SolarEdge string fusing

[Carultch]

OK then...until 2017 addresses it specifically(if it does)...this is what I have.

The ungrounded conductors of the output of a DC-to-DC converter are not specifically addressed in article 690 with regard to overcurrent protection.

So we must fall back to article 240 for guidance for overcurrent protection of these ungrounded conductors. Section 240.15(A) requires a fuse or circuit breaker in each ungrounded conductor and 240.21 requires it to be at where the conductor receives its supply.

So the DC-to-DC converter output needs overcurrent protection as required in article 240 where ungrounded conductors receive their supply.

What would "receives their supply" mean in this context? Serious question, because it could be interpreted as from the inverters or from the array.

 

[SolarPro]

Except that in this case, you want the OCP at the source of the fault currents. The PV array is still current limited, so putting OCP at the array is useless. So the OCP wants to be at the point of parallel connection where it can protect the dc-to-dc converter source circuit from the (nonexistent) backfeed currents.

 

[ggunn]

The only relevant requirement in 240.21 is 'at the point where conductors receive their supply' which is pretty much reiterated by 690.9's 'at the source'. (And nevermind that that we all ignore whatever this means and put the OCPDs at the point of parallel connection for very, very good engineering reasons that should be codified.)

Actually, we don't ignore what it means. In the case of a fault, which the OCPD is there for, "the source" is the service.

 

[jaggedben]

What would "receives their supply" mean in this context? Serious question, because it could be interpreted as from the inverters or from the array.

Actually, we don't ignore what it means. In the case of a fault, which the OCPD is there for, "the source" is the service.

Or from the parallel connected circuits.

Of course with optimizers we may have parallel connected circuits that, because of the optimizers firmware, will immediately shut down in case of a short circuit. I guess it just needs to somehow be proven.

 

[pv_n00b]

Basically these are treated just like standard strings for combining. The PDF the OP posted was for the market outside the USA and the links
shortcircuit2 provided are for the USA with NEC requirements.

The NEC has been very deficient in addressing the operation of MLE. It has also not accepted electronic components to replace ODCP. That's one reason we don't use diodes in strings instead of fuses when combining and we don't use solid state switches for GFDIs. I don't see the requirement for OCPD when combining strings going away anytime soon.

 

[jaggedben]

Basically these are treated just like standard strings for combining. The PDF the OP posted was for the market outside the USA and the links
shortcircuit2 provided are for the USA with NEC requirements.

That seems like a reasonable explanation, although the PDF in the first post doesn't actually say that.

...we don't use solid state switches for GFDIs....

Actually I believe we do now. That part has the advantage of being part of the listed equipment and thus not bound by any specific NEC language.

I don't see the requirement for OCPD when combining strings going away anytime soon.

Yeah, I'm pretty sure I can tell you it won't be in the 2017 code. Maybe someone should prepare a proposal for the next cycle.

 

[ggunn]

Yeah, I'm pretty sure I can tell you [the requirement for OCPD when combining strings going away] won't be in the 2017 code. Maybe someone should prepare a proposal for the next cycle.

Maybe so, but SolarEdge makes a good argument that in their case it is superfluous. 690.15(C) makes us put a DC disco on the combiner, too.

 

[shortcircuit2]

Maybe so, but SolarEdge makes a good argument that in their case it is superfluous.

SolarEdge's argument is based on claims in a company distributed application notes in which they use language like "have been verified" or "have been certified". If testing was done by a NRTL that supported these claims, statements in the installation instructions would guide us that overcurrent protection is not required. The instructions that come with the product now say..."If more than two strings are connected each should be properly fused on both DC+ and DC- according to NEC 690.35(B)"

They also have 2 conflicting applications notes! Which one are we to be guided by? The original poster was given the application note that pv_n00b states was for outside the USA market. So now how many installations were done with guidance from the wrong application note?

690.15(C) makes us put a DC disco on the combiner, too.

And 690.9(D) requires overcurrent protection to be "Listed PV overcurrent devices"

 

[Carultch]

Maybe so, but SolarEdge makes a good argument that in their case it is superfluous. 690.15(C) makes us put a DC disco on the combiner, too.

If you put the inverter immediately adjacent (max 6 ft) to the combiner, then the factory disconnect on the inverter will suffice. And thus a "bare bones" combiner would do the trick.

 

[pv_n00b]

I can't say I have heard of any GFDI system on an inverter with a grounded PV array that uses solid state switch in place of a fuse or CB. I'd like to know of one though if it exists. The newer non-isolated inverters with ungrounded PV arrays do use something different but it's not a GFDI system since there is no grounded conductor.

