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Sizing of Solar Fuses


DeepBass9

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I am finally getting around to putting fuses in my solar array (bad I know, I've had the fuses and holders for ages, but not got around to it). I'm not sure what is the correct size fuse though. I have 2 arrays, 2x2 300w panels and 2x3 300w panels, each into their own MPPT. The maximum current on the spec sheet is 8.2 A, so for the 2 panels strings that should be 16.4A and the 3 panel strings, 24.6A. I have enough fuses and holders for both ends of each of the 4 strings. The fuses that I have however are Bussman Solar PV . PV-15A, 10F. 15A-1000V DC. Does that mean the fuse will pop at 15A at 1000V, of is the fuse too small for the application? How do you figure out the right size fuse?

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Hi DB9

When you connect panels in series the voltage goes up (Voc * n where n is the number of panels) and the Amps stay the same.

When you connect panels in parallel the voltage stays the same and Amps go up (isc * n where n is the number of panels).

So each string of each array not produce more than 8.2A.  So the idea is to fuse each string  before their currents are combined. This will prevent a short circuit  on one panel from drawing the current from the other string(s). Remember current takes the path of least resistance so a short circuit  on one string and the current from the other string(s) will flow through it in preference to flowing through your MPPT doubling or tripling (and in my case quadrupling) the current in the affected string. One wants to size the fuse so that its Amp rating is low enough so that there is protection of your individual strings but not so low that there is nuisance blowing of fuses. The recommendation is to de-rate the fuses by 25% due to the warmer climate here in South Africa.

So 8.2A * 1.25 = 10.25A. You shoulld probably go with a 10 A fuse. My panels Isc rating is 8.79  (8.79A* 1.25 = 10.98 A ) and I have gone with 12A fuses. 15 A fuses will blow in your scenario but 10 Amp or 12 Amp would be safer.

I think if you look at your panels you have quoted I at Pmax  rather than Isc. One should use Isc  in sizing your fuses.

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Chris is spot on with his advice.

Just to state the obvious but is recommended that both the positive and negative conductors must be fused.

 

Nice guide : http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-an-10191-pv-app-guide.pdf

 

You wount find a better fuse to use in your setup than the current ones you have. The next size down after the 15 A is a 4 A.

 

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/product-datasheets-b/bus-ele-ds-1172-14x65mm-pv-fuses.pdf

 

There is a time current curve on that datasheet as well . If you look at it you will see that with only one string a short circuit will not blow the 15 A fuse .  On the TCC the 15 A line never crosses 8.7 A.

 

But you have strings in parallel, and all the other parallel string with contribute to the fault current.

 

Let assume 4 parallel strings  8.7 x 4  = 35 A will blow that 15 A fuse after about 15 seconds.

 

But its even worse as the Isc rating is for perfect conditions, so a short circuit in the early mornings or afternoons or even in cloudy weather and the fuse will take a longer time to blow.

 

So much for fast blow :D

 

 

 

 

Edited by Jaws
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OK, that's a problem then, so I should put the fuses on the parallel strings for a 15A fuse. I have two separate arrays, so I won't lose everything all at once.

So I have 16.4A on each array (although I must admit I see +20A on the MPPT, but I suppose that is after its done its magic), so the fuse will blow after 100s at midday? Better than nothing I suppose. 

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Fuses should not blow as the maximum current is Isc. Each string up until the combiner box should be fused separately  (or Y connector or what ever you are using to combine your PV strings). With one string there is no chance of reverse current due to a fault. With two strings the reverse current will always be lower than the forward current. Nevertheless I fuse even single strings as this provides one more additional place where one can break the circuit.

Note fuses are not disconnects and are there for a separate reason.

Below is a combiner box for my latest two strings.

1037012261_Combinerbox.thumb.jpeg.1584f657d0ffa2bb6abee403d6ef8628.jpeg

At the bottom of the image are two terminal blocks for the negative of the PV strings and followed by 2 fuse holders and two 25mm2 terminal blocks for the output of the combiner box and a SPD.

