[RE-wrenches] GFP and multiple charge controllers

Wed Apr 18 20:28:26 PDT 2012

Hi Kent, 

I wanted to add a little more re positive grounding. OutBack's communication structure, like many other manufacturers, is referenced to Negative. If you install multiple devices in a positive ground system and connect them together via a Hub and add disconnects/overcurrent protection like normal in the ungrounded (this time negative) conductor, any time the breaker opens current will try to return to the battery by any means possible, and that means through the comm cables. That pretty much takes out all the connected devices, it aint pretty.  Therefore, OutBack recommends using double pole breakers, in both positive and negative conductors, when using multiple connected current-generation devices in positive grounded systems.  Not a perfect solution, but it is what it is. 

To answer your question about upgrading older systems, if I were working on my mother's house and I had the option, I would not install any of the current generation GFDI devices. That might be heresy and not technically Code compliant, but on a battery based system I believe it to be the safer option.  If I had to install one, it could go on either the battery or the array side of the controller.  Once there is a better solution I would go back and retrofit Mom's system, because it's worth it. 

BTW, OutBack's double and quad GFDI can be used to simultaneously disconnect both positive and negative, per your desire. That's another advantage of a multi-pole device. 

Phil

On Apr 18, 2012, at 2:21 PM, Kent Osterberg <kent at coveoregon.com> wrote:

> 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
>>> 
>>> 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
>>>> Office: 907/457-1297
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