# Using AC Equipment in a DC car



## Mr. Sharkey (Jul 26, 2007)

Short answer:

No.

Long answer:

No, ABSOLUTELY NOT!!!

Detailed answer:

Any switchgear that you use in the traction battery circuit in your car MUST be rated for DC operation. The reason is that AC equipment is not designed to break a circuit that can sustain a plasma arc like a DC circuit can. AC systems need only snub an arc until the next zero-crossing point of the sine wave, at which time the arc will self-extinguish (1/100th of a second in 50Hz AC power systems). High voltage DC circuits will draw an arc across the contacts of an AC switch, relay, or circuit breaker, and the arc will sustain, as it does in an arc welder. This generally results in burned contacts, melted components, and in extreme cases, complete melt down and combustion of the component, and probably damage to the devices it is meant to protect.

Even fuses meant for high voltage DC circuitry protection must be specilised, many contain small plastic pellets which melt in the arc to kill the plasma.

Be sure that ANY devices you use in your car's traction battery pack circuitry is rated for DC current up to or exceeding the voltage and current extremes that you expect to encounter!!!


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## KiwiEV (Jul 26, 2007)

Very good. Consider it put back into storage!
Thanks for saving me a headache and a half.


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## rbgrn (Jul 24, 2007)

That's strange. I was sure I had seen breakers with both AC and DC ratings. There was a simple formula to convert between the ratings, like it can handle half the AC voltage for DC operation or something.


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## Mr. Sharkey (Jul 26, 2007)

Yes, you have seen breakers which are rated for both AC and DC. If a breaker is capable of handling a DC current, it is most likely very capable of doing so on a much more manageable AC current.

Many Aripax and Heinneman breakers carry dual ratings, and Square D makes breakers that clip into their standard household load centers with dual ratings (good for DC up to 48 volts).

It isn't just a matter of the distance of the gap that opens when the circuit is interrupted. High voltage, high current circuit interrupters of many types use additional techniques to snub arcs. Some use magnetic snubbers, others have internal baffling that breaks the path of the arc, while really high current switches and interrupters use compressed air to blow out the arc. Oil filled switches are also not uncommon, even in low voltage circuits (12 volts and up).

Even if your 200 ampere breaker was rated for DC, 500 amps is going to make it trip even during short period transients. Breakers typically have two trip modes, thermal and magnetic.

Most breakers will allow drawing more current than the trip rating for short periods of time. This is necessary to prevent nuisance tripping whith short term overloads from surges associated with motor and resistive load starting currents. They will sustain the higher current, but only for so long, the internal resistance of the breaker will cause it to heat up and it will eventually trip.

A short circuit, or a current draw of 1.5 times the rating of the breaker or more will cause a magnetic sensor inside the breaker to activate, tripping the breaker immediately. You might get by drawing 250 amps through a 200 amp breaker for a few minutes, but pull 400-500 and the breaker would let go at once.

What you have is a nice, big three-pole breaker, and a great paper weight, but look elsewhere for your DC disconnect equipment.

Oh yeah, and in case you wanted to see the results of DC arc-over, here's the field contactor out of my EV after it got a hit of regen current that was supposed to go into the batteries:










The top of the contactor has been removed. The black strips you see are soot and burned insulator, and the ends of the armature used to have large silver blocks, all shiny and bright. I managed to save this relay, but there wasn't much left after the fire. The same failure took out my field control board, turning it into a crispy cinder, and stranded me 190 miles from home in the middle of the desert in August.

Here's how I got the car home after that adventure:


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## KiwiEV (Jul 26, 2007)

Ok, so contacting equipment is out.

But what about fuses?

I can order in fuses very similar to these beauties from my work:








The biggest fuses I can get is 200amp at 500V AC.

What does anyone think? Yes/No?


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## rbgrn (Jul 24, 2007)

Kiwi - have you considered just using a few smaller fuses in parallel? I think it'd work fine. It's what big car audio rigs use that pull 100s of amps.


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