In a message dated 9/30/01, Ilan Cohen writes:
> This isolation between TNV1/3 and the DC power source can be waived as long > as there is Permanent bonding to earth of the SELV/TNV-2 and the TNV-1 > circuit. > Now to the problem…… > TNV-1/3 circuits must be a floating circuits. This means they cannot be > earthen by definition. In T1/E1 circuits there is ground provided through a > central tap of the T1/E1 transformer. Can such a circuit be considered > "bonded to earth circuit"? > Since the E1/T1 does not seem to be truly bonded to earth than Basic > insulation will be required. And much of the T1/E1 circuits residing in > central offices and applying DC voltages to the T1/E1 (wetting current) is > NON compliant?!? > Hi Ilan: I think you will be OK here, but a few things need to be clarified. First, are you talking about wetting current (typically 48 volts) or span power (up to 140 volts or so)? This affects the classification of the circuit. Second, your description of the requirements for earthing TNV circuits is not quite the way I would describe it. TNV circuits do not have to be floating. In fact, the conventional TNV-3 analog phone line is usually earthed at the central office. What EN 60950 says is that TNV circuits must be isolated from earth and SELV *unless* the earth and SELV circuits have a permanent connection to earth (other options are available, and Norway/Sweden have important limitations). In other words, it is not a requirement that the TNV circuit be earthed, but rather that the ground and SELV circuits from which the TNV circuit would normally be isolated must be earthed. So, if you can ensure that the earth and SELV circuits have a permanent connection to earth, you will not have to provide isolation from TNV circuits. In most industrial applications it is easy to provide the permanent ground. However, this grounding requirement is difficult to achieve if the equipment will be customer installed. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
