New BSS Requirement: 25mm2 cables.

[Ubergeekian]

The basic issue is that a short circuit down a long length of thin wire will potentially create enough heat for a fire.

I don't think it's quite that simple. Given a fixed run length, the resistance is inversely proportional to the cross-sectional area. Since power = V^2 / R, power developed is directly proportional to the cross sectional area: the bigger the wire the more the heat.

Against that, larger wires dissipate heat better, but that is generally proportional to diameter. As a result the equilibrium temperature tends to rise with wire diameter. In a short circuit, fatter wires get hotter.

Edit: this also assumes fixed supply voltage. Since lead acid batteries have low internal resistance, that's reasonable.

 

[mjf107]

I don't think it's quite that simple. Given a fixed run length, the resistance is inversely proportional to the cross-sectional area. Since power = V^2 / R, power developed is directly proportional to the cross sectional area: the bigger the wire the more the heat.

Against that, larger wires dissipate heat better, but that is generally proportional to diameter. As a result the equilibrium temperature tends to rise with wire diameter. In a short circuit, fatter wires get hotter.

Edit: this also assumes fixed supply voltage. Since lead acid batteries have low internal resistance, that's reasonable.

Yes.....But larger cables as you have stated have less resistance so therefore they can allow more power to flow in any given time,, thus the fuse circuit breaker operates quickly before the cable heats up by any significant amount. Conversely with significant over sizing of the fuse cable relationship you could arrive at a situation where the protective devise would not operate because of the resistance and voltage drop in the cable, this situation could happen with a long run of cable with a short near the far end of the cable run, the cable would then heat up ........... IxV Watts would give you the power/heat that would be dissipated by the cable.

Michael,

 

[Joker]

Since power = V^2 / R, power developed is directly proportional to the cross sectional area: the bigger the wire the more the heat.

Sorry. Schoolboy howler. The V in question is not the battery voltage, but the voltage drop across the cable itself. Low resistance cable, low voltage drop. This is why the filament of a light bulb gets hot but the connecting wires do not.

Better in this case to use I^2R for the power dissipated in the cable itself.

 

[Bilgediver]

I had my BSS inspection and the surveyor told me that in future all battery cables will have to be 25mm2. I have not been able to find more details, however, the following are my own thoughts.

Does this refer to new boats and those being rewired or updated?

Is this because damaging a thinner wire that is not fused could cause a fire throughout the length of wire, perhaps 20 ft or more? With a thicker wire a fire will be confined the the damaged area.

I intend to rewire Pendragon. As far as I can see this would mean terminating the 25mm2 cable in a main fuse. Each wire from this fuse to the fuse box will need to be capable of carrying the full current of the main fuse.

May I suggest you read up some books and do some research as your final paragraph could result in a recipe for disaster and give your insurance company sleepless nights if they read it.

The 25mm 2 wire is sized to carry the total output of the battery bank including starting capacity and the hotel load. Sometimes a high amp fuse is fitted adjacent to the battery but not always and its size is determined by the starting current requirements and usually terminates at the engine starter having come through the selector switch .from this point the feed to the distribution fuse box would be a smaller sized wire sized on the requirements of the boat and typically on many small boats something around 35/50 amps rating is suitable for the feed to the fuse box. The fuse box will contain the switches required to isolate the different on board services such as NAv lights, anchor lights, VHF . gps/plotter, cabin lights, bilge pump, fresh water pump . Each of these circuits will have a fuse or circuit breaker sized to suite the specific service and usually may be anything between 5 amps and 20 amps . You would not rely on the main supply fuse to protect these individual circuits as the circuitry would burn out first unless oversized.

If you come back with your ideas of what equipment you wish to fit in your boat and how many battery systems you want then you will receive good advice here however make sure you size the wire and fuses to suite each of the respective circuits used and not just one catch all fuse which my not offer protection due to its rating.

