The reason for the Part 15 residential (Class B) limit is to protect reception, and the levels prescribed are (arguably) low enough to do so. If we allow higher levels, we are asking for service calls and perhaps official attention. But (unless I am mistaken) it is now the USER who responsible for interference generated by his Class A devices, and it's perfectly legal for us to sell them to him.
I recently spent some years working for a company that makes telecomm equipment. I there encountered for the first time the "telco" point of view (which is probably not uncommon). In the telephone world, the service provider is responsible for everything up to the network interface. Everything beyond that is the responsibility of the customer. Therefore, some people assume that equipment installed prior to the NIC can be, and should be Class A for Part 15. I have argued, with success, that this is an error with potentially expensive consequences. Part 15 contains an exemption for equipment located within a facility - even just a locked room, cabinet or vault - controlled by the telco. There is an argument, which I make, that when we do this in a residential building, if we are NOT Class B compliant, we may wish we had been. (Even Class B is often not enough, and I have seen equipment meant for customer use whose specification was well below the FCC limit.) And though our employers' products may comply with Part 15 we are still liable for harmful interference. However, emissions may be suppressed by other means than installing only Class B equipment and this is often the way to go. The utility exemption does make this easier. I personally believe that one may make a case for the mechanical room being Class A. It often contains furnaces, motors, and many other unregulated devices which generate high levels of radio and television interference, and to impose a stricter standard on telecom equipment in the same place seems a bit of a reach. But look at the environment! Will emissions reaching a customer location be above the Class B limit? If so, then I would say due diligence requires suppressing them further. A vault in a steel reinforced building's basement is a different matter than a rooftop utility hut with TV antennas just 3 meters away. If deployment entails a wide range of installations, then it is probably best to suppress all of it to Class B, rather than install Class B retrofit kits on a case-by-case basis. This is a decision I believe has to be made when the product is proposed. Regards, Cortland Richmond (unemployed, and looking) ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
