Bob, A long time ago, before EDP (now ITE) equipment, the ground rules for maximum voltage of Class 2 circuits were defined at UL as 30 V rms, based on the Class 2 limits defined in the National Electrical Code at that time (This was before NEC Class 3 circuits). So that definition took care of bell ringing transformers which could be wired in your home without the conductors on the secondary being run in conduit.
When transistors came along they begat EDP equipment and the definition had to be enlarged to include 42.4 volts peak, to take care of the signals which are not sinusoidal on I/O ports (42.4 being 30 times the square root of 2, a reasonable method of getting to 42.4.). The step to 60 V DC came along when the NEC expanded Art. 725 to include Class 3 circuits. I was never clear on the reason for Class 3 circuits, but I suspect that someone else on this net will educate us both soon. As far as the policy of working on energized equipment in your company is concerned, in my opinion the only safe circuit is a Class 2 circuit, or one that is dead and to which I have the only key to the circuit breaker controlling that circuit in my pocket. All other circuits are hazardous, and extreme caution must be taken if the person must work on it live, including a person standing by to render help if needed. I still remember showing up at Zinsco on a Monday morning to run tests on one of their big transformers only to find out that their chief engineer electrocuted himself on Saturday while getting the transformer ready. He was working on a live circuit, alone. Regards, Gabriel Roy Hughes Network Systems MD The opinions expressed are mine and not those of HNS. P.S. ".... electrical safe work practices policy (working on energized equipment)..." isn't that an oxi-moron? ----------------------------------- snip----------------------------------------------- >Hello again, >I had previously asked for references to an alleged occupational requirement for a maximum touchable voltage of 24V and I thank you for your responses. I saw nothing concrete which gave legitimacy for a 24V limit. I found the references to the telephone company interesting. I have another somewhat related question. What is the reasoning for selecting 42.2V peak and 60Vdc as a limit? Does UL explain this anywhere? >The reason for this line of questioning is that I am involved in a debate on an electrical safe work practices policy(working on energized equipment). The issue of applicability involves who is covered. Ideally everything should be de-energized before performing work, but that is not always the case. There are plenty of situations where that will not occur. So, the question is at what voltage should the policy kick in. >Regards, >Bob Brister Digital Equipment Corp.
