Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cmcl2!phri!roy From: roy@phri.UUCP (Roy Smith) Newsgroups: sci.electronics Subject: Re: How can I recognize true ground? Message-ID: <3841@phri.UUCP> Date: 8 Jul 89 02:37:54 GMT References: <18425@mimsy.UUCP> Reply-To: roy@phri.UUCP (Roy Smith) Organization: Public Health Research Inst. (NY, NY) Lines: 36 In article <18425@mimsy.UUCP> spector@brillig.umd.edu.UUCP () writes: > I've been told that the center screw and casing of the outlet boxes is > generally grounded, [...] How do I (a vertual electronics know-nothing) > check to see if this is working in my case? I've seen enough electrical systems to know that you shouldn't take anything for granted. I wouldn't even assume that a 3-pin outlet is really grounded (from what I can tell, most electricians don't really understand that there is a difference between ground and neutral). Anyway, get a volt-ohm meter. Find a piece of copper plumbing (preferably a cold-water supply line) and measure the voltage between the supposed ground point and the copper pipe. It should be essentially zero (you might well have several volts between the neutral side of the outlet and the ground, however). Assuming you read zero volts between the ground point and the copper pipe, switch the meter to the lowest resistance scale and measure the resistance between the pipe and the ground. It should be very low, a few ohms at most. If it is, you've got a good ground. You might have to clean up a bit of the pipe with sandpaper to make a good connection. If you either get any appreciable voltage (I would guess even a tenth of volt on the ground is reason to be suspicious) or any appreciable resistance (more than a few ohms) between the ground and the copper pipe, your supposed ground is *probably* not really grounded. It is possible that the pipe is not really grounded -- there might be a piece of plastic pipe somewhere between it and the earth, or some teflon joint tape might be getting in the way, but by and large, if you've got solder-joint copper plumbing, you should be able to count on any point in the cold water supply being a good ground (but not the hot water supply; you don't usually have metalic continuity through the hot water heater). But, by-and-large, if your "grounded" outlet fails the tests above, I'd bet it was the outlet that was the problem, not the pipe. -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 {allegra,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu "The connector is the network"