Note the most recent thinking is that on ungrounded systems you fuse both the positive and the negative as even after a fault has occurred and the "positive fuse" has blown the remaining cable will be at the same potential as the negative side of the system and that is not necessarily the same potential as ground. Ungrounded systems = 2 fuses ground system 1 fuse. Most modern inverters specifically state that one should not ground the PV arrays. So technically  I should fuses both positive and negative but with 2 strings reverse current is less than forward current so the point is moot.

Addition: I have just had a COC inspection of a Goodwe with 2 strings each on a separate MPPT. Fortunately I have fused them even though the fusing is not necessary. It is easier to install R100 of fuses and holders than argue the merits thereof. I am like current I take the path of least resistance :lol:.

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27 minutes ago, Chris Hobson said:

Addition: I have just had a COC inspection of a Goodwe with 2 strings each on a separate MPPT. Fortunately I have fused them even though the fusing is not necessary. It is easier to install R100 of fuses and holders than argue the merits thereof. I am like current I take the path of least resistance :lol:.

I'm so going to do this and go with the least resistance option! :D

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Hi Chris

Im coming from a totally different background , and joined the forum to get a better understanding of solar systems and the protection thereof.

Due to PV panels having a Isc (short circuit current rating) only slightly higher than the In (nominal current rating)  in a single string scenario I don’t see how the fuse will blow. Maximum current flow is during a short circuit event  and then with the resultant Isc being so low compared to grid or battery supply systems the fuse in theory will never blow.  In my mind short circuiting  a solar panel won’t blow up anything, unlike other sources of generation like generator of batteries with high Isc . So why then have fuses on single string PV panels if it’s practically impossible to a protection device that will be able to discriminate between In and Isc  ?

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2 hours ago, Jaws said:

So why then have fuses on single string PV panels if it’s practically impossible to a protection device that will be able to discriminate between In and Isc  ?

I had a fuse installed, 10amps, soon as it got to about 10am, fuse popped. Had to put in a 15amp one.

Also this can happen with no fuse.

575fcdcd487c6_Tenesolpanelproblem.thumb.jpg.c8ba082c950e5fa44db144421ae01a8d.jpg

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2 hours ago, Chris Hobson said:

So technically  I should fuses both positive and negative but with 2 strings reverse current is less than forward current so the point is moot.

 

1 hour ago, Jaws said:

So why then have fuses on single string PV panels if it’s practically impossible to a protection device that will be able to discriminate between In and Isc  ?

Although the ISC and In isn't enough to blow the fuse, there is other Short circuit conditions that will create enough fault current to blow the fuse. We are focusing on Short circuit current and we think its only for a short between + and - 

What if:

  1. Something in the Inverter happens and the PV+ touches the 220VAC. 
  2. A trunking cover damaging the insulation of the PV + as well as the insulation 220V live wire causing them to short out.
  3. While producing 4kw the insulation on the PV wires breaks down due to UV damage and touches the normal earth wire.
  4. Or a clever electrician thinks the Red and Black PV wires should be connected to 220VAC 

I dont know exactly what will happen, but I can only imagine amps will be way in excess of 10 or 12 or even 15 amps. 

I will rather keep my 2 fuses per string and try to protect against all possibility's than take the stance that the fuse has no real value in any case.  

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3 hours ago, Jaco de Jongh said:

 

Although the ISC and In isn't enough to blow the fuse, there is other Short circuit conditions that will create enough fault current to blow the fuse. We are focusing on Short circuit current and we think its only for a short between + and - 

What if:

  1. Something in the Inverter happens and the PV+ touches the 220VAC. 
  2. A trunking cover damaging the insulation of the PV + as well as the insulation 220V live wire causing them to short out.
  3. While producing 4kw the insulation on the PV wires breaks down due to UV damage and touches the normal earth wire.
  4. Or a clever electrician thinks the Red and Black PV wires should be connected to 220VAC 

I dont know exactly what will happen, but I can only imagine amps will be way in excess of 10 or 12 or even 15 amps. 

