Fuse between solar panels and MPPT controller?

[TernVI]

I have just fitted a MorningSta Pro-Star Gen-3 30 amp solar controller and it specificlly states in the manaual a 40 amp fuse between the Controller and the Battery (within 6 inches of the battery) and a 40 Amp fuse between the controller and the solar panels, with breakers to disconnect the battery and the solars

And, by chance, last night I was shown a Vitron controller where the positive input from the solars to the controller has melted the controller plastic/solder. The only fuse in the whole circuit was in the controller itself which does not appear to have blown.

It's quite hard to blow a fuse with a panel which self-limits the current.
Sometimes fuses are mainly there to blow if something is connected the wrong way around.
Some equipment can have 'crowbar' circuitry, which deliberately blows the input fuse in certain circumstances.

 

[Kinsale373]

The fuse is there to limit current flow in the circuit to below the rating of the fuse.
Could the +Ve of the Panels end up shorted if water gets into connectors or if the cables get crushed- very possible senario on a boat. It is good electrical practice to have a fuse to protect the panels against a shorted output .
You could have a situation where because of a close by lightening strike ( it doesen't have to be a direct hit on your boat) you get a high energy pulse induced into the panel circuit /wiring. A fuse may not fully protect your Controller and the rest of your electrical circuit , but it will limit the energy pushed in.
Fuses are cheap and well worth fitting.
Kinsale 373

 

[PaulRainbow]

Plenty of posts in solar on a boat on Facebook showing various fires caused by panels, usually flexible ones on biminis. If you read my post, and I assume you didn't, I said that there probably aren't any related to fuses. I also not only didn't say to fit larger cables, I said don't fit larger cables.

I misread what you said about cables.

You then go on to say there's no need for a fuse since it's there to protect the cables.

If you read my post, and I assume you didn't, I said

"No need for a fuse between the panels and the controller for this setup. The fuse between the controller and the batteries should be at the batteries, it's there to protect the cables. "

I literally said the fuse is to protect the cables, and if someone upgrades the panels then the fitted cables may have an issue so £1 of fuse is probably a wise investment. These forums are littered with people upgrading panels who don't spare a second thought to also uprating their cabling. A fuse might protect them.

A fuse must be fitted at the batteries, to protect those cables. In this instance, there is no need to fit a fuse between the panels and the controller, it serves no purpose. If someone adds an extra panel, it won't make any difference to the cables going to the controller and if the fuse at the battery is rated for the controller output it won't make any difference there either, the Victron controller will limit the output based on its ratings.

There are cases where one might build in some redundancy, such as running overrated supply cables to the main distribution board, in case additional equipment is installed but this isn't one of those cases. The solar installation is carried out using cabling and fusing to protect the system you are installing, if someone comes along later and adds additional equipment that renders the installation unsafe, that's their error.

 

[PaulRainbow]

The fuse is there to limit current flow in the circuit to below the rating of the fuse.
Could the +Ve of the Panels end up shorted if water gets into connectors or if the cables get crushed- very possible senario on a boat. It is good electrical practice to have a fuse to protect the panels against a shorted output .

If you short the positive and negative wires on a solar panel, nothing happens.

 

[William_H]

If you short the positive and negative wires on a solar panel, nothing happens.

Quite so it is a good test of a panel to disconnect from controller and while in sunshine connect an amp meter across the output. Measures short circuit current and so how good the panel is. So no need for a fuse between panel and controller. Unless there is some possibility of battery current finding it's way back through the controller to a short in the panel wiring. I can't be sure but find this unlikely and anyway you have a fuse at the battery feed.
Now I have a solar PV system at home just 1500 watts. About 10 years old it has well paid for itself. I did find at one stage it was not working. Tracked it down to a pair of fuses on the roof one pos. one neg. They were 10 amp 1000v./ Quite absurd to me as max solar current is 5 amps. (at300volts). I can only assume a government requirement and only assume for some sort of lightning protection. Why did one of mine fail?I can only assume it was a bad fuse. ol'will

 

[scr0che]

No need for a fuse between the panels and the controller for this setup. The fuse between the controller and the batteries should be at the batteries, it's there to protect the cables.

