String fuses

[DomB]

Hi all,

Just running through the "Photovoltaics in buildings guide - 2nd edition", as you do, and can't get my head around the string cable requirements.

p 16, 2.1.5 says:

"For a system of N parallel connected strings, with each formed of M series of connected modules, string cables are to be rated as follows

a. Array with no string fuses (of three or fewer strings only)

Voltage: Voc(stc) * M * 1.15
Current: Isc(stc) * (N-1) * 1.25
​

b. Array with string fuses

Voltage: Voc(stc) * M * 1.15
Current: Isc(stc) * 1.25"
​

Are the Current ratings there not the wrong way round? - if I have 1 string then I use the calc's in 'a.' so the rated current would be zero (i.e. Isc(stc) * (1-1) * 1.25).

Cheers

 

[JulianC]

No, they're correct but only applicable to parallel connected strings (ie. more than 1)

a. basically says that each string cable has to handle the maximum Isc from all the other strings it's paralleled with being dumped down it in the event of a short on that string.

b. says that a string cable only has to cope with the max Isc of it's own string 'cos the fuses should protect it from fault currents from those around it.

On a domestic system string fusing is unlikely to be an issue as I don't think you'd ever go over 2 strings. On much bigger systems, with judicial choice of inverter and module, it's possible to avoid the need for them quite often.

 

[DomB]

Thanks there.

I figured a. was saying if you have "3 or fewer strings" in an array you don't need to install fuses. And to work out the Current Rating in that situation you use the formula given there "Isc(stc) * (N-1) * 1.25", where N is the number of strings. But with just one string then N=1 so that formula zeros out!

The formula in b. does work where N=1 tho, and the formula in a. does work for any number of strings greater than 1 - hence my thinking they got the formula's the wrong way round!? i.e. formula b. should be for 3 or fewer strings and formula a. is for 4 or more..

I'm missing a point here somewhere eh?lol