My comments tend to support the views of Mel Pederson rather than those of Rich Nute regarding their consideration as TNV rather than SELV circuits:
1. To meet SELV requirements the voltage must remain below 60 Vdc under fault conditions as well as tolerance limits. Compliance under fault conditions was not described in your inquiry. Otherwise the circuits within your equipment would be considered ELV circuits. If an ELV circuit, you run into problems with clause 2.10.3 restricting the use of ELV on interconnects. You may want to resolve this by specifying that your dc supply must be an SELV circuit. I would consider the coaxial cable output you described to be a TNV-1 circuit. I agree with Rich Nute, the standard does not mention "outdoors" in connection with TNV, but it does mention "overvoltages due to atmospheric discharges and faults in power distribution systems". You seem to be describing the feed to an antenna located transmitter. This sounds close enough to me to be considered a telecommunication network as described in 1.2.14.7. The note describes the internal circuit as TNV-1 and the interconnecting cable as telecommunication network (neither SELV nor TNV). Unfortunately the specifications for TNV circuits were written with telephones in mind, and do not do a good job of addressing cable TV, RF or other circuits such as yours. There is an IEC 950 WG8 antenna and cable TV project team currently addressing this issue. I would recommend applying the appropriate portions of Clause 6, explaining in your compliance report the rationale for implementing or waiving each clause. 2. IEC 950 does address lightning as mentioned above, but does so with the assumption of primary protectors on the telecommunication network. This is where the electrical code comes in. In the US the NFPA 70, 1999 edition, requires protection on these conductors. Refer to Chapter 8, particularly clause 810 and the new clause 830 on Network Powered Broadband Communication Systems. The test imposed by UL on connections such as yours from IEC 65 which consists of a 1nF cap charged to 10 kV is intended to represent lightning on cable. 3. There are requirements for Listed cable for outdoor coax cable in the US, as well as to get from the equipment through the building to the outdoors. Refer again to Chapter 8 of the NFPA 70. Obviously there are needs for insulation, flammability, ampacity and UV resistance. I have no knowlege of requirements outside US although Canada parallels many of the US code requirements for wire. Even for North American Class 2 circuits (i.e. clause 2.11 limited power) there are requirements for Listed wire. CL2, CL2P, and a host of other markings are possibilities. Check the code clause 725. Note that there is a limitation on the power source as well as the output power in these circuits. You may need redundant limiting circuits in these paths to meet the requirements. 4. IEC 950 has not yet tried to address dc supply circuits, or power feed on signal cables, except by accident. There are currently proposals in committees to introduce dc supplies and power feed, but at the moment your options seem to be to consider a 72 Vdc circuit as a hazardous circuit within the equipment. Sounds like you may have complications with interconnected accessible SELV portions. I tend to treat the incoming dc as a mains supply and provide it the same isolation as ac circuits within the switching supply which powers the product. > A CO equipment with a 48 Vdc nominal supply has coxial cable > connection to an outdoor transmitter. I have the following questions: > > 1. The coaxial cable output is conductively connected to the input. > It carries the same voltage and an amperage of 1 A (limited to 3 A > max). Assuming the input supply tolerance not to exceed 60 Vdc, > does everyone agree that this is SELV? Some of you may argue > that it is TNV-1 since it is exposed outdoors! Based on the 950 > definition of TNV, to the best of my understanding, the circuit is not > a telecommunication network voltage circuit. > > 2. 950 does not discuss protection of the equipment or user from > lightning. It should be noted that the unit also has user accessible > SELV interfaces. What type of tests or construction would you > recommend for the coaxial interface? I believe the Impulse Tests in > IEC 65 or in UL1492 should be highly recommended. > > 3. Are there any requirements for the outdoor coaxial cable? > Should it be Listed cable for North America and if so, what type of > Listed cable? Should it be Approved for Europe and if so, what type > of Approved cable? > > Assuming the power output to the cable was limited in accordance > to UL1950/EN 60 950 (ie output was limited to Class 2), should the > cable still be Listed or Approved? > > 4. Assuming the input tolerance now to be max 72 Vdc (a > secondary hazardous voltage circuit) which also makes the coaxial > output connection 72 Vdc, does anyone see a problem with this > type of circuit being transmitted through a coaxial cable to the > outdoor transmitter? --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
