nagy@bnlux0.bnl.gov (John Nagy) (11/10/88)
The purpose is safety of humans (and cats, etc.).
A 120V branch in a modern US residence is wired...
Function Wire Color Plug or Socket Shape
-------- ---------- --------------------
Hot Black small rectangle
Neutral White large rectangle
Ground Green round
The neutral and ground wires should be tied together at
the entrance box and both earthed.
The hot wire varies in voltage sinusoidally relative to
the neutral wire.
Equipment sees a voltage difference between the black and white
wires and it does its work. Whether your reference is black or
white means only a 180 degree phase difference.
Relative to a cold water pipe or a puddle of water on the floor
there is a big difference. A human simultaneously touching
a pipe and a white (neutral) wire should experience no shock.
Touching the black (hot) wire is something else. A 120V, 10A,
low-impedance source is often fatal.
Consider a lamp. If the house and lamp are correctly wired,
the center button of the screw socket for the lamp should be
hot and the shell neutral. Then someone changing a bulb with
the switch on is less likely to touch a "hot" part.
Checking house wiring...
Voltmeter Point A Point B Reading
------- ------- -------
small rectangle large rectangle 120 vac
small rectangle round hole 120 vac
large rectangle round hole 0
Only after passing the voltage test above, you can check with
an ohmmeter that the large rectangle and the round hole show
very low resistance between them and each of these to earth
(a continuous copper cold water pipe, for example).
For a lamp, check for continuity between the shell and the large
rectangular spade on the plug and between the button and the small
spade.henry@utzoo.uucp (Henry Spencer) (11/13/88)
In article <686@bnlux0.bnl.gov> nagy@bnlux0.bnl.gov (John Nagy) writes: >Relative to a cold water pipe or a puddle of water on the floor >there is a big difference. A human simultaneously touching >a pipe and a white (neutral) wire should experience no shock. Urk! No! Don't make this mistake! Why do you think there are separate ground and neutral wires anyway?!? The neutral wire should indeed be grounded at building entry, *but* remember Ohm's Law: that wire may be carrying substantial return current from running devices, and its resistance is not zero! At a point remote from the building entry, the neutral wire may have a substantial voltage with respect to ground. That is, if you touch pipe and neutral wire, some of that return current may try to flow through you rather than through the wire, because you are providing a shorter path to ground. This is particularly true in a large building. Last time I checked, I measured something like 10 volts between neutral and ground in my apartment building. Connecting (say) the chassis of a TV set to neutral is a whole lot safer than connecting it to the hot wire. However, it is **NOT** the same as connecting it to ground. NEUTRAL IS NOT GROUND! -- Sendmail is a bug, | Henry Spencer at U of Toronto Zoology not a feature. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
nebezene@ndsuvax.UUCP (Todd M. Bezenek KO0N) (11/14/88)
In article <1988Nov13.000833.21883@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: > At a point remote from the building entry, the >neutral wire may have a substantial voltage with respect to ground. ... >This is particularly true in a large building. Last time I checked, I >measured something like 10 volts between neutral and ground in my >apartment building. ... >-- >Sendmail is a bug, | Henry Spencer at U of Toronto Zoology >not a feature. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu I think that Henry has let us in on another of America's untapped energy sources. -Todd -- Todd Michael Bezenek, KO0N UUCP: nebezene@plains.nodak.edu Bitnet: nebezene@ndsuvax NU040889@ndsuvm1
dhesi@bsu-cs.UUCP (Rahul Dhesi) (11/15/88)
It seems to me that it would be far safer to have each of the two hot
wires be at +/- 55 volts relative to ground. A grounded center-tapped
secondary would do it.
Touching both would be just as lethal as before. Touching one and
ground at the same time would be much safer.
