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....
> 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...).
> 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.
> 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.
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.
However there is a lot of leeway on the "48V is the highest safe voltage" statement too. 48V has a special place in regulations because of its use in telco, but 60V DC is still very safe.
Doubling again to 96 is not safe, however. I’m not sure why they settled on powers of two. Something perhaps to do with noise filtration, and fewer new tricks to learn?
A lot of 48V infrastructure comes from the marine world - where 4 x 12V lead-acid batteries in series is common for this reason. So a lot of components already exist and can be made to work in the automotive sector with simple modifications. (In the marine world, it's much simpler to combine and split batteries in powers of 2 for cell balancing - you can take 4 x 12V batteries and charge them in parallel, discharge them in series)
It's convenient to build things in multiples and powers of 2 and 3. A nice two-tier or four-tier battery rack full of 2-volt cells just works out nicely.
Oh I hope nobody's doing that, rechargeables have been so good for so long. I'm just now retiring some that I bought in the 2007-2011 timeframe which have started to fail, but my 2015 purchases are still going strong with hundreds and hundreds of charge cycles on 'em. I haven't used alkalines since the 80s.
I have not had great luck with rechargeables. They have a lower voltage than regular AA, so I find that most sensitive electronics just start out thinking the battery is already low and then stop running before the battery is fully discharged. My Z-wave keypad lock is one of those. AA batteries last about 2 months, rechargeables start giving a low battery warning after a week and hardly last 2-4.
Aside from modern barrel jack supplies, none of it is actually powers of two, it's like, 13.8v lead acid or a little higher for lithium.
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.
60V DC is the upper limit for circuits that untrained personnel can work on and be exposed to as listed in IEC 62368-1. This is the standard that most consumer goods and IT stuff use to validate and verify electrical safety. 60V isn't going to be hazardous to a person in basically any situation.
Sure but they can feed the system through a regulator if they'd like. Do we have any reason to believe they are tapping directly on to a pack for this part of the system?
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.
Its actually difficult to for a "regulator" to do what battery does, the battery can output significant surge as needed and its resistance drops as voltage rises absorbing large spikes without issue.
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.
