PV On Grid Systems Protection - Fuses

[RRuts]

Hello everyone,

I m new to the forum, currently working with solar photovoltaic energy projection and instalation.

The protection for a PV On Grid System (according to almost all material I ve read) consists of:

AC Side:
Circuit Breakers (general system protection - cables, equipments, etc)
Surge Protection Devices (protection for enviromental discharges and grid voltage jumps)
Residual Differential Circuit Breaker (to make sure no current is leaking to earth)

DC Side:
Surge Protection Devices per Array (protection for enviromental discharges and grid voltage jumps)
Disconnect Switch (For maintanance and emergencies)
gPV Fuses strictly necessary when there are more than two strings (according to IEC 60364)

My question is why are fuses not necessary for one or two strings? What exactly are the fuses protecting and why?

I would appreciate a detailed answer if possible and/or material where I can find these answers.

Thank you very much!

[sensij]

The maximum current that a pv system can produce is known from its Isc (short circuit current). With a known upper current limit, there is no risk of overloading the PV conductors, so no fuses are required. In a multi string installation (>2), a fault path can exist that makes the string currents additive, generally exceeding the conductor rating and requiring fuses.

[Amy@altE]

Good timing, I'm scripting a video on this very topic, hope to have it released in a week or two. But meanwhile, to answer the why question, I have to quote NEC 690.9(A). Exceptions: (a)No external power sources like batteries or parallel or backfeeding inverters are in the system. (b) The current doesn’t exceed the ampacity of the conductors AND the max series fuse rating of the solar panel.So what does all of that mean? If there is just one inverter, and no batteries, so there is no chance of power coming from another source, you don’t need a fuse or breaker between the solar panels and the inverter IF the wire is sized big enough to handle the current, which you need to have anyways, AND the current between the panels and the inverter isn’t higher than the max series fuse rating of the solar panel. Since the max series fuse size is generally at least 1.56 times the Isc, if you have one string of solar panels is just Isc, so it is always going to be lower than the max fuse size. If you have two strings in parallel, that is Isc x 2, so it may be higher than the max fuse rating, and so over current protection would be needed.

Overcurrent protection is to protect the wires, not the equipment. It is to make sure if more current flows on the wires than they are able to safely carry, the current is turned off.

[sensij]

Amy, with only two strings, there is no fault path available that can cause the currents to add up to more than it would be in the normal condition. You need at least three strings to get 2x Isc. I can look for some links that walk through this in more detail, they are out there.

[Amy@altE]

Yah, I said *may* be needed for 2, I have heard of instances when it is needed for 2. But most of the times, not needed for 2. Definitely for 3. I'll clarify my wording to make it more obvious that it most likely would not be needed for 2. Thanks, I like running my material by here before I commit to it, you guys know your stuff.

[RRuts]

I thank you all for the quick and great reply guys!

Amy, could you give an example of when fuses may be needed for two strings?

I ve seen string boxes of one and two strings with fuses aswell, so there must be special cases still not taken into account here.

[ButchDeal]

I thank you all for the quick and great reply guys!

Amy, could you give an example of when fuses may be needed for two strings?

I ve seen string boxes of one and two strings with fuses aswell, so there must be special cases still not taken into account here.

if you have SolarEdge optimized system, you can even get three strings with no fuses.

[sensij]

I thank you all for the quick and great reply guys!

Amy, could you give an example of when fuses may be needed for two strings?

I ve seen string boxes of one and two strings with fuses aswell, so there must be special cases still not taken into account here.

There is a special case in which either the installer or the AHJ doesn't understand the requirements properly, and "unnecessary" components are added to satisfy a particular person's idea of what is right. For example, my system has an AC disconnect that is not required by either the city or the power company, but is part of the installers standard design. Extra fuses (in a typical grid tie system) don't really hurt, and usually don't add much cost.

Also, typically the residual current detector/interrupt is built into the inverter, and is not required to be separately installed. Surge protection is entirely optional on both the AC and DC side, unless required by local code (I suppose).

[Amy@altE]

I wrote an answer saying it was rare, but I just deleted it, but am reinstated it. I've seen some panels with a Max Series Fuse Size smaller than Isc x 2. But it is probably rare enough that I can just remove the caveat and say 3 strings.

[sensij]

I wrote an answer saying it was rare, but I just deleted it, but am reinstated it. I've seen some cheap panels with a Max Series Fuse Size smaller than Isc x 2. But it is probably rare enough that I can just remove the caveat and say 3 strings.

Here is a nice write-up:

http://dept-wp.nmsu.edu/swtdi/files/2015/11/iaei_may-jun_2008.pdf

It leaves open the possibility that the series fuse rating is less than 1.56 * Isc, but describes that as a UL issue. In properly approved panels, that shouldn't happen.

[Amy@altE]

Thanks sensji, I always love John Wiles' articles.

[RRuts]

Thanks guys! Helped a lot!

[Mike90250]

.... Extra fuses (in a typical grid tie system) don't really hurt......

Just to emphasize - ONLY in a typical grid tie system. GT systems generally run 10A or less per string.

Off-Grid systems, on the other hand, run say, 50A in a system wire, and the fuse and it's contacts all add resistance and heat, I've seen several times, a fuse contact become the faintest bit corroded and it builds up enough resistance to heat up and de-temper the springiness of the fuse clips, and the system fails, and the fuse holder itself has to be replaced.