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For battery-powered systems the nominal voltage is used. Telephone "48 volts" is 55.2 volts in practice, only falling near 48 if there's a power failure and the

For battery-powered systems the nominal voltage is used. Telephone "48 volts" is 55.2 volts in practice, only falling near 48 if there's a power failure and the office generators don't autostart in a timely fashion. That's never caused any regulatory problems for Ma Bell, despite OSHA saying 50v is the cutoff. And personally having spent roughly a decade of my career crawling all over such systems, 55.2 doesn't bother me one bit. Span-powered T1 at 130VDC, on the other hand.... that'll poke ya. That gets little plastic covers over all the terminals, but they have been known to fall off. So there is a meaningful threshold, and 55.2 is solidly below it. Which suggests to me that there's a good bit of leeway built into the standards, perhaps specifically so they don't have to wheedle about whether a battery system should be measured at its nominal voltage, its float voltage, its absorption voltage, its peak/equalization voltage, its....

Tempest1981 3y ago
> That's never caused any regulatory problems for Ma Bell, despite OSHA saying 50v is the cutoff I seem to recall getting a buzz when touching phone wires - while the line was ringing. I think I measured around 100 VAC. Apparently that's "ok", safety-wise.
- myself248 3y ago
  Ringing is 90VAC, 20Hz, current limited. And it's only on for 2 seconds and then off for 4, so it's not like you're gonna grab on and be unable to let go. So, yeah, as a momentary thing it's not a hazard unless it tickles you off a ladder or whatever. Punchdowns and terminal blocks are normally mounted where they can be worked on from ground level so that's unlikely to be an issue.
tzs 3y ago
> That's never caused any regulatory problems for Ma Bell, despite OSHA saying 50v is the cutoff. I'd be surprised if the wired phone system is not allowed to do a lot of things that would not be allowed under current codes and standards. By 1920 about 35% of US households had a phone. Upgrading all of that to keep up with evolving safety standards would probably have been way too expensive, and so I would expect that there was a lot of grandfathering.
dfox 3y ago
> That's never caused any regulatory problems for Ma Bell Telco lines generally are not considered safe voltage. One thing is that ringing voltage is fairly squarely above the threshold and another thing is the whole outside plant thing (ground potential differences, lightning strikes...).

Tempest1981 3y ago

> That's never caused any regulatory problems for Ma Bell, despite OSHA saying 50v is the cutoff I seem to recall getting a buzz when touching phone wires - while the line was ringing. I think I measured around 100 VAC. Apparently that's "ok", safety-wise.

myself248 3y ago
Ringing is 90VAC, 20Hz, current limited. And it's only on for 2 seconds and then off for 4, so it's not like you're gonna grab on and be unable to let go. So, yeah, as a momentary thing it's not a hazard unless it tickles you off a ladder or whatever. Punchdowns and terminal blocks are normally mounted where they can be worked on from ground level so that's unlikely to be an issue.

tzs 3y ago

> That's never caused any regulatory problems for Ma Bell, despite OSHA saying 50v is the cutoff. I'd be surprised if the wired phone system is not allowed to do a lot of things that would not be allowed under current codes and standards. By 1920 about 35% of US households had a phone. Upgrading all of that to keep up with evolving safety standards would probably have been way too expensive, and so I would expect that there was a lot of grandfathering.

dfox 3y ago

> That's never caused any regulatory problems for Ma Bell Telco lines generally are not considered safe voltage. One thing is that ringing voltage is fairly squarely above the threshold and another thing is the whole outside plant thing (ground potential differences, lightning strikes...).