BSS Inspection

[RonF]

Hi

Just had boat safty inspection, inspector passed it with some advice on one or two minor things, but what he queried was the fuse ratings which are as follows

Voltage sense 4 amp

Bowthruster Charge 70 amp

Cabin feed 70 amp

Central heating 40 amp

Alternator 150 amp

 

do these seem OK

Thanks

[Tony Brooks]

Hi

Just had boat safty inspection, inspector passed it with some advice on one or two minor things, but what he queried was the fuse ratings which are as follows

Voltage sense 4 amp

Bowthruster Charge 70 amp

Cabin feed 70 amp

Central heating 40 amp

Alternator 150 amp

 

do these seem OK

Thanks

 

 

No one can say unless you tell us the size of the smallest wire on each system. That is conductor cross sectional area in mm.

 

If he is querying the fuses then he can only do it by reference to cable size so he should know what to recommend.

 

 

Personally I think the cabin feed one a bit suspect UNLESS it is only protecting the cable from the battery bank to another fuse box.

 

I also feel the central heating one may be a bit too large but then I can not see the cable. I suspect this is a blown diesel combustion heater with a glow-plug.

 

Remember fuses protect the cables. If a piece of equipment needs protecting then the manufacturer should build that protection into the device.

[RonF]

Thanks for the reply. I'm not at the boat but will be going btomorrow. will check wire size.

[Tony Brooks]

Thanks for the reply. I'm not at the boat but will be going btomorrow. will check wire size.

 

 

Just a warning - that is not straight forward. I am not talking about the overall diameter or CSA but that of the conductor inside the insulation. There are two common types of cable - traditional PVC and what is known as tri-rate or thin wall cable. The latter has much thinner insulation.

 

You might be able to do it by taking known samples and comparing them otherwise it will be measure the diameter of a single strand (Could be 0.30, 0.20, or 0.40mm (or even 0.012"). Then calculate the CSA of the strand. Next count the number of strands and work out the total CSA of the conductors.

 

This is why I said the examiner should give the basis for his comment - he should be experienced in recognising common cable sizes.

[Chalky]

Just to pick up on Tony's comments the fuse / circuit breaker it there to protect the cable. The rating of the cable depends on its cross sectional area (CSA), the type of insulation, how its bundled with other cables and where it runs. Basically how hot its going to be when running and how hot it's going to get before the fuse blows (it should happen significantly before the insulation melts!).

The size of cable used on a boat is determined by a couple of things. The main one is not current rating but voltage drop - this is usually a more significant problem on a 12V system than the current carrying capability. It's possible that large cables have been used and then fuses fitted since it seems the right thing to do.

[Radiomariner]

Just to pick up on Tony's comments the fuse / circuit breaker it there to protect the cable. The rating of the cable depends on its cross sectional area (CSA), the type of insulation, how its bundled with other cables and where it runs. Basically how hot its going to be when running and how hot it's going to get before the fuse blows (it should happen significantly before the insulation melts!).

The size of cable used on a boat is determined by a couple of things. The main one is not current rating but voltage drop - this is usually a more significant problem on a 12V system than the current carrying capability. It's possible that large cables have been used and then fuses fitted since it seems the right thing to do.

 

But not the right thing to do. As mentioned earlier. The smallest wire in the system is the one to be protected. .

 

 

A 70A fuse would indicate that all the wiring in that circuit would have a greater than 70A capacity and the examiner would be immediately suspicious if the wiing looked a little "skinny" and will ask questions

[Tony Brooks]

But not the right thing to do. As mentioned earlier. The smallest wire in the system is the one to be protected. .

 

 

A 70A fuse would indicate that all the wiring in that circuit would have a greater than 70A capacity and the examiner would be immediately suspicious if the wiing looked a little "skinny" and will ask questions

 

 

And seeing "60 amp" cable is of a similar diameter to a pencil (or larger) I simply can not see that being used throughout the cabin wiring. There is too much information we do not have so anything we say is only conjecture. This is why I am surprised the BSS Examiner was apparently less than forthcoming with information.

 

The alternator one also looks suspicious because 150 amp cable (ignoring voltdrop which we must not on charging circuits) must have an overall OD of close to 10mm and that is getting on for car starter cable size which I rarely see on alternators. I also do not like the idea of fusing alternator main leads because of the danger of wrecking the alternator when they blow/make poor contact under load. But that is a question of deciding what risks one is willing to take and how to mitigate them I would prefer to keep a close eye on that cable being subject to chaffing.

[Bob18]

Be careful with reading cable ratings - "xAmp" cable is fairly meaningless, unless you know the construction (how many cores of what area) and the volt drop per metre.

 

At the top of this sub-forum there is a useful calculator, which some kind soul has pinned - it give a pretty good answer to what size cable you need for a given run. Just don't forget you have to include your return (negative) cable when working out the run.

 

 

(Where are those sample of 400mm "flexible" cable I was using as door stops?)

[Chalky]

But not the right thing to do. As mentioned earlier. The smallest wire in the system is the one to be protected. .

 

 

Agreed. The wording was meant to reflect what someone thought was correct rather than what actually is.

 

I also do not like the idea of fusing alternator main leads because of the danger of wrecking the alternator when they blow/make poor contact under load.

 

This is standard practice for several vehicle manufacturers.

[RonF]

went to check wire size, looked in owners manual first to see if size was in there answer NO. but fuse size in manual dont match whats fitted,

Bow thruster says 60 amp but 70 amp marked on fuse board

Cabin feed says 60 amp but 70 amp marked on fuse board

Central heating says 30 amp but 40 amp marked on fuse board.

So decided to look at fuse to see what was marked on them, all same as marked on fuse board, but two of the fuse holders broke so now i am fitting a new fuse board and will check the wire size before fitting any fuses.

[Chalky]

If you fit circuit breakers (of the correct type) there's no chance of the wrong value being substituted in the future...