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. ------------------------------------------------------------------------- lrk@k5qwb.lonestar.org lrk@k5qwb.UUCP 73, utacfd.utarl.edu!letni!rwsys!kf5iw!k5qwb!lrk Lyn Kennedy K5QWB @ N5LDD.#NTX.TX.US.NA P.O. Box 5133, Ovilla, TX, USA 75154 -------- "We have met the enemy and they are us." Pogo -----------------
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