Avoid open neutrals!

[Mattb4]

I should have been paying more attention category.

I am in the process of setting up a AIO powering a sub panel with critical loads. I wanted to be able to switch the panel back to the mains if there was a problem with the AIO setup. So I wired up a manual transfer switch with both the positive and neutral being switched. Only one problem I forgot to actually connect the wire to the neutral from the main. Should have let the coffee this morning work a bit longer. So when I flipped the power on for a quick test I had open neutral conditions. This in itself would have been OK since I had thought I had off all loads but I did not. There was a surge protector power strip with a wall wart charger that I had neglected to unplug.

Bzap! I immediately flip the power back off and start to figure out what I had screwed up. The surge strip is toast although the wall wart appears OK.

Not the most brilliant thing I have done.

 

[goldserve]

Did something similar. I believe I sent 240v to several appliances and I had to replace some input mov, etc.

 

[SparkyJJO]

Weird - an open neutral should have just resulted in an open circuit, which should have just made it not function. Not sure how you'd have 240V.

 

[goldserve]

I don’t know what happened exactly but the neutral that I was working on in the panel was part of a shared neutral where someone ran two phases and one neutral. I popped the breaker for one phase and then proceeded to disconnect the neutral.

 

[newbostonconst]

Weird - an open neutral should have just resulted in an open circuit, which should have just made it not function. Not sure how you'd have 240V.

Agree but maybe the surge suppressor in the outlet strip dumped the power to ground.

 

[Quattrohead]

Weird - an open neutral should have just resulted in an open circuit, which should have just made it not function. Not sure how you'd have 240V.

Unfortunately this is the most basic mis-understanding of the USA split phase system that is causing so many problems.
Ultimately, power will find it's way through other appliances and blow some shit up.

 

[SparkyJJO]

I've had an open neutral on a circuit before due to a wiring fault I was tracking down. The circuit simply didn't work. That's what befuddles me, how did something blow up because of an open neutral? Open neutral should result in an open circuit. There's no L2 anywhere on a circuit connected to only L1 and neutral (or vise versa).

If L1 and L2 are not both present, then there is no way for there to be 240V at that appliance.

I get electricity can do weird things though. It just doesn't make sense.

 

[John Frum]

I made this series of drawing to illustrate split phase power

Flowchart Maker & Online Diagram Software

tinyurl.com

Notice how current flows from 1 leg to the other via the common neutral.

 

[Mattb4]

Agree but maybe the surge suppressor in the outlet strip dumped the power to ground.

That is what I figured.

ETA: My knolwedge of surge suppresors is very little. I am not sure if the wall wart contributed to it going up in smoke or it was incidental. The surge suppressor is grounded plug but the wall wart (USB phone charger) is a non-polarity 2 prong plug.

 

[Mattb4]

Weird - an open neutral should have just resulted in an open circuit, which should have just made it not function. Not sure how you'd have 240V.

In my case it was only 120vAC.

 

[Hedges]

The loads on L1 and L2 are in series. Without neutral connected, their relative resistance determines how much voltage each sees.

umdberg / Example: Resistors in series

If you put 240V across 240 ohms, 1A flows.
If your two 120V phases each have 120 ohms load, the 240V source sees 120 + 120 = 240 ohms and 1A flows. Nothing flows in neutral.
With 1A through 120 ohms, 120V is created, so the loads divide the voltage and have same voltage between them as neutral, which is why no current has to flow in neutral.

If you put a 12 ohm heater on one phase and a 228 ohm resistor on the other, so long as neutral is connected, they carry 120/12 = 10A and 120/228 = 0.53A. Neutral carries 10A - 0.53A = 9.47A

If neutral is disconnected, 240V / (12 ohm + 228 ohm) = 240V / 240 ohm = 1A flows.
1A x 12 ohms = 12V across heater. 1A x 228 ohms = 228V across the small load, which is too much for it.

The surge arrestor similarly saw nearly 240V across it, higher than the 170Vpeak it expected, and carried current.
It was only expected to see spikes of 1 millisecond or 0.1 millisecond duration, couldn't handle continuous power dissipation.

 

[John Frum]

The surge arrestor similarly saw nearly 240V across it, higher than the 170Vpeak it expected, and carried current.

Because its a varistor.

Varistor - Wikipedia

en.wikipedia.org

 

[Mattb4]

Appreciate the explanation Hedges.

 

[SparkyJJO]

I understand how split phase works.

Pardon my quick MS Paint drawing. But you get the idea. Broken neutral connection = open circuit. There is 120V to ground (or neutral) from L1. Same from L2. 240V between L1 and L2. But, a broken neutral does not automatically give a connection to L2. There IS no both L1 and L2 at a traditional USA household outlet.


