Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!fluke!blew From: blew@tc.fluke.COM (Bob Lewandowski) Newsgroups: sci.electronics Subject: Re: How can I recognize true ground? Summary: Watch out for un-grounded equipment with power line filters!! Message-ID: <9756@fluke.COM> Date: 20 Jul 89 04:06:50 GMT References: <18425@mimsy.UUCP> <1989Jul7.155721.19105@utzoo.uucp> <10269@dasys1.UUCP> Sender: news@tc.fluke.COM Organization: John Fluke Mfg. Co., Inc., Everett, WA Lines: 62 In article <10269@dasys1.UUCP> aj-mberg@dasys1.UUCP (Micha Berger) writes: >spector@brillig.umd.edu.UUCP (?) 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? > {{Text deleted}} > >Barring lightning, I wouldn't panic about grounding. Especially if your ^^^^^^^^^^^^^^^^^ ^ ^^^^^ ^^^^^ ^^^^^ ^^^^^^^^^ >outlet strip has a fuse / circuit breaker. >-- > Micha Berger > I've watched this discussion go on for a while now and most of the advice has been good, BUT now to insert my $.02!! Many if not all PC's and a large portion of commercial grade electronics have some sort of power line filter to meet FCC and other RFI/EMI specs. Most of these filters have a pair of capacitors from each side of the line to "ground". (read chassis) Modern equipment probably use capacitor values such that the reactive current to "ground" is less than 0.5ma for 120V lines. However, I've seen equipment from not that long ago that had caps as big as .1uF from both lines to ground. When the chassis of such equipment is not connected to earth or mains ground, the chassis floats at about 1/2 the line voltage! I've frequently seen people (who should know better!) use "cheater" plugs (3 pin to 2 pin converters, with the little ground wire disconnected, or cut off) to "float" some piece of equipment, austensibly to eliminate ground loops. This may eliminate the dc ground loop, but the 60 HZ line current to "ground" is forced to flow through the signal leads, which usually doesn't help the problem. There are a lot of items in a home/shop in addition to plumbing and damp floors, that are directly or indirectly tied to ground, even in older buildings. A good example is heating equipment, radiators, floor or wall registers, sheet metal ductwork, etc. It can be a real surprise to be standing on a floor register in your bare feet and touch a "floating" appliance. The current may be limited to a few ma. or less, but shocks are dangerous under any conditions, especially 60 Hz. A 120V bulb can be rigged up with some care as a test light. One side is connected to the ground in question, and the other connected to the high side of the line (usually a black or red wire). If the bulb glows brightly the ground is of some value. If it doesn't light or lights dimly the ground is not good. Compare the brightness when the lamp is connected to the ground being tested to when it is plugged directly into the outlet. Comparative brightness is a good rough measure of voltage equality, because it varies significantly with small variations in voltage. This is *DEFINITELY* only a minimal test of ground integrity. The best advice is the licensed electrician. -----Bob -- Bob Lewandowski Domain: blew@tc.fluke.COM Voice: (206) 347-6100, Ext. 5368 UUCP: {microsof,sun}!fluke!blew U S nail: John Fluke Mfg. Co. / P.O. Box C9090 / MS 273G / Everett WA 98206