If you are running cables down from each panel separately (rather than joining them in parallel above deck, 6mm is perfectly adequate.

I was planning to join the 2 panels together at the top of the arch, so only running 2 x 10mm cables down below deck - or is the better option running 4 x 6mm and joining them below deck? If I do that I'll be <2m from the controller, so 6mm cable from the branch to the controller or 10mm?

An earlier post is quite right, these are Victron 175w panels, and the wiring diagram does show a circuit breaker between the panels and controller, so maybe I will just fit one, belt and braces. Should that breaker sit closer to the panel (an inline one) or below deck near the controller?

 

[lustyd]

Quite so it is a good test of a panel to disconnect from controller and while in sunshine connect an amp meter across the output. Measures short circuit current and so how good the panel is. So no need for a fuse between panel and controller.

So are you saying that if I hook up 3KW panels over 1.5mm wire that the panel will just reduce output because it knows the wire can't cope? That's certainly a new one on me.

 

[VicS]

So are you saying that if I hook up 3KW panels over 1.5mm wire that the panel will just reduce output because it knows the wire can't cope? That's certainly a new one on me.

William does not say that, nor is he suggesting that you use undersized wiring.

If a panels is wired to handle I_MAX nothing untoward will happen if you short circuit it .

In practice the wiring will be sized to avoid excessive volts drop so will be heavier than if sized for I_MAX

 

[VicS]

I was planning to join the 2 panels together at the top of the arch, so only running 2 x 10mm cables down below deck - or is the better option running 4 x 6mm and joining them below deck? If I do that I'll be <2m from the controller, so 6mm cable from the branch to the controller or 10mm?

An earlier post is quite right, these are Victron 175w panels, and the wiring diagram does show a circuit breaker between the panels and controller, so maybe I will just fit one, belt and braces. Should that breaker sit closer to the panel (an inline one) or below deck near the controller?

Where ? I still have not found it .

Are you sure they are suggesting an overcurrent protection device , not just a switch to enable the panels to be disconnected manually if necessary.

If its just a switch whever is convenient, but you not want it on deck!

If they really are suggesting an overcurrent protection circuit breaker I cannot see that it will matter where it is fitted as it will be serving no useful purpose . Leave it in its box.

 

[lustyd]

William does not say that, nor is he suggesting that you use undersized wiring.

If a panels is wired to handle I_MAX nothing untoward will happen if you short circuit it .

In practice the wiring will be sized to avoid excessive volts drop so will be heavier than if sized for I_MAX

But as I have said several times, if you increase the size of the panels, as is an extremely common upgrade on modern boats, then the wiring could become undersized without someone realising. Solar connectors make this even more likely since they appear plug and play. A fuse removes all danger from this scenario at what is effectively zero cost with no appreciable downsides. I don't understand why people considering themselves experts are so closed off to this extremely standard form of protection. All fuses are unnecessary right up until the point they are necessary!

 

[lustyd]

If they really are suggesting an overcurrent protection circuit breaker I cannot see that it will matter where it is fitted as it will be serving no useful purpose . Leave it in its box.

The purpose is to protect against overcurrent. It says it right there on the tin!

 

[scr0che]

Where ? I still have not found it .

Are you sure they are suggesting an overcurrent protection device , not just a switch to enable the panels to be disconnected manually if necessary.

If its just a switch whever is convenient, but you not want it on deck!

If they really are suggesting an overcurrent protection circuit breaker I cannot see that it will matter where it is fitted as it will be serving no useful purpose . Leave it in its box.

https://www.victronenergy.com/uploa...charger-12-50-1-inverter-375W-MPPT-100-30.pdf

 

[RupertW]

From all the forums I’ve read and tech advice the answer is almost always no, because no extra

The purpose is to protect against overcurrent. It says it right there on the tin!