I will rather keep my 2 fuses per string and try to protect against all possibility's than take the stance that the fuse has no real value in any case.  

I totally agree Jaco.! " Prevention is better than cure "

After spending money on quite expensive equipment, why trying to save a few bucks?

Fuses are really cheap and will make no change to the total cost of the system

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The other, more common scenario, is where a panel short circuits and you have several strings in parallel with it. Then you can get more than the short circuit current of one string flowing into one panel. Generally, a panel can handle a bit more than its short circuit current, but more than that can cause a fire. So it's essential to fuse or circuit break strings when more than two are paralleled.

Edit: The rule of thumb that two paralleled strings usually don't need fuses is because panels can generally handle a bit more than their short circuit current, but not twice or more. I had said that they can handle twice their own short circuit current, but this is not correct. Thanks Weber for sorting me out in email.

Edited by Coulomb
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1 hour ago, DeepBass9 said:

"I had a fuse installed, 10amps, soon as it got to about 10am, fuse popped. Had to put in a 15amp one"

I have 20 x 310Wp in 2s10p arrangement, with the +ve of each string fused with 12amp fuses in the combiner box. Some time after installation I noticed a drop in production, after investigating I found a blown fuse which turned out to a 10amp that had been mixed in with the 12amp fuses supplied.

I believe that 10amp is too low when using panels 300W and above.

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1 hour ago, DeepBass9 said:

"I had a fuse installed, 10amps, soon as it got to about 10am, fuse popped. Had to put in a 15amp one"

On what size array?

It was the installer, he said "Naaaa, 10amps is fine".

3 x 310w in series: Voc +-134.40 with Short circuit +-8.8. 

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Thank you for the response Jaco.

1. Something in the Inverter happens and the PV+ touches the 220 VAC. 

2. A trunking cover damaging the insulation of the PV + as well as the insulation 220 V live wire causing them to short out.

 4. Or a clever electrician thinks the Red and Black PV wires should be connected to 220 VAC 

Ok from the above scenarios  in a single string setup the fuses are actually there to protect the PV panel from other sources of energy as its own production of energy is not sufficient to cause any damage.

 

3.While producing 4kw the insulation on the PV wires breaks down due to UV damage and touches the normal earth wire.

I need to think about this a bit more but the first thing in my mind is that if the + or - wire of a PV string comes in contact with  earth nothing should  happen as in the case of IT earth system - Even if I touch it I should not get an electric shock as I’m not completing a circuit for current to flow.

 

Hi Chris-R

Im not advocating not to use fuses at all.  The question was posed how to size solar PV fuses. I had a quick look at SANS/IEC 60269-6 and they states 1.35x In.  My thinking was for a single string PV array as the only source of energy the fuses will not really add much in the form of protection. They need to be there I totally agree I was just trying to understand why they are there and not they are there "vir sommer maar net"

Thanks Coulomb - Yes that’s why I was always referring to single string arrays - when you have parallel strings the mode of protection actually becomes easier to achieve with the higher fault levels contributed by the other strings.

 

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14 hours ago, Jaco de Jongh said:

 

Although the ISC and In isn't enough to blow the fuse, there is other Short circuit conditions that will create enough fault current to blow the fuse. We are focusing on Short circuit current and we think its only for a short between + and - 

What if:

  1. Something in the Inverter happens and the PV+ touches the 220VAC. 
  2. A trunking cover damaging the insulation of the PV + as well as the insulation 220V live wire causing them to short out.
  3. While producing 4kw the insulation on the PV wires breaks down due to UV damage and touches the normal earth wire.
  4. Or a clever electrician thinks the Red and Black PV wires should be connected to 220VAC 

I dont know exactly what will happen, but I can only imagine amps will be way in excess of 10 or 12 or even 15 amps. 

