ECN Forum Forums General Topics of the Trade Alternative Energy RENEWABLE ENERGY: YOUR causes for concern?

I don't think the breaker in a grid tied system is truly "backfed" since the utility has to be present on the "line" side for the inverter to put out. (which will power the breaker circuitry).
I agree if you are truly backfeeding, like on a generator inlet, you will smoke them but that is not what is happening on a grid tie inverter.
The idea that net current is flowing one way or the other is meaningless since the current switches direction every 8MS anyway. The CT only makes sure they balance.

Vindiceptor,

Thank you for the link. Useful information.

You are kidding right?

Where am I wrong? Are you saying the grid on the line side will not provide power to the circuitry (the issue in the SqD white paper)

They never mentioned the direction of net current flow.

Every manufacturer is different and the link was only meant to explain why some GFI breaker protection can fail others can fail for similar reasons, but each is manufacturer specific.

We did 50-75kW arrays on three identical buildings in a business park and one of them had a 1600A GFI main that the manufacturer wouldn't guarantee in a backfeed. They all had the same model breaker, but one of them had a different GFI trip unit.

The AHJ in that area was anal about tapping existing gear and required third party recertification so we had to weigh the cost of that VS replacing the breaker. We wound up tapping ahead of the main and having it recertified as that was quite a bit cheaper than replacing then main in that case.

We were lucky on that building as the other two had such crowded electric rooms that we couldn't have tapped (within code distance) ahead on the mains.

We have had another case where we couldn't make the tap length 25' or less and the AHJ was OK with it.

So far we haven't encountered a large fused main with GFI protection and that may be entail an entirely different GFI scheme.

From reading the white paper, it sounds like they are saying the circuit breaker could open, the control circuit is not powered but somehow the coil remains powered and burns up.


In a grid tie situation using grid clocked inverters there is no situation where the line side voltage will be significantly lower than the load side voltage and you will not have a situation wnere the control circuitry of the XFCI is not properly powered .

This is not what we normally consider back feeding where the line side of the breaker remains hot with the breaker open.

Yes and I have been told it can happen in a fraction of a second.



I have seen variations of several volts and amps in meters on both sides of the POCC, the inverters are not always perfectly aligned



The GFCI protection only needs work as designed once and then it may be incapable of preventing utility fault from passing through it the possibility. See NEC-2008 690.64(B)(3) and NEC 2011 705.12(D)(3).

Newer breakers (made within the last 10-15 years) with integral and/or add-on GFCI devices should be listed for reverse feed applications, but few manufacturers indicate that specifically as it relates to the GFCI function, anything older and you may want to get written documentation from the manufacturer.

Lastly, we routinely combine the output from several large inverters at a distribution board with a GFCI main feeding back to the POCC, in that situation the line side is towards the inverters, should a fault occur between the inverters and said GFCI the GFCI protection may be burned up after the breaker is opened by the utility backfeeding the load side if the breaker and GFCI protection were not listed and tested for reverse feed applications.

My mentioning this to begin with was more a precautionary statement since we have dealt with it and I know of a couple others out here that have as well. Replacing a large GFI breaker can make a medium project significantly less profitable or put it in the red for small projects if you're going to eat the cost because of what you didn't know to look out for.

See page 5 of the following link:

http://www.osti.gov/bridge/servlets/purl/764900-aoqwfv/webviewable/764900.pdf

From the PDF


I suppose it all gets back to how good the anti islanding circuits are in the inverter.