Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!emory!stiatl!john From: john@stiatl.UUCP (John DeArmond) Newsgroups: sci.electronics Subject: Re: How can I recognize true ground? Message-ID: <5651@stiatl.UUCP> Date: 10 Jul 89 22:59:31 GMT References: <18425@mimsy.UUCP> <1547@unccvax.UUCP> <3283@kitty.UUCP> Reply-To: john@stiatl.UUCP (John DeArmond) Organization: Sales Technologies Inc., "The Procedure IS the product" Lines: 175 In article <3283@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: >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. Well, I considered it a bit too trite, especially since my dog knows more about electricity than most electricians I've met (yep, still got my IBEW journeyman card here somewhere :-). When I was an apprentice, the major thing I was taught was the three rules of electricianing: 1) The foreman is an a**hole. 2) Payday is on Friday. 3) One hand in the pocket at all times. (to keep current away from your chest if you get shocked) First, since the original poster is an admitted novice, some background. Power is supplied to you by the utility from a center-tapped 230 volt transformer secondary. This center-tap is connected to earth ground at the pole and at several other places. The end result is that you receive power via three wires - 2 are "hot" and one is ground or "neutral" The 2 hot wires present a voltage difference of about 230 volts and each hot wire presents about 115 volts to the neutral. Since you are generally at ground potential by virtue of standing on the ground or touching a grounded object, the general objective of safety grounds is to ensure that whatever you touch is always at or near the same voltage potential as the earth (or neutral lead since it is connected to ground.) A safety ground (what the third wire is for on your 3 prong plug) must a) be at ground potential and b) have enough amperage capacity to conduct any possible fault currents (currents resulting from a hot lead contacting a grounded object) while holding the voltage drop to an appropriate level, and c) be able to sink enough current to activate the short-circuit feature of your overload protection device (fuse or circuit breaker). The last part is important because if the ground does not have this capacity, the protective device may not trip or may trip later. In either case, the tremendous heat generated by the heavy-but-not-quite-enough current could start a fire. When the power comes into your distribution box in your house, the 2 hot leads connect to the main breakers (or fuses in older installations) while the neutral lead connects to a grounding strip. This strip is bonded via a heavy copper lead to earth ground (typically a water pipe or iron stake in the ground). The individual branch breakers are connected to the downstream side of the main breaker. These breakers distribute the energy to the individual branch loads (outlets, stove, air conditioner, etc) via branch wires, typically 12 to 14 gage wire. A normal 115 volt branch will have 3 leads: A black lead which is the hot wire connected to the branch breaker, a white lead, which is the neutral connected to the grounding block in the breaker box and a ground lead, normally bare or green if insulated, which connects TO THE SAME GROUNDING BLOCK in the breaker box. In other words, AT THE BREAKER BOX, the neutral and ground leads are connected together. For a normal three prong outlet, the black (hot) wire connects to the right (smaller) prong, the white (neutral) connects to the left (wider) prong and the ground wire connects to the round pin. This round pin is normally bonded via the mounting flange to the outlet box. (outlet viewed from this prospective:) | | | o Normally, power flows from the hot prong, through the load, and back to the neutral prong. The ground pin normally carries no current. Now consider what happens when a wire inside your electric drill fails and contacts the metal frame. The metal frame is normally connected to the ground pin. Line voltage would be applied to the frame. A properly functioning ground lead would conduct this current directly back to ground. In other words, a direct short circuit is created. All protective devices have a time vs overload response curve that is specified in a standard (ok, so I don't remember which one it is :-). What this means is that a circuit breaker or fuse will carry a small overload for a long period of time while tripping faster as the overload increases. At some point, defined as the short-circuit point and generally about 10 times the device's rating, it must trip instantaneously. Thus, for a 15 amp branch, the breaker must trip immediately at about 150 amps. The job of the safety ground is to ensure that this much current can be handled so that immediate tripping is guaranteed. A side benefit is that a high quality ground will conduct enough current that quite often the offending wire is vaporized by the arc, removing the fault even before the breaker can trip. If the ground were inadequate and capable of, say, only 60 amps under short circuit conditions, the breaker would handle the load for several seconds or minutes, meanwhile, your device is smoking and/or you are being shocked.. Our poster has the problem of only 2 wires - a hot and a neutral. But no safety ground. In these instances, the only real alternative is to run a safety ground to the outlet. In some cases, the neutral is connected to the outlet box/ "ground pin". This is worse than nothing at all. So a ground wire must be run - but how? Well, the ground rules (pun intended) are that the ground should have at least the ampacity of the branch wiring which means it should be of the same AWG as the branch or at a minimum, one AWG smaller. Thus, a branch wired with AWG 14 wire should have a ground lead of AWG 14 or minimally, AWG 16. Rule of thumb: it should handle the full load of the branch with no more than 5 volts rise as measured between the ground prong and a known good earth ground. I don't remember if this is in the code but it's a parameter I've always used. How to run this wire? In a modern house, you will probably want to snake it along the same path as the branch wire. This is sometimes difficult. Another alternative is to run the wire OUTSIDE the house, especially if the house has siding which could hide the wire. In an old apartment building I once owned in PA, I ran a ground bus (awg 8) wire around the outside of the house up under the overhang of the siding. At each outlet, I drilled a hole through the wall and siding and ran a ground wire out. The wire was soldered to the ground bus. The ground bus was routed back to the grounding block on the breaker panel. After sealing the holes and painting the bus, it was almost invisible. So how to test a ground? Well, the little outlet testers mentioned in other postings verify that a connection exists but will not verify the ampacity of the ground. What I have done is build a tester as follows: Three prong plug push button switch (15 amp) | |---------------/ ----------------| | | o LOAD (hair dryer, Iron, etc) | | |------------------------------------| "a" What this circuit does is place a heavy load - typically 1000 watts or better - between the hot leg and the safety ground. Voltage is then measured between the safety ground pin ("a") and a known good earth ground such as the grounding strip in the breaker box. The voltage here should be less than about 5 volts. *** WARNING - If the ground is defective, the outlet box will become electrically hot as long as the button is pushed. Keep this in mind while testing. While most electricians don't do this test, I've found enough marginal grounds to warrent it. Doing your own electrical wiring is not hard or unsafe if you follow established standard, are careful and THINK. I'd suggest that the original poster get a copy of the current National Electrical Code (and any local codes) and a book on home wiring. As a person who was almost electrocuted by a defective battery charger plugged into a 3 prong outlet with no ground wire attached, I can personally testify to the value of a good grounding. Were it not for me falling down as I was being shocked and thus loosened from the short, I'd probably be dead. John -- John De Armond, WD4OQC | Manual? ... What manual ?!? Sales Technologies, Inc. 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