Solar Panel Ground Fault

[timm333]

Just trying to figure out the ground fault protection of solar panels as required by NEC-2014 sections 690.5 (A)(2) and 609.5(B)(2). It looks that if a ground fault occurs, both phase and ground conductors need to be opened in order to interrupt the fault current. I thought ground conductor should never be opened? Thanks for help!

 

[jaggedben]

First, you should be calling it a 'grounded conductor', to be clear your not talking about an equipment grounding conductor or an grounding electrode conductor.

Second, if you're using a listed inverter or charge controller or other assembly, it probably already has a proper ground-fault protection device and you just follow the instructions. There's nothing to figure out unless you mean that you want to understand how it works, which of course we're happy to help with.

To summarize PV ground-fault protection as concisely as I can...

5 years ago almost all inverters used a ground-fault fuse to connect the grounded conductor to the equipment ground. Since the fuse detected a ground-fault by tripping and opening, it ungrounded the grounded conductor. This is permitted in the sections you cited because it was a common method and the best anyone had come up with. In these systems a disconnect is not supposed to open the grounded conductor but the ground-fault device disconnects it from ground.

Nowadays most inverters do not ground either DC conductor, and use fancy electronic ground-fault detection. The ground-fault protection disconnects both conductors from everything else when it's electronic circuitry says so.

To reiterate, both methods are part of the inverter and don't require a separate device. Some systems, e.g. off-grid inverters, might have a distinct ground-fault device that you install yourself, but the code still requires it to be a listed device.

 

[timm333]

Thanks. So does it mean that now a days an electronic ground fault relay is used inside the equipment, which senses the difference of current between positive and negative wires; and in this way we are no more required to ground the negative terminal of the batteries (which we did in the past) ?

 

[jaggedben]

Thanks. So does it mean that now a days an electronic ground fault relay is used inside the equipment, which senses the difference of current between positive and negative wires;

If you've only got a grid tied inverter, then yes, that is most typical.

and in this way we are no more required to ground the negative terminal of the batteries (which we did in the past) ?

With lead-acid batteries you are likely still grounding a terminal (usually negative although the NEC doesn't care). Requirements in 250.162 still apply. However if you look at 690.71(G) there are some conditions where you can have ungrounded battery conductors.

Consult with your inverter or charge controller manufacturer to see if there is already a ground fault detection device in their equipment or if you need to provide one, and how they see themselves complying with requirements. I'm pretty sure in some systems there is a ground fault circuit breaker tied to the main battery conductors, and if there's a ground fault current the breaker trips and ungrounds both the PV and the batteries to interrupt the fault. It's been years since I worked on a battery system.

I'm using 2011 NEC references cause that's the book I have at work.