Author Topic: Why no disconnect on batttery negative?  (Read 142 times)

chowderhead

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Why no disconnect on batttery negative?
« on: May 09, 2020, 01:08:45 PM »
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

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Re: Why no disconnect on batttery negative?
« Reply #1 on: May 09, 2020, 06:02:47 PM »

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

jnh

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Re: Why no disconnect on batttery negative?
« Reply #2 on: May 12, 2020, 02:14:48 PM »
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.
« Last Edit: May 12, 2020, 02:23:18 PM by jnh »