On Feb 3, 1:25 am, Don Foreman wrote:
On Sun, 03 Feb 2008 05:26:20 GMT, "Carl Byrns"



wrote:

"Stephen Robinson" wrote in message
...

That suits me. Under the right conditions, 48 volts is plenty enough
to kill. Therefore, one inevitable result of 48 volt electrical systems
in cars will be the occasional dead mechanic.

How will 48v kill people, except for causing a fire, which 12 v is quite
capable of doing that also.

Do you own an arc welder? Look at the open circuit voltage(should be on the
case somewhere). Now look at all the shock hazard warnings.

If you don't own an arc welder the open circuit voltage is usually around 48
volts and arc voltage is around 25 volts (sound familiar?) and at the right
amperage that is enough to kill you.

-Carl

The user handholds the live stinger of an arcwelder. BTW, the O.C.V
on many arcwelders is more like 60 to 80 volts.

Your house is wired with 110 volts, except for stove, dryer, A/C and
perhaps some stuff in the shop which is 220V hot-to-hot. In Europe
all domestic sockets are 240V.

The issue is cost of change. Some NEC wiring codes kick in at (I
think) around 48 volts so 36/42 volts sounds good for automotive.
Insulation is considerably cheaper and lighter than copper. Another
cost issue is switching and fault management of DC, which tends to
hold an arc at higher voltages. Fuses and contacts don't break
higher-voltage DC current well unless specifically designed to do
that. There's also the issue of the lightbulb mfrs worrying about
cost of moving to thinner, longer more fragile filaments.

These issues can and will be solved, eventually if not sooner. Silicon
in high volume keeps getting cheaper and more capable. When the cost
of not changing becomes higher to the mfr than cost of change, then
change will happen.

The trip time of a "fast blow" MIL STD circuit breaker at 1000% over
current is specified around 880 milliseconds. You have to design the
current path for what may be a very low operating current for fault
current. Going from 12 v to 48 v will quadruple the fault current
keeping everything else the same (it's only the source and wire
resistance that limits this). When you use circuit boards to carry
this current, the circuit trace has to support this current until the
circuit breaker trips. Empirically we figured this to be 25AWG of
copper for a 28 volt aircraft DC system. What's interesting is if the
circuit trace "pops", the circuit breaker never does, and FR4 (fire
retardant PWB material) converts to carbon from this, and since ground
planes are used, this burnt trace makes a connection to ground. The
leakage current starts the board to glow orange where this occurs, and
emit smoke, and it burns like a firecracker fuse until all the board
carbon is converted to ash. The circuit breaker never trips as this
current is well under the 2-10 amp ratings.
ignator