Ron, you are correct that 6.4.1 makes testing between a TNV-1 or -3 circuit and an SELV circuit moot, if there is a common earthing connection between them. With regard to determining the reliability of the earthing of the TNV-1 or -3 circuit, the method of 2.5.11 would be used, with the test current set to 2A. UL, CSA and TUV Rheinland NA have endorsed the 2A value.
Regards, Peter L. Tarver, PE Senior Regulatory Engineer Homologation Engineering Nortel, MPK ESN 655 V: 408-565-2453 F: 408-565-2575 > -----Original Message----- > From: [email protected] [SMTP:[email protected]] > Sent: Thursday, July 09, 1998 4:57 PM > > Alan, > > You stated that: > >Paragraph 5 of clause 6.4 of EN60950/A4 which follows items a) b) > >and c) states the following: > > > >'These requirements do not apply where circuit analysis and > >equipment investigation indicate that safety is provided by > >other means, for example between two circuits each of which > >has a permanent connection to earth.' > > Your reference to 6.4 above should actually be 6.4.1 which specifically > refers > to electrical separation (see also Table 19), not the impulse test of > 6.4.2.1. > However, if a TNV-1 or TNV-3 circuit and an SELV circuit are permanently > connected to earth, then there may be no separation requirements between > them, > therefore, no testing should be required, if I'm reading the standard > correctly. > > If an overvoltage fault occurred (such as an impulse of 6.4.2.1 that you > mentioned) between unearthed TNV-1 or TNV-3 circuits or conductive parts > and an > SELV circuit (in this case), that fault current must be reliably routed > to > earth (see 2.3.3.3). In particular, the "permanence" of the SELV > connection to > earth is described in 2.5.11, particularly the 2nd to last paragraph. See > also > 6.3.2. > > I hope this answers your questions and that you will be soon out of your > misery. :-) > > Comments anyone? > > Best regards, > Ron Pickard > [email protected] >
