Can you explain how your external Ground Fault breakers work?

[Halfcrazy]

The idea behind MidNite Solar's present DC-GFP's are to stop the current flow in the event of a
short from the PV array. Our device is a custom made high current breaker or switch internally
ganged with a 1/2 amp breaker section. The high current section is connected in series with the
PV+ circuit. The 1/2 amp section is connected between battery minus and earth ground. The device
monitors current flowing from battery minus to earth ground. There should be no current flowing
under normal conditions. In the event that a fault occurs from either PV+ or PV- and earth ground,
the 1/2 amp section will trip thus opening the PV+ circuit and stopping current flow. This type of
fault is what the NEC is trying to prevent as it could potentially cause a fire.
We place a high value resistor across the 1/2 amp section to help keep potentially high stray
voltage down on the battery negative circuit while the device is in the tripped condition. This
resistor in not a requirement of the NEC, but is installed as a safety precaution.
The NEC specifically disallows the ground fault interrupter from being the PV+ disconnect as it
effectively un-grounds the system when opened. The NEC does specifically allow the system to be
ungrounded in the case of a ground fault however. In order to satisfy these requirements and still
provide the NEC required PV disconnect and over-current device, there must be another breaker in
series with the PV+ circuit. This additional breaker is to be used as a disconnect and is not part of
the DC-GFP.
Neither the PV+ or the high current section of the DC GFP are actually used as an over-current
device as a double fault would be required in order to have excessive current flowing in the PV
circuit. A Solar array is a current source which means it can only produce a limited amount of
current. The NEC requires the wiring and over-current (disconnect) device to be sized 1.56 times
the short circuit capability of the PV array. This is high enough that it cannot trip even under a
fault. To make the PV+ disconnect breaker trip would require not only a short in the PV array, but
also a direct short in the solar charge controller from PV+ in to battery + out. This condition would
effectively put a direct short across the battery at the PV array. All solar charge controllers must
have an over-current /disconnect between the battery + and the output of the controller. This is
the device that trips when a PV controller shorts. Most controllers do actually short from battery
plus to battery minus in the event of a catastrophic failure, so the battery plus breaker is essential.
If this breaker is not installed in your charge controller output and the controller fails, a fire WILL
result!! All DC breakers are polarity sensitive. When dealing with the high fault currents associated
with batteries, it is very important to get it right. The "+" sign on our din rail mount breakers and
the "Line" side of our panel mount breakers are intended to be at the highest potential during a
fault condition. The battery plus connection will always be the highest voltage potential during a
fault.