Re: Room grounding
Hmmm, forgive me if I am wrong but I wonder if the following will help? unknown wrote.. #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: Same here in the UK with IEE regs. # #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. However, if the seams are good, then there will be no leaks. Of course if you earth the screened room, then some RF will flow across a joint to get to the earthing point but a _good_ Faraday cage does not need to be earthed in order to provide screening. # #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. To provide electrical safety, the screened room must be connected to the _earth_ or _ground_ of the electrical supply. This is particularly important when the room is supplied with a filtered single phase supply via filters with high leakage current. (The filter on live and neutral will bring the screened room to half line potential without the earth!) Of course, any RF _noise_ from the building or environment will get into the room via any penetration - so make sure the seams are tight and that you do not take an earth bolt _through_ the screened room wall. If the room is good, there will be no leakage of the external environment to the inside. # #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. Hmmm... possibly, but is depends on what the _metallic connection_ was providing. If the connection is just a fortuitous connection - or a room support, then it should be made of insulating material. If you need a, say, water or compressed air supply in the screened room, then there are two ways of bringing it in. First (and my preference) is to BOND the pipe to the outside of the screened room. I mean weld, braise, solder or clamp (in that order) around 360 deg. of the periphery and then do the same on the inside! Second is to use a section of plastic pipe through the room wall. However - since the plastic does not stop RF and there is a hole, the room will leak RF at this point. The degree of leakage will depend on the size of hole, the thickness of the metal wall(s) and the frequency. I NEVER have different grounds in the area. I have the electrical ground (earth) ONLY. This is connected to the outside of the room at the point where the filters are mounted. I pick up the ground (earth) again on the inside. This point inside and outside the room is often called the common reference point (C.R.P.). # #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. If the filters are not connected directly to the room, then the _clean_ wires will need to be protected from the environment by screening or in conduit. Don't forget, that many filters have a high leakage current and so should be fed directly from the main supply to the building or floor - that is - don't supply sockets off the same feed. The other supplies for the lab outside the screened room _must_ be on the same phase as a room supplied with a single phase supply.. # #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. It is not a good idea to use the _clean_ room supply to power external equipment, 1/ because the external equipment can generate noise on the _clean_ supply line and 2/ the equipment can act as an antenna - feeding the external environment onto the power lines. At DC / 50 / 60Hz the arrangement I have given will provide a common ground. If however, the room filters have very high leakage or the room supply is very long, then you might need to provide an isolation transformer and bring the protective earth from the room C.R.P. # #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
Re: Shield Room Grounding
Bob: Barry is trying to say that a dummy power load is only resistive at the power frequency, and may not truly represent the EUT at all of the other frequencies within the test range. He's right. Bob suggests a powerline filter be added to my El Cheapo lightbulb load bank to more closely approximate an EUT. He's right, too. My bank of lightbulbs is versatile, but it IS spread out over a few square feet. There are stray lead inductances and capacitances, and the lamp filaments are also inductors. At a low frequency starting point of 10KHz (or 30Hz for me), this is negligible. But yes, as we go up to 10MHz or 30MHz, these undefined and uncontrolled variables will start causing some weird effects. A better load bank could be constructed of non-inductive bulk silicon carbide resistors, but these are expensive ($150 each) and not as versatile. (OTOH, I have NEVER had one burn out during a test!) Practically, I assume that the load is mostly resistive, having had the experience of using ordinary light bulbs on the end of a coax cable for loading amateur transmitters in the HF and VHF regions. Lamp filaments do a good job of converting RF to heat and light, so I trust that they retain a considerable resistive component of the impedance at any of these frequencies! But, I don't think we need to know what the impedance curve of my loadbank is, because there is no standard to compare it against. What we are groping toward here is that there seems to be a need to define a standard dummy power load. This load would have an impedance similar to the typical EUT. Placing this dummy load into the test setup would then allow test conductors to verify the quality of the supplied power and the sensitivity of the measurement equipment. So, if there is a need to have a defined dummy load impedance, then we'll have to start with a survey of typical EUT's to find the value and variation of the impedance. Maybe we'll need several categories of dummy loads (appliance, computer, entertainment)? But, as far as I'm concerned, I still have a few cases of light bulbs to use up first! ;-) Ed From: Robert Macy m...@california.com Subject: Re: Shield Room Grounding Date: Fri, 9 Oct 1998 16:27:11 -0700 (PDT) To: b...