Safety (Combiner) Box Question

[007 RED]

A new question from the dumbo who is considering a small DIY solar system on the roof of his carport.

 

From everything that I have read, and seen on various videos, it is wise to install a ‘safety’ box between the solar panels and the inverter.  My understanding is that these normally contain the following components:  a)  DC fuse;  b)  DC breaker;  c)  DC surge arrestor.  (number of each depending upon the number of strings.

 

FYI….. The system I’m contemplating will comprise 3 or 4 x 415W panels (Vmp = 41.7V; Imp =9.96A) which will be connected in series (single string) to a Sofar 2200W GTI (Max input voltage = 500V; max input current MPPT = 12A).

 

I’ve seen a number of circuit diagrams for various types of ‘safety’ or combiner boxes and some advertises online but these tend to have been multi entry string entry units.

 

Q…. how do I determine the correct type/size of components needed if I wanted to construct or purchase a single entry ‘safety’ box for the system mentioned above?

 

Once again, your informed contributions will be much appreciated.

[Crossy]

When the rain stops I'll post a photo of ours ????

 

It could be a while.

 

[Crossy]

Here we go.

 

Cheap DIN rail consumer unit from MegaHome, 30A AC MCB same place.

DC MCBs, DC MOVs and AC MOVs from Lazada.

 

Do NOT try to save a few $$$ by using normal MCBs on the DC, conflagration may result if one opens when the panels are active.

 

This is for two strings (inverter has 2 MPPT inputs) so you could lose one DC MCB and a pair of DC MOVs.

 

[007 RED]

1 hour ago, Crossy said:

Here we go.

 

Cheap DIN rail consumer unit from MegaHome, 30A AC MCB same place.

DC MCBs, DC MOVs and AC MOVs from Lazada.

 

Do NOT try to save a few $$$ by using normal MCBs on the DC, conflagration may result if one opens when the panels are active.

 

This is for two strings (inverter has 2 MPPT inputs) so you could lose one DC MCB and a pair of DC MOVs.

 

Once again, many thanks for your input.  I hope that you didn't get to wet taking the photo on my behalf ????.

 

I note that you have not used any DC fuses on the string inputs, is there a reason for not using?  Also is there any 'secret' formula to determine the values of the MOVs?

[Crossy]

No fuses, the DC MCBs should do the trick, in reality they are there to allow you to turn off the string. The panels are self-limiting at the ISC of 10A or so.

 

MOVs need to be rated for "a bit more" than the maximum string voltage, the 1kV DC ones were what was available.

 

The inverters have their own internal protection but it does no harm to give it a bit of help.

 

The runs to the panels are short, only a few metres but the AC run is 40m so I put protection on everything. The steelwork is also solidly grounded.

[Muhendis]

If you only have one string of solar panels then you have nothing to combine and therefore do not need a combiner box. A simple DC protection/isolator and a surge arrestor is all you need. If you make it same as Crossy's neat box you can't go wrong. 

If you were to have several strings then you would need to be able to independently isolate and fuse each string. Also each string would need to have a series blocking diode to prevent feed from an unshaded string going to a shaded or otherwise weaker string. I have attached a good video which will explain all.

 

[007 RED]

4 hours ago, Muhendis said:

If you only have one string of solar panels then you have nothing to combine and therefore do not need a combiner box. A simple DC protection/isolator and a surge arrestor is all you need. If you make it same as Crossy's neat box you can't go wrong. 

(snipped)

Many thanks for your comments and the clip video which is very informative.  I note that according to the 'presenter' he indicates "that with a single string setup there is no need for any kind of over current protection".  Really?  I think I would rather a fuse blows or a breaker trips that the expensive inverter 'bowing a gasket'.  What is it?  "Belt & Braces".

 

FYI..... I will be following Crossy's neat 'model', suitably modified for single string input.

[Crossy]

7 minutes ago, 007 RED said:

I think I would rather a fuse blows or a breaker trips that the expensive inverter 'bowing a gasket'.

 

A single string will stabilise at ISC of the panels (10A ish) so your 15A fuse wouldn't be troubled. The MCBs in my box really serve to turn off the panels so I can muck about with the surge arrestors etc. without getting zapped. 

[007 RED]

9 minutes ago, Crossy said:

 

A single string will stabilise at ISC of the panels (10A ish) so your 15A fuse wouldn't be troubled. The MCBs in my box really serve to turn off the panels so I can muck about with the surge arrestors etc. without getting zapped. 

Thanks for your observation. My comment about "blowing a fuse" was more a figure of speach than practical proposition.

 

As l said, l will be following your 'neat model', suitably modified for a single string input with a DC MCB and surge arrestors for feed to the inverter. I'll also include an AC MCB and surge arrestors for the AC output.

 

I take your comment about being zapped very seriously.  My father, bless him, always kept telling me when l was young, you can't see electricity, you can't smell it, but by god you can feel it. Wise words that l've always remembered.

[Crossy]

13 minutes ago, 007 RED said:

I take your comment about being zapped very seriously.

 

The problem is that you can't turn solar panels off, they generate power in almost any illumination.

 

Feed your panels to the DC MCB then to the surge arrestors, then the inverter. This means that you can isolate the string whilst working on the surge arrestors (replacing the cartridge) and inverter.