RE: Internal telephone system requirements 1. What are the specific North American and European safety requirements (isolation, power-cross) relating to a telephone exchange supporting on-premise extensions (usual DC and ringing voltages), powered from a 2-pin 12 VDC mains adaptor (and therefore unearthed), and equipped with a 10BASE-T port? The supply voltages are generated using a switching regulator mounted within the product, supplying the logic (5 volts) and the telephone extension feeds (current-limited 24-volt off-hook and 60-volt for current-limited trapezoidal-waveform ringing). We intend to design the circuitry with a common return line for the 5-volt, 24-volt and 60-volt lines, which means that there will be no isolation between them. Furthermore, we do not intend to provide isolation between these voltages and the unregulated 12-volt input. Isolation will be provided by the external mains adaptor.
Wow, that is one very long question. The applicable standard is IEC EN60950 in both Europe and North America. The European version is a CENELEC standard of that number, the North American version goes under the UL1950/CSA950 designation. (It is a bi-national standard, one has a UL cover, the other a CSA cover, but the content is identical.) The North American version has some "deviations" from the IEC version, that you should pay careful attention to. Many of the differences have to do with satisfying provisions of the National Electrical Code (NEC) in the USA and the Canadian Electrical Code (CEC) in Canada. Some have to do with "Not Invented Here". There is no European PABX standard that I know of, but there is a North American PABX standard. However, that document does not deal with electrical safety aspects for which it refers to UL1950/CSA950. The IEC document deals with both the European and the North American operating voltages in separate annexes, as I recall. 2. Are there any North American or European mandatory telephony requirements relating to the connection of the above exchange to standard subscribers' line equipment (telephone sets, modems, fax machines, etc.), or is this purely a market-driven product quality factor (with voluntary requirements such as EIA and BELLCORE requirements)? Yes, in the USA, the equipment must meet the requirements of Part 68 of the FCC rules and Regulations, in Canada it must meet the requirments of CS-03. The two are almost identical in their technical requirements. As already mentioned there is a TIA Standard for PABXs, going under the number ANSI/EIA/TIA-464-A. my catalogue gives 1992 as the latest version, but that may need updating. 3. Are the answers to the above influenced by the range (distance from the exchange to the subbscriber's line equipment) of the extensions and whether they are permitted to pass from one building to another within the customer's premises? Yes, if you serve a campus environment, the extensions may be subject to the same environmental conditions as terminals serving a public exchange. 4. If voice signals derived from the telephone exchange and its extensions are delivered to the public network (analog PSTN or E1/T1 digital trunks) by means of off-the-shelf PC-based line interface boards with FCC Part 68 and European approval (such as Dialogic boards), does the above picture change? I am not sure what you mean by that question as you now seem to be mixing operational issues with safety issues. The issue here is whether there is a dielectric barrier between what goes on behind the PBX to what goes out to the public exchange. If you can show that no matter what hits the terminal behind the PBX, cannot ever go out on the trunk going to the Central Office, you're OK. In most modern PBXs that is the case. In the old Strowger switch-through PBXs, that was not the case. 5. If a certain item of subscriber's line equipment is intended solely for connection to a PBX extension, and never to the public telephone network, is the equipment subject to exemption from North American and European telephony requirements and regulatory procedures, and to the power-cross requirement, and therefore subject to sale purely on the basis of UL (except power-cross) and FCC Part 15 compliance (North America), or CE mark (European safety and EMC requirements only)? There are special provisions for terminal equipment that connects only behind PBXs that provide the required barrier and a specila registration code that indicates the equipment can only be used in such an application. In most cases such equipment is tested and approved "as part of the PBX or KTS". In all cases, you should look at the intent of the regulations, not at the letter of the standard. In the final analysis, the only thing that counts is whether you are satisfying the essential requirements of the regulation, not whether you satisfy the "letter" of the standards. That is what the official requirement in Europe is (satisfying the essentail requirements in the Directive) and you'll find that the North American regulators will accept well-reasoned arguments that demonstrate that the intent of the regulations is met. Best regards, Vic Boersma
