Floating Neutral

[jeremysterling]

Today I happened to work on a circuit in a single phase panel fed from 25 kVA 480/240V XFMR. The voltage from neutral to ground was 11.2 volts. I suspect X0 has not been connected to building steel.

Tomorrow, an outage is scheduled at 6:00 AM to shut down and install the GEC.

My question is: What is the best way to explain to the customer why this needs to be corrected?

I told the manager that, "This electrical system is abnormal. It has what we call a floating neutral. This can result in equipment damage and may be a hazard to personnel."

My answer is generic. Is there anecdotal evidence to back it up?

I think saying that it is a code violation is insufficient or even immaterial (to a manager). What have you out there said to explain the importance of retrofitting the GEC?

 

[chris kennedy]

You could see a system bonding jumper installed but no grounding electrode system or grounding electrode conductor?

 

[LarryFine]

In an extreme condition, while the line-to-line and line-to-neutral voltages will remain consistent, the overall voltages to ground could become almost anything (say, from a primary-to-secondary fault) and possibly exceed equipment and insulation ratings.

You might also bring up the fact that the NEC requires the bond.

 

[jeremysterling]

You could see a system bonding jumper installed but no grounding electrode system or grounding electrode conductor?

I did not open the XFMR, but my belief is that neither the SBJ or GEC has been installed.

In an extreme condition, while the line-to-line and line-to-neutral voltages will remain consistent, the overall voltages to ground could become almost anything (say, from a primary-to-secondary fault) and possibly exceed equipment and insulation ratings.

You might also bring up the fact that the NEC requires the bond.

Good thought, Larry. I did not conceive of consequences of fault conditions. Thank you.

 

[Chev]

At a division of GM where I used to work a unistrut grid was constructed to support utilities and a suspended ceiling for a new cafeteria/restaurant. The person who wired the transformer neglected to bond the transformer to building steel. The result was the entire grid was giving people shocks!

 

[Rick Christopherson]

Today I happened to work on a circuit in a single phase panel fed from 25 kVA 480/240V XFMR. The voltage from neutral to ground was 11.2 volts. I suspect X0 has not been connected to building steel.

First off, this voltage reading alone does not indicate a lack of bond. How far away from the transformer was your voltage reading taken? What is the amperage in the neutral? What is the amperage in the Ground (probably not zero)?

Even though we don't normally consider conductors to be resistive, they are, and you will have some voltage drop across the neutral between the point of bonding and the point of load. Because the ground has very little amperage, it won't have this voltage drop for the same distance, and that is why you will have a voltage difference between the two conductors.

I told the manager that, "This electrical system is abnormal. It has what we call a floating neutral.

It is not correct to call this a floating neutral (which is much more serious). A floating neutral is a break in the neutral between the source and load.

 

[don_resqcapt19]

... It is not correct to call this a floating neutral (which is much more serious). A floating neutral is a break in the neutral between the source and load.

I don't agree. I have never heard the term "floating neutral" used to describe a break in the neutral. That would be an "open" neutral and a "floating" neutral is one where there is no system bonding jumper.

 

[Rick Christopherson]

I don't agree. I have never heard the term "floating neutral" used to describe a break in the neutral. That would be an "open" neutral and a "floating" neutral is one where there is no system bonding jumper.

No, a missing bond causes the whole system to float. The difference in voltage between ground and neutral will also be the same offset voltage for the phases too. Everything floats at the same amount. For example, if your gnd-neut voltage is 20 volts, then your gnd-A voltage will be 140V and gnd-B will be 100V. However, Neut-A and Neut-B will still remain 120V

A true floating neutral has no immediate tie to the rest of the system (except via the loads), and as such, its voltage relative to the phases will vary, depending on the load between the phases and the neutral. In this case, your neut-A will be 100V and Neut-B will be 140V. (But your gnd-A and gnd-B will likely remain at 120V because the bond is still present upstream from the break in the neutral.)

An open neutral is a different condition, and doesn't become a floating neutral unless there are multiple loads (on opposing phases) sharing the same neutral, but still no return path back to the system, except via the loads through the opposing phases.

