PV fed into sub board...?

[SPARTYKUS]

Hi- not a PV spark just a normal one (!) but have a PV question if I may.

A friend has had it installed, & is happy. I am quite familiar with the fixed wiring at the house & recall her mentioning that the PV equipment has been "connected to the fusebox upstairs in the airing cupboard". This is a massive house with 3 sub boards. Its connected to one of these.

Is that ok? Should it not be connected at or near the meter and DB1? Rather than feeding back through the sub board, sub main, or is it fine like that.

Not looking for faults just wondered if that was right?

Cheeers.
House is Tt (but gets a TNCs -like Zs due to shared water service with Tncs next door!)

 

[BruceB]

From MIS3002:
"In particular, attention is drawn to the unique combination of hazards associated with installation
of PV systems highlighted in clause 1.3 of the above document. Furthermore an inverter supplied
from a PV array must be connected via a dedicated circuit, to a spare fuseway in the main
distribution unit, or to a fuseway in an additional dedicated distribution board."

So taken literally, you should not connect through a sub-board which is also a distribution board for other circuits. That goes beyond 7671 requirements. However, I suspect many MCS installers (including myself) are willing to connect through a sub-board with sufficient protection when it is disproportionately uneconomic to do anything different.

The bigger issue for your friend is likely to be TT combined with a sub-board. Look at 712.411.3.2.1.1. Its tricky to interpret, but in essence you cannot have pv on the load side of a mcb (and for TT also an rcd if relying on ADS) that is protecting other current using equipment. In other words PV must be on its own circuit. With careful duplication and/or discrimination of rcds you can make it work sharing a distribution circuit (not final circuit), but it needs to be designed carefully. Without knowing what rcds/rcbos are where in his system I could not say whether it is satisfactory.

Regards
Bruce

 

[SPARTYKUS]

Thanks Bruceb ! Digesting....

 

[SPARTYKUS]

Ps the Main Db was installed as if installation is TN, no Rcds up front. 2 of 3 sub boards have sone Rcd protn, my work there has never been to fix the world, I've geadually improved, so to speak when authorised.

This has always been a dilemma & suspect was for original fitter of said board, TT with ze of 17 ohms but Zs at nearest socket of 0.4 (via undersized 4/6mm main bond to water main).

Recommended ( years ago) upgrading to be sure as strictly speaking am sure its to be seen as "TT" ! But no go on that one (more umportant things like PV to sspend on)

 

[SPARTYKUS]

Please excuse my speelink, typing on an iPhone

 

[BruceB]

If trying to work out/follow the rules is too difficult, you could go back to first principles:
- will a fuse/mcb blow/trip in time for a L-N short anywhere in the installation?
- will a fuse/mcb/rcd blow/trip in time for a L-E short anywhere in the installation?
- for those circuits where additional protection is required, will a high resistance L-E contact trip an rcd (and not be compromised by the PV which has up to 5s to disconnect in the event of mains failure)?

 

[moggy1968]

You can't assume on a TT system that you will meet the disconection times just because your Ze is giving you a result like TNS because the quality of earth will vary with climatic conditions, so it may not be as good in operation as when you test it. Also you should not be relying on the bonding to achieve those results, the bonding should be disconected before taking the Ze.
All circuits on a TT system must be RCD protected in order to guarantee tripping within the required time. In the case of PV that really means a type B RCD IMHO although there has been considerable debate on here about that.
Once you've got your hands on the installation you take responsibility for that installation so the lack of RCD protection could come back on you if there was a problem.
Fundamentally this is an unsafe installation that fails to comply with the 17th or 16th edition (and possibly the 15th but I don't go back that far!)
TT and PV is a PITA because of the RCD requirements.