DC string fusing puzzle

Zee

Senior Member
Location
CA
What size fuse is prudent in this situation?
Or, are fuses even required?

2 systems on a single house roof feeding 2 meters.
2 strings to one inverter feed one meter and 1 string to another inv. and meter.
All three strings go to a single combiner box on roof. The first 2 strings are combined, while of course the third remains separate. They proceed to a single conduit (even though separate systems) down from there to two dc discos, one for each system. The first two strings are combined into a #10. The remaining uncombined circuit is #12.

Source circuit: 5.62 A Isc
(@156% = 8.78A)
Max series fuse rating from spec sheet : 10 A. That sucks.
1@10 A fuses on each of three strings. (grounded system, so single fuse on positive conductors only)

Now, were it three strings combined to one combined output circuit, clearly fuses are required. But here?
I would love to just put in some higher Amp fuses. Not really for over current protection, but to complete circuit and be done.
This is assuming fuses were not needed. Otherwise I do not know how I may exceed panel rating of 10A........
Say 15 or 20 or even some 12's i have left over.....kinda like a solid metal slug.

In general, I have been finding that after several years in operation, and using the smallest allowable fuse size in a rooftop comb. box, it results in nuisance burn outs and troublesome replacements of fuses.
In all cases the fuses were sized above 156% Isc.
(In one case 12 A fuses were appropriate (about 7 or so A Isc) but burned out so i replaced with 15A as wiring and max series fuse rating allows 15A.)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What size fuse is prudent in this situation?
Or, are fuses even required?

2 systems on a single house roof feeding 2 meters.
2 strings to one inverter feed one meter and 1 string to another inv. and meter.
All three strings go to a single combiner box on roof. The first 2 strings are combined, while of course the third remains separate. They proceed to a single conduit (even though separate systems) down from there to two dc discos, one for each system. The first two strings are combined into a #10. The remaining uncombined circuit is #12.

Source circuit: 5.62 A Isc
(@156% = 8.78A)
Max series fuse rating from spec sheet : 10 A. That sucks.
1@10 A fuses on each of three strings. (grounded system, so single fuse on positive conductors only)

Now, were it three strings combined to one combined output circuit, clearly fuses are required. But here?
I would love to just put in some higher Amp fuses. Not really for over current protection, but to complete circuit and be done.
This is assuming fuses were not needed. Otherwise I do not know how I may exceed panel rating of 10A........
Say 15 or 20 or even some 12's i have left over.....kinda like a solid metal slug.

In general, I have been finding that after several years in operation, and using the smallest allowable fuse size in a rooftop comb. box, it results in nuisance burn outs and troublesome replacements of fuses.
In all cases the fuses were sized above 156% Isc.
(In one case 12 A fuses were appropriate (about 7 or so A Isc) but burned out so i replaced with 15A as wiring and max series fuse rating allows 15A.)
You don't need string fuses at all if the maximum number of strings you are combining into any single input is two.
 

pv_n00b

Senior Member
Location
CA, USA
Depends on the version of the code, 2014 had a significant rewrite.

2011:

690.9(A) requires OCPD and the exception says it's not needed under certain circumstances. Your design being one of the circumstances.

2014:

690.9(E) states that OCPD is to be used were required. It would not be required for your design.
 

pv_n00b

Senior Member
Location
CA, USA
Two strings times 5.62A Isc is 11.24A Isc combined, which exceeds the 10A max string fuse rating on module.

Am I missing something?
A fault on a string results in (N-1)*Isc current flowing into the fault from the other strings with N being the total number of strings. Therefore with 2 strings the fault current would be Isc.
 

pv_n00b

Senior Member
Location
CA, USA
In general, I have been finding that after several years in operation, and using the smallest allowable fuse size in a rooftop comb. box, it results in nuisance burn outs and troublesome replacements of fuses.
In all cases the fuses were sized above 156% Isc.
(In one case 12 A fuses were appropriate (about 7 or so A Isc) but burned out so i replaced with 15A as wiring and max series fuse rating allows 15A.)
If the combiner is on the roof in the sun then it's hot and over the 40°C the fuses are rated to operate in. Hot fuses trip at lower current so most likely the rating of those 12A fuses after correcting for temperature was less than the current from the string, sometimes, causing intermittent tripping.
 

