I too recall the days before dirt was converted to top soil, and the Euro
stds at that time had SELV for three phase equipment with the upper limit
of 24 VRMS. Manning Rose, [email protected], NCR Dayton, OH
----------
>From: DenBleyker, R. (919-543-7251 TL 441)
>To: emc-pstc
>Subject: In response to the 24 volt SELV level--
>Date: Tuesday, July 09, 1996 12:49PM
>
>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
>
>
