48Vdc is supposedly the highest voltage that is still considered safe. (https://electronics.stackexchange.com/questions/267789/how-s...)
It certainly sounds like a smart move on the copper savings alone.
It certainly sounds like a smart move on the copper savings alone.
48V is somewhat of a dinosaur from the days when you designed things with a 20% margin since that's what was cheaply attainable. Regular PoE is 54V these days (10% margin), and some more specific applications creep even closer to 60V. (I've seen 57V and 58.5V, which is of course 5% and 2.5%)
A lot of the satellite and wireless equipment I've worked on has inherited that and runs off 48 too.
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....
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...).
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.
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.
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.
3 and 6 were the standard voltages for consumer stuff back in the days of people needing to buy multiple packs of AA cells every year
And non techies. They always have a few disposable battery flashlights hanging around it seems like.
I think we just stay close to the numbers we do, because we just really like multiples of 12 and numbers with lots of divisors. 24 hours in a day, 360 degrees, etc.
Most numbers people really like a lot seem to be on the 7-smooth numbers(https://oeis.org/A002473) list and related/overlapping sets like highly composite numbers.
Which I think is super cool, because it means you can choose one and there's a good chance someone else chose it too for something similar, unless they were using renard numbers or something instead.
For a 12v system, of course. But on a modern 48v system who knows what they decided. Having a switching regulator with built in current limits might be helpful.
In the Cybertruck there would need to be a DC-DC buck that could handle the 48v inrushes properly from the 800v traction battery and spikes would have no where to go as the DC-DC would not be bi-directional or even if it was could not react fast enough.
For some reason people assume it's 48v DC or 50v dc. But it's double that. That said, I feel significantly more scared dealing with 100v DC than I do 48 or 24v.
Really underappreciated safety aspect. The currents required for your average doodad at 48V leave you with a MUCH lower chance of unscheduled welding.
Remember folks, everything is a fuse if you put enough current through it, as a rule of thumb it's good to keep "enough" pretty low.
--old EE lore
Also, those numbers are for ripple-free DC, which you're not going to find in a car. They're cut roughly in half for ripple peaks.
We used to call getting shocked "getting a taste" - like getting a taste of ice cream, except it's more like a microsecond blackout.