Why no disconnect on batttery negative?

Started by chowderhead, May 09, 2020, 01:08:45 PM

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chowderhead

Rather than necro bump http://midniteftp.com/forum/index.php?topic=4725.0:

Perhaps I'm misunderstanding: Apparently NEC requires ground-fault protection which only works when PV negative is not bonded to ground, necessitating over-current protection and separate disconnects ahead of the charge controller for both legs of a PV string. However, the MNDC disconnects only provide a single breaker for battery positive. Why not another for battery negative?

boB


It just costs more is all because you need two breakers that are ganged together and at the correct voltage rating.

I think the new ground-fault requirements are for disconnecting both PV+ and PV- though.

So, what you are talking about will be standard if not already used in some systems already.

I know that we are working on this or may already have something ?

My brain isn't really headed in that direction so that's not on the top of my thoughts lately.


K7IQ 🌛  He/She/Me

jnh

#2
I'm thankful to have had my system signed off on before DC GFP requirements went into effect (for ground-mounted arrays... they may have already been mandated for roof-mount in 2008).  It seems like a lot of trouble and potential drawbacks for very little real safety benefit, especially with the lower PV string voltages typically used in off-grid & hybrid systems.  How does opening the circuit *downstream* from a detected fault (unlike with AC GFCI's), and deliberately unbonding PV- from GND help in clearing a potential hazard?

Still, given this dumb rule is here to stay, has there been thought given to integrating GFP capability within the Midnite charge controllers, as was done for arc-fault protection?  I guess this would preclude having DC- and PV- internally wired together (are Classics are the same as Outback CC's in this regard?), requiring 4 wires plus ground into each controller rather than three, but it would at least save some breaker positions.

Edit: or is DC GFP already a feature of some/all Classics?

I have PV- bonded to ground rods at both ends of  a long buried cable run, in hopes this would help reduce damage in the event of a lightning strike (with MNSPD's between PV+ and GND)... assuming this would no longer be possible with GFP's in the system.