Actually I believe we do now. That part has the advantage of being part of the listed equipment and thus not bound by any specific NEC language.

 

[shortcircuit2]

As far as I have been able to ascertain, it hasn't, and that's what my local AHJ wants to see to sign off on the exemption from fusing on three strings into SolarEdge. A white paper issued by Solar Edge is not enough. If anyone knows of any NRTL documentation supporting SolarEdge in their assertion, please post a link.

Curious on how you made out on fusing with 3-strings at this job?

 

[ggunn]

Curious on how you made out on fusing with 3-strings at this job?

Fused them.

 

[pv_n00b]

SolarEdge should just push to have this as part of the 1741 listing and change the NEC accordingly. It's a good idea if they can sell it to CMP 4

 

[shortcircuit2]

SolarEdge should just push to have this as part of the 1741 listing and change the NEC accordingly. It's a good idea if they can sell it to CMP 4

Do you mean that Solaredge should have its inverter listed as not requiring fusing of its strings? That their Isolation Moniter Interupter (IMI) device provides the overcurrent protection?

 

[ggunn]

Do you mean that Solaredge should have its inverter listed as not requiring fusing of its strings? That their Isolation Moniter Interupter (IMI) device provides the overcurrent protection?

The fusing of multiple strings has nothing to do with the inverter. The idea is that in the event of a fault in a string the other strings can feed the fault. If there is more than one other string then the available fault current will exceed the current tolerance of the faulted string.

SolarEdge says that in their circuit topology that cannot happen. "My" AHJ says that they do not accept that purely for the reason that it is the equipment provider who is saying it, not an NRTL, so we fuse when there are three or more strings. It doesn't happen that often, though, so it's not a big deal.

 

[Carultch]

The fusing of multiple strings has nothing to do with the inverter. The idea is that in the event of a fault in a string the other strings can feed the fault. If there is more than one other string then the available fault current will exceed the current tolerance of the faulted string.

SolarEdge says that in their circuit topology that cannot happen. "My" AHJ says that they do not accept that purely for the reason that it is the equipment provider who is saying it, not an NRTL, so we fuse when there are three or more strings. It doesn't happen that often, though, so it's not a big deal.

SolarEdge says that in their circuit topology, it cannot happen, but they have conceded that to build a code compliant system, the fuses are still needed.

In the inverters where they anticipated more than two strings, they've modified their design to build string fuses into the wiring compartment. Examples are the 10kW & 11.4 kW single phase, the 14.4 kW 208V three phase, and the 33.3 kW 480V three phase inverters. You still need to be aware that this is a new feature, so old inventory of those particular single phase inverters at your distributor may cause the need for a 3rd party basic string combiner. The 14.4kW & 33kW inverters developed with the fuses as a standard feature, since the inverters first came on the market.

 

[jaggedben]

SolarEdge says that in their circuit topology that cannot happen. "My" AHJ says that they do not accept that purely for the reason that it is the equipment provider who is saying it, not an NRTL, so we fuse when there are three or more strings. It doesn't happen that often, though, so it's not a big deal.

One possible way around the code argument is that it isn't a string. i.e. it's an inverter input circuit, not a photovoltaic source or output circuit. Inverter input circuits are not mentioned in 690.9 so that section does not apply and there's no overcurrent protection requirements.

The code definitely has a giant blank spot that needs to be filled in when it comes to rules for optimizer systems. At least the DC-to-DC converter definition was added last cycle. Maybe I'm forgetting I don't recall seeing much to address this in the 2017 draft.

 

[shortcircuit2]

One possible way around the code argument is that it isn't a string. i.e. it's an inverter input circuit, not a photovoltaic source or output circuit. Inverter input circuits are not mentioned in 690.9 so that section does not apply and there's no overcurrent protection requirements.

We discussed this earlier in the thread and if nothing in 690 applies...then the first paragraph of 240.21 would apply.

 

[shortcircuit2]

What is the maximum overcurrent protective device rating for a SolarEdge dc-to-dc converter?

Here are some specs...but I don't see any maximum overcurrent protective device listed...

http://www.solaredge.com/sites/default/files/se-p-series-add-on-power-optimizer-datasheet-na.pdf

 

[jaggedben]

What is the maximum overcurrent protective device rating for a SolarEdge dc-to-dc converter?

Here are some specs...but I don't see any maximum overcurrent protective device listed...

http://www.solaredge.com/sites/default/files/se-p-series-add-on-power-optimizer-datasheet-na.pdf

I'd say there isn't one.