Fuse Alternator Cable???

[Richard10002]

I have a Blue Sea Dual MRBF Terminal Fuse Block that I'm ready to fit.

 

http://www.rjsmarine.co.uk/2151-dual-mrbf-terminal-fuse-block-30-to-300a

 

However, I've ready that, if the fuse blows when the engine is running, and supply from the alternator is cut, the alternator will be damaged, possibly burning out and causing its' own fire. If this is the case, the cable from the alternator should be connected directly to the battery terminal, before fitting the terminal fuse block.

 

Is this correct?

 

If so, it seems to fly in the face of the stuff I've recently become aware of, which relates to fusing cables within about 6 inches of the battery. This is to minimise the risk of a short, where the battery supplies a current exceeding the cable rating, thus starting a fire.

 

If the alternator cable isn't fused, and shorts, surely it catches fire with the same consequences as any other cable?

 

On the rare occassions that a short might occur, wouldn't we prefer a damaged alternator to a cable fire that could destroy the boat, and kill someone, (Me!!).

 

Just trying to make some sense of things

 

 

[Tony Brooks]

Burning out and causing a fire? Highly unlikely. Causing a voltage spike and cooking the electronics perfectly possible but may not.

 

Alternator direct to battery terminal - OK and possibly a good idea if you are a sea boat. On the inland waterways if interpreted strictly a BSS fail. I put mine on the switched side of the isolator but I bet if I put it on the battery side few examiners would check it. If my isolator is switched off with the engine running then the alternator might be damaged.

 

We had some Broads boats once with fused alternator main supplies but I would not do it. If you wire your boat in a way that protects the cable then how many instances of alternator wires shorting do we hear or see?

 

Also consider any connection in close proximity to the battery (even sealed ones) has a greater chance of corrosion.

 

Remember that on split charge boats far too many have the main alternator output wired sub-optimally if a relay is used for the splitting. In that case the main output would ideally to to the domestic battery, not the engine battery via the starter.

 

[nicknorman]

Normal practice is to not fuse alternators. The BSS requires the alternator to route through the battery isolator, but I don’t like that for 3  reasons: unnecessary voltage drop, risk of turning the isolator off and trashing the alternator, the fact the having turned off the isolator, all the boat’s services remain live if the engine is running. I have the alternator direct onto the battery side of the isolator. If a BSS person doesn’t like it I can move the connection to the other terminal.

 

As to the “common sense” part, yes the BSS rules do result in battery live cable being unprotected, however it is normally a short run of  cable and  visible. I took care to ensure it was installed in such a way that the chances that of damage, chafing etc was extremely remote.

[Richard10002]

OK... I'm certain that my alternator cable connects to the switched side of the battery isolator.

 

Can I fit my new fuse block, then fit any cables currently connected to the terminal, to the new block?

 

Any reservations?

 

I chose a dual block so I could have a 300A fuse for the inverter/charger cable, and a 70A fuse for the other cables.

Edited July 13, 2018 by Richard10002

[nicknorman]

That sounds fine. Assuming you mean to leave the alternator as is.

Edited July 13, 2018 by nicknorman

[Richard10002]

5 minutes ago, nicknorman said:

That sounds fine. Assuming you mean to leave the alternator as is.

Yes... and thank you

[Chewbacka]

Same as Nick, I don’t like having the alternator connected to the boat side of the isolator switch so I wired the alternator direct to the batteries with an isolator switch, meets the BSS requirements and you don’t even need to ever switch it off.