Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sunybcs!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: sci.electronics Subject: Re: How can I recognize true ground? Summary: Whether or not the outlet is grounded... Message-ID: <3283@kitty.UUCP> Date: 9 Jul 89 20:56:20 GMT References: <18425@mimsy.UUCP> <1547@unccvax.UUCP> Organization: Recognition Research Corp., Clarence, NY Lines: 111 In article <1547@unccvax.UUCP>, dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) writes: > > Question from a novice: My house has "two prong" outlets. I've been told > > that the center screw and casing of the outlet boxes is generally grounded, > > so that I will have a correctly grounded outlet if I attach the wire of a > > "3 to 2 adapter" to the center screw. How do I (a vertual electronics > > know-nothing) check to see if this is working in my case? > > Basically, you must consult a licensed electrician. Some readers may think that the above is a trite answer, but it is really the best advice, which I trust you will understand by the time you have read this article. > There is no guarantee that the centre screw, or the outlet > box, is in fact grounded appropriately. Although licensed electricians > are supposed to wire the house while under construction, and city > inspectors are supposed to inspect the thing, there is no guarantee > that it was done correctly. ( I'm not old enough to remember what > was done with two-prong outlets in new construction...Larry Lippman, > are you out there???) You rang? :-) One of Life's Great Mysteries is whether there is in fact an adequate "equipment" ground on a given electrical outlet. An inexpensive "outlet safety tester" which utilizes three or so neon lamps to ascertain if an outlet is correctly wired should NOT be relied upon in situations where health, safety and critical equipment is involved. A neon lamp is a pretty crude voltage indicator; typical neon lamps (like the NE-2) fire at 65 to 70 volts, so a non-illuminated neon lamp on such an outlet tester can STILL allow 60 volts to be present! Think for a minute: one could fool those neon lamp outlet testers with a piece of 30 AWG wire as an equipment ground, although there will obviously be little current-carrying capacity in such an arrangement! The efficacy an outlet ground can only be ascertained through careful wiring inspection and/or through the use of a special ohmmeter intended for that purpose called a "ground resistance meter" (manufactured by such companies as Associated Research, Biddle, etc.). The real test involves measurement of resistance between the electrical outlet equipment ground and earth ground - NOT just measuring potential difference between the outlet wires in an energized state. While I have identified the correct technique for outlet equipment ground testing using resistance meaurement, I would urge caution on the part of anyone attempting this with other than a ground resistance meter. Many VOM's have ohmmeter circuits whose accuracy may be adversely affected by the presence of AC potential differences between the equipment ground and earth ground (say, a water pipe) arising when measurements are taken on an system with energized loads. If someone is determined to try this method using a conventional VOM, at least try to open the main disconnect to preclude any chance of such error. Just because a duplex electrical outlet has a grounding screw does NOT mean it is grounded. Consider the following techniques which have been used in residential wiring, along with some corresponding caveats: 1. Before 1930, most one and two-family homes were wired using a technique known as "knob and tube", in which case there were just two rubber/cloth-insulated wires run to each outlet (neutral and hot). The outlet box, while metal, was NOT grounded - so neither was the outlet or grounding screw! While many houses built during that time period have since been rewired for 240 volt service, there is no guarantee that every single branch circuit was in fact rewired to provide an equipment ground. Do not assume anything in an older home! 2. "Better" one and two-family homes built before 1930 used rigid metal conduit or metallic-sheathed cable (BX), and most other one and two-family homes built between 1930 and 1950 used BX. Metal outlet boxes and outlets used in this construction are grounded, PROVIDED that the BX sheath was properly teminated and that BX and conduit connections have not become loose or corroded. 3. Romex did not see general use until after 1950. Early Romex used an impregnated cloth sheath, while Romex manufactured after about 1960 or so used an extruded plastic sheath (as today). Many one and two-family homes built between 1950 and 1960 used only two-wire Romex with NO EQUIPMENT GROUND WIRE. This means that a home built during the 1950's may NOT have grounded outet boxes, even though they are metal (plastic outlet boxes did not become common until will into the 1960's). 4. Three-wire grounded outlets became mandatory for NEW construction sometime in the mid-1960's. At this time, any Romex had to carry an equipment ground wire in order to satisfy this requirement and ground the outlet. 5. Then there came aluminum wire, which pervaded the residence wiring market from the mid 1960's to the early 1970's. The equipment ground wire in alunimun Romex has no insulating sheath to give it mechanical protection, and hence it is very fragile. It is not uncommon to see an outlet grounded using aluminum Romex to have NO ground due to a broken or loose aluminum ground wire! There are three morals to my article: 1. Do not assume anything with respect to outlet grounding! 2. Do not place implicit trust in a neon lamp "outlet safety tester"! 3. Have a competent person inspect your electrical wiring where the issue of equipment grounding is important! <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"