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Re: 240 volt blasts appliances #43986 10/28/04 04:57 PM
Joined: Oct 2003
Posts: 156
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dereckbc Offline
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shortcicuit asked: Is there any protection that could be added at the main service to a dwelling to protect against a lost neutral...

Yes, install an isolation transformer. Not exactly what you wanted hear, right? [Linked Image]

Edited out rhetorical comment

[This message has been edited by dereckbc (edited 10-28-2004).]

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Re: 240 volt blasts appliances #43987 10/28/04 05:54 PM
Joined: May 2002
Posts: 1,716
R
Roger Offline
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Dereck
Quote
Yes, install an isolation transformer. Not exactly what you wanted hear, right? [Linked Image]


And preferably an ungrounded (illegal [Linked Image] ) isolation system in the sense of only leg to leg voltage. [Linked Image]

Roger

[This message has been edited by Roger (edited 10-28-2004).]

Re: 240 volt blasts appliances #43988 10/28/04 06:14 PM
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pauluk Offline
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Quote
Does't sound like there will be much to salvage from pauluk's post

I didn't mean to sound too pessimistic, just pointing out the worst-case scenario.

Sometimes equipment can survive surprisingly high overvoltages: It depends very much upon the tolerances which were incorporated into the design, along with other factors such as the duration of the excess voltage.

It's just very hard to be specific until you actually open it up and start testing.

Re: 240 volt blasts appliances #43989 10/28/04 06:49 PM
Joined: Jun 2004
Posts: 220
T
trekkie76 Offline
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wouldn't the 120 loads respond to the ground being there? Are you saying that if the neutral is lost, the 120V loads wouldn't try to get back to ground through the GEC? The circuit neutrals still being connected to the GEC, wouldn't the flow still be there?

Re: 240 volt blasts appliances #43990 10/28/04 07:19 PM
Joined: May 2002
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Roger Offline
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Trekkie,
Quote
Are you saying that if the neutral is lost, the 120V loads wouldn't try to get back to ground through the GEC?
current doesn't flow (or try to get back) to ground, it MUST flow to the source.

The ground may become a poor additional conductor back to the source but not enough to consider.

Roger

Re: 240 volt blasts appliances #43991 10/29/04 10:04 AM
Joined: Oct 2003
Posts: 156
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dereckbc Offline
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Trekkie76 asked:

Q1. Wouldn't the 120 loads respond to the ground being there?

A1. Yes

Q2. Are you saying that if the neutral is lost, the 120V loads wouldn't try to get back to ground through the GEC?

A2. It would try to get back to the source (center tap of the transformer), via earth, but the impedance of earth is too high.

Q3. The circuit neutrals still being connected to the GEC, wouldn't the flow still be there?

A3. It would try, but what impedance is the ground electrode to the transformer center tap (the source)? When you remove the neutral you have changed the circuit efectively from parallel to series. Normally the single phase loads are connected L1-N and L2-N forming 2 parallel loads. Once you remove the neutral the two loads are in series, and the voltage will divide in proportion to the resistance of the two loads.

Hope that helps. Dereck

Re: 240 volt blasts appliances #43992 10/30/04 10:49 AM
Joined: Jun 2004
Posts: 220
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trekkie76 Offline
Member
If electricity isn't trying to get back to ground then why do we ground things? If current is simply going back to the source why is the neutral grounded, if not to provide a good path to ground for the current? I understand that without the center tap it would be a straight 240v secondary, but I can't beleive that the potential between one phase and the neutrals in the panel wouldn't still be 120v. Wouldn't the path of least resistance now be to the GEC through the neutral bond? I am not being argumentative here, I am genuinely confused.

Re: 240 volt blasts appliances #43993 10/30/04 02:02 PM
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pauluk Offline
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Quote
If electricity isn't trying to get back to ground then why do we ground things?


The ground connection is so that a circuit is solidly referenced to ground. There are several reasons for this, such as improved protection against lightning strikes and because an ungrounded circuit of any length will set up its own "floating" reference point with respect to ground due to capacitance between the wiring and the earth.

In a system where there is just a single point grounded, no current actually flows to ground through that connection. The ground rod simply insures that the neutral is at zero volts with respect to ground.

When a ground-fault occurs, the current which flows is not just "returning to ground," but is in fact returning to its source. It's just that with the neutral grounded at source, the ground happens to be a convenient path for the current to take.

Re: 240 volt blasts appliances #43994 10/30/04 05:22 PM
Joined: Nov 2002
Posts: 558
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ga.sparky56 Offline
Member
I remember sombody(I believe it was rescapt) posting an experiement to try.

Drive a ground rod,put a 15a breaker in a panel,and run a piece of 12 or 14 thhn and clamp it to the rod,and turn the breaker on.

I tried it,and with an amprobe, read about 8 amps or so...but it never tripped.

Russell

Re: 240 volt blasts appliances #43995 10/30/04 05:24 PM
Joined: Dec 2003
Posts: 524
Attic Rat Offline
Member
... I got the scare of my life when,we were working in a house with a recent addition,and I had to add to a junction box in the attic...(where else right??)and I'd taken the wirenut off of the neutrals to splice mine in,and as I pulled the nut off,the existing neutrals were not twisted together,and thus,came apart,..with a nice arc I might add.My helper yelled to me from down below that the lights went "super-nova".I realized why,and reconnected them in a hurry.Luckily for us, the addition had no furniture or appliances,and the problem circuit(s)were isolated to the 2nd floor.A near tragedy was averted...Next time,I'm killing the circuits at the panel...Promise!! [Linked Image] [Linked Image] [Linked Image] [Linked Image]
Russ


.."if it ain't fixed,don't break it...call a Licensed Electrician"
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