Why can 5 amp GFDI (utility inverters) be configured with PV panels with 1.5 isc stri

Status
Not open for further replies.
C

CajunTech

Member
Location
Watson, LA, USA
I have commissioned several sites on east coast that a single panel generated an isc value (usually 8 amps) which would allow a single panel to cause failure of the gfdi fuse (per utility scale allowing 5 amp fuses). Now having commissioned sites on the west coast they use panels of 1.5 amps isc with the same 5 amp gfdi fuse. This would mean there could be many square yards of charged area of possible metal racking and wire combination creating a uninterrupted current sourced circuit till other circuits also fail then adding to the fault current till enough combined current can open the fuse. I have looked for code compliance's that would mandate a coordination of the gfdi fuse be rated for the isc of a single string but can not find such code.
 
You must be using thin film panels if they have currents that low. Not really an east-vs.-west coast thing, just the projects you happen to be working on. Anyway...

It's more of a UL standard issue than a code issue. The code just requires the GFDI to be listed. The thing is, maybe those 5amp GFDI fuses shouldn't be approved by UL anymore. See this article. (And note how old it is.) This is a big reason why we've moved towards transformerless inverters with electronic GFDI, although I gather the move has been more complete in residential scale inverters than with the big ones.

It may be advisable to switch out that fuse for a 1amp fuse regardless, especially with your low-current panels, but ask the manufacturer. Yes, nuisance tripping is annoying and costly, but not as annoying and costly as burning a building down. (Hopefully your system is a ground mount?)
 
Here are some more recent articles on the topic:

The Heat is On: Fault Detection and Fire Prevention

Resolving Fire Hazards from the Ground-Fault Detection Blind Spot

The fault energy required to open a 1A GFDI fuse is 3,000 times greater than that required to trip the detector in a non-isolated string inverter. That delta is probably even larger when you are talking about central inverters with a 5A fuse-based GFDI.

Unless you are adding additional hardware, such as Bender current sense monitors, an inverter with a fuse-based GFP system will not meet the requirements of 690.5(A) in NEC 2014. This hardware also exposes the developer to unnecessary liability in the event of a "thermal event" caused by the so-called ground-fault detection blind spot.
 
Yes sites are multi megawatt sites ranging in size of 5 mw to 80 mw. using either fixed angle or tilt tracking racking systems.

Yes I am aware of the standard allowing the 5 amp fuse but is there a specific codes requiring a coordination of fuse rating to isc of string?
 
SolarPro said:
No.
Click to expand...
The panels themselves will state a maximum series fuse size, for protection against reverse direction fault current.
That value is always more than Isc and is typically 2-3 times that number.
But I do not know of any similar standard for GF fuses.

mobile
 
Again, as long as your inverter is designed with advanced detection systems, the 5 A threshold is not the one that matters. From the article mentioned previously:

The ground path provided by the damaged connector produced only 120 mA of ground-fault current, substantially less than the trip threshold of the 5-amp GFDI fuse in the inverter deployed at this site. In a worst-case scenario, this latent ground-fault condition would have persisted until a second ground fault occurred, shorting the array around the GFDI fuse and causing a fire similar to the incident in Bakersfield, California. Fortunately, the 120 mA fault current was above the 100 mA detection threshold the EPC firm had programmed into the CSM equipment. This enabled the team to remotely identify and quickly remedy the ground-fault condition in the roof-mounted PV array.
Click to expand...

Any inverter designed to comply with NEC 2014 requirements, should provide this functionality. If you see Bender current sense monitors or similar in the enclosure, they may well be part of the advanced ground fault protection system.
 
There was never a requirement that the GFDI fuse be tripped by a single module or string so there is no coordination between GFDI fuse rating and module Isc.
 
These systems do not have a bender, simply a 5 amp gfdi fuse. I understand a GFDI fuse has a different purpose than an OCPD (over current protection device). GFDI fuses are intended to fault when some portion of the string between the most negative part and the most positive part physically touches ground ( ground can be racking, or the frame of panel if that type that connected to the rack should then connect with a dedicated ground conductor of that string back to the combiner box and through the combiner box to the inverter and bond to one side of the GFDI fuse, that then basically bonds with the common negative buss ( these are negatively grounded ). During non operational points of inverter (early morning before set point voltage as allocated within inverter configuration or that early evening when voltage nolonger supports operation, its during that time current leaves the most negative point of string in fault travels through to sting harness back to the combiner box then on to the common negative buss. The current is limited by the circuit resistance between this most negative point back to the other side of the fuse. Again as I understand solar panels and solar systems, panels being a current sourced power source can never generate any more current than the ISC rating, so even if there was no inherent resistance ( lets call it a dead short ) between the most negative point and other side of fuse, the ISC if only 1.5 amps rated for panels of the strings can never reach the 5 amp threshold of the 5 amp rated fuse when only one string fails, it will take multiple strings to fail to allow the paralleled current adding together to force GFDI fuse failure. The OCPD is to protect a different type of circuit failure. Basically the dedicated ground is used to act as a temporary positive conductor to what ever the failed point within string. This then puts now temporary positive connecting through the GFDI fuse to the most negative point of said failed string.
 