Now the exception could be if you had a circuit like my old house's kitchen used to be wired, where L1 went to the top half of a duplex outlet, and L2 went to the bottom half, and they shared the neutral, and then that neutral got broken, AND you had something plugged into both top and bottom of the duplex outlet. THEN you could have some funky stuff possibly happen, yes. Maybe that's what happened here? That isn't a common way of wiring things, and I don't know if they still do that anymore or not. When I redid the kitchen in my old house I changed the wiring to eliminate that split wiring as it was just asking for trouble.

 

[Mattb4]

I understand how split phase works.

Pardon my quick MS Paint drawing. But you get the idea. Broken neutral connection = open circuit. There is 120V to ground (or neutral) from L1. Same from L2. 240V between L1 and L2. But, a broken neutral does not automatically give a connection to L2. There IS no both L1 and L2 at a traditional USA household outlet.
View attachment 105508

Now the exception could be if you had a circuit like my old house's kitchen used to be wired, where L1 went to the top half of a duplex outlet, and L2 went to the bottom half, and they shared the neutral, and then that neutral got broken, AND you had something plugged into both top and bottom of the duplex outlet. THEN you could have some funky stuff possibly happen, yes. Maybe that's what happened here? That isn't a common way of wiring things, and I don't know if they still do that anymore or not. When I redid the kitchen in my old house I changed the wiring to eliminate that split wiring as it was just asking for trouble.

My outlets are wired with just the one leg. That is why I would think like you that I had only 120vAC availble. As I mentioned my knowledge of how surge suppressors work is very little. Stuff like that is in the ET (electronic tech) realm.

 

[efficientPV]

I had that experience twice when a tree trimmer let a tiny branch land on my incoming grid power. It took out the refrigerator, dishwasher, microwave, DVD player and a clock. That would have been pretty expensive for most people. What happens is existing loads to neutral unbalance L1 and L2. One line can see 200V and the other 40V. UL changed the ruling years back when many used 150V MOV and raised it to a much higher voltage. Too many of these were failing.
* months later my lights were flashing and got the electric company back because the incoming lines weren't balanced. I could see from a ladder the tape on one crimp crimp was cracked. When the guy came he could pull all three wires out of the crimps. Prior repair had used the wrong size crimp. That guy said he was retiring in a week.

 

[Hedges]

But, a broken neutral does not automatically give a connection to L2. There IS no both L1 and L2 at a traditional USA household outlet.
View attachment 105508

Now the exception could be if you had a circuit like my old house's kitchen used to be wired, where L1 went to the top half of a duplex outlet, and L2 went to the bottom half, and they shared the neutral, and then that neutral got broken, AND you had something plugged into both top and bottom of the duplex outlet.

Open neutral as you've shown it would open circuit, of course.
Shared neutral, either at a modified duplex outlet as you mention, or branch circuits with shared neutral wire, would put loads on two phases in series.

But consider what OP said he did:

I am in the process of setting up a AIO powering a sub panel with critical loads. I wanted to be able to switch the panel back to the mains if there was a problem with the AIO setup. So I wired up a manual transfer switch with both the positive and neutral being switched. Only one problem I forgot to actually connect the wire to the neutral from the main.

He forgot neutral from main, so L1 and L2 from main went to sub panel. With multiple circuits connected to sub panel, that is a bunch on L1 and a bunch on L2, but no neutral. So they negotiate who gets how much voltage.

 

[Hedges]

Was switching neutral necessary? Advisable? Do you bond neutral to ground locally?

It gets switched in mobile applications, along with neutral/ground bond being made locally, when shore power is disconnected. Or, could be shore power failed but still connected, can't tell the difference, so want to separate from neutral's (presumably ground bonded) shore power when bonding locally.

For my fixed application, I have neutral and ground always coming from main panel, only switch L1 & L2.

 

[Mattb4]

Was switching neutral necessary? Advisable? Do you bond neutral to ground locally?

It gets switched in mobile applications, along with neutral/ground bond being made locally, when shore power is disconnected. Or, could be shore power failed but still connected, can't tell the difference, so want to separate from neutral's (presumably ground bonded) shore power when bonding locally.

For my fixed application, I have neutral and ground always coming from main panel, only switch L1 & L2.

My reasoning for switching neutrals was based on having only the neutral come from the AIO to the sub panel when operating. Possibly not really needed. My neutral ground bond is at the main panel. The sub panel is not bonded.

BTW once I hooked up the neutral wire I have no further problems.

 

[SparkyJJO]

Which inverter is it?

My Radian has Grid, Gen, and Output separately labeled but it is just for convenience and simplicity when looking at the wiring. All the neutrals are common.

Given how I've seen generator installs and such things, I don't know if switching your neutral is necessary. Of course, defer to the documentation of your inverter but most seem to keep the neutrals always connected.