------------------------ From: Scott Roleson <[email protected]> Subject: Re: Shield Room Grounding Date: Thu, 8 Oct 1998 14:17:56 -0700 To: [email protected] Cc: [email protected]
> > Barry Ma forwarded to me a message from Peter Hays that said: > > > Can someone tell me what is the best method to find out and ensure > > that a screen room is adequately grounded? (SNIP) > There is some debate on this single-point ground approach. I know > some people who think it doesn't matter, so long as the room doesn't > have any gaps so that ALL potentially interfering currents stay on > the outside surface of the room. This may be true, but in practice > it's not always possible to have a room without any holes or gaps. > > -- Scott Roleson > Scott: Grounding for safety is a must, since a shielded enclosure will almost always have a set of low-pass powerline filters which bring the 60Hz power into the equipment within the room. These filters usually have a few large (15uF or so) capacitors from line to filter case to bypass RF currents. Since the capacitors also have a modest Xc at 60 Hz, there will also be a 60 Hz current component to the filter case. If the room isn't grounded, a hazardous voltage potential can exist on the room wall. A person standing on the concrete floor of the parent structure can get a very serious shock just by reaching out to open the door handle or to connect a coax cable to a port. So, for almost every situation, we have to ground the room to protect the people around it. But grounding for RF shielding effectiveness isn't needed. A copper spherical Faraday cage floating in mid air (what a sight!) would make an fine RF shield. The shielding would be limited primarily by the gaps, intentional seams and accidental cracks and gaps. But... it's not very useful. Let's land that baby and now think of it as a shielded room. To be of any use, this room has to be big, say 10 feet tall. And maybe 30 feet wide. Now, let's get very simplistic. To an RF wave, propagating along happily in air, your room looks like a little old antenna. I mean, it's conductive, and it has a height above ground. An "effective height." Right, it looks like a stubby, broadband vertical monopole above a ground plane. And that RF wave gives that antenna a present; it induces some RF current into the conductive structure. Now, that RF current would like to flow, along the outer surface of the conductor (skin depth effect), somewhere. By providing a single, well defined ground path, you prevent that current from flowing along paths which would create problems. What problems? Well, imagine the current flowing to ground through the outer jacket of a coax cable connected to a grounded spectrum analyzer. The noise currents would sum with the valid RF currents on the analyzer coax. Now, with all that said, let me tell you a story. At both General Dynamics and Cubic in San Diego, I needed a large shielded room (for the EUT) and a smaller shielded room or antechamber (for the program support equipment. (Support equipment is notorious for being built just barely able to work, with a rat's nest construction and no thought to EMC.) At GD, I had a solid, welded main room and a modular, 8' cube antechamber. At Cubic, I have a modular main room and a modular, screen antechamber. Both locations used the same technique to join the large and small rooms; a penetration port was located in each facing wall, and a steel "tunnel" was fabricated to bolt onto each room's penetration port bolt pattern. In effect, the shielded volume turned into a "dumbbell" shape, with the tunnel at the waist of the dumbbell. Then, a penetration port cover plate, equipped with multiple signal line filters, is bolted across the tunnel throat at one end. (This isolates the two test chambers from each other.) Each shielded room has it's own set of powerline filters (400Hz three phase, 60Hz three phase and two DC lines). The modular panel (on each room) that carries the filter sets has it's own ground well. The two powerline filter sets were over fifty feet apart in both examples. I can't remember when anybody would have recommended a design like this. I know I wouldn't. But the GD example grew from merging existing facilities, and it worked. It worked good enough that we NEVER had a trace of any ambient signals in the many emission tests that were performed there over about 15 years (using Eaton receivers and HP spectrum analyzers, often with active antennas). It was good enough to do TEMPEST testing at the facility. It worked good enough that I decided to deliberately emulate the design here at Cubic. And it's working again, good enough that my fancy HP-8571A Receiver (a re-worked 8566B), even operating with external pre-amps in some bands, doesn't see any ambient distractions. So what's the lesson in all of this? Well, I was scrupulous about seam quality and using very good powerline filters and custom penetration ports. But, just maybe, we may have been worrying too much about how multiple grounds affect the RF shielding of a shielded enclosure. Best Regards, Ed -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 10/08/1998 Time: 14:22:54 -------------------------- --------- 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).