And what could possibly cause the overcurrent?

 

[PaulRainbow]

But as I have said several times, if you increase the size of the panels, as is an extremely common upgrade on modern boats, then the wiring could become undersized without someone realising. Solar connectors make this even more likely since they appear plug and play. A fuse removes all danger from this scenario at what is effectively zero cost with no appreciable downsides. I don't understand why people considering themselves experts are so closed off to this extremely standard form of protection. All fuses are unnecessary right up until the point they are necessary!

Solar panels at the end of the cables are not the same as having a power source, such as a battery. If the wiring between the battery and the controller shorts it will melt/catch fire, so it's fused at the battery to protect the cable. If the cable between the panels and the controller shorts it does not overload the cable, so there is no need for overcurrent protection.

 

[Graham376]

I was planning to join the 2 panels together at the top of the arch, so only running 2 x 10mm cables down below deck - or is the better option running 4 x 6mm and joining them below deck? If I do that I'll be <2m from the controller, so 6mm cable from the branch to the controller or 10mm?

An earlier post is quite right, these are Victron 175w panels, and the wiring diagram does show a circuit breaker between the panels and controller, so maybe I will just fit one, belt and braces. Should that breaker sit closer to the panel (an inline one) or below deck near the controller?

I have 2 x 160w panels on Victron 100/30. I prefer cable joints to be in the dry whenever possible so have brought all four cables below deck and don't have fuses/cb/switch between panels and controller.

 

[PaulRainbow]

I was planning to join the 2 panels together at the top of the arch, so only running 2 x 10mm cables down below deck - or is the better option running 4 x 6mm and joining them below deck? If I do that I'll be <2m from the controller, so 6mm cable from the branch to the controller or 10mm?

An earlier post is quite right, these are Victron 175w panels, and the wiring diagram does show a circuit breaker between the panels and controller, so maybe I will just fit one, belt and braces. Should that breaker sit closer to the panel (an inline one) or below deck near the controller?

I'd run 4x6mm cables to below decks, no worry about waterproofing joints. 6mm cable from the joint to the controller will be perfect if it's 2m or less.

I think you'll find the device in the Victron diagram is just an isolator. If you want to fit one, by all means do so. Mount somewhere convenient below decks, it is not there to protect the cables from current overload, it's there so you can isolate the panels for maintenance.

 

[VicS]

https://www.victronenergy.com/uploa...charger-12-50-1-inverter-375W-MPPT-100-30.pdf

Thanks.

There is no mention of this in the manuals for either your solar controller or the solar panels

But what is it ? Is it an overcurrent protection device or is it merely an isolator ?

.

 

[noelex]

This topic comes up frequently.

In very simple terms if you have three or more panels the combined current of two or more panels can in some fault conditions and some wiring configurations be enough to overheat the internal wires embedded within a solar panel. A fuse between the battery and the controller will not protect against this fault, indeed there is no need for a battery connection at all, the excess current can be produced by the other solar panels in the array.

As the OP only has two solar panels in his array the above scenario will not occur so fusing on the solar panel side of the controller is not required in this case, but it is required for some installations.

 

[VicS]

The purpose is to protect against overcurrent. It says it right there on the tin!

How do you get an overcurrent from a solar panel ? The quoted max current is the short circuit current and is only slightly greater that the current at maximum power ( less than 10% as has already been stated)

What tin are you reading ?

 

[lustyd]

If the cable between the panels and the controller shorts it does not overload the cable, so there is no need for overcurrent protection.

So you're saying that if I put a too small fuse accross the terminals of a too large panel on a sunny day that the fuse won't blow? Where does the power go, and how would the panel know to stop providing current?

How do you get an overcurrent from a solar panel ? The quoted max current is the short circuit current and is only slightly greater that the current at maximum power ( less than 10% as has already been stated)

What tin are you reading ?

If you match the current on install to the cable and then upgrade the panels you'll have more current.