Short circuit current

CALLD · March 2015

Not PV isc, but batteries!

Something compelled me to test the DC breaker I have installed on my batteries last night, and would you believe the horror that filled me when I failed to get it to trip! It's a 130amp breaker, and putting a 5 second load of 3640watts on my inverter (it can do 3750w for 30secs and 5000w for 2secs) could not get the switch to trip. I don't have a shunt to measure the current but conservative calculations lead me to believe the current was at least 150amps. 3640/23.8v=153amps.

I took it off and shorted it across an old 12v car battery and could not get it to trip either - the 35mm2 Awg #2 cables did get warm so I know there was plenty of current flowing.

Anyway, I have to take it back and complain and get a new one - which I will test for sure!

Just out of interest sake - does anyone know what a 2600Amp short circuit at 24v could do? That is apparently what my AGM batteries could deliver if a dead-short were to occur for some reason. Just interested, not that I would ever allow that to happen, but what if it did?

I've heard people in the gun fraternity talk about accidental discharges and how there are basically two groups of people - those who have had one happen and those who are still going to have one happen! I sincerely hope the same doesn't go for people who work with batteries and accidental short circuits...

Photowhit · March 2015

Relax a little bit, Breakers are NOT designed to trip instantly, at a slightly higher amperage they may well hold for hours!

Now lets look at the situation, You have a 24 volt system, while you were actually well above your calculation, "3640/23.8v=153amps" doesn't take into account the inverter loss. Add at least another 5%, but it started at 25+ volts and dropped over time to 23.8 volts.

Now lets look at your 12 volt battery shorting, your using #2 gauge wire, breakers are designed to protect wire from over heating. Since #2 is rated at around 120 amps at best your test isn't valid since your loosing some amperage as heat. To protect #2 wire you should be using a smaller breaker. You might not be passing as much amperage through as you think dissipating/using some of it to heat the wire. I doubt you were pushing as much amperage as you think.

I don't think you really have a failure, If you must push the limits I might try shorting across the AGM battery bank, I'd rather trust quality breakers, but Robin at Midnite has said they are good for 5000 trips (magnetic type) so have at it... Don't hurt yourself!

Mountain Don · March 2015

What you experienced is why I use a large breaker AND a type T fuse in series. The fuse is very close to the battery positive connection point. The breaker is closer to the inverter and is there to be used mainly as a disconnect. The fuse is larger capacity than the breaker. In case of a dead short the fuse will blow for certain.

CALLD wrote: »

Just out of interest sake - does anyone know what a 2600Amp short circuit at 24v could do?

You could weld metal as in arc welding.

waynefromnscanada · March 2015

CALLD wrote: »

Just out of interest sake - does anyone know what a 2600Amp short circuit at 24v could do? .

I agree with Mountain Don's answer, but will add to that - - - - - it's also excellent (in the back yard electrician's situation) for spraying molten metal (copper wire for example) around the room, starting fires in several places at once, igniting the combined oxygen and hydrogen at lead-acid batteries, causing them to burst and spray acid about the area, and of course molten metal is useful for vaporizing human skin and flesh. Other than that, nothing serious will happen.