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Posted By Topic: Solar Panels On Motorhomes

north1
Jul 31 2019 21:13

Can some one direct me to the standards relating to solar panel installs onto motorhomes.
E.g

At what size solar array is it necessary for a electrician to became involved.
Isolation switching /fused load /battery side.

Thanks
North1

   

AlecK
Aug 01 2019 08:40

There is official guidance available fin the PV array Standard: ASS/NZS 5033. Regulations require PV arrays that are part of any electrical installation to comply with the Standard. As long as the PV array has an open-circuit voltage in the range of extra-low voltage, ie below 120 V, no licence is required.

However the Standard excludes very small arrays from coverage; ie if the array is less than 240W and less than 50 V. So in effect the requirements of the Standard become only guidance for these small arrays.

That doesn't mean they can safely be ignored, and fusing is a good example. Ideally there should be fusing at both ends of the array downlead, as there are in effect 2 sources of energy (the array itself, and the battery) so to ensure or other short-circuit doesn't start a fire protection is required at both ends. And protection is required for each string of modules within the array, which on a motorhome is likely to mean each module; this guards against a short-circuit within one module being fed by other modules. These array fuses need to be special type, as a typical array can't produce enough current for standard types of fuse to operate reliably.




   

north1
Aug 01 2019 11:10

Thank You Alex for your prompt reply.
It reinforced my thinking re best practice
   

AlecK
Aug 01 2019 12:24

current revision of "3001" includes adding some guidance for ELV, including PV arrays.
And trying to get the lower limit for application of "5033" raised a bit.

But will be a while before that gets through the process, and then gets cited by ESRs
   

DougP
Aug 01 2019 15:15

The DC from the panels would normally be going directly to a controller of some sort. So I don't see how there could be any backfeed from the batteries requiring protection at the panel end, unless there was an unlikely sequence involving multiple faults.
As per usual advice, we generally only protect against single faults.
   

AlecK
Aug 01 2019 15:45

It's not about back-feed; it's simply that there are 2 sources of supply, and each has to be dealt with.

The cable intended for feeding into the battery can equally carry current in the other direction. Eg if that cable suffers a short-circuit, protection at the battery end is necessary. That's dealing with the battery as one source.

in the other direction, PV modules are limited-current devices so 'short circuit' protection in the normal sense isn't needed to protect the downlead itself. Most arrays are multi=module, but modules in series remain current-limited. But as soon as there's more than one string in the array , there's the possibility of a fault in one module being fed by energy generated in other string(s; which has led to a significant number of fires. Hence the need to over-current protection for each string; and to allow it to operate on the low PSSC available from PV modules, the fuses need to have a suitable operating characteristic.

Shoving a charge controller in doesn't change the need for protection at both ends, but may affect the choice of rating for both.


   

DougP
Aug 01 2019 22:04

Thanks for the clarification Alec.

Seems that we had different ideas in mind. I was thinking of just 1 or 2 panels in parallel for a basic 12V motorhome system. Whereas you seem to be talking about multiple panel strings in parallel?

I thought there was blocking diodes in the panels to prevent one string being shorted out by another faulty string?
But maybe there are more possibly configurations than I'm aware of? I only have limited experience with larger solar systems TBH.
   

evanh
Aug 02 2019 01:39

The diodes inside the panels are there to prevent cell damage and allow generating current to continue when some cells in a segment are not generating - in shadow. The weak cells will be driven into reverse voltage. There is typically three such diodes arranged across a segment each. The segment diode limits any reverse volt to only what can be generated by the remaining cells in that segment.

   

evanh
Aug 02 2019 03:38

Blocking diodes don't cover all failure modes so pretty much don't help while also reducing efficiency.

   

AlecK
Aug 02 2019 09:55

consider a panel that's faulted so badly it's essentially a short circuit. it then acts as a load for any other panel(s) that are connected in parallel.
Starting as a low-current arc, the damage increases, and is constantly fed by the rest of the array in keeping the fault-loop going until the sun sets - and then starting up again next morning.

Matters more with higher string voltage, but even at 12 V - as typical for a motorhome - the possibility still exists. Even a 12 V arc can start a fire.

haven't had to deal with one, but have seen the pics of serious damage to a number of domestic arrays - and wouldn't want it to happen on the roof of the polystyrene box (motorhome) I was sleeping in.

The appropriate fuses and fuse holders are now becoming available; that will operate at just above FLC - from memory the fusing point is only 1.1 x I.