PV on metal corrugated roof - earthing question

[sepiot]

Hi

I will soon have 3 panels on a metal corrugated roof (outbuilding). They are on the ground, on plastic boxes at the moment. I plan to use the plastic boxes on a different install, and attach the panels directly to the metal roof with mini rails. From what I've read, it's a good idea to run a cable from the panels to the rod, due to possible build up of static in the roof, malfunction, shorting etc. I have installed the earth rod next to the outbuilding and run an earth cable to the hybrid all-in-one inverter/battery system via a consumer unit with a type B RCD.

So my questions are mainly related to the details of running the cable, plus anything I may done wrong in the install, if anything.

I used a 6mm (7m long) single core multistrand earth cable for the inverter and CU. Would it be ok to run a similar cable to the panels?

Should I cut the cable in 4 and add terminals to attach to the 3 panels and roof?

The method would be drilling the mini rails/roof, and use nut and bolt to attach cable terminals.

Would there be any corrosion; and how to minimize it? I think the rails and roof are aluminum (?), and the nuts & bolts steel (?)

I do not know the resistance of the earth rod, and cannot think of a cheap way to measure it. Is this a must? If so, any suggestions?
Should I proceed with the roof install without knowing this info?

If I use the renusol plastic boxes on a different, flatter, roof (on a different offgrid system), do I need to earth those panels to a rod as well? The panels would not be touching the metal roof, though the PV cables will likely touch the roof at some point.

I am a DIYer (and new to electrical stuff until last year!). I have been reading some forum posts and found that a couple of people have a very similar system to mine: binky and Ucathmydrift ('RCD tripping problem' thread). I'd love to hear from you, and I have a few more question relating to the AC input of the hybrid inverter (which is not connected at the moment), but will leave that for another thread.

I think that's all. Thanks in advance!


Offgrid System specs:

3 x Panels:
Peak power 410W
Vmpp 31.25V
Voc 37.25V
Impp 13.12A
Isc 13.88A

Iconica Hybrid 3000W 24V pure sine wave inverter
6 x 100Ah AGM deep cycle

 

[binky]

just bond to the roof, no need to do individul panels, but if you are felling very keen, you will find the panels have holes in the frames for bonding.

6mm will be fine.

 

[Ucatchmydrift]

Hi

I will soon have 3 panels on a metal corrugated roof (outbuilding). They are on the ground, on plastic boxes at the moment. I plan to use the plastic boxes on a different install, and attach the panels directly to the metal roof with mini rails. From what I've read, it's a good idea to run a cable from the panels to the rod, due to possible build up of static in the roof, malfunction, shorting etc. I have installed the earth rod next to the outbuilding and run an earth cable to the hybrid all-in-one inverter/battery system via a consumer unit with a type B RCD.

So my questions are mainly related to the details of running the cable, plus anything I may done wrong in the install, if anything.

I used a 6mm (7m long) single core multistrand earth cable for the inverter and CU. Would it be ok to run a similar cable to the panels?

Should I cut the cable in 4 and add terminals to attach to the 3 panels and roof?

The method would be drilling the mini rails/roof, and use nut and bolt to attach cable terminals.

Would there be any corrosion; and how to minimize it? I think the rails and roof are aluminum (?), and the nuts & bolts steel (?)

I do not know the resistance of the earth rod, and cannot think of a cheap way to measure it. Is this a must? If so, any suggestions?
Should I proceed with the roof install without knowing this info?

If I use the renusol plastic boxes on a different, flatter, roof (on a different offgrid system), do I need to earth those panels to a rod as well? The panels would not be touching the metal roof, though the PV cables will likely touch the roof at some point.

I am a DIYer (and new to electrical stuff until last year!). I have been reading some forum posts and found that a couple of people have a very similar system to mine: binky and Ucathmydrift ('RCD tripping problem' thread). I'd love to hear from you, and I have a few more question relating to the AC input of the hybrid inverter (which is not connected at the moment), but will leave that for another thread.

I think that's all. Thanks in advance!


