DC cabling

[kopkings]

Hi All, Need some advice please as I'm new to PV and going for my MCS accreditation now. I'm installing on my own property and because of the position of the house, the best position to put the panels is on the garage roof. This is a separate building to the house with the CCU on the other side of the house to the garage. I ideally want to put the invertor next to the CCU in the house, so my thinking was to put the panels on the garage, a DC isolator near the panels in the garage and a DC isolator near the invertor in the house, connecting the two via SWA. I then got thinking about how I earth the armour of the SWA if it is carrying a DC current and was not sure what the answer was. I was thinking I don't earth it in the usual way if carrying an AC current, but am pretty sure I still need to earth it - I would really appreciate some advice on this. Thanks all.

 

[malcolmsanford]

I would earth the SWA at the CU in the house to be honest, as the SWA should not be in contact with the DC side of things, as SWA would just be an exposed conductive part. I would assure that you use a SWA that has the rated 1000v DC for this.

Can I ask why you are considering 2 DC isolators, the normal type are the rotary kind that can be locked in the open position, so Why not just put 1 at the inverter.

 

[JD6400]

Sounds like you are going to have horrendous dc losses to me , i would be looking at the garage for the inverter and then the sub at ac back to the head .

 

[kopkings]

Thanks for the speedy reply. I was considering 2 DC isolators only because that is what I was taught when installing a system that has a considerable amount of space between it and the invertor, so you can isolate at either point. Regarding earthing the SWA, is it ok to earth the SWA as normal, connecting it to the earthing bar in the CCU, with it carrying a DC current?

 

[kopkings]

It's an option, but I'm thinking slightly ahead in that if I decided to put another system on another part of the property, I would have the reverse problem of getting the cable to the invertor in the garage... The garage will take a system of about 2kw and when funds allow, I was thinking of possibly adding another small system later.....trying to work out best approach.

 

[malcolmsanford]

The SWA armour is not carrying the DC current, you do not need an earth in the DC circuit, as the panels are class II and though the SWA is not double insulated, it is double sheathed so is good. The SWA is then classed as a exposed conductive part and needs earthing.

Saying all this are you using a transformer type inverter??

 

[JD6400]

I am sorry fella , i have never looked into doing it this way so are as in the dark as you .
I would strongly recommend a shorter dc run with the inverter in the garage , i am not trying to be condescending with you , honestly.
But were you not taught about how much more prolific your losses can be on dc over ac .

 

[kopkings]

The SWA armour is not carrying the DC current, you do not need an earth in the DC circuit, as the panels are class II and though the SWA is not double insulated, it is double sheathed so is good. The SWA is then classed as a exposed conductive part and needs earthing.

Saying all this are you using a transformer type inverter??

OK - understand now regarding the SWA - so I can earth in the normal manner to the earthing bar. Makes sense now. Regarding invertors, not sure... only that I was looking at the Sunnyboy and Mastervolt.

 

[JD6400]

Sorry missed your third post .

 

[kopkings]

No worries at all. I'm grateful of all advice you pro's can give me. I'm right in the middle of spec'ing and planning what to do, so every little bit of advice and direction is good. Yes, I was taught about the comparing losses and I'm wondering if I just rest with one installation and e happy with that.

 

[JD6400]

No worries at all. I'm grateful of all advice you pro's can give me. I'm right in the middle of spec'ing and planning what to do, so every little bit of advice and direction is good. Yes, I was taught about the comparing losses and I'm wondering if I just rest with one installation and e happy with that.

There is alot to be said for trying to walk before you can run .
Why pay more now for a over specked under efficient inverter that may not end up getting added too , and anyway you could put the next inverter in the house then .
Good luck with it , and to be honest i have found even the bigger and longer established boys are still on nearly the same learning curb as us all !
A word of warning it is no longer the gravy train that some thought it was meant to be !

 

[kopkings]

There is alot to be said for trying to walk before you can run .
Why pay more now for a over specked under efficient inverter that may not end up getting added too , and anyway you could put the next inverter in the house then .
Good luck with it , and to be honest i have found even the bigger and longer established boys are still on nearly the same learning curb as us all !
A word of warning it is no longer the gravy train that some thought it was meant to be !

Thanks to all for your time and advice. I'll come back on this thread to feedback what I did. Cheers.

 

[edexlab]

The SWA armour is not carrying the DC current, you do not need an earth in the DC circuit, as the panels are class II and though the SWA is not double insulated, it is double sheathed so is good. The SWA is then classed as a exposed conductive part and needs earthing.



Malcom I have been thinking this over recently and cannot come to a conclusion of what is the correct thing to do (this would also apply to the use of steel conduit for Dc runs which is done a fair bit in US and other countries)
I may be completely wrong in this but I'm just wondering whether connecting the swa armour to the Ac earthing system is the correct thing to do Bs7671 says connect all exposed conductive parts
no problem with that
yet with an isolating transformer inverter connecting the Dc swa armour back to the house earthing system could lead to Ac voltages appearing on the array frame
which I thought was part of the reason the transformers were used to prevent parasitic capacitance, ie Ac voltages transferring over to the Dc side which would then mean that the array would need to be bonded as in a TL system
I have spoken to the Niceic about this and one opinion was that in most cases the armour should be bonded to the array frame in TL systems another was that it should be left floating ,there was no definitive guidance only opinions from people not that familiar with Pv systems
I've not used swa for Dc as yet but we do have a project where this may be a possible method of installation be interested to see others views

 

[pvconnections]

I frequently install SWA for the DC element. Why should you need to earth the armour? it is totally independant of any earth fault monitoring system ? It's not extraneous.

 

[kopkings]

I frequently install SWA for the DC element. Why should you need to earth the armour? it is totally independant of any earth fault monitoring system ? It's not extraneous.

Just that I have always had to earth SWA traditionally and as I am now preparing for my MCS inspection, it was something I was not sure about as it carried a DC current