Hot connected to neutral

Status
Not open for further replies.
T

Tank11

Member
Location
CO
A 400-amp, single phase service was cut by an excavator. The electrician then reconnected the service feeder and within 24 hours, two fires started downstream. There is an argument that one of the hot conductors was connected to the neutral in error. Wouldn't this cause an immediate short and a catastrophic event? There was no main breaker but the utility transformer was not damaged.

There were reports of people getting mild shocks before the fire started.

My theory is that a connection was pulled loose when the line was cut and a loose neutral condition occurred.

Any thoughts?
 
Tank11 said:
A 400-amp, single phase service was cut by an excavator. The electrician then reconnected the service feeder and within 24 hours, two fires started downstream. There is an argument that one of the hot conductors was connected to the neutral in error. Wouldn't this cause an immediate short and a catastrophic event?
Click to expand...
No. Assuming the service neutral was earthed at the utility transformer, and earthed at the customer service, there would be a current path through the earth, with potentially a lot of current flowing. That could easily cause excessive heating that would lead to fires.

But the earth path has a high enough impedance that it would not trip any breakers or be an "immediate" short or catastrophic event.

Cheers, Wayne
 
The "mild shock" could indicate a phase-neutral swap. NO reported damage to any appliances, lighting. etc. ??
 
Tank11 said:
There is an argument that one of the hot conductors was connected to the neutral in error. Wouldn't this cause an immediate short and a catastrophic event? There was no main breaker but the utility transformer was not damaged.

There were reports of people getting mild shocks before the fire started.
Click to expand...

Tank11 said:
The wire was direct burial cable and there was no earthing at the customer service
Click to expand...

If there is no connection between the premises neutral and the utility neutral, then there is no short circuit.

It is very common when there are metal underground water pipes that the pipes can serve as this connection. CATV cables can also provide a neutral path. But without these metallic paths a hot to neutral swap would not carry much current.

-Jon
 
With direct burial, usually the neutral is smaller, so identifying it should be easy when reconnecting. Either bad splice on the neutral when they reconnected, or was broken long enough to damage the load. Did they complain of getting shocked after it was fixed?
 
There were reports of getting shocked after it was fixed. However there were no reports of 220-volt equipment not working nor were there reports of equipment breaking down due to too much voltage, which I believe would have happened if the neutral and hot were switched at reconnection.
 
With what you've described, I think more likely than hot and neutral being swapped is a poor reconnection of the neutral, either high impedance or intermittently open.

A swap would only cause an immediate short if there were a separate low impedance connection between utility ground and premises ground, which should not be assumed. However, it's unlikely that 240V appliances would work on 120V, or that the normally 120V circuits receiving 240V would not get noticed immediately.

A high impedance or intermittently open neutral connection, on the other hand, might not be obvious until there was either high current draw, or when the neutral connection opened somewhat or entirely. Once one of those things happens,especially an entirely open connection, it would likely burn up 120V devices. The connection doesn't have to completely open; a poor connection (low conductance, high impedance) would still likely result it very unbalanced voltages and damage to stuff on the higher voltage leg. Heat or vibration could be an immediate cause of change.
 
If the hot conductor was connected to the neutral terminal, and the neutral conductor was connected to the hot terminal, wouldn't every outlet in the house have reversed polarity?
Also, If that's the case, a majority of the current would be traveling back to the transformer via the low impedance conductor, but a trickle will also be traveling back via the ground. (Electricity takes all paths of resistance). Also, a single ground to neutral short anywhere in the building, would cause some current on lots of bonded metal parts. Unless I'm thinking of this wrong. Check the outlets with an outlet tester for polarity swap.
 
Sea Nile said:
If the hot conductor was connected to the neutral terminal, and the neutral conductor was connected to the hot terminal, wouldn't every outlet in the house have reversed polarity?
Click to expand...

It's AC, so 'reversed polarity' is of limited meaning. The 'neutral' (white) wiring in the house is still bonded and/or grounded at the service, so that's still your grounded conductor for testing purposes inside the house, and the only thing that really changes is the voltages.

Let's assume that any earth conductance back to the utility's grounding of the neutral is negligible; ( i.e. let's leave aside any shock hazards to actual 'earth' rather than to EGCs and bonded metal.) . And let's say that before the swap the neutral was properly bonded to the EGCs at the service. And let's specify that it was L1 (black) and neutral that were swapped, but we'll continue to refer to L1, L2 and N as they are labeled in the house. What you get then within the house is 120V between the L1 and N, 240V between L2 and N, and 120V between L1 and L2. And N is still bonded to EGCs, and thus still no voltage between N and G.

Therefore testing with one of those simple three-prong outlet testers would give the following results:
On receptacles connected to L1, the tester reads like it would before.
On receptacles connected to L2, the tester LED circuits receive 240V and will thus be either twice as bright as normal or burn up. But otherwise the light patterns still as before.

