Piggybacking fuse holder?

[BabaYaga]

This photo below shows the 80 amp ANL fuse protecting my house bank circuit. On the right stud is the wire leading to the positive bus bar, on the left stud a very short wire coming from the positive battery post.
The second wire on the left stud is leading to an 'emergency switch', enabling the engine to be started from this bank, if necessary. Since this wire bypasses the fuse, the connection is unprotected.
I intend to put this right and because of space restrictions I am thinking of attaching a terminal fuse block like this:
https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A
to the left stud.
The reason for two fuses on the same bank is the much higher amperage fuse required for engine starting, in my case 250 A.
Is piggybacking one fuseholder to another like this OK?
Any thoughts and comments much appreciated!

 

[pvb]

Can't see why that shouldn't work OK.

 

[VicS]

Can't see why that shouldn't work OK.

+1

Since piggy backing the emergency starting connection anyway can't see any problems

 

[prv]

Seems reasonable to me too, though if the run to the commoning switch is short and well-protected (ideally all inside the battery bay) then I wouldn't see a fuse on it as mandatory anyway.

Pete

 

[lw395]

Seems like taking the high current path through unnecessary terminals to me?
Why not go battery > Fuse > Switch?
Or even battery > 250A fuse > 80A fuse?

I'm a bit sceptical of "emergency start from house bank" switches, I have some sympathy with the simpler school of thought that says swap the batteries with a spanner in the unlikely event your engine start battery lets you down. A switch that's unused for 3 years on a boat has a tendency to oxidise a little, then not pass 250A. For the price of all that complication, you could replace the engine battery well before it failed.

I generally don't trust very high rating fuses, a nominal 250a fuse can pass 500A for long enough to light up quite a lot of cable. Or it can blow at an awkward moment. Do the math as the septics say, is that fuse guaranteed to blow before anything pyro happens, or is it just decoration?

Might be nice to see the unfused terminal covered too?

 

[pvb]

I generally don't trust very high rating fuses, a nominal 250a fuse can pass 500A for long enough to light up quite a lot of cable. Or it can blow at an awkward moment.

Do you know much about them? Do you realise that a battery short can result in huge currents, enough to blow a 250A fuse in milliseconds?

 

[prv]

I'm a bit sceptical of "emergency start from house bank" switches, I have some sympathy with the simpler school of thought that says swap the batteries with a spanner in the unlikely event your engine start battery lets you down. A switch that's unused for 3 years on a boat has a tendency to oxidise a little, then not pass 250A.

It's an interesting point.

Fortunately my "emergency connection" (which I agree is unlikely to ever be used) is a very large relay which is opened and closed every time the engine is started and stopped, so it gets plenty of practice

Pete

 

[prv]

Do you know much about them? Do you realise that a battery short can result in huge currents, enough to blow a 250A fuse in milliseconds?

I guess the point is, what if a particular short only results in "quite large" currents rather than "huge" ones?

I don't know much about them, though, so I don't know how likely that is nor how much you can actually put down a 50mm² cable before it melts.

Pete

 

[pvb]

I don't know much about them, though, so I don't know how likely that is nor how much you can actually put down a 50mm² cable before it melts.

An awful lot!

 

[lw395]

Do you know much about them? Do you realise that a battery short can result in huge currents, enough to blow a 250A fuse in milliseconds?

Or not?
I know a little about fuses, enough to know you have to do the sums if you're depending on them. To blow a '250A' fuse in milliseconds needs the wiring to be really low impedance, if the battery is only 12V.
It pays to understand exactly what faults you hope to be protecting against.
It's best to start with the data sheet of the actual fuse you are considering.
And it's all moot if the short is upstream of the fuser, as invited by an uninsulated terminal....

 

[pvb]

Or not?
I know a little about fuses, enough to know you have to do the sums if you're depending on them. To blow a '250A' fuse in milliseconds needs the wiring to be really low impedance, if the battery is only 12V.
It pays to understand exactly what faults you hope to be protecting against.
It's best to start with the data sheet of the actual fuse you are considering.
And it's all moot if the short is upstream of the fuser, as invited by an uninsulated terminal....

I'm a big fan of MegaFuses. They protect; they work. On my last boat, I once foolishly replaced the alternator wiring incorrectly. There was an instant pfft and the power went off. The 300A MegaFuse on the battery circuit had blown, protecting the wiring, as it was intended to do.

 

[BabaYaga]

Thanks all for your comments so far. I think I can safely go ahead.

I am aware that the solution I am considering means one extra bolt connection (increased resistance) compared to going directly from the battery clamp to the fuse holder. Since the over all cable runs are pretty short and the cables 50mm2 I hope this could be acceptable.

I find the discussion about the merits of fuses and emergency switches interesting. It is not all black or white. But on balance I am in favour of both.
Shifting out battery cables by spanner is also a risk, especially in an emergency situation.

Agree about covering the terminals.

 

[Sarnia]

Can't see why that shouldn't work OK.

When I upgraded my old alternator you were good enough to give advice about the options available and the wireing methods etc. working very well now thank you.

I am fairly sure, will be able to confirm when I return to the boat, my wiring to the starter from the battery does not have an inline fuse, and as I have never fitted one would have been that way for some 12 years.
I have a OFF-DOM-BOTH switch, newly replacing the previous doublw switch arrangement. I have 360amp/h domestic, and a 80 amp/h starter. After starting I switch to domestic.
If I decide to install a fuse, what would you recommend.

 

[pvb]

When I upgraded my old alternator you were good enough to give advice about the options available and the wireing methods etc. working very well now thank you.

I am fairly sure, will be able to confirm when I return to the boat, my wiring to the starter from the battery does not have an inline fuse, and as I have never fitted one would have been that way for some 12 years.
I have a OFF-DOM-BOTH switch, newly replacing the previous doublw switch arrangement. I have 360amp/h domestic, and a 80 amp/h starter. After starting I switch to domestic.
If I decide to install a fuse, what would you recommend.

If you want to install a fuse, something like a 250-300A MegaFuse would be ideal for the starter circuit on most smallish boat engines.

 

[William_H]

It's an interesting point.

Fortunately my "emergency connection" (which I agree is unlikely to ever be used) is a very large relay which is opened and closed every time the engine is started and stopped, so it gets plenty of practice

Pete

This seems to me to be a very bad arrangement. If I read it correctly when you operate the starter motor the relay comes in to parallel the house battery to the engine battery. That will give you a very brisk start every time. But when the engine battery dies, as it will eventually, you would not know it because the house battery will take up the slack. So in effect no point in having an engine battery. You need a small switch in the coil circuit of the relay so that it only goes into this mode if engine battery dies. Normally starting on engine battery alone. If I read you correctly. good luck olewill