Fuse rating question...

icarus

I just added 300 watts of PV to a off grid system, 3 strings of 3 12 volt panels.  Each string is breakered with a 12 amp midnight breaker, and the home run is fused just before the Midnight Kid with a 30 amp conventional screw in fuse.

the midnight kid sizing tool suggests in the best (worst in this case!) scenario is that the array COULD put out 31.2 amps.  My question is, this system is remote and is unattended all winter.  Do I worry that leaving all three strings connected risks blowing the 30 amp fuse.  (I can’t replace the fuse with out replacing the box as screw ins only come as big as 30 amps as far as I know) It can be very cold (-40 or colder) with lots of reflection off snow, so I suppose it could actually reach the 31.2 amps, but I would be surprised.

I know I could simply turn off 1/3 of the array when it is unattended, but thenI am relying on someone else who may not know, to turn the last breaker on.  Do I take the risk and leave it on or play it safe?

Tony

Brain fart...  I just realized that the Kid is showing battery current, not PV current.  Since the PV is wired in series at ~36 VDC the maximum SC current is only ~18 amps!  Duh!

GT

BB.

The fuse after the combiner box is not really required. The input current is less than the max (panel current, not fuse rating current).

The only reason for the third fuse is if the down stream wiring from the combiner to the Kid is rated for less 36 Amps (i.e., if wiring is >10 AWG, the wire can never overheat). Of course, since your 3x Isc * 1.25 of the existing panels is less than the final feed current rating, you don't need the fuse unless you are planning on adding more panels later.

Fuses and breakers are the Unreliable part of the system (they open first before anything wiring fails/catches fire. So, I would simply get rid of the fuse and just use your series breakers with the panels (especially if you want a reliable/unattended system).

Of course, there is the other fox in the hen house--Any Lightning possibilities? Midnite DC surge suppressor on the array side of the Midnite? Possibly on the AC output of the AC inverter (if the system has an AC inverter).

-Bill

littleharbor2

Screw in fuses? are you referring to the old style residential AC  fuse panel type? I forgot about those decades ago until I read this. I wouldn't think they were DC rated

icarus

I may be wrong, but I don’t think in the case of a fuse (of this general size) it makes any difference.  The difference between a fuse and a breaker is that the breaker has to open, and sustains an arc, causing the breaker contacts to fuse (potentially) so that it won’t open.  As a fuse simply melts and opens with over current.  

Feel free to correct me if I am wrong...

Tony

PS a bit of google search reveals that the DC rating of MOST fuses is ~1/2 that of the AC rating.BB. said:

The fuse after the combiner box is not really required. The input current is less than the max (panel current, not fuse rating current).

The only reason for the third fuse is if the down stream wiring from the combiner to the Kid is rated for less 36 Amps (i.e., if wiring is >10 AWG, the wire can never overheat). Of course, since your 3x Isc * 1.25 of the existing panels is less than the final feed current rating, you don't need the fuse unless you are planning on adding more panels later.

Fuses and breakers are the Unreliable part of the system (they open first before anything wiring fails/catches fire. So, I would simply get rid of the fuse and just use your series breakers with the panels (especially if you want a reliable/unattended system).

Of course, there is the other fox in the hen house--Any Lightning possibilities? Midnite DC surge suppressor on the array side of the Midnite? Possibly on the AC output of the AC inverter (if the system has an AC inverter).

-Bill

BB. said:

The fuse after the combiner box is not really required. The input current is less than the max (panel current, not fuse rating current).

The only reason for the third fuse is if the down stream wiring from the combiner to the Kid is rated for less 36 Amps (i.e., if wiring is >10 AWG, the wire can never overheat). Of course, since your 3x Isc * 1.25 of the existing panels is less than the final feed current rating, you don't need the fuse unless you are planning on adding more panels later.

Fuses and breakers are the Unreliable part of the system (they open first before anything wiring fails/catches fire. So, I would simply get rid of the fuse and just use your series breakers with the panels (especially if you want a reliable/unattended system).

Of course, there is the other fox in the hen house--Any Lightning possibilities? Midnite DC surge suppressor on the array side of the Midnite? Possibly on the AC output of the AC inverter (if the system has an AC inverter).

-Bill

Bill,  My reason for the fuses is it gives me an easy disconnect 100’ from the combiner box, and it protects the wire in the event of some catastrophic CC failure that might release “reverse”(battery) current into the wire.  My theory has always been to protect wiring at each end if there is a potential current source at both ends.  Overkill?  Probably.

Lightning is an ever present possibility.  I don’t have any surge protection, and in 20 years haven’t needed any, but tomorrow is another day.

Tony

mvas

Per Little Fuse ...

• The DC Voltage rating of a fuse is not the same as the AC Voltage rating
• Typically, the DC Voltage rating is about 50% of an AC Only Voltage Rating
• The DC Voltage rating, is a safety rating that should never be exceeded
• If used in a DC circuit, the fuse must be DC Rated 

Extinguishing a DC arc is much more difficult than an AC arc ... an AC arc is self extinguishing 120 times per second.

mike95490

If you are simply maintaining batteries over winter, I don't see a problem.   A full battery bank in the morning, will start charging slowly as the sun angle changes, by the time the sun is high enough that the panels could produce 31A, the batteries are fairly full and cannot accept 31A

 The failure mode is 5.4 cloudy days, and the sky clears at solar noon.   Boom, full power into the batteries

littleharbor2

mvas said:

Per Little Fuse ... 
If used in a DC circuit, the fuse must be DC Rated.  Extinguishing a DC arc is much more difficult than an AC arc ... an AC arc is self extinguishing 120 times per second.

Exactly why I question the use of these fuses for this purpose. It would be interesting to see if the DC arc would be able to jump the distance in one of these blown fuses. Being that they have a "window" you would likely be able to see the arc.

mcgivor

@icarus said
Bill,  My reason for the fuses is it gives me an easy disconnect 100’ from the combiner box, and it protects the wire in the event of some catastrophic CC failure that might release “reverse”(battery) current into the wire.  My theory has always been to protect wiring at each end if there is a potential current source at both ends.  Overkill?  Probably.

Lightning is an ever present possibility.  I don’t have any surge protection, and in 20 years haven’t needed any, but tomorrow is another day.

Tony

Fuses are not a reliable means of disconnect especially under load, wether DC or AC rated. Consider a MIG welder which operates at ~18V DC, the arc can span a distance of 5mm, melting steel at a modest 30A. As the fuse is removed, unscrewed in this case, the action is slow which is the perfect conditions to develop an arc, DC circuit breakers have a quick action along with arc quenching to reduce or eliminate the possibility of arc induction.

DC circuit breakers are not expensive, my recommendation is to replace the fuse with a DC circuit breaker, for safeties sake. Also consider the polarity of the circuit breaker, the positive terminal, should be connected to the side with the highest potential energy, in order for it to provide optimal protection, not all have polarization however.

icarus

All good points, thank you all.

My described fuses are in an old fashioned service box with a handle disconnect.  The fuses are just in the fuse holders, so I can disconnect the PV current with the service handle.  I think I will just lest sleeping dogs lie...until I have a failure.  

Tony