DC CB Wiring

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Re: DC CB Wiring

Postby davidg » Sat Aug 02, 2014 3:48 pm

Cherokee Solar wrote:They often wire them in series so that you can double the current which will pass through the circuit breaker. I have a triple pole circuit breaker on my inverter for example.

Correction:- They are wired in series for VOLTAGE not for current, ie if one pole is rated for 250Vdc then two in series would be 500Vdc and so on.

Lets take another way, if two poles of a 4 pole were rated at 250Vdc and both the wires from the + & - were each run through one pole each that would be for 500Vdc and if the extra 2 poles were added in series in each wire then it would be 1000Vdc total.

In parallel, theory is that the current rating of each pole is added together, in practice, not always true, be very careful if your thinking of doing this. I recommend against doing this, just obtain a properly rated CB or ceramic fuse system or if allowed, isolator instead.

If it for fault currents really HRC fuses are a better option and will always work for short circuit currents, in DC. One thing that lots of people forget is that PV is a current limited source (assuming single string), no point in putting CB's in single strings, they will never do anything, if they do "pop out" then it is almost certainly underated for the job or faulty.

So if you have an array and the total short circuit current for one string is say 8.6 amps, then one would expect to install lets say 12A fuses with multiple strings, the rating of the fuse should be based on what the modules state eg if they say a maximum of 15A then a fuse rating of that or less should be used for each string, these provide protection against a string short and that string turning into a "heater" then doing a complete melt down. HRC fuses will always work and are highly accurate in their rating, often a better choice than CB's. Always make sure you have spares sitting there, nothing worse than getting caught without any. I always supply spares with any HRC fuses in systems I install for that very reason. They have never been needed however, it saves real hassle if they ever were. The same goes for fuse switches on battery systems, always have suitable spares sitting there, chances are they will never be needed.
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