I absolutely disagree about having a separate ground for the shielded room. Any ac fault inside the room can potentially cause a personnel hazard, as will any lightning strike to the vicinity. A separate ground for the shielded room is acceptable ONLY if it is bonded to building ground. The risk may be minimal, but why take any at all.
Mike Hopkins [email protected] > -----Original Message----- > From: [email protected] [SMTP:[email protected]] > Sent: Tuesday, October 06, 1998 3:36 PM > To: [email protected] > Subject: Room grounding > > Chaps, > > The grounding of a shield room has always been a tricky subject. In the US > we > have the NEC code that basically want's everything connected, which > contradicts some EMC requirements. The Shield room hence needs some > tricks: > > 1) One of the functions of the outer skin is to terminate impinging > fields. > Current is generated on the shield surface that will flow across joints > and > seams ( and hence leak into the chamber ) unless it's routed somewhere.... > Earth. > > 2) While buildings usually have a good earth, they typically have a noisy > earth. The last thing we need is building noise on our room, so an earth > dedicated to the chamber is provided. > > 3) To stop building noise from using the shield room earth, all metallic > connections are cut, and plastic used instead. This is where the NEC folks > can > get upset. Their concern is that the two grounds could lift with respect > to > each other.... I've never seen that happen, and I've gone looking for it. > So > that I can sleep at night, I ensure that an operator can't touch metal > referenced to the different grounds at any one time. > > 4) Power supplied to the room has to be directly connected. To stop > building > noise from entering the room through this wiring, the noise is removed to > the > case of a filter. I've located my filters very close to the room single > point > room earth connection, so the noise can get there easily without crossing > one > of my room seams or joints. > > 5) Any equipment I use with the room is referenced to the room ground. > Power > for this equipment is filtered at the same point the room power is. Most > instrumentation used in EMC is quiet.... so they don't supply much noise. > > I don't believe there is guess work involved with room grounding. The > above is > based on conversations with many room installers. I suggest that if you > have > specific questions, contact the folks that made yours. > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