Smart $

Esteemed Member
Location
Ohio
A fault on a string results in (N-1)*Isc current flowing into the fault from the other strings with N being the total number of strings. Therefore with 2 strings the fault current would be Isc.
I know many industry documents use the (N-1)*Isc calculation for maximum array fault current into one string, but doing a quick web search I could not find the reason behind the N-1 part. Traditional thought, at least what I consider traditional thought, says all strings can contribute to fault current. I can certainly see where N-1 is the contribution from outside the string, but the string itself can contribute to the fault, can it not?

Even if (N-1)*Isc is industry standard, where is it indicated in the NEC that we can use this value rather than N*Isc? The exception we are discussing says modules and conductors sized in accordance with 690.8(B)... and y'all know what that says (which is not [N-1]*Isc).
 

SolarPro

Senior Member
Location
Austin, TX
Traditional thought, at least what I consider traditional thought, says all strings can contribute to fault current. I can certainly see where N-1 is the contribution from outside the string, but the string itself can contribute to the fault, can it not?
You need to do a Google search for an I-V curve. At Isc, the PV source voltage equals zero. Since power is the product of current and voltage, power at Isc also equals zero.
 

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Smart $

Esteemed Member
Location
Ohio
You need to do a Google search for an I-V curve. At Isc, the PV source voltage equals zero. Since power is the product of current and voltage, power at Isc also equals zero.
That would be true of the array as a whole during a short-circuit fault, would it not?
 

SolarPro

Senior Member
Location
Austin, TX
Paralleled source and PV output circuits will back feed to the fault, which generally requires OCP. But if you only have two paralleled source circuits, the maximum possible fault current is simply Isc, because the shorted string does not contribute to the fault. Since the wiring is designed to carry Isc, no OCP is required where two strings are paralleled. Generally speaking, 3 or more paralleled source circuits require string fusing because the available fault current exceeds the conductor or module ampacity.
 

Smart $

Esteemed Member
Location
Ohio
Paralleled source and PV output circuits will back feed to the fault, which generally requires OCP. But if you only have two paralleled source circuits, the maximum possible fault current is simply Isc, because the shorted string does not contribute to the fault. Since the wiring is designed to carry Isc, no OCP is required where two strings are paralleled. Generally speaking, 3 or more paralleled source circuits require string fusing because the available fault current exceeds the conductor or module ampacity.
Please explain. How does the "shorted string" not contribute to the fault? Is it at all dependent on where the short occurs?
 

SolarPro

Senior Member
Location
Austin, TX
Let's takes a step back. This does a good job of explaining why there is no need for OCP with a 2-source circuit fault:

Fault Analysis for Combined PV Circuits

Where are all the sources of current in a PV system? It is a rare PV system where a fault on a conductor does not draw current from sources on both ends of the conductor. Therefore, just as your parents taught you to look both ways before crossing the road, you must look for possible sources of current from both directions on a conductor: the normal current and the reverse current. Reverse current, also called backfeed current, is the current that can flow through a conductor to a fault in the opposite direction of the normal current flow; this is usually the current that the conductor needs to be protected against.


Single source circuit. In a PV system consisting of a single source circuit, where the conductor is connected to the PV string on one end and to the inverter on the other, a fault on the conductor has two possible sources of current, as shown in Diagram 1. One source is the maximum PV source-circuit current, which NEC Article 690.8(A)(1) defines as the shortcircuit current multiplied by 125%. Therefore the fault current (IFAULT) at Point A in Diagram 1 is calculated as follows:

IFAULT = ISC x 1.25
= 8.19 x 1.25
= 10.2

The conductor at Point A is already sized to carry the maximum circuit current. The other potential source is the inverter backfeed current, or I BACKFEED at Point B. Most transformer-based inverters have zero backfeed current, which is assumed in Diagrams 1–3.

Note, however, that some transformerless inverters may have backfeed current. If IBACKFEED is greater than the conductor rating, then either an overcurrent protection device (OCPD) is required at the inverter connection or the conductor has to be sized for the backfeed current. If IBACKFEED exceeds the module’s series fuse rating, then overcurrent protection is required regardless of how the conductors are sized. In this case, the OCPD is not required in order to meet Article 110.3(B) of the NEC, which requires that equipment is “installed and used in accordance with any instructions included in the listing or labeling.”

