Re:DC Grids
by vlovich123 on Friday August 25, 2023 @08:07AM
(#37260762)
Or just use thicker insulation? Not aware of any AC unit that uses such a thin power cord.
Re:DC Grids
by londons_explore on Friday August 25, 2023 @08:30AM
(#37260947)
It is unlikely any future high voltage DC system would be sufficiently safe without tracking leakage current as I outlined. Partly because it is almost impossible to stop someone cutting through the insulation, however thick (eg. with a kitchen knife, lawnmower, fire, etc).
3000 volts DC is a "definitely dead" voltage, as opposed to current 110 volt AC systems which are "you'll probably survive" if you use a kitchen knife to cut through the insulation.
Given that you need the protection systems in place anyway, there isn't much point in thicker insulation, unless you like your cord being more cumbersome, heavier and more expensive.
Re:DC Grids
by vlovich123 on Friday August 25, 2023 @09:32AM
(#37261529)
Thanks for the explanation. I wonder if you can detect integrity issues in the insulation instead and use that to shut down the system instead.
For example, imagine I want a 3000 volt DC wire to power a portable air conditioner. The air conditioner will be 10 kilowatts, so 3.3 amps. The wire can be thinner than headphone cables (two 0.3mm conductors, +-1500 volts, 150um PTFE coating) if desired.
Obviously, with such a thin insulation, the system needs to be human safe when chewed through by a baby. To ensure that, the current flow through the baby must be under 1 milliamp, or 10 milliJoules through the baby's heart. That can be ensured by tracking the current through each conductor, accurate to 1 milliamp, and shutting off the supply if there is ever more than 1 milliamp unaccounted for (either to earth, or to the other conductor). When the shutoff occurs, it must therefore happen within 1 microsecond (assuming the worst case fault, that is all three amps direct to the baby's heart). That in turn puts capacitance and therefore length limits on the cable - it wouldn't be possible for this cable to be safe longer than ~1000 feet.
TL;DR: It is very possible, with today's technology, to design very high voltage DC systems safe enough for use within a home. However, no hardware available off-the-shelf yet can do this, due to no demand.