@namg.us.anritsu.com, Bailin Ma@unspecified-domain Cc: emc-p...@ieee.org Lightbulbs make great AC loads, but use a line filter in series with them and you'll probably duplicate most EUT's since they'll be using filters. - Robert - On Fri, 9 Oct 1998 b...@namg.us.anritsu.com wrote: Hi Ed, I appreciate your kindness to share your experience with us about constructing equivalent load: I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductance and capacitance. But all I want to do is draw a few amps DC I have two questions: (1)What is the impedance of your bulb array at 30 MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? i.e., Ze=? @30 MHz (DETAILS SNIPPED) Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by using an Impedance Analyzer, e. g., HP4191A(?). Thank you. Please correct me. Best Regards, Barry Ma -- Ed Price ed.pr...@cubic.com Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: emc-pstc@listserv.ieee.org Date: 10/12/1998 Time: 10:47:22 -- - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Re: Shield Room Grounding
From: Scott Roleson sc...@hpsdde.sdd.hp.com Subject: Re: Shield Room Grounding Date: Thu, 8 Oct 1998 14:17:56 -0700 To: emc-p...@ieee.org Cc: sc...@hpsdde.sdd.hp.com 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
RE: Room grounding
'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 r...@itsqs.com The opinions expressed are my own and not necessarily those of my employer. -Original Message- From: Jon D. Curtis [SMTP:j...@curtis-straus.com] Sent: Thursday, October 08, 1998 11:52 AM To: lfresea...@aol.com Cc: mhopk...@keytek.com; emc-p...@ieee.org 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. lfresea...@aol.com 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 majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr
Re: Shield Room Grounding
Hi Ed, I appreciate your kindness to share your experience with us about constructing equivalent load: I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductance and capacitance. But all I want to do is draw a few amps DC I have two questions: (1)What is the impedance of your bulb array at 30 MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? i.e., Ze=? @30 MHz If we are not sure Zb=Ze @30 MHz, I am afraid, it's hard to say the spectrum analyzer would receive the same RF emission at 30 MHz from noise sources other than EUT, although the bulb array draws the same current at 60 Hz as EUT does. In other words, Zb=Ze @60 Hz is one thing, and Zb=Ze @30 MHz would be another. Let's see an example, assuming Ze=Re+jXe, where Xe=Omega*Le, and Omega=2*Pi*F. Re=20 Ohm, Xe=0.1 Ohm @60 Hz, Ze=20+j*0.1=20 Ohm Be=20 Ohm, Xe=5 Ohm @30 MHz,Ze=j*5 Ohm Conclusion: As far as the equivalent load is concerned, we can only pay attention to the equivalence of resistance part of Zb and Ze @60 Hz. At 30 MHz, however, we should pay more attention to the equivalence of reactance part of Ze and Zb instead. Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by using an Impedance Analyzer, e. g., HP4191A(?). Thank you. Please correct me. Best Regards, Barry Ma - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Shield Room Grounding
Hi Barry et al, I'm I missing something here? Is this exercise worth the trouble? If one doesn't have a clear margin to any commercial limit line with any kind of resistive dummy load attached to your LISN shouldn't you examine the your system setup? Accurate determination of your measurement system noise floor with a known impedance through the frequency range is really only of academic use in this case, isn't it? This should have no bearing on pass/fail of an EUT as the measurement system noise floor and the limit line should be well separated for conducted emissions in a shielded room. Regards, Kevin Harris -Original Message- From: b...@namg.us.anritsu.com [SMTP:b...@namg.us.anritsu.com] Sent: Friday, October 09, 1998 1:33 PM To: emc-p...@ieee.org Subject: Re: Shield Room Grounding Hi Ed, I appreciate your kindness to share your experience with us about constructing equivalent load: I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductance and capacitance. But all I want to do is draw a few amps DC I have two questions: (1)What is the impedance of your bulb array at 30 MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? i.e., Ze=? @30 MHz If we are not sure Zb=Ze @30 MHz, I am afraid, it's hard to say the spectrum analyzer would receive the same RF emission at 30 MHz from noise sources other than EUT, although the bulb array draws the same current at 60 Hz as EUT does. In other words, Zb=Ze @60 Hz is one thing, and Zb=Ze @30 MHz would be another. Let's see an example, assuming Ze=Re+jXe, where Xe=Omega*Le, and Omega=2*Pi*F. Re=20 Ohm, Xe=0.1 Ohm @60 Hz, Ze=20+j*0.1=20 Ohm Be=20 Ohm, Xe=5 Ohm @30 MHz,Ze=j*5 Ohm Conclusion: As far as the equivalent load is concerned, we can only pay attention to the equivalence of resistance part of Zb and Ze @60 Hz. At 30 MHz, however, we should pay more attention to the equivalence of reactance part of Ze and Zb instead. Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by using an Impedance Analyzer, e. g., HP4191A(?). Thank you. Please correct me. Best Regards, Barry Ma - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Room grounding
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 mhopk...@keytek.com -Original Message- From: lfresea...@aol.com [SMTP:lfresea...@aol.com] Sent: Tuesday, October 06, 1998 3:36 PM To: emc-p...@majordomo.ieee.org 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 majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Re: Room grounding
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 majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
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 majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
