Dear Treggers, Can anyone explain the meaning of protective earthing when applied to an SELV or TNV circuit (or any circuit for that matter). I ask this question with the following in mind. 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.' The part of the circuit which would normally be connected to the protective earth is the current return leg. The rest of the circuit will not be at the same potential as this point due to circuit impedances. Therefore this point and this point only will be at protective earth potential. Bearing this in mind, if, during the test of 6.4.2.1 (1.5kV impulse), breakdown occurred between the unearthed parts of the TNV-1 or 3 circuit and the SELV circuit earthed return leg the fault current would be dumped directly to ground via the protective earth and there would be no raised potential at this point. However if the breakdown occurred between the unearthed parts of the TNV-1 or 3 circuit and any other point in the SELV circuit, the potential at this point (which may be user accessible) would be raised due to circuit impedance between this point and earth.
It would seem that only the parts that are directly connected to the protective earth as stated under table 0.1 condition 6, last paragraph are the earthed SELV points and any other parts of the SELV circuit are unearthed SELV. Do I have a point or am I talking rubbish? Please put me out of my misery. Sorry about the lengthy nature of the enquiry. Thanks in advance. Alan Booth. Equipment Engineering Group Fujitsu Telecommunications Europe Limited Solihull Parkway Birmingham Business Park Birmingham B37 7YU Telephone +44 121 717 6492 FAX +44 121 717 6014/6018