Two source circuits. If two PV source circuits are combined at a common point and a fault occurs in a currentcarrying conductor, as shown in Diagram 2 (above), then the fault current carried by the conductor to the fault from the PV string is once again the maximum circuit current (IMAX) at Point A. At Point B, the available fault current through the common connection point is the sum of the current from points C and D:

IFAULT = IMAX + IBACKFEED

where IMAX is equal to 1.25 x ISC. Assuming that IBACKFEED is zero, as it most often is, the conductor at Point B has to carry only IMAX for which it is already rated, and no OCPD is required.

Three source circuits. If three PV source circuits are combined at a common point and a fault occurs in any of the strings, as shown in Diagram 3, then the fault current from the PV modules on the faulted string at Point A is the maximum circuit current (IMAX) as before. In this case, however, the fault current traveling in reverse from the common connection point is two times the maximum circuit current plus the inverter backfeed current—up to 15 A, at which point the fuse would open.

If there were no fuses, the fault current could be larger still:

IFAULT = (2 x IMAX) + IBACKFEED
= (2 x 10.2 A) + 0 A
= 20.4 A

As more strings are added, the difference between the fused and unfused currents increases. If the fault current is greater than the rated current of the conductor, then the designer has two choices: increase the size of the conductor so that it is rated to carry the fault current, or place an OCPD in each of the PV input circuits at the common connection point to limit the fault current that can travel back over the faulted conductor. Because the first option quickly becomes cost prohibitive, and PV modules require series fuse protection, system designers commonly rely on dc combiners to provide a convenient place to locate OCPDs in ungrounded current-carrying conductors in PV circuits. This allows for the use of smaller conductors while protecting PV modules from fault current in excess of the module’s fault-current rating.

With PV source circuits, it is always necessary to protect strings of modules according to the products’ series fuse rating. This ensures the integrity of the product in the event of a fault and mitigates fire hazard. In the absence of inverter backfeed, as shown in the previous examples, fault current in series strings is equal to the number of parallel-connected source circuits minus 1 multiplied by 125% of the short-circuit current:

IFAULT = (n – 1) x 1.25 x ISC

For more information on this topic, refer to the latest version of John Wiles’ Sandia Report, “Photovoltaic Power Systems and the National Electrical Code: Suggested Practices” (see Resources).
 

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Smart $

Esteemed Member
Location
Ohio
Let's takes a step back. This does a good job of explaining why there is no need for OCP with a 2-source circuit fault:
As I suspected. The (N-1)*Isc is maximum backfeed fault current... which means it is location dependent. It's not that the faulting string does not contribute to fault current but rather the maximum directional current and that which would flow through the OCPD during the fault.

Note that to comply with the NEC the calculation must be (N-1)*Isc*125% as exhibited toward the end of your quoted material.

PS: Thanks for posting that info. :thumbsup:
 
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SolarPro

Senior Member
Location
Austin, TX
:thumbsup:

Yes, the fault current is location/design dependent. And since PV is a current limited source, some faults are incapable of tripping an OCPD immediately or over time. In a 2-source circuit design (w/out an external source of backfeed current), a string fuse won't ever see enough current to trip, assuming it is sized properly.
 

Smart $

Esteemed Member
Location
Ohio
While we're on the subject, is there anything set up in Code to provide adequate protection of PV output conductors?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
While we're on the subject, is there anything set up in Code to provide adequate protection of PV output conductors?
From what? Unless you are recombining them there is no need to protect output circuits. In a correctly designed system with no recombining there is not enough fault current available from the modules to endanger the conductors. I haven't done the math, but I suspect that recombining output circuits would be the same as combining source circuits, i.e., fuses are necessary only if you are recombining three or more of them.
 

Smart $

Esteemed Member
Location
Ohio
From what? Unless you are recombining them there is no need to protect output circuits. In a correctly designed system with no recombining there is not enough fault current available from the modules to endanger the conductors. I haven't done the math, but I suspect that recombining output circuits would be the same as combining source circuits, i.e., fuses are necessary only if you are recombining three or more of them.
Combined, not recombined. For example, consider a fault between string OCPD's and the combiner bus, that short horizontal line in the diagram below. I know it is shown as fusing and a combiner combo, but the string fuses are not required to be connected directly to a combiner bus, right? Is that conductor ampacity required to be sized N*Isc*125%? And even if it is, there may not be a need for additional fusing, but a short circuit in that area will never trip a string OCPD no matter how many strings are combined.

May not be a problem... even if it is, could be one that hasn't presented itself in practice.


 
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