Why An Open Neutral Kills 120V Devices

Re: School is in

Langston Holland said:
The genny then magically decided to send way too much voltage to about half of the small sound and lighting rig the moment he threw the switch to energize the system.
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Without knowing any other facts, I'm going to guess that it was a 4-wire hookup to the generator, and somebody mixed up one of the hots and the neutral. One side still had 120V (just hot and neutral reversed, which contrary to Emil's first post is not a problem by itself), but the other was looking at two hots, or about 240V.

Unfortunately, my guess is not based solely on theory. There's some painful personal experience in there too.:(~:-(~:sad:

GTD

After listening to the video, I see that's not the suspected cause of the failure. I guess I'll have to wait and hear the full story.
 
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Re: School is in

Very easy to understand.. With an open neutral the devices with the smallest loads gets the higher voltage.. Learned something new today thanks.. I always carry one of these around with me to any venue before I plug up equipment.. Trust no one..
 
Re: School is in

Jason Joseph said:
Very easy to understand.. With an open neutral the devices with the smallest loads gets the higher voltage.. Learned something new today thanks..
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can't believe i'm going to admit this but...

very early in my Pro Sound Career i was trying to troubleshoot a buzz in the system and got the hair-brained idea that maybe it was being induced by the neutral somehow. so, with everything completely hooked up and running, i proceed to disconnect the neutral Camlock from the PD panel. thankfully, our PD panel had nice big analog Voltage meters, which proceeded to go all kinds of haywire and so i immediately reconnected the neutral. elapsed time of transgression probably was less than one second, which is the only reason why every bit of our gear didn't go up in smoke, along with my fledging job/career. i never did tell anyone i had done this because it was obvious in hindsight that it was a Very Stupid Thing.

While i learned that day that removing the neutral was VERY BAD i never really did know or understand why. thanks to this little demonstration i now totally get it. so thanks once again to the good people of this forum for at least teaching an old dog the reason why he should never DO this new trick....
 
Re: Why An Open Neutral Kills 120V Devices

Emil Gawaziuk said:
I still fail to see how this can destroy a device. Seeing as you have one device plugged into one leg that is minus a neutral, and you plug a second device into another outlet that is also minus a neutral, you would still have no return path on a pair of 2 wire devices. THis is assuming loss of the neutral at the service entrance to the secondary panel, or even the primary panel. Now perhaps you still have the green ground hooked up. That green ground is bonded in the panel to the neutral. That will provide the return path to the devices.

Perhaps I am misunderstanding something here.
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Here is what you would need in order for this to happen:

The two circuits, with a working neutral, are normally independent from each other. The two 120v hots would need to be opposites of each other (240v potential, not 0v) in this case.

The neutral would need to be COMMON to both circuits. In fact in many cases neutrals are often connected together (in the wiring) before they even reach the neutral bus-bar on the breaker box.

So let's say you disconnect a neutral wire at the breaker box. That might affect two different circuits, as described above. The two devices on opposing circuits will still be connected together by their respective neutral wires further down in the wiring. You now have a 240v SERIES circuit (instead of two independent 120v circuits).

I have had this exact situation happen in my house before, fortunately just with two lighting circuits, and it was a most unusual event. :)
 
Re: School is in

Geoff said:
After listening to the video, I see that's not the suspected cause of the failure. I guess I'll have to wait and hear the full story.
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I'm impressed Geoff. I've violated my promise to make my next post the one with the circuit analyzer contest questions, but this is some serious insight. :)

I'd go ahead and tell you what I've concluded about the neutral failure with the genny hookup, but my two employees have $50 each riding on giving me a good answer to this by Aug 1st. I get $50 from them if they don't figure it out. :)

I can't believe I'm about to do this, but here are some clues:

1. The genny was fine, but had no neutral to ground bond because it had just been used near a building with electrical service.
2. The genny lug bolts for the electrical cables had no color coding or understandable L/N/G markings.
3. They had a good meter with a (not helpful in this case) 10MΩ input impedance.
 
Re: School is in

I agree, sounds like a neutral/ground swap. If I understand correctly, without neutral and ground bonded, swapping them would have an effect similar to lifting the neutral, since the current would not have a return path back to the genny other than through the second hot. The ground rod (if there was one) might be able to absorb some energy, but probably not enough to hold the neutral at 0 volts.
 
Re: School is in

I'm gonna throw out the possibility that the generator has ground and neutral possibly reversed internally, and until they were separated for the special case of connecting it to a building nobody noticed or cared because the rest of the time they were bonded together inside the generator.

Jason
 
Re: School is in

I have my own idea, but I'll wait to see what develops....

But I will say that it's a Very Good Idea to meter voltages at ones Cam tails and then at the end of the feeder run. At a gig in May, we had a genset that *appeared* (by metering at the tails) to have N/G bonded when in fact they were not. The on-site technician was unable to explain or rectify the bonding issue internally. That genset developed other issues that eventually required that it be replaced.
 
