Back in the early days, just before dirt was invented, we all used UL-498 for information technology equipment. This spelled out 24 volts peak as the allowed limit. In these early times we also had a telephony industry operating under their own rules--an ok situation since the industry was very tightly regulated and 'craftspeople' were permitted access to -48 nominal (almost 60 with fully charged batteries) circuits.
With deregulation of the telephony industry came a horrible awakening--unless something changed all the telephone battery circuits would require the same degree of isolation/insulation as primary ac-mains! Things like phone jacks would be outlawed. So the rules were relaxed to meet the telephone battery circuits in IEC-950, see para 1.2.8.5 in the 2nd edition. I have no reasoning behind the 60 VDC and 42.4V peak relationship. They are proportional to root 2, but this has no technical meaning in a DC to peak AC wave. FYI, in my opinion this change made a great deal of sense and did not reduce the safety level presented to the user. Bob Den Bleyker, BOBDB @ VNET.IBM.COM