Sea Nile said:
Also, If that's the case, a majority of the current would be traveling back to the transformer via the low impedance conductor, but a trickle will also be traveling back via the ground. (Electricity takes all paths of resistance).
Click to expand...
This is true anyway if the conductors aren't swapped. If they are swapped, the difference would only be seen if the ground path is of sufficiently low impedance then it will essentially create a short back to grounding connection at the utility transformer. That should trip the house main breaker.


Sea Nile said:
Also, a single ground to neutral short anywhere in the building, would cause some current on lots of bonded metal parts. Unless I'm thinking of this wrong. Check the outlets with an outlet tester for polarity swap.
Click to expand...
No, it wouldn't carry any more current than such a fault would normally cause. Because in both cases it's the same parallel path between the the fault and the MBJ at the service. (The only caveat is that if the fault is on a 120V circuit that is swapped to see 240V, the path would be carrying roughly twice as much current if the load didn't fry open.)
 
Thanks, what I meant by reversed polarity is the large slot of the receptacle would be hot and the small slot would be neutral.

After I posted, I wanted to make an edit after I thought about it more, but it was too late.

I don't disagree with what you said, but just for S's&He's this is what I wanted to say:

I think what would happen, depending on where a branch circuit is connected to the panel, the 120 volt receptacles will either be reversed polarity or have 240v applied to them.

All 240v branch circuits would be running at 120v.

And there would be a substantial voltage gradient around the buildings grounding electrode and the grounding electrode at the transformer. As well as many other places such as bonded gas lines, etc.
 
220508-1000 EDT

Suppose that coming to the main panel that ---

1. One hot 120 V line relative to the utility transformer neutral, and earth at the utility transformer is correctly wired to one hot phase of the main panel.
2. The other 120 V line, but of opposite phase, is connected to the maim panel neutral bar at the main panel, and assume the main panel is not earthed at the main panel. This means earth is only at one point, that ground point at the this assumption is only at the utility transformer.

This will put 240 V between the neutral bar in the main panel and the correctly connected 120 V hot wire. Clearly that will produce major problems.

Now lets put a ground rod at the main panel from the main panel neutral bar. This puts 120 V between the two ground rods. Typical ground resistance to remote earth, not too many feet, is about 20 ohms. But even assume it is 10 ohms or 5 ohms per rod, then current flow thru the two earthed rods is 120 / 10 is 12 A, not much.

So in reality one could get major shocks, and equipment damage, including possible fires because one 120 V phase really has 240 V between hot and the neutral bar.

,.

.
 
Sea Nile said:
Thanks, what I meant by reversed polarity is the large slot of the receptacle would be hot and the small slot would be neutral.
Click to expand...

The large slot of the receptacle would not be 'hot', if 'hot' means ungrounded and/or having voltage to the EGC. It would be the same as before with respect to the receptacle ground. 'hot' is obviously not a technical term, but the closest technical term would be 'ungrounded'. The small slot would still be ungrounded and the large slot would still be grounded with respect to the house electrode and EGCs.

The small slot would be 'neutral' in the sense of being 120V to the other two conductors, but it would also be 'hot' to the EGC. So it would be both 'neutral' and hot.

In other words, we normally use 'neutral' and as a shorthand for 'neutral and grounded' because the code requires the former to be the latter. But the swap would undo that, and make the neutral ungrounded and another conductor grounded (violating code as well as function) and thus create a lot of confusion as to what you mean by 'neutral.'

Sea Nile said:
After I posted, I wanted to make an edit after I thought about it more, but it was too late.

I don't disagree with what you said, but just for S's&He's this is what I wanted to say:

I think what would happen, depending on where a branch circuit is connected to the panel, the 120 volt receptacles will either be reversed polarity or have 240v applied to them.

All 240v branch circuits would be running at 120v.

And there would be a substantial voltage gradient around the buildings grounding electrode and the grounding electrode at the transformer.
Click to expand...
True. And to some degree this might make all slots on some receptacles 'hot', i.e. having a voltage to the earth.
Sea Nile said:
As well as many other places such as bonded gas lines, etc.
Click to expand...
Probably not, unless they are in contact with the earth.
 
Not to state the obvious, but digging up a direct-buried service lateral with an excavator, is highly likely to cause damage at both ends as well as the point where it was hit, along with possibly other places along it’s length.
 
**it happens.
In regards to excavation.
That's one good reason to install marking tape at the proper depth if used at all.
One of the reasons why I insist on being present while the excavator is back filling. Too many times I see electricians & plumbers install marking tape directly over the cable or sand.
 
We had one a couple of years ago the utilities neutral had a light blue stripe on it. One of the guys hooked it to the hot. Burnt up the fridge, couple other appliances. I'm not sure how many they made up.
 
Status
Not open for further replies.
Top