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.

[sgt_woulds]

Don't forget, if you are running cables from the loft to a position elsewhere in the house, they need to be SWA (or double insulated cables in Copex with a draw cable to the inverter if an electrician is doing final fix at a later date).  

 

If an inverter is not being fitted in the loft then a seperate DC isolator as close to the entry point is always required. 

 

32 amp - DC specific - double pole isolator.  From experience I wouldn't recommend any DC isolator made by ABB...  although I haven't touched one for 10 years so hopefully they are more reliable now!

 

A competent (sensible) sparky will work from the AC location up to the DC isolator and make the final connection to the isolated switch.

[Dillsue]

3 hours ago, sgt_woulds said:

Don't forget, if you are running cables from the loft to a position elsewhere in the house, they need to be SWA (or double insulated cables in Copex with a draw cable to the inverter if an electrician is doing final fix at a later date).  

 

If an inverter is not being fitted in the loft then a seperate DC isolator as close to the entry point is always required. 

Is that requirement for an isolator as close to the point of entry as possible a mandatory requirement or a recommendation??

 

On the use of SWA for DC PV I read recently that SWA generally doesn't meet the requirement for DC PV as the regs require double insulation between the conductors which standard SWA doesn't have. I beleive there are some specialist SWA cables available but the common or garden SWA shouldn't be used if you want to meet the regs

[Beelbeebub]

25 minutes ago, Dillsue said:

On the use of SWA for DC PV I read recently that SWA generally doesn't meet the requirement for DC PV as the regs require double insulation between the conductors which standard SWA doesn't have. I beleive there are some specialist SWA cables available but the common or garden SWA shouldn't be used if you want to meet the regs

Yes, you are correct - normal SWA won't do.

 

You do need to be able to isolate the DC supply from the inverter but you *don't* need a separate DC isolator *if* your inverter has a suitible one built in (which most modern ones do) 

(The Institution of Engineering and Technology - Code of Practice for Grid-connected Solar Photovoltaic Systems) 

 

Statistically speaking, DC isolators have been the biggest cause of fires. 

 

And as such the latest guidence says to avoid them (as long as you have a built in one). 

 

https://www.thefpa.co.uk/advice-and-guidance/free-documents?q=RC62 Recommendations for fire safety with PV panel installations

 

 

[sgt_woulds]

Sorry, yes I should have said it will need to be Solar DC specific SWA. We didn't fit it very often but there was a specific brand we found that was SWA with insulated and sheathed cables.    I remember that our Sparx still had to note it as a variation as technically you are not supposed to use a wire armoured cable for unearthed DC.  

 

Copex is the easier / cheaper / more compliant option.

 

Elimination of DC isolators is generally accepepted, but unless the cables are surface mounted and identified with labels all the way to the inverter location you need a method of isolating the hidden DC cable run.  If you rely on the built in DC isolator in the inverter - and espescially if that inverter is fitted elsewhere in the property - that still leaves the entire run of field cables live.

 

Interesting figures on the failure rates of isolators.  I imagine this is mostly due to people not paying for regular electrical inspections.  For some reason the negative DC cable always seems to work loose over time in any screw connction, and its important to check and re-torque at regular intervals.   (The neutral connections on the AC side are prone to this too)

 

If this was taken out of the equation then the biggest failure boint would be the DC connectors - which sounds right to me as changing from MC3 to the MC4 'compatible' type connector was the biggest mistake ever made in the solar industry.  

[jimseng]

The thing that confuses me over the SWA discussion is that I plan to use PV ultra cable, which now comes in an SWA version. I was planning on running it from the loft to the ground floor and to the inverter inside the service voids/posi joists using safe zones. This is only a 3m drop. 

This is from Doncaster Cables:

 

Quote

PV-Ultra® provides a quicker, easier and neater install and can be installed using normal cable accessories, cleats and/or clips and gives additional protection without the need for conduit installations.

Quote

PV-Ultra® allows for direct connections from the solar panels to the DC isolator/invertor every time, without the need to assess the route for whether conduits will be required, and without the need for junction boxes.

This strongly suggests I don't need to use SWA which adds about £3.00/ metre. I'll use SWA if I have to but unless "without the need for conduit installations" means "must be run in conduit" then I'll go for the cheaper option.

[cjsparkey]

The steel wire is for mechanical protection, if you can route the DC cables so this isn’t needed that’s an option or copex etc wherever it needs mechanical protection.

[Dillsue]

4 hours ago, sgt_woulds said:

Elimination of DC isolators is generally accepepted, but unless the cables are surface mounted and identified with labels all the way to the inverter location you need a method of isolating the hidden DC cable run.  If you rely on the built in DC isolator in the inverter - and espescially if that inverter is fitted elsewhere in the property - that still leaves the entire run of field cables live.

If you have the inverter in a ground floor plant room and that inverter has an integral isolator or there's a free standing DC isolator next to it you've surely met the requirement @Beelbeebub posted without putting a second isolator in the loft. The loft is remote and rarely quickly or easily accessible so the isolator in the loft is going to be unused. If you need to work on the cables in the void, shut the inverter down, switch off the inverters DC isolator so there's no load in the cables then disconnect the MC4s in the loft to isolate the void cables. That's in line with the PV string disconnection method in @Beelbeebubs table. Wait til its dark if your concerned about voltage on the plugs. I really dont see the point in putting an additional isolator tucked away in the loft where it's going to get forgotten about, particularly if it's seen as an unnecessary additional fire risk.