[RE-wrenches] GFP and multiple charge controllers

Wed Apr 18 14:21:02 PDT 2012

William,

Square D used to make a QO breaker body without the overcurrent trip 
mechanism. Internally it just had the switch contacts and it was 
intended for use as a disconnect in locations where overcurrent 
protection was already provided. The reason I mention this is that the 
80-amp PV GFP should not be thought of as overcurrent protection. In 
fact it would be most appropriate if the 80-amp side of the GFP didn't 
have an overcurrent trip mechanism. It /should/ only be sensitive to 
ground faults. If it is sensitive to overcurrent, then it may be 
activated when there isn't a ground fault and unnecessarily disconnect 
the negative conductor from ground.

Code prohibits the use of a GFP as a disconnect for the PV array. I 
believe that's because they intend the power system to remain bonded to 
ground except in the event of ground fault. It follows that the GFP 
should not be used as a disconnect between the batteries and charge 
controller either. That's my interpretation. The Code doesn't mention 
the possibility of putting the GFP between the batteries and charge 
controller as Outback's instructions for the GFDI require. So, even 
though Outback's instructions for the GFDI don't show it, put an 
appropriate breaker for overcurrent protection between the battery and 
the GFDI, then use wire that's appropriate for the charge controller and 
overcurrent protection.

In my opinion putting the GFDI between the batteries and charge 
controller is a really bad idea. Nearly all charge controllers warn that 
the battery should be turned on first and turned off last. It puts much 
more stress on the charge controller when the battery circuit is opened 
under load than it does when the PV circuit is opened. In fact opening 
the battery circuit may well cause the charge controller to fail. 
Outback warns that if their charge controllers are connected to a 
positive ground system, the communication port of the charge controller 
or the Hub may be damaged. If there is a fault in the PV array that 
causes the GFDI to open, it very well may leave the system in a positive 
ground condition.

When upgrading an older PSDC Outback system which instructions are more 
appropriate to follow? The ones for the PSDC that show the GFP before 
the charge controller. Or the newer instructions for the GFDI that show 
it after the charge controller. Except the the label and instructions 
the GFP and the GFDI are identical. In case you don't remember, the PSDC 
came with a schematic showing all possible options attached to the back 
of the front panel. When it comes time to troubleshoot sometime in the 
future, that schematic is still going to be there.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
t: 541-568-4882

On 4/17/2012 6:42 PM, William Miller wrote:
> Friends:
>
> Here is a related problem:  If we use an 80 amp GFD (ground fault 
> detection) breaker, even for a very small array, we have to use 3AWG 
> CU wire from the battery buss to the breaker and from the breaker to 
> the FM or MX60.  This seems like a big hassle for 5 or 10 amps of 
> charging.  A variety of sizes of GFD breakers would be helpful.
>
> Here is a bigger problem:  GFD breakers don't really work.  All they 
> do is turn off the charge function in hopes that someone will find the 
> problem.  In the meantime, the negative-to-ground bond is removed and 
> module mounting framework could be energized.  PV GFD breakers are 
> completely different from standard AC GFDI (ground fault detection and 
> interruption) receptacles and circuit breakers.  The AC versions 
> interrupt power upstream, the PV versions interrupt power /downstream/ 
> from the fault.
>
> See: 
> http://millersolar.com/MillerSolar/case_studies/ground_fauilt/_ground_fault.html 
> for some musings on the subject.
>
> As they say on TV: "There had to be a better way!"
>
> William Miller
>
>
>
>
>
> At 05:54 PM 4/17/2012, Kent Osterberg wrote:
>> Nathan,
>>
>> I don't think anyone makes a 100-amp PV GFP. The panel mount breaker 
>> model that's being used for the PV GFP doesn't include a 100-amp breaker.
>>
>> The Classic has a built-in GFP so you shouldn't need an external GFP 
>> for it. You will need to add a GFP for the FM60 - a single 80-amp GFP 
>> would work.
>>
>> You can use the dual 80-amp GFP and shut down both PV arrays at the 
>> same time if the GFP is wired to disconnect the PV array from the 
>> charge controller input. That's where Outback's instructions used to 
>> show the GFP, and it is a better place for it than on the charge 
>> controller output anyway. Midnite's instructions show the GFP on the 
>> charge controller input, but their 80-amp GFP is a single.
>>
>> Kent Osterberg
>> Blue Mountain Solar, Inc.
>> www.bluemountainsolar.com  <http://www.bluemountainsolar.com>
>>
>> On 4/17/2012 11:08 AM, Nathan Stumpff wrote:
>>> Wrenches,
>>>
>>> In a job coming up I have a customer with an existing FM60 
>>> controller,  520 watt (!!) array. We are doing a ton of work, 
>>> including adding a 4 kW solar array with Classic 150 charge 
>>> controller. I am trying to figure out how to handle the GFP.
>>>
>>> For the Classic, I need a 90 or 100 A output breaker minimum. 80 A 
>>> is not an option. The OutBack dual GFP is 80 A, and so it not an option.
>>>
>>> Is there a 100 A GFP breaker assembly I have just never seen? Is 
>>> there a way to use the Classic's GFP to run an external relay via 
>>> AUX (to trip the FM60 output breaker at the same time)? Any ideas?
>>>
>>> Thanks,
>>> -Nathan
>>>
>>> --
>>> Nathan J. Stumpff - Arctic Sun, LLC
>>> NABCEP Ceritified PV Installer #091209-175
>>> nathan at arcticsun-llc.com <mailto:nathan at arcticsun-llc.com>
>>> Office: 907/457-1297
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>
> Miller Solar
> Voice :805-438-5600
> email: william at millersolar.com
> http://millersolar.com
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