>Yes, the building ground is likely to be noisy, unless you actually connect directly >back to its earth spike
No signal, ground or supply is noisy. It is the designer that defines a certain conductor to be clean, and references all other voltages (probably to be filtered) to that potential. Inside a screened room, there is only ONE ground (reference potential), and that is the conductive shell around the EUT (so the chamber wall itself). Electrical safety inside a room must be referenced to that potential, as well as all signals measured inside. All cable screens are therefore connected to the conductive wall before leaving. It is fully irrelevant to what you connect the outside of the chamber , as the skin effect effectively separates all current on the external surface from the internal surface. For extremely low frequencies, wall thickness may be insufficient, and wall current on the external surface can show up on the internal surface and create potential differences on different parts of the internal wall (and possibly fields inside). The only remedy is increasing wall thickness , dual walls, increase conductivity or apply a layer of magnetic conductive shield (ferro metal Ur > 1) . One should prevent LF current to flow on the outside. Still it makes no difference to what ground point you connect the room (just do not make 2 different connections). One last problem may be the presence of LF magnetic fields on the outside, they may create currents op to several amps on the outside surface, leading to the problem sketched above. (try connecting a 50-60Hz current clamp around the metal tubing of your central heating and be surprised: the loops in these tubes and the mains magnetic field easily creates a few amps of "thougth" ground faults) . Electrical safety at the outside of the room is obtained by simply connecting the wall to the building's safety conductor. Gert -----Original Message----- From: John Woodgate [mailto:jmw1...@btinternet.com] Sent: Wednesday 19 July 2017 09:31 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Chamber grounding [General Use] It's not my consensus! Yes, the building ground is likely to be noisy, unless you actually connect directly back to its earth spike. Even then, the wire you use is an inductor and may have noise induced in it. I am quite sure that BS 7671 and other Code standards do not recognize that a screened room is outside their scopes. With best wishes DESIGN IT IN! OOO - Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England Sylvae in aeternum manent. -----Original Message----- From: Price, Andrew (Leonardo, UK) [mailto:andrew.p.pr...@leonardocompany.com] Sent: 19 July 2017 07:16 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Chamber grounding [General Use] Thanks all for the replies. It seems that there is some divided opinion. Some of the test houses I have spoken to state that they definitely ground their chambers via ground spikes. This is to remove low frequency noise that causes issues with the low frequency emc tests that are required in mil-std, def stan and rtca/do-160. One test house I spoke to stated that as a result of cost cutting an earth spike was not fitted so earthing was taken to building ground, as a result of this the test house has issues with low frequency noise. The consensus of the forum is that a ground spike should not be required and the building ground used should be enough. This then probably raises the question of how good is the building ground? Is there a requirement for this? If there is noise on the building ground how can this be eliminated? 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