Ľ


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