Questions about Fusing, Micro Circuit Breakers, AC/DC Disconnects

[Bendesa1962]

Hi Guy's,

I have some questions about Fusing, MCB's and AC/DC Disconnects


I've browsed the internet and learned a lot about these topics but also some things confuse me. Here come my questions

First about sizing a Fuse or MCB. I've seen the explanation many times as follows:

Take the Isc of your PV Module For example two strings of 6 PV Modules in Parallel.

For the combiner box we connect the two minus wires of the two strings to the minus busbarr
The two plus wires are connected to the plus of a circuit breaker.

The size of the circuit breaker is calculated as follows:

Isc 8,7A x 2 (because 2 parallel) x 1,56 (I also did run into sides where they claim it's not 1,56 but 1,25) = the MINIMUM breaker size In this case 8,7 x 2 x 1,56 = 27,14 A
They say alway minimum breaker size, so 63 A would be ok also?

I don't understand this. The breaker is installed to protect the wire right?
So if for some reason the current becomes higher than 27,14 A then the MCB will trip and prevent the wiring from burning right?

If this is the case how can they say minimum? Higher will be wrong then. If for some reason the current becomes 50 and it will burn up the wiring but the MCB of 63 A still will not trip so it's not pprotecting anything? Where do I go wrong?

Other question is if you use MCB's do you still have to use AC and DC disconnects. Some say the MCB can function as a disconnect as well and some say that's wrong?

And when would you decide to use Fuse and when MCB?

Thanks a lot

Regards

[SunEagle]

The over-current device is mostly there to protect the wire.

You go through the calculation to determine the minimum over-current device rating to isolate a short caused by the panels and then make sure the wire is rated for that value. If you decide to use a large over-current rated device you need to increase the wire size accordingly. Although you now can have too high a rating which would no longer protect the panel(s) if they shorted.

Some MCB's are "switch rated". Which allows you to use them as a disconnect. Most MCB's are not and can fail mechanically after a number of operations. That is where you would install a disconnect switch that have a better mechanical integrity and can handle a much higher number of cycles.

[Mike90250]

Breakers in Combiner Boxes, are to protect panels. If a parallel array of 6 strings, each producing 5A. has a panel in a string go bad, suddenly there are 25A of PV (5 good strings) backfeeding a bad panel (1 string) The panel and it's components are only rated for 5-8 amps, and 25a will make a mess of things. That's what the Min Series Fuse spec is to protect for, a bad panel from any more than "5" amps
Generally, switch rated DC breakers ( you can turn them on and off like a switch) are very handy for trouble shooting a PV array, Midnight Solar markets them for about $20 ea, Fuses are not able to switch , and by the time you buy fuse holders and fuses and a couple spares, you are at the cost of breakers. Some automotive style blade fuse panels have been used, but they may not hold up well to 5 hours of daily power heating them, and then cooling at night.

I used a six position controller for 3 strings, and then the spare positions for the wires from Controller .

20160821_132247cc.jpg

[SunEagle]

Breakers in Combiner Boxes, are to protect panels. If a parallel array of 6 strings, each producing 5A. has a panel in a string go bad, suddenly there are 25A of PV (5 good strings) backfeeding a bad panel (1 string) The panel and it's components are only rated for 5-8 amps, and 25a will make a mess of things. That's what the Min Series Fuse spec is to protect for, a bad panel from any more than "5" amps
Generally, switch rated DC breakers ( you can turn them on and off like a switch) are very handy for trouble shooting a PV array, Midnight Solar markets them for about $20 ea, Fuses are not able to switch , and by the time you buy fuse holders and fuses and a couple spares, you are at the cost of breakers. Some automotive style blade fuse panels have been used, but they may not hold up well to 5 hours of daily power heating them, and then cooling at night.

I used a six position controller for 3 strings, and then the spare positions for the wires from Controller .

20160821_132247cc.jpg

Great info on those DC circuit breakers Mike.

Although I believe the OP has a 12 panel system with 2 strings of 6 panels wired to the combiner box. Technically he doesn't need either "string" fuse since there are only 2. But whatever OC device used will still need to be less then what the wire can handle.

[bcroe]

Frequent practice is to use circuit breakers for lower voltage operation, fuses for high voltage
strings (400VDC). Its harder to break a high voltage DC current, since it doesn't stop flowing
like AC every half cycle. Bruce Roe

[SunEagle]

Frequent practice is to use circuit breakers for lower voltage operation, fuses for high voltage
strings (400VDC). Its harder to break a high voltage DC current, since it doesn't stop flowing
like AC every half cycle. Bruce Roe

Another good point to go with a CB. They also have springs to quickly clear the contacts under load as oppose to removing the blades from a fuse.