Correct earthing of off-grid PV systems?

[mikepepler]

I'm sure Adam will be able to comment here, and perhaps others...

With the forthcoming upgrade of our PV system, I thought I ought to get the earthing sorted out, given there will be a lot more power flowing. So, here's my questions about what should be done:

1. Should the module frames be earthed? Advice I've found from DECC indicates not, as it can create more shock risks while working on the roof. I'm not concerned about lighting in our location.

2. The Morningstar controller has an earth terminal, which I'm assuming I should connect to the mains earth for the house? The manual warns that system negative must NOT be connected to this terminal: "DO NOT bond system electrical negative to earth ground at the controller. Per NEC requirements, system negative must be bonded to earth ground through a GFPD at only one point."

3. The inverters have earth terminals, I assume these should also connect to the mains earth for the house?

4. Is it OK to NOT earth any part of the solar feed (operating at ~30V normally, 40V open circuit)?

5. Is it OK to NOT earth the 12V battery system?

For 4 and 5, note the point in 2 - the controller earth must not be linked to the system negative. The manual does mention using a GFPD though - would this be an RCD installed into the solar +/- system? Or something else? Is it really needed? If I don't earth the PV module frames, how could a serious ground fault occur?

Any practical advice much appreciated!

[adam2]

I would be inclined to earth the battery negative, that should be enough to prevent any part of the system reaching a dangerous voltage to earth even under fault or failure conditions.

I would not normally earth the module frames unless there was a special reason to so do.

I doubt that you will find an affordable RCD for DC operation at the low voltages involved and would not worry about this.

It would be advisable to protect the mains voltage output of any power inverter with an RCD.
Opinions differ as to the merits of earthing the "neutral" of the inverter output.

[mikepepler]

Thanks Adam!

I found another document from the Morningstar website which says:

The TriStar is oriented towards use in a negative ground system since these are most typical. The negative terminals of the TriStar are electrically common and charge control switching is done in the positive leg of the circuit. The negative terminals can be grounded at various points in the system since the bus is common and it will operate as designed.

So I can just earth a negative connection somewhere in the system and that will do all parts of the system.

However, the chassis ground is isolated from this negative system bus:

Electrical negative is isolated from chassis ground in the TriStar. Morningstar recommends earth grounding the chassis per NEC guidelines. See the installation section in the Owner's Manual for more details.

and the manual says about the ground terminal:

Do not connect the system negative conductor to this terminal. NEC requires the use of an external ground fault protection device (GFPD). The TriStar MPPT does not have internal ground fault protection. The system electrical negative should be bonded through a GFPD to earth ground at one (and only one) location. The grounding point may be located in the solar circuit or the battery circuit.

I'll send them a message to ask for more explanation, but it looks to me like even following their instructions you end up with system negative and chassis ground connected, albeit through earth, it's just that there's a GFPD in the system negative leg?

So do you think I can just wire negative to earth and also the chassis to earth and not worry about it? I get the impression that the GFPD requirement is due to some PV systems in the US causing fires when they develop an earth fault, but if I don't earth the module frames I don't see how that can happen...

Opinions differ as to the merits of earthing the "neutral" of the inverter output.

Actually, I was talking about using the earth terminal on the inverter chassis, which is connected to the earth mains output of the inverter.

[adam2]

Whilst you could in principle earth the negative conductor at any point in the system, it would be usual to earth the battery negative lead, fairly close to the battery.
Earthing elswhere carries more risk of loosing the earth connection during repairs or modifications, but earthing the battery negative lead should be permanent.

If the case of the controller is not connected to the earthed battery negative lead it is probably best to earth it, but ommision of such is most unlikely to have any serious consequences.

If the case of the inverter is internally connected to the earth of the 230 volt output, then this should be connected to earth.

I would install an earth rod, and connect this to an earthing block.
To the earthing block connect all the following
Battery negative
Charge controller frame
Inverter frame

[mikepepler]

Cheers Adam. The current mains setup looks to be earthed to both the metal armour around the incoming mains cable and also a copper gas pipe (though I know the gas pipe under the road is plastic, as they replaced it recently). Is the current earth OK, or is it still worth adding an earth rod?

[adam2]

I would not rely only on an earth connection provided by electricity supplier.
There is the small but real risk that this could be lost, if for example your service cable was damaged or dissconnected.

Gas pipes should be connected to earth (known as bonding) but should not be used as the means of earthing an installation.


Water supply pipes likewise should be connected to earth but not used as the means of earthing.

[mikepepler]

Right, I heard back from Morningstar, and they said that if I didn't have a GFPD, I should earth the system -ve, and not the chassis.

They gave a link to a relevant GFPD though, from Outback. The manual is here: http://www.outbackpower.com/docman/1401 ... _REV_A.pdf

Page 9 of that shows the circuit diagram, basically:
- the chassis of the controller (and presumably any inverter) is wired directly to ground.
- the system -ve is wired to ground through the GFPD
- if the GFPD detects current flowing from -ve to ground, it operates two circuit breakers, one to stop the current flowing to ground and the other to break the PV positive current.
- a 43K resistor keeps system -ve connected to ground after a trip.

What do you make of that Adam? (best see the diagram to make sense of it) Midnite make a similar product: http://prismsolar.co.uk/shop/index.php? ... uct_id=283

It seems these devices are not on sale very much in the UK in general though. I wonder why they are so popular in the US and not here?

[Catweazle]

Are you in a location where lightning could be a problem ? If so, then earthing the module frames might save the inverter.

[mikepepler]

Are you in a location where lightning could be a problem ? If so, then earthing the module frames might save the inverter.

No. We do get storms here sometimes, but the panels are along the lower edge of a dormer roof, so there are many higher things to get hit first, and also houses higher up the hill from us.

So far I've earthed the inverter chassis and system -ve, but not the module frames and not the controller chassis. I read somewhere that earthing the module frame could be asking for trouble, as it creates an extra opportunity for shocks and short circuits.

[mikepepler]

Video of the controller installation and earthing is now online. http://www.youtube.com/watch?v=4VmGm5BCFeA