Offgrid System specs:

3 x Panels:
Peak power 410W
Vmpp 31.25V
Voc 37.25V
Impp 13.12A
Isc 13.88A

Iconica Hybrid 3000W 24V pure sine wave inverter
6 x 100Ah AGM deep cycle

I used these to bolt the panels to the shed roof: (cheapest way i could find) works well:
https://amzn.eu/d/3rh3xTy
Then used a ground wire from the metal shed to an earth rod that goes about 1 meter deep. I then used another earth rod about 3m away from the other earth rod to ground the ac load side of the inverter - as the instructions say.. Your inverter instructions may be different though..

 

[binky]

I used these to bolt the panels to the shed roof: (cheapest way i could find) works well:
https://amzn.eu/d/3rh3xTy
Then used a ground wire from the metal shed to an earth rod that goes about 1 meter deep. I then used another earth rod about 3m away from the other earth rod to ground the ac load side of the inverter - as the instructions say.. Your inverter instructions may be different though..

You should bond to the AC side, otherwise it's a separate earthing system, but as you have 2 rods close together, I doubt it makes much difference.

These are good fixings https://www.tradesparky.com/solarsp...ImVJOKfel-Ttqa_51bsL4D3a2WgBU2ghoCaj8QAvD_BwE

 

[sepiot]

Thanks both for the mounting links. I wish I'd have ask here months ago, as I spent an age trying to find something appropriate and in stock. I have bought something already, but will keep your suggestions in mind in case it does not work well.

Inverter manual only says 'Connect to a permanent grounded wiring system, and comply with local regulations'. I don't know if there could be a risk of (significant) leakage from the roof earth wire to the wire going to the CU, if both were connected to the same rod and there was some static build up in the roof. For the moment I'll connect both to the same rod, and test for voltage with a multimeter every now and then.

There is bonding between the earth and N terminals on the inverter's AC output. Not sure if you are referring to that bonding? (ok, re-reading, i think you are talking about Ucatchmydrift system).

 

[Ucatchmydrift]

Thanks both for the mounting links. I wish I'd have ask here months ago, as I spent an age trying to find something appropriate and in stock. I have bought something already, but will keep your suggestions in mind in case it does not work well.

Inverter manual only says 'Connect to a permanent grounded wiring system, and comply with local regulations'. I don't know if there could be a risk of (significant) leakage from the roof earth wire to the wire going to the CU, if both were connected to the same rod and there was some static build up in the roof. For the moment I'll connect both to the same rod, and test for voltage with a multimeter every now and then.

There is bonding between the earth and N terminals on the inverter's AC output. Not sure if you are referring to that bonding? (ok, re-reading, i think you are talking about Ucatchmydrift system).

I think my inverter has temporary earth/nuetral bonding when grid power is supplied, but i forget now tbh, the post you are referring to will tell you all i know about that! These all in one inverters (non grid tie) supposedly should not have an rcd on the incoming grid supply. I have one, and it did trip once on a very damp foggy morning, presumably from the metal roof/solar panels finding its way through the bonding in the inv to the rcd? - but just a guess,, as im no electrician...

 

[sepiot]

Well, that's a subject I am very interested in, because at some point I may connect the AC input of the all in one hybrid inverter to grid. But being a total noob, I decided to go step by step, and not connect the AC input yet, as I don't quite understand all the implications and the risks of doing so.

From what I have read, you are right in that the inverter should not sit downstream an RCD, which would be my case. Since I am in no rush or have the knowledge to open my home's CU and change cabling, RCDs etc, I might never do it. Or perhaps, like you, I may just connect it regardless of the RCD issue. I mean, if it's 'just' the tripping of the RCD and not dangerous, I could probably live with it, if it's not too often.

One question though: do I need to worry/check the specs of the mains cable going to the outbuilding? This cable goes from my house CU, to another CU in the outbuilding, from which I would feed the inverter's AC input. I know the cable is thick armored, but I don't know if it can feed the inverter (plus other outlets in the outbuilding). In other words, can the inverter be treated as just another appliance in terms of AC load?
The manual says i should use 12AWG 4mm cabling on the AC input. I guess that should include both the exterior cable, and also the cable I would run from the shed's CU to the hybrid inverter. Problem is, I can't see the specs of the armored cable anywhere.

