Surge protection, fuses and MCBs in loft from PV array.

[jimseng]

Hi All

When I come to install my panels on my roof I was thinking of installing one of these in my loft (or something similar). I will have two strings:

 

Much of my thinking is that it would give me a way of terminating the solar cables and allowing me to fully connect up the array before I continue to do the rest of the cabling to the inverter on the ground floor. (Eliminating un-terminated high voltage cables seems like a good thing). I just want to check that this is a sensible thing to do. I have some nagging doubts:

The inverter already has class II surge protection.

Is the current protection and fusing necessary given that the arrays are inherently current limiting in their output?

If anybody wants to suggest putting in a DC islolator how does that square with the current advise not to fit isolators and rely on the inverter DC isolator?

Is there any alternative I should consider if any of this is unnecessary such as a simple termination bus bar. (Does such a thing exist?)
I am considering that I could probably do away with having to crimp my own MC4 connectors given that the Growatt PV input seems to be gland and screw terminal, not MC4 for the PV input.

Comments?

[JohnMo]

Just put the inverter in the loft, terminate direct to the inverter, then an AC isolation switch and then down to consumer units. Then you have PV array plugs and inverter plugs nothing else on DC side.  Or just use longer DC cables and take then direct to the inverter location planned? 

[-rick-]

9 minutes ago, JohnMo said:

Just put the inverter in the loft, terminate direct to the inverter, then an AC isolation switch and then down to consumer units. Then you have PV array plugs and inverter plugs nothing else on DC side.  Or just use longer DC cables and take then direct to the inverter location planned? 

 

Thought that was recommended against these days. In any case, best to keep the active electronics outside of the hot (in summer) loft if you can.

[jimseng]

I also have two 16k batteries which won't be going up there.

[JohnMo]

15 minutes ago, jimseng said:

I also have two 16k batteries which won't be going up there.

So just do long cables, with no breaks between panels and inverter.

[jimseng]

Quote

So just do long cables, with no breaks between panels and inverter.

So do people see the need for fusing/spd/MCB between the panels and inverter, or as @JohnMo says, nothing between panels and inverter.

If it is nothing then I am either going to have to fit MC4 connections to the PV coming through the roof or find some suitable method of terminating them safely, giving me a method of continuing the cable run at a later date. The panels are going on the roof sooner than the rest of the build.

Edit: I should say MC4 connections to the ends of the PV cables that have come through the roof into the loft space.

Edited Saturday at 20:12 by jimseng

[LaChab]

Personally, I just put DC Isolaters in the attic to terminate the cables from the panels. No MC4s needed.

Then you can sort out the "on going" safely at any time.  Similar to @JohnMo's approach I think.

I find it hard to see "never switched" DC Isolaters  as a fire risk. Less than an inverter I would have thought.

[cjsparkey]

11 minutes ago, LaChab said:

Personally, I just put DC Isolaters in the attic to terminate the cables from the panels. No MC4s needed.

Then you can sort out the "on going" safely at any time.  Similar to @JohnMo's approach I think.

I find it hard to see "never switched" DC Isolaters  as a fire risk. Less than an inverter I would have thought.

+1

Terminate the PV cables into a DC isolator, go for a branded isolator and oversize 32A rather than 16/20A the larger enclosures make terminating easier with 6mm cables.

[Nickfromwales]

1 hour ago, LaChab said:

Personally, I just put DC Isolaters in the attic to terminate the cables from the panels. No MC4s needed.

Then you can sort out the "on going" safely at any time.  Similar to @JohnMo's approach I think.

I find it hard to see "never switched" DC Isolaters  as a fire risk. Less than an inverter I would have thought.

All this nonsense about issues / danger from using an isolator is just a mountain out of a possible molehill. 
 

We fit isolators early on, as roofs go on way ahead of the plant / M&E going in, and we just lock them shut with cable ties.

 

Just put a sticker on to say “isolate only outside of sunshine hours” if you have any risk of a random 3rd party getting their idle thumbs on this equipment, eg it’s rented out, but anyone else working on this kit should be competent or yourself supervising, so the risk from misadventure is easily manageable.

 

Anyone who disconnects a DC array under load will be very unlikely to do that a second time…..

[Dillsue]

3 hours ago, jimseng said:

So do people see the need for fusing/spd/MCB between the panels and inverter, or as @JohnMo says, nothing between panels and inverter.

If it is nothing then I am either going to have to fit MC4 connections to the PV coming through the roof or find some suitable method of terminating them safely, giving me a method of continuing the cable run at a later date. 

Maybe Im missing something but the Isc is likely very close to the Imp so you'd need a very precise fuse/mcb that can differentiate between normal operation and a short circuit. You'll be sizing your PV cable for the Isc so adding fuses/mcbs isn't going protect anything as a short will just take the circuit to Isc unlike a grid powered circuit which can hit very high currents more or less instantaneously and where fuses/mcbs are needed to protect cabling.

 

How you temporarily make the PV cable ends safe depends on whose gonna do the final connection. If it's you and you can connect up when it's dark then put the ends in a JB and move them to the inverter when its dark. If it's an electrician that's likely only going to want to work in the daytime then terminate them in a DC isolator assuming your electrician would be happy terminating in the dead side of an otherwise live isolator....probably best asking if you're using an electrician.

[jimseng]

Thanks for the replies. I'll fit an isolator. That's another one ticked off the list.