why i HATE shared NEUTRALS!!

[LarryFine]

Here's a picture:

Should the neutral be broken at the Black phase, #3 fixture, how does the computer get damaged?

Just as in any properly-wired system, I see no place where the neutral is carrying any current not also carried by one or more accompanying phase conductor.

 

[iwire]

Brother you would not have a "POOF" in this scenario, this "true neutral" can be removed without affecting anything.

Roger's post brings up another point.

Even when we do not have a 'true neutral' opening this neutral is not always damaging to the equipment.

On a 240/120 system a broken neutral may end up with 150 on one side and 90 on the other. Take care of this fairly quickly and you may have little damage.

It is almost imposable for a broken neutral to result in full line to line voltage at any one piece of equipment.

 

[LarryFine]

From his link:

On October 31, 2005, at the Hanford Fast Flux Test Facility, an electrician found voltage on the neutral for a switch after he had isolated electrical power and placed his Authorized Worker Lock on a lighting circuit. The electrician was preparing to replace a broken switch. As part of his zero-energy check, he disconnected the neutral and tested it with a voltmeter. When he detected voltage he stopped work. The power source involved with the shared neutral was traced, de-energized, and locked out. The electrician’s actions identified an electrical legacy issue not shown on facility drawings.

If properly wired, either (A) there should have been no current on the neutral (which did not really have to be opened to replace a switch) or (B) the energizing conductor should have passed through the switch box, meaning visible to the electrician.

Looking at this image,

http://www.mikeholt.com/code_forum/attachment.php?attachmentid=5&stc=1&d=1148938617

While the drawing seems to show Ckt #1's hot coming from another direction, if properly wired, it would have been routed with Ckt #2's hot and the neutral. The box where the neutral was opened should also have contained Ckt #1's hot wire, alerting the electrician.


The point is that the neutral of even a single, stand-alone, neutral-unshared-with-anything-else circuit can be hazardous to open if someone connects the neutral of another hot to it downstream. A multi-wire circuit is no more inherently dangerous than a single circuit is.

 

[winnie]

These appear to be non-contact _current_ sensors, which _presumably_ would be able to detect current flowing in a grounded conductor, even though the voltage would be close to zero. I have no experience with these devices, but they should be able to function as very inaccurate clamp meters.
http://www.electrical-contractor.net/the_store/EX/DVA30.htm
http://www.electrical-contractor.net/the_store/EX/DA30.htm

-Jon

 

[ramsy]

Re: my tic tracer doesn't work on neutrals

To be clear, are you talking about an intact neutral? I thought no grounded conductor would be read by a tick tracer, reagrdless of how many amps are passing through the conductor. There is no potential to ground (normally) so there would be nothing for the tick-tracer to detect.

George I think your right. Thanks for the multi-tasking, lots going on here today.

So phase to ground voltage is not the same as neutral to ground voltage. Even if this Common Mode Voltage is present on any circuit, from 0.1 to 4.0 vac, thats far below what tic tracers can detect.

The best Tic Tracer sensing range I've seen had a 12 VDC minimum, up to 1000vac max. So, if any voltage potential existed on any wire below the sensing range, it will not register.

 

[brother]

Roger's post brings up another point.

Even when we do not have a 'true neutral' opening this neutral is not always damaging to the equipment.

On a 240/120 system a broken neutral may end up with 150 on one side and 90 on the other. Take care of this fairly quickly and you may have little damage.

It is almost imposable for a broken neutral to result in full line to line voltage at any one piece of equipment.

Brother you would not have a "POOF" in this scenario, this "true neutral" can be removed without affecting anything.

NOT always damaging??? Well I, just like to ELIMINATE any possibilities of damage. You make good points roger, however even in the possibility of the 'series' voltage, remember in reality amps do fluctuate, (turning on/off) so i still think there is a 'possibilty' there no matter how small.

Thanks for the formulas refresher. Its always good to hear others points.