Jack, I suppose there are, in fact, no truly absolute unearthed circuits given such parameters as the earth's magnetic fields, numerous parasitic capacitances and atmospheric conditions. ;-) However, within certain defined specific conditions (e.g., isolation voltage, spacings, insulation resistance, etc.), isolation would then exist and then a circuit would then become unearthed if that circuit is not physically connected to earth. Your suggestion for using capacitors might be valid, but only at F=0 (DC). IMO, a circuit is considered connected to earth via a capacitor because capacitors have impedance of 1/(2*pi*F*C) [remember your basic circuits classes]. And, when was the last truly DC circuit you observed? This is why DC hipot testing is recommended for circuits with capacitors across tested barriers (see note 2 of 5.3.2 of IEC60950). Leakage current requirements of 5.2 of IEC60950 might also need to be considered when using such capacitors, although SELV voltages would likely not present a safety concern. As for optos and relays, they offer AC and DC electrical isolation/separation from their primary to secondary circuits up to certain specified voltages (most transformers also apply here, too) and therefore, offer unearthed connections. These are reasons why I feel that capacitors should not be considered as offering unearthed connections. Well, so much for my temporal soapbox. And of course, comments are invited. Best regards, Ron Pickard [email protected]
______________________________ Reply Separator _________________________________ Subject: RE: protective earth Author: <[email protected]> at INTERNET List-Post: [email protected] Date: 7/14/98 2:05 PM >From another angle: When is a circuit considered NOT connected to earth ground? In other words, if the only connections to earth in a SELV circuit are through isolating components such as 3kV capacitors (from point B to PE in the drawing), is that circuit considered unearthed? I understand that a circuit isolated from protective earth by optos and relays is considered unearthed. Thanks, Jack Netaccess, Inc.
