48 volt car elect not going to happen

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. Would the advantages of higher voltage systems be worth that result?

Vaughn

So, what voltage do the hybrid vehicles run at? I thought is was over

100, possibly 200 volts.

Paul

Yes, but that is "apples and oranges". (correct me if I am wrong) In a hybrid car, most of the electrics are still 12 volt, the high voltage battery just runs the hybrid part, and those wires are made very obvious.

The original post was about the potential conversion of ALL the electrics in conventional cars to 48 volts.

Vaughn

So, in an electric car, the starter is 12 volts?

i

On Sat, 02 Feb 2008 20:34:00 -0600, with neither quill nor qualm, Ignoramus17662 quickly quoth:

I wonder how many cheeks you'll catch with that big hook, Ig.

None so far, but I am waiting...

i

Carl et all

The 48 volt figure is the nominal battery voltage, 4 X 12, closest to

52 volts which is the nominal voltage at which one gets shocked. IOW the maximun safe voltage to work with before the voltage becomes a shock hazard. There are of course exceptions to this. The kickback from an inductance load can be many times the 12 volts that we presently are accostomed to working with.

Also the 48 volt figure is not the maximun charging voltage when charging a 48 volt system. eg the charging voltage for a 12 volt system is 13.5 to 14.X something.

I read all this stuff perhaps 20 years ago when talk of higher voltage Automobile Electric Systems was started. I don't remember the source of the documentation. Probably a government publication.

Bob AZ

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

Why are they not considering lower voltages, such as 24 nominal, which is used in commercial trucks?

i

I thought the electric cars were hand-cranked.

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

"Stephen Robinson" wrote: You need the VOLTAGE to to push the CURRENT in order to kill

^^^^^^^^^^^^^^^^^^^^^^^ It is refreshing that SOMEONE seems to know this. However, I have a couple of reservations about your second sentence. The resistance of the body is almost entirely in the skin, which is quite high when dry. If, somehow, you got across some voltage while wet, or perhaps bleeding from an accident, the resistance could be very much lower. Secondly, the amount of current that can be lethal depends on the path through the body. The worst would be across the shoulders, which places the heart in the circuit. The path from one hand to the other, or from one hand to some lower part of the body could also be quite dangerous.

Not the case if a wet human body is holding on to car frame, while grabbing firmly some unlucky item energized to 48 volts. Resistance depends mostly on how contact is done.

You need the VOLTAGE to to push the CURRENT in order to kill someone(approx 30 mA).The human body is a very high resistance object, which needs very high voltage to provide that current. 48v ain't gonna be enough.

Agreed, I still don't think 48 v is going to be very dangerous.

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

48 volts DC is not very dangerous. But because of the conditions car mechanics work under, especially backyard mechanics, it could be contribute to a more dangerous situation. I think we'll see a lot more off the cuff "arc welding" of Craftsman adjustable wrenches.

Not mine ( I checked) 48 and 25 are the numbers. The stinger _is_ insulated, no?

-Carl