Hello Larry, I have heard this question before, and it remains a mystery to me. What is required in the vast majority of cases for a separation between TNV and SELV is SUPPLEMENTARY INSULATION for a PRIMARY CIRCUIT at 250Vrms (this will change in many places with Amd. 4). Really, the TNV to SELV requirements are BASIC INSULATION, however, to protect the network from unreliable earthing, SUPPLEMENTARY insulation is needed. This is found in clause 3.5.2 of BS6301, 4.4.2 of EN41003, and 6.3.3 of EN 60950. This is a 1.5kV barrier.
Consequently, we sell many BABT certified telecom line transformers that are specified as providing SUPPLEMENTARY INSULATION and only provide 1.5kV of isolation. This is sufficient. Possibly one would need REINFORCED INSULATION if you need to separate the network from a Hazardous voltage, and the only isolation you have between the network and that hazardous voltage is provided at the line transformer. That would be 3.0kV of isolation. We sometimes rate our REINFORCED INSULATION transformers at 3750VAC for one second, and 3.0kV for 1 minute. We often test 25% over the one minute test voltage for our one second production line test so we sleep better at night, but I still have no idea where the 3750 VAC came from. Maybe a country other that the UK? Also, with regard to the 2.5kV surge, the electrical strength test option in clause 6.4.2.2. calls out 1.5 kV from the network to user accessible parts. Mel Pedersen Midcom, Inc. Homologations Engineer Phone: (605) 882-8535 [email protected] Fax: (605) 886-6752 ---------- From: Larry Barnette[SMTP:[email protected]] Sent: Wednesday, July 30, 1997 8:56 AM To: [email protected] Subject: 3750VAC seperation Hello Treggers, I'm looking for a standard that prescribes 3750VAC seperation from circuitry to be connected to telecom networks. I know BABT rated line transformers are tested at this magnitude, as published in most telco transformer manufacturers spec sheets. The rating in EN41003/En 60950 (impluse test voltage) is 2.5kV dc to non-network ports/and handsets (through foil) . From my notes, I understand that BS6301 was replaced by EN41003 in 93, so the 3750V rating may be from that standard. Who can help shed light on this? Thanks in advance. ----------------------------------------------------- Larry Barnette Compliance Engineer Instrument Associates 2455 Harbor Ave. Memphis, Tenn. USA 38113 [email protected] -----------------------------------------------------
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