[sci.electronics] What's acceptable voltage ground-neutral?

jscott@mandata@uunet.uu.net (Jeff Scott) (04/13/91)

I measured the voltage between the neutral and ground on the outlets
around my home.  Some read .25 VAC and some read .67 VAC (one or the other,
nothing inbetween).  Any opinions on whether this is OK?  
(All this talk about leakage current got me on this in the first place.)
				
				jeff scott
				jscott%mandata@uunet.uu.net

lrk@k5qwb.lonestar.org (Lyn R. Kennedy) (04/15/91)

jscott@mandata@uunet.uu.net (Jeff Scott) writes:

> I measured the voltage between the neutral and ground on the outlets
> around my home.  Some read .25 VAC and some read .67 VAC (one or the other,
> nothing inbetween).  Any opinions on whether this is OK?  
> (All this talk about leakage current got me on this in the first place.)
> 				
> 				jeff scott
> 				jscott%mandata@uunet.uu.net

But did you read the voltage between your hands with a good DVM? :-)

This should be OK around the house, the ground is mostly for safety
and the important part is that it is acuually connected. There are
simple testers that are supposed to tell you if the wires are connected
and if correctly. If you find >10 VAC start getting real worried.


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tonya@hpldsla.sid.hp.com (Tony Arnerich) (04/17/91)

I've often seen around 4V GND-NEUTRAL at work - that got me a bit worried.
Too much use of extension cords is the typical cause. It seems that about
1 in 10 people don't get the message...

If you know the current that corresponds to a particular voltage drop, you
can get the wiring resistance.

If you get anything over a couple of volts, you might try making
the same measurement at your electric panel to see where the voltage
drop is occuring:

A) If it's in your part of the wiring, you might have a high-resistance
   connection at a receptacle. This could be a fire hazard (Remember when
   some builders buried aluminum wiring inside house walls?).

B) If it's in the power company's wiring, don't worry about burning
   down your house, but shock hazards can still be a threat. Just think
   about all those polarized plugs that make you believe you'll only get
   exposed to "0V".

tonya@sid.hp.com

jeh@dcs.simpact.com (04/17/91)

In article <1991Apr13.131621.225@mandata@uunet.uu.net>,
 jscott@mandata@uunet.uu.net (Jeff Scott) writes:
> I measured the voltage between the neutral and ground on the outlets
> around my home.  Some read .25 VAC and some read .67 VAC (one or the other,
> nothing inbetween).  Any opinions on whether this is OK?  

What you are measuring is actually the voltage drop in the neutral wire
between wherever you're measuring and the tiepoint at the main box where
the two are bonded together.  Current flows in the neutral wire, hence
there's a voltage drop.  No current (is supposed to) flow in the ground 
wire, hence no drop... which is to say that you'll get the same reading
with one probe of the DVM at the outlet neutral and the other probe at
the ground/neutral tiepoint.  

The voltage drop will obviously vary with the amount of current running through
the circuit.  Say you're drawing 1 amp, if you just have 0.1 volt drop in the
neutral wire, you are looking at only 0.1 ohm back to the service entrance.  If
10 amps were flowing in the same circuit, you would measure a full volt of
drop.  This is quite a reasonable value.  The standard wire tables will tell
you that it would take many miles of 14 AWG copper to add up to a tenth of an
ohm, but of course, nearly all of the R comes from the splices, not the
wire itself.  

Paradoxically, if you ever measure ZERO voltage between neutral and ground
anywhere but at the service entrance, with a heavy load on the circuit, 
this means trouble -- specifically, that some clown has connected neutral and 
ground together other than at the service entrance.  Of course, a higher 
voltage drop means trouble too:  A high-resistance path in the neutral line.  

	--- Jamie Hanrahan (x1116), Simpact Associates, San Diego CA
Internet:  jeh@dcs.simpact.com, or if that fails, jeh@crash.cts.com
Uucp:  ...{crash,scubed,decwrl}!simpact!jeh

aalonzo@eagle.wesleyan.edu (04/17/91)

In article <1991Apr13.131621.225@mandata@uunet.uu.net>, jscott@mandata@uunet.uu.net (Jeff Scott) writes:
> I measured the voltage between the neutral and ground on the outlets
> around my home.  Some read .25 VAC and some read .67 VAC (one or the other,
> nothing inbetween).  Any opinions on whether this is OK?  
> (All this talk about leakage current got me on this in the first place.)

