Fuse or not to fuse, that is the question.

[ExpatOilWorker]

All solar DC combiner boxes in Thailand have a fuse, breaker and surge protection, but in many parts of the world they skip the fuse and just have a 20A breaker and surge protector.

 

Is the fuse really needed? What protection is the fuse offering that a breaker can't do?

 

 

[Cardano]

The fuse only provides protection against short circuits whereas the MCB provides both short circuit and overload protection, where is the fuse located in the circuit and what is the fuse and MCB ratings?

[johng]

I think the MCB is a mechanical device and so could fail...the fuse is backup.

[JBChiangRai]

A fuse is redundant, the panels are incapable of delivering more than the rated current.  Even a short circuit is no problem.

 

The breaker’s main purpose is to be able to isolate the panels from the inverter, it’s never going to trip because of an overcurrent situation.

[Crossy]

I'm with @JBChiangRai unless your "combiner" actually combines two or more strings into a single output.

 

in the case of multiple parallel strings each individual string should have a suitable fuse, then all those fuses feed a single breaker.

 

The reasoning is thus. If you have, say, three 10A strings in parallel then in the event of a failed (short) panel in one string (reducing its output voltage) the other strings will push current "backwards"  through that string possibly resulting in a hotspot and conflagration.

 

I suspect the fuses in the "not actually combiner" units are simply a result of just taking out the extra strings from a real combiner design without really understanding what's going on.

[SLOWHAND225]

12 hours ago, ExpatOilWorker said:

All solar DC combiner boxes in Thailand have a fuse, breaker and surge protection, but in many parts of the world they skip the fuse and just have a 20A breaker and surge protector.

 

Is the fuse really needed? What protection is the fuse offering that a breaker can't do?

 

 

A fuse and a breaker are the same thing just a different style. They protect against over loads and shorts. Just use whichever you prefer.
Make sure you have a spare breaker though, they do go bad at the worst time

[daveAustin]

Fuse will blow faster than breaker?

[SLOWHAND225]

1 minute ago, daveAustin said:

Fuse will blow faster than breaker?

No Sir but a breaker is resettable. They perform the exact same function

[JBChiangRai]

1 hour ago, Crossy said:

I'm with @JBChiangRai unless your "combiner" actually combines two or more strings into a single output.

 

in the case of multiple parallel strings each individual string should have a suitable fuse, then all those fuses feed a single breaker.

 

The reasoning is thus. If you have, say, three 10A strings in parallel then in the event of a failed (short) panel in one string (reducing its output voltage) the other strings will push current "backwards"  through that string possibly resulting in a hotspot and conflagration.

 

I suspect the fuses in the "not actually combiner" units are simply a result of just taking out the extra strings from a real combiner design without really understanding what's going on.

Is there a diode in each panel preventing back flow?

[Crossy]

20 minutes ago, JBChiangRai said:

Is there a diode in each panel preventing back flow?

 

Sometimes, but rarely on the larger panels, they tend to be included in small panels intended as battery maintainers or similar applications.

 

The problem is that every time a diode is introduced into the circuit there's a volt-drop of around 0.8V (power schottky device). With 10 panels in series that's 8V that's not getting through (80W @ 10A).

 

Some do advocate adding a single blocking diode to each string and they are also sometimes included in the combiner box.

 

As always, there are many ways to skin a cat.

[ExpatOilWorker]

1 hour ago, Crossy said:

 

Sometimes, but rarely on the larger panels, they tend to be included in small panels intended as battery maintainers or similar applications.

 

The problem is that every time a diode is introduced into the circuit there's a volt-drop of around 0.8V (power schottky device). With 10 panels in series that's 8V that's not getting through (80W @ 10A).

 

Some do advocate adding a single blocking diode to each string and they are also sometimes included in the combiner box.

 

As always, there are many ways to skin a cat.

Very good 👍 idea.

 

[Muhendis]

It's worth remembering that the panels, by their very nature, are current and voltage limited.

The fuses, surge absorbers and breakers normally fitted are more to protect against lightning EMP (electromagnetic pulse) events.

The fuses fitted are normally "quick blow" which is considerably faster than a circuit breaker.

The scenario in the event of a lightning surge is:

The EMP will cause a high current flow in the solar panel wiring.

The surge absorber conducts this high current to ground.

This high current causes the fuse to blow very quickly.

A second strike nearby will go nowhere because the fuse has already blown.

House and inverter et al should never see any dangerous and damaging surges.

Having said all that I've never heard of such a disastrous event happening but sods law being what it is .........

[TronxII]

5 hours ago, ExpatOilWorker said:

Very good 👍 idea.

 

Idea is good, but take a suitable diode, not one of these toys. At 0.8V and 10A, the diode needs to dissipate 8W.  Put this into your MC4 connector and you get a nice ignition device to burn down your whole installation. 

 

[daveAustin]

8 hours ago, SLOWHAND225 said:

No Sir but a breaker is resettable. They perform the exact same function

It sure is but beg to differ. Dunno about this system specifically, but fuses are incorporated into electronic circuitry routinely in order to protect components against shorts. A fast blow fuse will drop out a lot quicker than an mcb, which is chiefly to protect from overcurrent. 

[Muhendis]

On 5/28/2025 at 9:35 AM, Muhendis said:

It's worth remembering that the panels, by their very nature, are current and voltage limited.

The fuses, surge absorbers and breakers normally fitted are more to protect against lightning EMP (electromagnetic pulse) events.

The fuses fitted are normally "quick blow" which is considerably faster than a circuit breaker.

The scenario in the event of a lightning surge is:

The EMP will cause a high current flow in the solar panel wiring.

The surge absorber conducts this high current to ground.

This high current causes the fuse to blow very quickly.

A second strike nearby will go nowhere because the fuse has already blown.

House and inverter et al should never see any dangerous and damaging surges.

Having said all that I've never heard of such a disastrous event happening but sods law being what it is .........

 

Just to be on the safe side I have sketched up a schematic of a combiner box.