 

[Rick Christopherson]

I forgot that I have a diagram showing a simple a floating (unbonded) system. If I were to redraw this a a floating Neutral, the L! and L2 would remain locked to Ground, but the Neutral would be the "Float".

Oops, it is not clear. This is a Floating System, not a Floating Neutral.

 

[Rick Christopherson]

This is what a floating neutral would look like

 

[iwire]

Rick first it looks to me like you and Don are saying the same thing.

Second is if I measure almost 12 volts between neutral and ground at a panel my first stop is to check the bond.

Yes it could be just VD but 12 votls would be excessive and unusual in my experience.

OTH it is pretty common to find bonding jumper missing, one of my guys found one just last week and is going in this Saturday to correct it during a shutdown.

 

[Rick Christopherson]

Bob, I don't disagree, just that care needs to be taken in drawing conclusions. For example, a knee-jerk reaction to my postings may be that I am an arse, but some people are gullible enough to believe that I am some sort of nice guy.....see, they can be wrong! :grin:

I wouldn't find 12 volts to be excessive if the distance justified it. Like it or not, the voltage is expected, even though nothing is technically wrong.

 

[don_resqcapt19]

Rick,
I still see the second drawing in post 10 as an open neutral and the second drawing in 9 as a floating neutral. At least that is what they are called around here.

 

[LarryFine]

I wouldn't find 12 volts to be excessive if the distance justified it.

I would on anything other than a line-to-neutral 2-wire load.

 

[don_resqcapt19]

...
I wouldn't find 12 volts to be excessive if the distance justified it. Like it or not, the voltage is expected, even though nothing is technically wrong.

I would find 12 volts between the grounded conductor and the grounding conductor to be excessive voltage drop. That would only be the voltage drop in the grounded conductor and there will be additional drop in the ungrounded conductor. That is close to 5% even on a 277 volt system. Yes there is no code rule on voltage drop, but I would find that to be an excessive drop and possibly evidence of a neutral that has a poor connection...one that may soon become an open neutral.

 

[don_resqcapt19]

Rick first it looks to me like you and Don are saying the same thing. ...

I don't think Rick and I use the terms in the say way.

That is not to say either of us is incorrect...just that we have to use the same meaning of a term to have any type of technical discussion. His definition of a "floating" neutral indicates a serious problem and mine is not such a serious problem.

 

[LarryFine]

His definition of a "floating" neutral indicates a serious problem and mine is not such a serious problem.

True. An intentionally-unconnected neutral on the primary of a Y-D transformer is an example.

 

[jeremysterling]

Follow up to "Floating neutral/system"

Follow up to "Floating neutral/system"

In the first pic you see the neutral leaving the XFMR going straight to the neutral bar in the secondary's Main Breaker Enclosure. The only other "bonding" wires present are the 1? & 2? EGC's going to the double barrel mechanical lug that is screwed to the XFMR enclosure.

In the MBE, you can see the green and yellow taped #4 also on the Neutral bar. Downstream the yellow and green lands on an isolated ground bar in the load center. Upstream from the MBE, the green and yellow taped #4 is capped off and shoved hidden into the back of the XFMR!!!!!

In this state, the bonding of the XFMR neutral to earth would be accomplished through the lone IG receptacle fed from the load center. That is, if something like a cash register with a bonded metal chassis were plugged into the IG and the chassis or the unlucky person touching the chassis touched earth.

I know what your thinking. Table 250.66 says we're to use #4, not a cashier's hand, to bond the neutral!

I moved the IG to it's own isolated tap block in the MBE and then bonded it to the XFMR chassis. Then I installed the GEC and the SBJ.

To Rick: I did consider voltage drop to be a possible cause, but the electrical room was only 100 feet away and the load was ~15A, so the likely cause was the missing GEC & SBJ. I see your point of calling this condition "floating system" in regards to explaining to customer what the problem is.

 

[don_resqcapt19]

I would on anything other than a line-to-neutral 2-wire load.

I would find the 12 volts between the grounding conductor and the grounded conductor even more excessive on a two wire line to neutral load. That would indicate a circuit voltage drop of 24 volts, assuming that there are no problems with a neutral.