What Size Alternator Fuse ?

[CharlieJ]

Don-

If a fuse in the alternator-battery line blows, with a battery sensed alternator regulator like the Balmar 612, you are not going to have a short time transient.

That's not really the way the load dump, aka "spike", works. The spike is generated by the inherent inductance of the alternator. When the load on the alternator disappears, via an opening OCPD or the opening of a switch, the energy that is inductively store dissipates in a 60-120 VAC spike with a nominal 5 msec rise time and a decay of several hundred msec. BTW, I remember, but cannot find, a source stating the spike could go as high as 600 VAC.

In summary, it does not matter where the voltage sensing to drive the regulator is connected to. It is the energy stored in the inductive windings of the alternator that cause the diode damage. The zapstopper has a good chance of mitigating these spikes.

Best regards,
Charlie

[donradcliffe]

Charlie,
I think I understand how the voltage transient occurs in a regulated alternator when the load disappears--it is of short duration because the regulator senses the voltage increase and drops the field current, so all you have to deal with is the energy stored in the windings. However, a battery-sensed regulator actually sees a drop in voltage when the OCPD opens, and will increase the field current to compensate. Then you will have to dissipate whatever energy the windings continue to generate, which could be on the order of a kilowatt for a 100 amp alternator. With nowhere for the current to flow except the zapstop, something's going to melt pretty soon.

This is the alternator equivalent of a Chernobyl versus a Three Mile Island reactor accident. Both reactors lost cooling water, but the Three Mile Island reactor ceased to generate more energy, and all they had to deal with was the decay heat, which caused some of the core to melt, but wasn't all that big a deal. The Chernobyl reactor design actually put out more power when they lost coolant, and it continued to power up until it literally blew apart.

It would be the same effect as if you had an alternator running with the output disconneted and gave it full battery voltage on the field coil. I would be happy to demonstrate with someone else's alternator....

[CharlieJ]

Don-
The zapstop is there to shunt the spike to ground and probably save the alternator output diodes. The series fuse that is upstream of the reversed bias diode that is the heart of the zapstop is there to protect the conductor between the alternator's output terminal and B-. It is typically a 20 A fuse. If the battery voltage regulator does not reduce the field current when the regulator senses 0 A output, then, yes the 20 A fuse in the zapstop will open. And the output diodes will have probably been saved in the meantime.

Since the OP wanted advice on what size alternator OCPD to put in the output of his alternator and the zapstop is tertiary at best, I hope we have finished discussing zapstops.

Chernobyl and TMI...not so much.

[cal40john]

Originally Posted by CharlieJ

Don-
The zapstop is there to shunt the spike to ground and probably save the alternator output diodes. The series fuse that is upstream of the reversed bias diode that is the heart of the zapstop is there to protect the conductor between the alternator's output terminal and B-. It is typically a 20 A fuse. If the battery voltage regulator does not reduce the field current when the regulator senses 0 A output, then, yes the 20 A fuse in the zapstop will open. And the output diodes will have probably been saved in the meantime.

Since the OP wanted advice on what size alternator OCPD to put in the output of his alternator and the zapstop is tertiary at best, I hope we have finished discussing zapstops.

Chernobyl and TMI...not so much.

I attempted to point out earlier that I think the OP mis-interpreted his wiring diagram. The 30 amp fuse in a standard Yanmar harness protects the starter relay, alarms and lights. It does not carry the alternator output.

John

[Wotname]

Originally Posted by AaronJ

Hi, I have a 35A alternator on my Yanmar 2GM20F, according to the manual. It has a 30A inline fuse from the output to the battery. My question is why hasn't the 30A fuse blown? Does the alternator not actually put out 35 amps; is it weakened when converted from AC to DC?

Thanks,
Aaron

Originally Posted by David M

In that circuit you have two sources of current, the battery and the alternator. The idea is to put a fuse as close to the source as is practical to protect the wire further down the circuit. Given you have two sources of current in that circuit, you need two fuses.

Originally Posted by GordMay

I’d use a minimum wire size of #8 AWG (>8mm) for a 35 A alternator, which ABYC rates to 68A & over (NEC & I etc only rate it to 40A).
In my view (not necessarily that of other authorities) the prime purpose of fusing an alternator output is short circuit protection, which is a very high catastrophic fault current.

Accordingly, I be happy with a fuse, at the alternator, sized at anywhere between 40A and 60A, for a 35A alternator /w #8 wire.

Although I understand Charlie’s concern, I (FWIW) don’t view the battery as a power source to the alternator, and wouldn’t require a fuse at the load end (batt).

Originally Posted by cal40john

I attempted to point out earlier that I think the OP mis-interpreted his wiring diagram. The 30 amp fuse in a standard Yanmar harness protects the starter relay, alarms and lights. It does not carry the alternator output.

John

Not withstanding the possible mis-interpreted wiring diagram and the three pages of varying concepts of circuit protection, AM I MISSING SOMETHING.

I "know" I should know this but isn't there two power sources here connected by wire.

The alternator and the battery are both power sources. One (the alternator) can supply around 35 amps and the other (the battery) quite a lot more say several hundred amps for a short duration.

Given the primary function of the fuse is to protection for the wiring, we should size and postion the fuse accordingly.

If the wire can carry in excess of 35 amps then it doesn't need a fuse at the alternator power source as the alternator won't deliver much more than this before frying and then delivering none.

The battery end of the wire should be fused if there is any likehood of the wire becoming grounded either by faulty wiring or a dud alternator.

So given Gord's figures of an 8 awg wire between the two, I would put a say 50 Amp fuse near the battery end.

Or have I missed something basic??

[CharlieJ]

Wotname-

If the wire can carry in excess of 35 amps then it doesn't need a fuse at the alternator power source as the alternator won't deliver much more than this before frying and then delivering none.

The battery end of the wire should be fused if there is any likehood of the wire becoming grounded either by faulty wiring or a dud alternator.

So given Gord's figures of an 8 awg wire between the two, I would put a say 50 Amp fuse near the battery end.
Or have I missed something basic??

Only thing you missed is the ABYC Standard that requires an OCPD at each connection to a source.

Put it this way: if you have an electrical fire that is even remotely connected to this system, from an insurance claim perspective, wouldn't the prudent boat owner want to be in compliance with the standard?

Or, put it this way: why in the world has this discussion gone for four pages when the resolution will cost the Owner $35 per OCPD (see posting about MRBF upstream) and he can do it himself?

Or have I missed something basic??

[Rick]

What's missing regarding OCPD applications with reference to ABYC recommendations is that all alternators are current devices and, therefore, are by nature current limited and need no current protection at the source end!

As long as the wire is rated to carry continuous current equal to the maximum possible alternator current only ONE current protection device is needed (at the load or battery end).

[Wotname]

Thanks Rick, that is how I understood it but wasn't sure if I had missed some theory along the way