Thread: Why An Open Neutral Kills 120V Devices

Beautifully explained in a Powerpoint file by a guy named Ken DePue, see the 2nd post in this thread.

My conversion to PDF format with corrections made to include the greek Ω symbol and some text edits for clarity:

http://soundscapesweb.com/files/SAC/Open_Neutral.pdf

Thanks for the pdf, Langston.

Golly. Even I can now understand it. That's going some.......

Thanks for posting that.

Ahh. So that is what happened. 2 open neutrals, 1 dead Crown K1 and 1 dead Crown Macrotech 2400. The moral of this story....METER EVERY SOCKET EVERY TIME NO EXCEPTIONS. Just because the uniformed "house electrician" for a large mall comes and turns the pavillion power on and says everything is good does not mean anything. I knew better and it bit me. The little $5 socket tester will show an open neutral. I carry that and a Fluke 77 so I have no excuses. This is a mistake even a beginner shouldn't have to make.

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.

Originally Posted by Emil Gawaziuk

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.

Neutral to ground will only really read zero volts at the main service entrance, where they are bonded. Code does not allow neutral and ground to be bonded anywhere else (as in, it should NOT be bonded in subpanels). So, as you get farther and farther away from the main service, and as unbalanced current is being drawn, there will be voltage on the neutral relative to ground, but it is normal.

Originally Posted by Eric Cagle

The little $5 socket tester will show an open neutral. I carry that and a Fluke 77 so I have no excuses. This is a mistake even a beginner shouldn't have to make.

Hey Eric,

I think you need to include a wiggy (solenoid voltage tester) in your tool kit. The high impedance of the multimeter can mask all sorts of problems (like an almost open neutral).

Of course the one and only time I had an intermittent neutral (loose neutral bar in another soundco's distro) I didn't have my wiggy with me!

Originally Posted by Emil Gawaziuk

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.

No, the point that was shown on the PDF is that your 2 wire devices are what will be damaged if the neutral line gets disconnected. You need 2 devices, on separate legs for anything to happen. If the neutral is broken, and you only plug stuff into one leg, it won't work, as the circuit can't complete. However, as soon as you plug something into the second leg, the circuit now can complete. Of course, for this to work, the neutrals have to be bonded at some point. If you just lose a neutral wire at the outlet, no big deal. But if the neutral for your distro gets disconnected, then you've got problems.

Originally Posted by Marlow Wilson

Hey Eric,

I think you need to include a wiggy (solenoid voltage tester) in your tool kit. The high impedance of the multimeter can mask all sorts of problems (like an almost open neutral).

Of course the one and only time I had an intermittent neutral (loose neutral bar in another soundco's distro) I didn't have my wiggy with me!

Thanks for the advice! Just ordered the Klein Wiggy. I will try not to blow up any more amps now.

Originally Posted by Silas Pradetto

Neutral to ground will only really read zero volts at the main service entrance, where they are bonded. Code does not allow neutral and ground to be bonded anywhere else (as in, it should NOT be bonded in subpanels). So, as you get farther and farther away from the main service, and as unbalanced current is being drawn, there will be voltage on the neutral relative to ground, but it is normal.

Code requires that neutral and ground be bonded at the point of origin, which is often the service entrance, but may also be a delta-Y transformer (common in larger buildings).

Originally Posted by Brian jojade

No, the point that was shown on the PDF is that your 2 wire devices are what will be damaged if the neutral line gets disconnected. You need 2 devices, on separate legs for anything to happen. If the neutral is broken, and you only plug stuff into one leg, it won't work, as the circuit can't complete. However, as soon as you plug something into the second leg, the circuit now can complete. Of course, for this to work, the neutrals have to be bonded at some point. If you just lose a neutral wire at the outlet, no big deal. But if the neutral for your distro gets disconnected, then you've got problems.

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.

Originally Posted by Emil Gawaziuk

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.