Finally, I dont' know if my inverter has a temporary bonding or not (nothing about it in the manual), and not sure how to check it. I know that in theory the bonding should only happen once, else it is not good practice/safe.

There is a guy in youtube Roland W that makes very helpful videos about this sort of stuff. Have already asked a few questions, but don't want to pester him about the smallest details and issues. HIs playlist 'Inverter earthing grounding bonding' is a good reference for any noobys out there..

 

[Ucatchmydrift]

Well, that's a subject I am very interested in, because at some point I may connect the AC input of the all in one hybrid inverter to grid. But being a total noob, I decided to go step by step, and not connect the AC input yet, as I don't quite understand all the implications and the risks of doing so.

From what I have read, you are right in that the inverter should not sit downstream an RCD, which would be my case. Since I am in no rush or have the knowledge to open my home's CU and change cabling, RCDs etc, I might never do it. Or perhaps, like you, I may just connect it regardless of the RCD issue. I mean, if it's 'just' the tripping of the RCD and not dangerous, I could probably live with it, if it's not too often.

One question though: do I need to worry/check the specs of the mains cable going to the outbuilding? This cable goes from my house CU, to another CU in the outbuilding, from which I would feed the inverter's AC input. I know the cable is thick armored, but I don't know if it can feed the inverter (plus other outlets in the outbuilding). In other words, can the inverter be treated as just another appliance in terms of AC load?
The manual says i should use 12AWG 4mm cabling on the AC input. I guess that should include both the exterior cable, and also the cable I would run from the shed's CU to the hybrid inverter. Problem is, I can't see the specs of the armored cable anywhere.

Finally, I dont' know if my inverter has a temporary bonding or not (nothing about it in the manual), and not sure how to check it. I know that in theory the bonding should only happen once, else it is not good practice/safe.

There is a guy in youtube Roland W that makes very helpful videos about this sort of stuff. Have already asked a few questions, but don't want to pester him about the smallest details and issues. HIs playlist 'Inverter earthing grounding bonding' is a good reference for any noobys out there..

I should imagine a 4mm armoured cable would be ok to power the inverter, (4mm should be good for 6kw) but depends on how many kw your inverter is.. Mines a 3kw and has a 4mm armoured supplying it which is plenty big enough. You may find that the inv draws a constant draw from the grid when its wired up, mine does, a constant annoying 10w. So i fitted a contactor switched via a wifi smart plug to control charging battery banks via an app at off peak times only. Im now trying to get it all linked to home assistant so i can automate the battery charging by pulling data from the met office to put the right amount of charge into the battery banks depending on current soc and what the weather will be doing the next day etc. You should check out diysolarforum.com, its the best place for diyers like us - lots of extremely knowledgeable diy solar people on there...

Heres my build thread (needs updating as been busy since):

https://diysolarforum.com/threads/first-build-3kw-panels-3kw-inv-14-4kw-storage.52783/#post-672398

 

[binky]

I should imagine a 4mm armoured cable would be ok to power the inverter, (4mm should be good for 6kw) but depends on how many kw your inverter is.. Mines a 3kw and has a 4mm armoured supplying it which is plenty big enough. You may find that the inv draws a constant draw from the grid when its wired up, mine does, a constant annoying 10w. So i fitted a contactor switched via a wifi smart plug to control charging battery banks via an app at off peak times only. Im now trying to get it all linked to home assistant so i can automate the battery charging by pulling data from the met office to put the right amount of charge into the battery banks depending on current soc and what the weather will be doing the next day etc. You should check out diysolarforum.com, its the best place for diyers like us - lots of extremely knowledgeable diy solar people on there...

Heres my build thread (needs updating as been busy since):

https://diysolarforum.com/threads/first-build-3kw-panels-3kw-inv-14-4kw-storage.52783/#post-672398

4mm will happily run 32A, it's roughly 4Amps per kW so rough figure says it's good for 8kW. However, standard power supply circuit is allowed 3% voltage drop along the cable, but for solar we like to keep voltage drop at only 1%. Higher voltage drops can cause the inverter to increase it's output voltage, if you are unlucky enough to live in an area with higher grid voltages, then the two combined can cause the inverter to shut down. Only seen that actually happen on very rare occasions, but it can.