Just my opinion, but I think the fuse method of GFDI is simply inadequate. Someone once showed me a calculation on what's really required to blow a 1a fuse and it strongly suggested that many ground faults in such systems won't be caught. Never mind a 5A fuse. I never understood why the fuse was allowed to not trip on a single-string ground fault. As the links above show, a single string fault that doesn't get caught sets up a very dangerous situation. To repeat, yes, nuisance trips may be annoying, but not as annoying as burning down a building.

CajunTech said:
... Again as I understand solar panels and solar systems, panels being a current sourced power source can never generate any more current than the ISC rating, ....
Click to expand...

Solar panels can produce more than ISC if conditions vary from standard test conditions. For example high irradiance at low temperatures. Most likely real world occurence is when there's reflectance from snow in front of an array. But this doesn't really dispute your point, because wiring and fuses are supposed to be oversized to account for this sort of thing.
 
Like most things the GFDI fuse ratings were a compromise between something that would trip on the smallest ground fault current and something that would not have false trips. Keep in mind that there is always ground fault current in a grounded system because there is capacitive coupling between the array and ground, even in DC systems. In large thin film arrays this current can be several amps or more, hence the larger GFDI fuse in larger inverters. Even relatively small arrays mounted close on a metal roof can cause capacitive coupling problems for the new residual current monitoring systems.
 
jaggedben said:
Correct me if I'm wrong but isn't the capacitive current much higher on a system with a grounded conductor?
Click to expand...
How can there be "capacitive current" on DC conductors? If there is current to ground, it is leakage, and that is resistive, not capacitive.
 
Excuse me... Imprecise shorthand for "current due to capactive coupling".

And I realize that pv_noob did refer to it as something that happens in grounded systems specifically.
 
jaggedben said:
Excuse me... Imprecise shorthand for "current due to capactive coupling".

And I realize that pv_noob did refer to it as something that happens in grounded systems specifically.
Click to expand...
I understand, but I do not believe there can be "current due to capactive coupling" in DC circuits. Capacitance is a reactive element of impedance and if there is no dV/dt there is no current flow.
 
Anytime you have two conductors at different potentials separated by an insulator you have a capacitor. The energy is stored in the electric field between the conductors. Capacitors leak so you get some current flow in AC and DC systems. In DC systems they just leak a bit of current because some electrons are going to make the jump, in AC systems the capacitor becomes an impedance due to the constant change in the electric field so you have reactive power flow plus leakage current. And that’s about as deep as I what to get into it.

End result, PV arrays make poor capacitors but there is always going to be some leakage as long as there is a difference in potential. The greater the surface area of the capacitor, the smaller the gap, the greater the potential difference the greater the leakage. Hence an array over a metal roof mounted down close will have a higher leakage than an array mounted on an asphalt and wood roof.

I think the reason thinfilm modules have high leakage is because the modules themselves tend to have a high Voc which gives a higher potential difference in each module.
 
Last edited:
Safety I believe is the biggest concern.

Safety I believe is the biggest concern.

Many projects use solar (pv) panels that have an ISC of 7 amps or 8 amps, this much current could allow a fault within a single panel to blow a 5 amp rated ground fault fuse instantly, in my opinion a preferred way to design a solar power plant at least in regards to safety. However these solar plants that use these panels only rated at 1.5 amps ISC would then mean multiple faults must combine before enough paralleled current could blow the 5 amp fuse. This means the hazard area now goes from square feet (3.5 x 5.5 = 19.25 sq-ft approximately) that of a single panel to that of multiple panels with a combined surface area of square yards. The single panel with a higher ISC than the rating of the gfdi fuse activates the gfdi fuse instantly while the systems built with lower ISC than the rating of the gfdi fuse could fester for years at a threshold just below the required 5 amps. Fire departments should be made aware of this elevated hazard for any solar power plant designed with this configuration of the gfdi fuse rated higher than the ISC of the panels. I designed my 20 amp solar powered dedicated emergency circuit (to power fridge and fans when we lose power from hurricanes) with a 1 amp gfdi fuse and here I did use 1.5 amp ISC solar panels, notice gfdi fuse rating is less than the panel ISC rating.
 
Last edited:
Status
Not open for further replies.
Top