--
Rahul Dhesi UUCP: <backbones>!{iuvax,pur-ee}!bsu-cs!dhesiembick@tetra.NOSC.MIL (Edward M. Embick) (11/16/88)
In article <4759@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes: >It seems to me that it would be far safer to have each of the two hot >wires be at +/- 55 volts relative to ground. A grounded center-tapped >secondary would do it. > Rahul, This is the way the 240 VAC is set up. Each hot wire is 120 VAC relative to ground. The 120 VAC in you house is usually the neutral and one of the two phases from a 240 VAC feed from the utility to your main power panel. The 240 VAC then is used for electric range, dryer, hot water heater, or other appliances without having to get additional service from the utility. Your 120 VAC recepticles are usually divided between the two phases to balance the loading on the 240 VAC source. ------------------------------------------------------------------------------ Ed Embick (If God wanted me to write legibly, He wouldn't have invented email) Computer Sciences Corporation ____ ____ ____ 4045 Hancock St. MILNET: embick@tetra.nosc.mil // // // San Diego, CA 92110 \\___ ___// \\___ (619) 225-8401 x287
howard@tp2.Waterloo.NCR.COM (Howard F. Steel) (11/16/88)
In article <4759@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes: >It seems to me that it would be far safer to have each of the two hot >wires be at +/- 55 volts relative to ground. A grounded center-tapped >secondary would do it. > >Touching both would be just as lethal as before. Touching one and >ground at the same time would be much safer. Safer, yes. Safe, No. It has been concluded that voltage levels below 30Vrms (42.4V peak) are "Touch-Safe", otherwise designated as Safety Extra Low Voltage (SELV). The level of 55Vrms in your scheme is still almost twice this limit. In addition, to the above you must consider the energy hazard; 240VA and above is considered to be such a hazard. Considering a normal 15A outlet, you have in excess of 800VA, 3-4 times the limit; this represents a fire hazard if exposed. In order to reduce this hazard, the service would have to be limited to less 5A, (run your heater on that if you will). I have only seen one safe outlet/appliance combination, that considers even the dolts among us who like sticking their tongues in outlets, and that was devised by Manning Rose of NCR (Dayton). Rather than go into the details of it here, suffice it to say that it all boils down to a matter of cost; the cost in changing the present standard to a new, albeit better system. Maybe if Manning is reading this he could shed some expert advice on the whole affair. -- Howard.Steel@Waterloo.NCR.COM :-( I Think, Therefore I AM, aka: Howard "The Duck" Steel :-) I think
henry@utzoo.uucp (Henry Spencer) (11/16/88)
In article <4759@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes: >It seems to me that it would be far safer to have each of the two hot >wires be at +/- 55 volts relative to ground... >Touching both would be just as lethal as before. Touching one and >ground at the same time would be much safer. This assumes that you are as likely to touch one as the other. Not so; in things like hot-chassis TV sets, not to mention ordinary lamp sockets, the odds of touching one side are much higher than those of touching the other side. So it makes sense to approximately ground one side, and try to make that the more exposed one. -- Sendmail is a bug, | Henry Spencer at U of Toronto Zoology not a feature. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
howard@tp2.Waterloo.NCR.COM (Howard F. Steel) (11/17/88)
In article <197@sunset.MATH.UCLA.EDU> hgw@math.ucla.edu (Harold Wong) writes: >It just dawned on me that those little two prong to three prong electrical >converters might be a potential hazard. >Take the same converter, turn it upside down (now you have the grounding >tab on top) and plug it into the bottom half, now you have a reverse plug. >Is this true? YES! -- Howard.Steel@Waterloo.NCR.COM :-( I Think, Therefore I AM, aka: Howard "The Duck" Steel :-) I think
jgo@mcgp1.UUCP (John Opalko, N7KBT) (11/17/88)
In article <4759@bsu-cs.UUCP>, dhesi@bsu-cs.UUCP (Rahul Dhesi) writes: > It seems to me that it would be far safer to have each of the two hot > wires be at +/- 55 volts relative to ground. A grounded center-tapped > secondary would do it. Your house feed is already on a grounded center-tapped secondary. That's how you get 120V for your lights, etc., and 240V for your water heater, etc. The neutral (white) wire on your 120V outlets is connected to the distribution transformer center tap. The hot (black) wire is connected to one or the other end of the secondary winding. If you look at a 240V outlet, you will see two hot wires (typically one black and one red, but they can be any color except white or green), a ground wire (green or bare) that MUST NOT carry current, and maybe a white neutral that can be a current carrier. The ground is required for safety. The neutral is needed only if you require dual voltages from the outlet. John Opalko