Fixing installation where 1 string should be 2

[jerriais]

Hi,

On a recent installation a new subbie wired up 1 string of 16 (CS6P-250P) where there should have been 2 strings of 8. Scaffolding came down yesterday but now system has an over-voltage shutdown, not surprising really.

Question is ... can it be fixed by asking them to split the incoming single string into 2 in the loft? It seems that this would halve the voltage and solve the problem, it is a dual MPPT inverter (SB 3600TL-20) but no shading so not a problem if independent MPP tracking is no longer possible. The max system voltage of the single string will be 668V which is within the panel and cable limits (1000V IEC) so I don't see any problem. Am I missing something? It almost seems too simple. Obviously I'd rather do it this way than get the scaffolders out again and rewire on the roof.

Cheers,

Matt

 

[Gavin A]

how do you mean split it in the loft? Presumably there are only 2 wires coming in to the loft from each end of the string, so unless there's a convenient skylight someone's going to have to go back out on the roof to split it into 2.

 

[jerriais]

Hi Gavin,

Glad to see you're keeping busy on Good Friday night! ;-)

My idea is to split it downstream of the DC isolator. Somehow it doesn't seem right but I can't put my finger on it. But this is the theory behind DC combiner boxes although we haven't used them before.

 

[whinmoor]

I'm not sure how this would/could work. I'm sure it could easily go the other way to take four cables in and two out.

Is it possible to switch inverters? How about a Kaco 4000?

Otherwise, someone might need to get their harness out...

Let us know your solution as others could run into this issue in the future.

 

[Gavin A]

Hi Gavin,

Glad to see you're keeping busy on Good Friday night! ;-)

My idea is to split it downstream of the DC isolator. Somehow it doesn't seem right but I can't put my finger on it. But this is the theory behind DC combiner boxes although we haven't used them before.

View attachment 18160

that would halve the ampage, not the voltage, which is the bit that you couldn't put your finger on.

 

[Gavin A]

ps I'd view that as short duration work, and use a ladder / roof ladder to resolve it if you've got space for the roof ladder up the side.

 

[zeljuga]

Time to gear yourself with some decent Static climbing rope , Harness and rest of rope access equipment.
Throw the rope over ridge to the other side ,fix it to safe annker (Concrete post or car) on the ground level.
They you are. Couple of good Jumars and you can fiddle around the roof safely as you like with both hands free for any possible future issue.
Al do staff should be trained properly before they can use equipment safely.

 

[Worcester]

That would reduce the current in the cables by half, but not the voltage. Think series/ parrallel batteries.

How have they been wired ? up / down / along / up ... or along...... up . along.... if the later then you just need to disconnect the cable joing the top to the bottom row on the end panels and then run a pair of cables connected to these back into the loft and then along the rafters to the same point that the other wires come in, so as not to create a big loop of dc cable (see new guide for reasons)

And of course if they followed the previuos good practice on edge distances, they got room up the side to put a roof ladder ....