Re: Why An Open Neutral Kills 120V Devices

Emil Gawaziuk said:
In a 3 wire 240V system, I can see this being a problem (as per the example given in the PDF).

One should be more concerned with miswired plugs (hot and neutral reversed. Somewhat common). A 2 wire device such as a laptop power supply or lamp would not be damaged, but a 3 wire device expecting a panel bonded safety ground would get a nice voltage surprise. The more important thing to observe when metering provided outlets or tie ins is voltage difference between the safety ground and neutral (indicating poor panel bonding. Ideally, should read zero volts), and that individual phase to both the ground and neutral is correct.

Just a thought.
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The more important thing to observe when metering provided outlets or tie ins is voltage difference between the safety ground and neutral (indicating poor panel bonding. Ideally, should read zero volts)

That's not exactly right, unless you're measuring in the service panel at the N-G and Ground Rod bonding point. The neutral wire will typically have a volt or two drop in any loaded branch circuit, unless it's feeding perfectly balanced 240-volt loads like a stove or hot water heater. If there's any unbalanced load on a hot wire in a branch circuit, there will be a corresponding and out-of-phase voltage drop on the neutral return wire. Since the safety ground wire should not be part of the neutral current return path, it should stick at earth potential. So if you meter between the ground and neutral in a wall outlet you should see the voltage vary by up to several volts (and up to half of the total voltage drop on a loaded branch circuit) depending on the unbalanced load in that branch circuit, depending on length of run, wire gauge, etc.... If you don't see at least some AC voltage (from a few tenths up to a volt or two) between the neutral and ground contacts, there's a good chance you have a bootleg ground with N-G bonding close to the outlet.

A 2 wire device such as a laptop power supply or lamp would not be damaged, but a 3 wire device expecting a panel bonded safety ground would get a nice voltage surprise.

That's an old-wives tale and incorrect. I've done this experiment hundreds of times in my No~Shock~Zone seminars using every type of appliance with a grounded power plug such as guitar amps, active speakers, and toaster ovens. See attached picture below for my test bench. Modern devices with a safety ground really don't care about the voltage difference between the safety ground and neutral and will operate normally. So a swapped Neutral and Hot really won't harm anything, but it's a serious code violation for many safety reasons.

On the other hand a RPBG (Reverse Polarity Bootleg Ground) where the Neutral and Ground are at 120 volts and the Hot wire is at 0 volts is another story. These are often found in older renovated buildings where an old ungrounded wall outlet was "upgraded" to a grounded receptacle, and the ground screw was bonded to the neutral wire as a quick fix. If the hot and neutral wires were swapped in the walls (more common than you think), then the entire appliance can have a 120-volt hot-chassis, including your mixing console, guitar amps, microphones, and instruments.

Interestingly, a RPBG outlet can't be found by a 3-light outlet tester, or even by simply metering from H-N, H-G and G-N. It will indicate as being wired correctly using any standard test procedure that doesn't have an earth reference, but this RPBG outlet will electrify the chassis anything with a grounded power plug. See Reverse Polarity Bootleg Ground Testing - YouTube for a video of this failure. By adding a cheap NCVT (Non Contact Voltage Tester) to your tool kit, you can find and avoid this costly (and deadly) mistake. I can post circuit diagrams and pictures of destroyed gear as examples later.
 

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Re: Why An Open Neutral Kills 120V Devices

Thanks for your input and work in this area Mike.

Our industry spends so much time thinking about proper equipment selection and use, but given that electricity is as ubiquitous as tap water, we've come to treat leaks in either with a similar yawn.

The approx. $1,000 hit I took at a gig a few weeks ago combined with a shocking (sorry) lack of understanding from my crew as to why it happened spawned this thread. A deadly serious respect for how the L/N/G wires are supposed to be connected and why is non-optional in our work. If you haven't already, there is a day coming when you'll find out that you've probably saved someone's life.

Carry on. :)
 
Re: Why An Open Neutral Kills 120V Devices

I wish ignorance was limited to mains wiring, but alas it is more widespread than that (different subject).

Burning up gear is bad, killing talent is generally worse (just kidding, it is always worse). :-)

Mike has done some good work in raising awareness about human safety issues..

JR
 
Re: Why An Open Neutral Kills 120V Devices

Just to confirm what many folks figured out already about what turned out to be a non-profit gig:

1. The genny was fine.
2. The genny didn't have its neutral and ground bonded.
3. The genny was switched correctly to 120/208V 3-phase.
4. The hots were connected to the genny correctly.
5. Our neutral cable was connected to the genny ground and our ground cable was connected to the genny neutral.

A take home lesson:

Metering the genny with a high quality multimeter prior to connecting our cables didn't reveal the not-so-obvious problem. Metering between the (3) hot lugs and the neutral lug showed a solid 120V. Metering between the (3) hot lugs and the ground lug very probably showed a near 120V that varied by several volts over time. It's also likely that the meter probes were applied for too short a time to see this variation.

