wtm@neoucom.UUCP (05/06/87)
Hi, Sorry, but I was going to reply to the person from Virgina that wanted information about grounding in [electro] physiology, but our system crashed right in the middle of your article, and it disappeared into the bit bucket in the sky. If you'll write or call me, I'd be happy to relate some of my experiences working in a lab doing electrophysiology. In short, if you wire everything up as per the manuals for whatever electrophysiological amplifier you have, the fact that the AC ground wire is bonded to the neutral lead in multiple places in your building should not matter. What you do have to keep in mind is that if your experimental subject is a person, you gotta gotta gotta gotta gotta use a Ground Fault Interrupter (GFI) circuit breaker right at the end of your recording equipment's line cord. This will detect any current that accidentally flows in the GREEN ground lead, indicating that your subject is in danger of being electrocuted. Most GFIs trip at a 10 mA current flow in the GREEN wire. They'll trip within 16 mS of sensing the fault. 10 mA flowing in your victim, err ah subject won't fell real good, but should be harmless if it happens. The easiest way to add a GFI is to replace the wall plug in your lab with a GFI receptacle. Most hardware stores have them for $15 to $20. The expense is well worth it to avoid the possibility of facing a technical malpractice suit! The hard part will be getting your physical plant staff to appreciate the import of having a GFI and installing it for you. Bill Mayhew Division of Basic Medical Science Northeastern Ohio Universities' College of Medicine Rootstown, OH 44272 USA phone: 216-325-2511 (wtm@neoucom.UUCP !cbatt!neoucom!wtm)
ornitz@kodak.UUCP (05/06/87)
Ground fault interrupters do not trip by sensing current in the Green wire; they trip by detecting the difference in current between the hot and neutral wires. Leakage from the hot wire (Black) may be to externally grounded objects, thus sensing current in the ground wire (Green) does no good. In Galen's situation, the ground wires are actually lifted from ground by the impedances of the wiring. Current returning in the neutral line will create a voltage drop in the wiring. Connecting the ground to the neutral at the socket will impose this voltage on any equipment connected to the ground wire. Touching this equipment and another, perhaps better, ground such as a cold water pipe could create a shock hazard which would not be detected by a GFI. I suggested to Galen that he use an Ultra-Isolation transformer with the equipment ground not being connected to his neutral but to an external ground. Since neither side of the isolation transformer secondary is grounded, there should be an added measure of safety. With the proper use of differential inputs on amplifiers, etc., most of the noise problems can be eliminated but the safety problem will remain unless the neutral and ground line are connected only at the actual "ground" location (practically speaking - the service entrance where the physical "ground" connection is made). ----------------- | ___ ________ | | | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz | | / / | | Eastman Kodak Company | |< < K O D A K| | Eastman Chemicals Division Research Laboratories | | \ \ | | P. O. Box 1972 | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | -----------------