Fusing an alternator on a house bank

[BjarneK]

I was thinking about how to fuse an alternator in the scenario where it is used to charge a house bank and an echo-charge or similar device is used to keep the starting battery charged. You may argue that an alternator on an engine battery does not need to be fused, but I would definitely want a fuse if connecting an alternator to a house bank.

On the house battery there will typically be a large fuse very close to the battery to protect the heavy wiring going to inverters and other high current devices. Depending on the alternator wiring size, a smaller fuse may be needed to protect that wiring from the battery side (in case of a short between ground and positive in the wiring leading to the alternator).

The alternator will not put out enough current to overload the wiring between it and the battery so a fuse in the alternator end will not be needed. (This can be argued, but let us do that in another thread).

My first thought was to use heavy wire to the alternator and connect its wiring on the load side of the large fuse close to the battery. But what happens if that large fuse blows for some reason unrelated to the alternator? The alternator will put out a high voltage spike like when you disconnect it without disconnecting the field wire first.

If the alternator is not somehow protected, its diodes will be fried. That is of course bad enough, but what about everything else on the same circuit (which is pretty much everything electrical on the boat)? As far as I can see, all those device will also receive a high voltage spike, risking basically breaking everything. With a protection circuit on the alternator output, the voltage spike will be limited which may save the diodes in the alternator, but the voltage will still be high enough to break a lot of other devices.

The input capacitors on some devices will help mitigate the voltage spike, but this still seems like a very bad situation to me. Note that putting the alternator on a (potentially smaller) fuse after the big one will not change anything since the large fuse blew for a reason unrelated to the alternator and the alternator will still be connected to everything else though its own fuse that did not blow.

The only solution I can come up with is to put the alternator on a separate fuse that is directly connected to the house battery. Given the potential short circuit current of a large house battery, this should be of the same high quality (e.g. class T) as the main fuse on the house bank.

What are your thoughts on this?

[donradcliffe]

I think a separate fuse would be overkill. You need go with the method that used to design nuclear power plants. Develop a Maximum Credible Accident and design to survive that.

[team karst]

There is merit in fusing alternator output lines. Cars have done this for many decades. Oh, and I have done designs for commercial nuke dc and ac power systems.
On my boat, both alternators run thru diode isolators, lowering this risk.

[GreenWave]

Go look up marine how to.com and find his article on alternator wiring.

[Recy]

You can install something like this.

https://balmar.net/product/apm-12/

https://sterling-power.com/products/...tection-device

[Mariner84]

"The only solution I can come up with is to put the alternator on a separate fuse that is directly connected to the house battery. Given the potential short circuit current of a large house battery, this should be of the same high quality (e.g. class T) as the main fuse on the house bank."


Yes, absolutely this. Consider an event whereby the alternator output wire terminal fails, allowing an exposed conductor to contact nearby metal. I've seen this, actually - fortunately without catastrophic consequences. Much more important to protect the boat from fire than the alternator from failure.
Installation of a fuse rated ~120-125% of alternator capacity is my chosen method of protection. That and good wiring, periodic inspection of said wiring and terminal boots on all high amperage connections. Should go without saying, but......


https://www.bluesea.com/products/519...k_-_30_to_300A



@don: We call it a Maximum Credible Incident (MCI) in our business. Assuming similar concept here.

[CarlF]

Give the alternator its own cable and fuse. Replacing alternator diodes is chump change compared to the cost of a fire. And most modern alternators have avalanche diodes that almost always survive a disconnect.

With nothing else on the cable, the odds of the fuse blowing are extraordinarily low. Remember a fuse is sized to protect the wire - which should be big to minimize voltage drop. So the fuse will be much bigger than the alternator amps. Just be sure the cable is run with care to avoid chafing. Then - about the only other possible reason to blow the fuse would be a short inside the alternator -- in which case you'll need a new alternator anyways.

And always good to put an alternator protection device on any alternator -- as belts and suspenders. Balmar has a new one that is improved over their previous effort. And Sterling has one. They aren't expensive.

Alternators don't really require a Class T fuse. They aren't capable of generating that many amps. Certainly nothing like a large inverter or lithium battery bank.

[smac999]

Fuse at battery with own fuse. Fuse 150% of alt size so it can’t blow by mistake. Doesn’t mater which battery it goes to it needs a fuse. No fuse needed at alt end.

I use Mrbf fuses a lot.

This is all abyc guidelines.

Yes if it blows it might kill the alt. But if it’s blowing there is already a big problem somewhere.

Buy a alternator protector if you are worried.
https://sterling-power.com/products/...tection-device

The echo charge is irelivent. It would also have its own fuses on both battery ends.

[team karst]

Agree 100%

[rgleason]

Hi BjarneK,

I realize that this diagram there is a 160a fuse for the alternator in the Lynx distribution bus. This was reviewed by many, including goboatingnow, and svsagres and wholybee.

I had never thought of the event you describe, but that is a reasonable scenario.

Hope you meet up with Mathias! I would love to hear the discussion by two mathematical anchoring gurus!

Best
Rick

[coopec43]

I was going to use a 1/2/Both switch but decided against it and plan to use Voltage Sensing Relay (VSR) to charge the starting battery

Maybe these would be helpful? (I'll have to look at them myself too)

https://www.youtube.com/watch?v=ZJWPYYRZMuY

https://www.google.com.au/search?q=A...4-EPnL6gmAs_67

[goboatingnow]

Correct dedicated alt to battery , fuse at battery end to protect wire. Fuse over Alt Mac current

That’s it no further thinking required.

By definition if the fuse blows the Alternator is not output anything and won’t blow anyway

[BjarneK]

Thanks everybody for your thoughts.

Originally Posted by rgleason

Hi BjarneK,
I realize that this diagram there is a 160a fuse for the alternator in the Lynx distribution bus. This was reviewed by many, including goboatingnow, and svsagres and wholybee.

I had never thought of the event you describe, but that is a reasonable scenario.

I think you have to be a member of the DIY Solar Power Forum to see the diagram. Can you post it here, so I can see it?

Originally Posted by rgleason

Hope you meet up with Mathias! I would love to hear the discussion by two mathematical anchoring gurus!

Thanks, we are spending some time together right now in San Blas. It is always great to meet people in real life that you have only known through the internet before.

[rgleason]

BjarneK

Here is a jpg of it.

[s/v Jedi]

Yes, you need a dedicated fuse for the circuit to the alternator. The fuse should be as close to the positive main busbar as possible. This is the closest you can get and it doesn’t even need a cable: https://www.bluesea.com/products/519...k_-_30_to_300A

It has a 10kA interrupt capacity so that’s good enough for the alternator circuit

Attached is my reference diagram showing this alternator circuit (right side of diagram) incl. this fuse. Note that there is a switch near the alternator as well. This is the alternator service switch, which provides safety while working on the engine for maintenance, repair etc. because it’s rather easy to create a short there.