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brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548

Posted: 11:46am 11 Jul 2013

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I have these "miniture" cicuit breakers that are very common in most electrical boards and equipment these days ...

My question is a strange one ,probably only known to the very experienced installers.

"Is there a definite way to connect the incoming leads-- remembering lead ,say, no.1 is 'from' the battery bank (the supply) , and lead No.2 is the one going to the inverter,say,"load" .

there are no indications on the breaker except it is an 'ac' breaker . OK, before you jump on me , I know ,,, should be a dc breaker ...

But ,the question is ,does anyone know much about the connections,,considering there are springs and bi-metallic strips , Etc inside these little fellas that may need certain ways of connection ..

Bruce

Bushboy

BobD

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Joined: 07/12/2011
Location: Australia
Posts: 935

Posted: 01:07pm 11 Jul 2013

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Brand & model? Have you tried Dr Google. Is there no legend on the side of the breaker? And that which you seem to know, they are built for AC not DC. They may not open with sufficient air gap to quench a DC arc. This could lead to overheating and .....

brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548

Posted: 06:36pm 11 Jul 2013

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OK, thanks Bob.. Yep, dc does develope a "larger" arc ....

Probably because these are ac mcb's ,might I add -100amp, then just a maybe ,they are ok for my application --48v @ ,say 30 amps ,,dc that is .. I'm mainly using these "because I had them" brand new,pity to waste them if they will do the job?.

Bruce

Bushboy

BobD

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Joined: 07/12/2011
Location: Australia
Posts: 935

Posted: 07:38pm 11 Jul 2013

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Bruce, it pays to be very careful with arcing DC if there is a substantial current flow and low source impedance. A long time ago in a place far away .... I used to work for the NSW railways in the section to do with train electrical power distribution. Voltages ranged up to about 1800 V DC (nominal 1500 V DC) and up to 10,000 amps. I saw the results of a procedural failure where a wrong circuit was opened and it had a load (a train). The resultant ionisation and arcing melted 13mm steel plate and 4 parallel 4x0.25 inch copper busbars. That is effectively a piece of copper 100x25mm and the load just turned it molten.

Ever since then I have been careful with DC especially where it has a low source impedance (lead acid batteries etc.). If there is a failure the results can be very bad. If that had been AC then there may have been a few sparks but probably nothing to get too excited about. As it was, the incident destroyed half of the sub station. Not sure about current technology but way back then (the 1960s) when a DC circuit breaker opened under load there used to be a blast of compressed air to blow out the arc.

If your CBs are AC rated then I would not use them on DC but your choices.

Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333

Posted: 08:30pm 11 Jul 2013

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One method you might consider is to gang 2 breakers together and use them as a dual breaker (their leavers are joined together as if they were a 2 phase breaker)then power into one breaker, looped to the second breaker, then power out from the second breaker, this method gives 2 break points and reduces the arcing, its common for high voltage solar DC to be done this way.

Although 100 amp DC is a lot of current, and i can easy weld with that, so think about the arc a welding electrode produces.

The rule of thumb for DC switches and AC inverters is to switch off the AC output before switching off the DC input, this way there is little load on the DC and almost no arcing.

Pete.

Sometimes it just works

brucedownunder2
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Joined: 14/09/2005
Location: Australia
Posts: 1548

Posted: 09:13pm 11 Jul 2013

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OK ,thanks guys

bruce

Bushboy

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Joined: 09/06/2012
Location: Australia
Posts: 39

Posted: 12:18am 12 Jul 2013

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You will have to look up the spec's for your mcb. Some are rated for DC voltage up to 48vdc but not marked as such.
However for the cost I have found it easier to get ones that are clearly marked as dc rated to avoid issues with inspectors.

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