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Why fuse SCC wire at "far" end?

jameshowison

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Jul 30, 2021
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In all the diagrams I see the fuse to protect the wire connecting a busbar and the solar charge controller (SCC, MPPT etc) is at the busbar end of the wire, not near the SCC. Given that the power flows from the SCC to the busbar, it seems odd to me that the fuse is at the "far" end. I'm not arguing with it, but I'd like to understand. Any links or threads that discuss this (I tried to search but no luck)? The Victron Wiring Unlimited book shows things this way (but with a marine circuit breaker on the positive of the MPPT) while in the same paragraph saying "Each individual consumer needs to have an individual fuse." See p 31 of https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

I'm guessing that maybe the protection is really for a situation in which power flows from the busbar to the SCC, but I'm not sure of when that might be (other than a battery short circuit which should be managed by the battery Class T which is placed as close to the battery as possible). Maybe it's just that a SCC is a "consumer" even if it's just voltage from the battery to run the SCC when the sun isn't shining. Maybe the failure situation that the fuse is protecting for is a short in the circuit to the SCC, such as the wires rubbing against each other or the positive with tear in insulation to a chassis creating a short circuit (in which case the power would be flowing from the busbar, through the fuse, to the short, back to the grounding lug, back to the battery (or something).

I notice that some circuit breaker providers (e.g., Midnite solar) advertise "non-polarized" breakers, which I think means they work bi-directionally, but they are still usually depicted closer to the busbar than to the SCC.

So, I think the SCC just isn't able to generate enough current to cause real problems (given appropriate wiring for the panels), and the main cause of problems is the battery, therefore fuse as close to the battery as possible. Is that the thinking? Sure, someone could wire way too many panels up to the SCC but then the SCC will blow up) but that's not the issue being protected for.
 
Remember that a fuse primarily protects wiring. The wire from a charge controller is rated at say, 200A. When your 60A MPPT shorts out, it produces at most 60A - no big deal for those wires. However, the battery can produce much more current than that, and a short inside the MPPT for example would mean all the potential the battery can produce now flows through that short, burning the wires. Putting the fuse close to the battery means any potential short in the MPPT wiring won't cause a huge amount of current to flow though a cable that isn't rated for it.
 
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When your 60A MPPT shorts out, it produces at most 60A


If the SCC has a dead short fault it would short the full battery capacity through the SCC, That short circuit is not limited by the maximum capability of the SCC to charge the battery, The current into the short could be in the 100’s of amps.
 
If the SCC has a dead short fault it would short the full battery capacity through the SCC, That short circuit is not limited by the maximum capability of the SCC to charge the battery, The current into the short could be in the 100’s of amps.

Did you read the rest of my comment?
 
Did you read the rest of my comment?
I read the whole thread, I am pointing out an incorrect wording.....

Please reread it and then tell me that if the SCC has a dead short circuit fault that it would only short 60 amps from the battery

The fault current is in no way limited by the maximum capability of the SCC to charge the battery.

No battle here, you used the wrong phrase.
 
Please reread it and then tell me that if the SCC has a dead short circuit fault that it would only short 60 amps from the battery

I didn't say that - I sad that the SCC short circuit is that much, but because it's coupled to the battery, it's the battery current going into a short at the SCC one should worry about.

Maybe just bad wording on my part... nm.
 
I think that we are all here to share and learn, your wording is not correct. The fault current in either direction is not limited by the maximum capability of the SCC to deliver current in a non faulted condition.
 
I absolutely agree that (quoting you) a short inside the MPPT for example would mean all the potential the battery can produce now flows through that short, burning the wires.

BUT i do not agree that (quoting you) When your 60A MPPT shorts out, it produces at most 60A

In fact the capacitors within the SCC can and does hold more energy than the SCC can output in a non-faulted condition.....in fact that current could be in the hundreds of amps, true , not of long duration but in any case it is not limited by the maximum current capability in a non faulted condition
 
BUT i do not agree that (quoting you) When your 60A MPPT shorts out, it produces at most 60A

In fact the capacitors within the SCC can and does hold more energy than the SCC can output in a non-faulted condition.....in fact that current could be in the hundreds of amps, true , not of long duration but in any case it is not limited by the maximum current capability in a non faulted condition

A regular fuse/breaker would not even blow with the transient created by shorting the output of an MPPT. The capacitors are tiny compared to the ones found in e.g. an inverter and definitely pose no threat to the wiring.
 
Correct, your statement there is true but my statement is also true, I have seen that fault and recorded it on my data logger.

Are we done with this, You are putting out some good information there and I do appreciate it. There are a lot of newcomers to this arena who do need some guidance from those who have real time expereince in the feild.

Im certainly no new comer to living off the grid, Im from Alaska, born off the grid.....probably as far north as you are
 
Thanks all, makes sense to me. So it's all about identifying the largest potential sources of current and protecting against those. Almost always the battery is the largest source. But I guess with a really massive solar array they are fused in the combiner box or otherwise en route to the SCC.
 
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