I am not an electrician, but I would expect this voltage to be normal.  It can
be explaned by Ohm's law, and I think there is even a name for it.  I believe
it is called "IR loss".  This name comes from the fact that the voltage you are
measuring is generated by current flowing through the neutral wire, back to the
fuse box.  The neutral wire does have a slight resistance between the fuse box
and the point you are measuring.  The ground wire should be attached to the
nuetral wire in only one place .. the fuse box.  Essentially what you are
measuring is the voltage differance across the length of the nuetral wire of
the particular circut you are testing.  Using Ohm's law, current*resistance =
voltage.  Most home circuts are designed for 15 to 20 amps.  If you take the
measurements you made, and run them through the formula, you could calculate
the aproximate resistance in the neutral wire.  (.67V / 15A = .045 Ohms) 
Actually, .045 Ohms sounds a little low, indicating that less current than 15
Amps was flowing.  Try shutting off the circut breaker for the circut you are
measuring, this will stop all current flowing in the nuetral wire, and thus
should result in a much lower voltage reading.  (0 Volts).

______________________________________________________________________________
Allen Alonzo
Wesleyan University
AALONZO@EAGLE.WESLEYAN.EDU
_______________________________________________________________________________

jgk@osc.COM (Joe Keane) (04/18/91)

This topic comes up so frequently it should be in a frequently-asked questions
list.  The answer is that a difference of a volt or two is not abnormal.

For example, i figured out the neutral-to-ground impedance of the power at my
place, and it comes out to 18 milliohms.  So if i draw 20 amperes, which isn't
that much, there will be a 360 millivolt difference between ground and
neutral, and an equal drop on the hot line.  I would guess that this is pretty
typical.
--
Joe Keane, amateur mathematician
jgk@osc.com (...!uunet!stratus!osc!jgk)

norton@manta.NOSC.MIL (Scott Norton) (04/22/91)

To take this thread off on a tangent, I want to point out to anyone
who is designing for US Navy shipboard applications, that on
ships, neutral and ground are purposely isolated.  The reason
for this is to make the power system more survivable; a ground
in any phase causes no disruption to service.

But because of this arrangement, neutral and ground can float a
couple of hundred volts apart.  In a casualty that grounds one phase, 
neutral will move to 380 volts from ground in the 440 primary 
power distribution, or 103 volts for the 115 volt branches.

As a result of designers failing to account for this possibility,
at least one brand of surge supressor has been known to short
out and catch fire when used on a ship.  Apparently, the
surge capacitors or MOVs were not spec-ed for the voltage float.

LT Scott A. Norton, USN  <norton@NOSC.MIL>

ken@wcsn.csms.com (Kenneth D. Fox WCSN Operations Engineer) (04/26/91)

In article <1926@manta.NOSC.MIL>, norton@manta.NOSC.MIL (Scott Norton) writes:
> 
> But because of this arrangement, neutral and ground can float a
> couple of hundred volts apart.  In a casualty that grounds one phase, 
> neutral will move to 380 volts from ground in the 440 primary 
> power distribution, or 103 volts for the 115 volt branches.
> 

Yah! 
The above statement brings back a bad memory of my 
U.S. Navy days. While stationed on an old ship, (U.S.S. 
Fulton, to be decommisioned this year) myself a electronic 
technician and an electricians mate found out the hard way that 
some stupid fool rewired our test equipment shop benches with the 
neutral and ground tied together!!!

KEN 

johne@hp-vcd.HP.COM (John Eaton) (04/29/91)

<<<
< To take this thread off on a tangent, I want to point out to anyone
< who is designing for US Navy shipboard applications, that on
< ships, neutral and ground are purposely isolated.  The reason
< for this is to make the power system more survivable; a ground
< in any phase causes no disruption to service.
----------
Darn! I always though that it was to make the power into a balanced system
which reduced the possibility of 60 hz emissions. 

John Eaton
!hp-vcd!johne