'Single point ground' in this situation is used for the same purpose as we would in electronics design. When using a shielded room for making low frequency measurements (particularly MIL-STD), multiple ground connections can make significant power frequency and harmonic noise. Generally this is not so much of a problem for commercial operations because emission measurements are not as restrictive (either in frequency or in limit). Because,as Jon says, the conduit, etc. is not always reliable (particularly at the higher frequencies which filters are designed to shunt) an effort is made to provide ground through an independent means. I have used enclosures where great care has been taken (multiple 12' ground rods in close proximity, with Cu Sulfate 'salting') to provide a very 'robust' ground. This is then connected to both the enclosure and the building ground.
In some cases, you have to convince the electrician and/or the inspector that your room provides at least as good a ground as the conduit in order to be able to take advantage of the isolation provided by the PVC. It goes against their grain to connect a metal box without using metal conduit. Bob Martin Sr. Technical Manager Intertek Testing Services (978)263-2662 fax(978)263-7086 [email protected] The opinions expressed are my own and not necessarily those of my employer. -----Original Message----- From: Jon D. Curtis [SMTP:[email protected]] Sent: Thursday, October 08, 1998 11:52 AM To: [email protected] Cc: [email protected]; [email protected] Subject: Re: Room grounding As I understand it, Lingren connects the shielded rooms they build back to the main building ground point by a separate ground wire. The conduit is disconnected by use of a plastic sleave. I assume they do this because the AC filters incorporated in the shielded room include LARGE capacitors to the shielded room walls. Effectively the leakage through these capacitors turns the ENTIRE room surface into an AC electrode with respect to building ground. This leakage current is potentially leathal unless returned back to the source where the neutrals of the building are tied to ground at the circuit panel. Not grounding the room (if it incorporates standard room filters) should not be considered acceptable. If there is an regular outlet on an adjacent building wall, then a lethal hazard will exist between a metal test instrument pluged into that outlet and the room surface. I would hazard a bet that most room installations are not well enough controlled to insure that building ground and a separate room ground are NEVER allowed to meet. Consider that no sparks will fly if it happens, but hearts may stop. People automatically consider dead metal as ground. I would also caution against those in this thread who rely on conduit. Conduit breaks, is removed, etc. For high leakage threats only a dedicated ground wire of suitable gage to carry the total fault current of the supply should be employed. This is not about a single fault problem. You have a hazardous condition with NO fault because of filter leakage if you do not ground the room to the building ground at the circuit panel. Watch out for LISNs Also. The design of all lisns incorporate LARGE capacitors to ground for filtering. Without a ground connection on the LISN case, high leakage threats exist. Most use LISNs bonded to the ground plane which addresses this threat as long as the ground plane is connected to the building ground. The debate on reduction of noise and effects on EMC results should continue, BUT personel safety comes FIRST and should not be compromised. [email protected] wrote: > Mike, > > sorry you disagree. > > Inside the room, all equipment is referenced to the room itself, there is no > new safety risk introduced by the room being grounded differently. > > Outside the room, again, all equipment is referenced tightly to the room, so > the operator does not see any differential. > > Should lightning strike the building, then true, the building earth potential > may lift, but the operator is protected because he is referenced to the room > which will not move much because the energy has been dissipated by the > building earthing system. > > I state again this is for performance reasons, and is accepted practice. In a > true Faraday shielded room, earthing the room is not even neccessary. Mind > you, since these don't exist off the shelf, I'll stick to grounding using my > original guidlines. NEC inspectors, when the rationale is explained to them > have little problem. However, I have come across situations were the two > unique earths were tied by a very heavy inductor.... > > Best regards, > > Derek N. Walton > > --------- > 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). -- Jon D. Curtis, PE Curtis-Straus LLC [email protected] Laboratory for EMC, Safety, NEBS, SEMI-S2 and Telecom 527 Great Road voice (978) 486-8880 Littleton, MA 01460 fax (978) 486-8828 http://www.curtis-straus.com --------- 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).
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