The reason a high impedance multimeter can read what appears to be close to normal voltage between hot and an unbonded ground is due to the capacitance between that ground and nearby neutral. Thus what is needed for proper electrical source measurements is a load - but not your sound system! :) A cheap electric trouble light or heater or the like will zero that capacitive voltage reading. A fancy way to apply a load is using a circuit analyzer such as the one I'm giving away to the first guy that answers the questions correctly (see my next post). These circuit analyzers apply a 15 to 20 amp resistive load on the circuit for 1/2 to 1 cycle of the 50Hz or 60Hz sine. This allows them to use a low wattage resistor and have the device ready (cooled down) for another measurement after 20 seconds.

The problem with using an electrical source without neutral and ground bonding:

If you connect the cables correctly, everything should work properly, thus you may think all is well. I wonder how many times we've used gennys without neutral/ground bonding successfully and not known it? Regardless, it won't happen to me again. How about you?

If a powered piece of equipment malfunctions with the hot connecting to chassis there is no return path through the ground wire to trip the breaker, thus the chassis remains electrified. This obviously is potentially deadly to anyone that touches the malfunctioning component. The component may still work fine with a hot chassis.

If this happens to be a genny gig and the genny doesn't have continuity with the ground due to its rubber tires and the trailer tongue is insulated from the ground in some fashion, the electrified chassis may not shock the person touching it. But don't worry - you can still die - what is required is completing the circuit with your body by touching the hot chassis and another component with an issue.
 
The Test

The first one to answer the (18) questions correctly wins the circuit analyzer or ONE MILLION DOLLARS[SUP]*[/SUP]

Aleš, John and Mike are disqualified just because and my budget for shipping is about $20. :)

Assumptions: (refer to open neutral demonstration board schematic).

B = Incandescent light bulbs rated at 120V, 25 watts. All are in working order and maintain constant resistance regardless of applied voltage.
TP = Test Point that will accept multimeter probe.
Switch closed = Circuit connected (solid line)
Switch open = Circuit broken (dashed line)
N = Neutral
L1 = 120V relative to N, 240V relative to L2.
L2 = 120V relative to N, 240V relative to L1.
The dash between like components means "through".
Each bulb means all (10) bulbs! :)

Concepts:

Series means one after the other. The bulbs B5 and B6 are connected in series. If a bulb in series stops passing electricity, the other bulb turns off.

Parallel means each bulb is wired identically to the others. The (4) bulbs B7 - B10 are wired in parallel. If a bulb in parallel stops passing electricity, the others still work.

Relevant portions of Ohm's Law: (once R is known, solve for I, then E)

Resistance = R = E^2/W
Current = I = E/R
Voltage = E = IR

Questions:

  1. What is the resistance of a single bulb?
  2. What is the series resistance of B5 and B6?
  3. What is the voltage between TP1 and TP3?
  4. How much voltage does B5 see?
  5. How much voltage does B6 see?
  6. What is the parallel resistance of (2) bulbs?
  7. What is the parallel resistance of (3) bulbs?
  8. What is the parallel resistance of (4) bulbs?
  9. With S1 - S5 closed, how much voltage does each bulb see?
  10. With S1 - S4 closed, S5 open, how much voltage does each bulb see?
  11. With S1 open, S2 - S4 closed, S5 open, how much voltage does each bulb see?
  12. With S1 - S2 open, S3 - S4 closed, S5 open, how much voltage does each bulb see?
  13. With S1 - S3 open, S4 closed, S5 open, how much voltage does each bulb see?
  14. With S1 - S4 open, S5 open, how much voltage does each bulb see?
  15. With S1 open, S2 - S4 closed, S5 closed, how much voltage does each bulb see?
  16. With S1 - S2 open, S3 - S4 closed, S5 closed, how much voltage does each bulb see?
  17. With S1 - S3 open, S4 closed, S5 closed, how much voltage does each bulb see?
  18. With S1 - S4 open, S5 closed, how much voltage does each bulb see?

* At my discretion.
 
Re: The Test

PM sent

PS: for those who are hoping we're both wrong, or are doing the exercise for the educational value (highly recommended). you don't have to calculate the current at all. treat S1-4/S5-10 as a resistor divider (when S5 is open) and multiply the ratio by 240V to figure out what portion of the 240V each half gets
of course, the point of the exercise is that once S5 is closed (neutral properly bonded) it is no longer a divider, and each side gets 120V regardless of how many lights are on.

Jason
 
Re: The Test

Both Cody's and Jason's answers were flawless, but Cody was first...

But Jason's comments about the test are so good that I'm going to send him one too. :)

You two need to PM me with your shipping addresses - thanks.

---

The rest of you really should do the test yourself as the questions are purposefully designed to bring mastery to your understanding of the open neutral concept applied to what we do.

PM me for the answers once you're ready to check your work. Though I rather not post all the answers outright, feel free to post with specific questions including your preliminary answers and Cody or Jason will help you. :)
 


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