I will rather keep my 2 fuses per string and try to protect against all possibility's than take the stance that the fuse has no real value in any case.  

If I may, 

 

  1. The 220V and PV+ lines, inside the inverter won't touch, but even if they did, chances are you would have bigger problems than a fuse installed further up the PV wires could protect against. What I am saying, is that fuses probably won't protect your equipment if there is an internal fault. 
  2. The 220V and PV wires should not run in the same trunking, nor should the 220V wires run across the roof. 
  3. Proper solar cables have a 20year warrentee, especially against UV. They normally have two layers of insulation. Which is why people shouldn't use cheap "speaker wires", or twin flex for their PV side wiring. 
  4. All wires should be properly labeled to avoid mistakes like this. 

 

P.S. Jaco, I am not attacking you, but just adding to your statement. 

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8 minutes ago, SilverNodashi said:

The 220V and PV+ lines, inside the inverter won't touch, but even if they did, chances are you would have bigger problems than a fuse installed further up the PV wires could protect against. What I am saying, is that fuses probably won't protect your equipment if there is an internal fault. 

No, but it will protect my panels.. 

 

8 minutes ago, SilverNodashi said:

The 220V and PV wires should not run in the same trunking, nor should the 220V wires run across the roof.

Should not and reality is 2 different things. Look at all the Member showcases and see all the single trunking.

8 minutes ago, SilverNodashi said:
  • Proper solar cables have a 20year warrentee, especially against UV. They normally have two layers of insulation. Which is why people shouldn't use cheap "speaker wires", or twin flex for their PV side wiring. 
  • All wires should be properly labeled to avoid mistakes like this. 

Not everybody does this, some owner installs are not labled.

Not even going to try and justify my statement above, I want to protect my panels from any possibility of 220Volt being  sent back to the panels.  

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4 minutes ago, Jaco de Jongh said:

No, but it will protect my panels.. 

 

Should not and reality is 2 different things. Look at all the Member showcases and see all the single trunking.

Not everybody does this, some owner installs are not labled.

Not even going to try and justify my statement above, I want to protect my panels from any possibility of 220Volt being  sent back to the panels.  

I get that, but fuses won't protect your panels under the circumstances you mentioned. A 1000V/12A fuse won't blow if you push 220V to the panels, as the panels won't necessarily draw 12A or more, in order for the fuse to blow. 

Chances are the diodes in the panels will blow with a few milliamp at 220VAC. 

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43 minutes ago, Jaws said:

I need to think about this a bit more but the first thing in my mind is that if the + or - wire of a PV string comes in contact with  earth nothing should  happen as in the case of IT earth system - Even if I touch it I should not get an electric shock as I’m not completing a circuit for current to flow.

A short Video I saw a while back.

 

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29 minutes ago, SilverNodashi said:

I get that, but fuses won't protect your panels under the circumstances you mentioned. A 1000V/12A fuse won't blow if you push 220V to the panels, as the panels won't necessarily draw 12A or more, in order for the fuse to blow. 

Chances are the diodes in the panels will blow with a few milliamp at 220VAC. 

In my case you will be sending 220VAc to 12 panels in series, that is 18volts for each panel. The SB1240 diode in @The Terrible Triplett picture above can handle 30Volt to 100 Volt and its Peak forwarding SURGE current (IFSM) is rated at 280Amps(In Halve wave rectification). Look at the direction of the Diodes, it will be a Dead Short. 

If you feel your panels can handle that, when can we do the test, But please can I be there, I want to make the video.  

EDIT: Forgot to mention the 1240 stands for 12Amp (Average Forward Current) 40 Volts, so I am pretty certain in my setup a booboo like that will create a fault current of way above 12Amps, and that is where I rely on a  fast blow fuse to give before the diode will.

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Guys, fuse are not a nice to have, money making racket.

Put fuses on the blerrie wires, positive and negative, as Chris pointed out and move on. :D

If you don't and need a CoC for the panels, you ain't getting one.

It must be done. "Finish and klaar".

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