You do have a neutral path - through the device on the other leg. When the neutral wire is correctly attached to the service, it holds the voltage at 120 volts from either hot to ground. If the neutral wire is disconnected, the entire 240 or 208 volts are divided between the relative loads on the two phases. A large load like an amp or lightbulb on one side and a small load like a mixer or rack device on the other side will cause the voltage across the large load to drop well below 120 volts, and the voltage across the lightly loaded side to rise potentially well above 120 volts. In an extreme case of a near short circuit on one side will cause the load on the other side to see basically the entire 240 or 208 volts from hot to "neutral", causing equipment death.

Originally Posted by Eric Cagle

Thanks for the advice! Just ordered the Klein Wiggy. I will try not to blow up any more amps now.

Note that if you have a GFI protected outlet and you try to test between the hot and the ground you WILL trip the GFI. For us old fogys that learned most of our electrics before GFIs, the reflex of checking the hot to both ground and neutral gets us to have to reset the GFIs all the time :-)

Hello boys and girls!

I'm having a little bit of difficulty achieving that gleam of understanding in the eyes of my employees in reference to how a grand worth of damage occurred in 2 seconds at a genny powered gig recently. The crew leader (not an employee and never to work for me again) metered the genny as one of my employees watched. 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.

He called me in full CYA mode blaming the genny - I was a couple of states away at another gig. This was a very painful event and I'm determined never to allow it to happen again. Contrary to the A1's faulting the genny, no voodoo occurred and I'm convinced the genny was fine.

Of course I'm not going to tell you exactly what happened quite yet, I'm going to use you guys as guinea pigs (additional students) to help me figure out how to bring permanent understanding to my employees.

In a soon coming post, I'm going to ask some questions that will require you to do a little math just like what you saw in the PDF file linked to in my first post in this thread. The prize for the first one that answers all the questions correctly will be an Extech CT70 circuit analyzer. It's quite good, but not as good as the Suretest 61-164, thus I'm giving this one away with maybe 10 minutes of use. I'll ship anywhere in the US:

Extech CT70 Circuit Analyzer Brochure

To Begin:

I just put together a little high school level (if that) science project to bring it home to my crew. I demonstrated this thing to them today and they got quite a bit further toward the goal of mastery on this open neutral topic, but they're not quite there yet.

Here's what it looks like:



Here's how it works:



To be continued...

Kool

Mr wizard... move over...

JR

Well then, I used my iPhone 4S to video our little open neutral demonstration today.

No edits where made and we did it in two takes instead of one because I accidentally stopped the filming looking for a non-existant pause function.

The end of the second video is wonderful - I was completely taken back by the result of turning the last bulb off because I didn't realize that I'd broken the circuit. Had a good laugh and then decided to do the schematic that you see in my earlier post. :)

Show Time:

Open Neutral Demo #1: http://soundscapesweb.com/files/PSW/..._Neutral_1.mov

Open Neutral Demo #2: http://soundscapesweb.com/files/PSW/..._Neutral_2.mov

The next post will contain the opportunity to win the circuit analyzer. I have events beginning tomorrow through the weekend, so I might not get to this until next Monday. Then again, you never know. :)

To be continued...

Second video really made my day ... Thanks for a good laugh, Lang.
And please, DO make the last bulb blow

Respectfully

P.s.: Maybe you should have used different kind of loads .... PAR64s ??

Brilliant... :-)

I wonder how tolerant a LED or CFL lamp is to 2x over voltage. I notice CFL do not vary as much as incandescent with normal mains variation, so maybe stick a CFL lamp in just the last holes of each row to add some excitement. It will try to regulate for a while until it exceeds it's input voltage range (my speculation).

It would actually be pretty representative of electronic products (ASSuming it releases smoke) because the CFL has electronic circuitry inside.

Of course watch out for mercury if the CFL explodes. While LED lamp is probably too expensive to sacrifice. :-)

JR

Originally Posted by John Roberts

While LED lamp is probably too expensive to sacrifice. :-)

A friend of mine has been buying them direct from China through Ebay for very little money and they are still going strong. I would contribute one in the name of science. Here in Quebec many places are still heated with electricity (mine included, though only supplementing a woodstove) so the heat of an incandescent is mostly appreciated in the winter . I do have CFL's for lights that see a lot of use though.

Very cool demo Langston!

Langston,

Outstanding!

At the end you should have said "Inconceivable!". Never go up against a sicilian when death is on the line.