As for RCDs, better of without those supplying on the AC inverter supply, and certainy not sharing a RCD with other circuits. RCDs shutdown in milliseconds and at less than 30mA should you do comething daft like run over a cable with the lawn mower, Grid tied inverters are designed to shut down on loss of grid supply, but they are allowed 3 seconds to do that, so basically your solar system could kill you backfeeding a shared circuit with Amps. To cause heart and lungs to stop (defribrillation) only takes around 50mA for a healthy adult.

 

[Ucatchmydrift]

4mm will happily run 32A, it's roughly 4Amps per kW so rough figure says it's good for 8kW. However, standard power supply circuit is allowed 3% voltage drop along the cable, but for solar we like to keep voltage drop at only 1%. Higher voltage drops can cause the inverter to increase it's output voltage, if you are unlucky enough to live in an area with higher grid voltages, then the two combined can cause the inverter to shut down. Only seen that actually happen on very rare occasions, but it can.

As for RCDs, better of without those supplying on the AC inverter supply, and certainy not sharing a RCD with other circuits. RCDs shutdown in milliseconds and at less than 30mA should you do comething daft like run over a cable with the lawn mower, Grid tied inverters are designed to shut down on loss of grid supply, but they are allowed 3 seconds to do that, so basically your solar system could kill you backfeeding a shared circuit with Amps. To cause heart and lungs to stop (defribrillation) only takes around 50mA for a healthy adult.

Whats the reason rcds are not usually placed on the grid supply to the inverter binky (im talking about non grid tie, ive never had experience with grid tie)? Mine is not on a shared circuit, but i put one there anyway just incase i cut through the cable or something silly lol. Its only ever tripped once on a very damp winters night.

 

[binky]

Whats the reason rcds are not usually placed on the grid supply to the inverter binky (im talking about non grid tie, ive never had experience with grid tie)? Mine is not on a shared circuit, but i put one there anyway just incase i cut through the cable or something silly lol. Its only ever tripped once on a very damp winters night.

It's a sub main, so RCD not necessarily needed, and the way inverter relays 'chatter' on start up can trip RCDs. Plus most inverters have some form of built in RCD protection on the AC side, and something similar on the DC side. RCDs are for the protection of people who may have done something stupid like banging a nail through a cable hidden in a wall, or cutting a cable in the garden. So as long as the cable run does not force installation of a RCD to comply with regs, then there's no need for it.

 

[sepiot]

I should imagine a 4mm armoured cable would be ok to power the inverter, (4mm should be good for 6kw) but depends on how many kw your inverter is.. Mines a 3kw and has a 4mm armoured supplying it which is plenty big enough. You may find that the inv draws a constant draw from the grid when its wired up, mine does, a constant annoying 10w. So i fitted a contactor switched via a wifi smart plug to control charging battery banks via an app at off peak times only. Im now trying to get it all linked to home assistant so i can automate the battery charging by pulling data from the met office to put the right amount of charge into the battery banks depending on current soc and what the weather will be doing the next day etc. You should check out diysolarforum.com, its the best place for diyers like us - lots of extremely knowledgeable diy solar people on there...

Heres my build thread (needs updating as been busy since):

https://diysolarforum.com/threads/first-build-3kw-panels-3kw-inv-14-4kw-storage.52783/#post-672398

Ok, so 4mm is good. Still don't know what mine is. It's 15mm diameter, but that could be the armoured part. I 've no idea of the conductor inside or the amp specs.
I like your creative thinking around charging the batteries etc. I am taking this as a bit of a fun hobby, though I can see I am way behind you!
I have read and posted on diysolarforum, but being mostly US based, I prefer to get answers from UK based people.

Basically, I need to find out more information about my own system, before I can tackle the AC input of the inverter:
Specs of my armoured cable.
Specs of my inverter (temporary bonding or not? Internal RCD or not?)
Is the outbuilding RCD in a shared circuit or not? (I can probably find this out after a bit of research on my own house CU )

Looks like a bit of a headache at the moment. But who knows in the future...