 

[Ubergeekian]

Sorry. Schoolboy howler. The V in question is not the battery voltage, but the voltage drop across the cable itself. Low resistance cable, low voltage drop. This is why the filament of a light bulb gets hot but the connecting wires do not.

Thank you, Georg Ohm. Oh, how I wasted those years researching electrical power systems. Or maybe not...

I don't think it's quite that simple. Given a fixed run length, the resistance is inversely proportional to the cross-sectional area. Since power = V^2 / R, power developed is directly proportional to the cross sectional area: the bigger the wire the more the heat.

Against that, larger wires dissipate heat better, but that is generally proportional to diameter. As a result the equilibrium temperature tends to rise with wire diameter. In a short circuit, fatter wires get hotter.

Edit: this also assumes fixed supply voltage. Since lead acid batteries have low internal resistance, that's reasonable.

 

[Joker]

Are you assuming there's only one wire in the circuit?
If so, then yes.
Are you saying then that thicker wires are more dangerous than thin?

 

[Ubergeekian]

Are you assuming there's only one wire in the circuit?
If so, then yes.

Since I was discussing short circuits in reply to a post about short circuits then yes, clearly.

Are you saying then that thicker wires are more dangerous than thin?

They certainly can be. Would you rather short a car battery with a bit of 16 swg or a spanner?

 

[Joker]

Since I was discussing short circuits in reply to a post about short circuits then yes, clearly.

They certainly can be. Would you rather short a car battery with a bit of 16 swg or a spanner?

Let's make our boats really safe by having very thin pieces of wire.

 

[huldah]

Thanks for the interesting replies.

When Pendragon was purchased the wiring was seriously unsafe. Many switches were connected with twisted wire. There was also a large tangle of wire, much not connected. Tracing wires was a nightmare, but eventually everything was safe. There was also a lot of redundant instruments.

I intend to update both engine and domestic circuits.

I like VicS's suggestion of a fuse near the battery, though I have now found a way to connect 25mm2 cable to my fuse/circuit boxes.

One thing we have not yet discussed is the alternator battery charge cable. If we are talking safety, then this wire needs to be protected at the battery end as well. The alternator would die if this fuse was to blow!

I am not an electrician, but I have spent many decades problem solving complex equipment. When finalized the job will be done correctly.

 

[pvb]

One thing we have not yet discussed is the alternator battery charge cable. If we are talking safety, then this wire needs to be protected at the battery end as well. The alternator would die if this fuse was to blow!

But if the cable is sized to handle more current than the alternator can produce, and if the fuse is sized to protect the cable, it won't risk damaging the alternator unless there's a serious fault. And, in this eventuality, would you rather blow the alternator or set fire to the boat??

 

[huldah]

Yep PVB, I agree.

 

[LittleSister]

I am certainly no expert on this, but as I read it much of the above (and what the examiner is quoted as saying) is based on an oversimplification of the Boat Safety Scheme (BSS).

The scheme has a list of General Requirements, and it is only these which are stricly obligatory. These are brief and very general. The relevant one for the cable size issue is simply 'All electrical systems must be designed, installed and maintained in a way that minimises the risk of explosion or of fire starting or spreading'

You will normally meet these requirements by complying with the detailed expectations set out in the guide, e.g. the 25mm cables discussed above, and this is what the examiner will be looking for. However, the scheme does make provision for the possibility that you can meet the General Requirements another way, albeit the process for this is rather bureacratic. You can get the BSS office to assess whether an alternative meets the General Requirements by submitting 'documentary evidence' (see Section 1.17 of the Guide).

The examiner has to examine against the standard expectations, but, it seems to me, should be aware and ready to tell you that there are potentially other ways of meeting the General Requirements, even though they can't sanction them themselves.

 

[Ubergeekian]

Let's make our boats really safe by having very thin pieces of wire.

I'll go for having the right sized pieces of wire, myself. Of course when a circuit is fused or otherwise protected the wires can be as thick as you like.

 

[lw395]

Let's make our boats really safe by having very thin pieces of wire.

We call these thin bits of wire 'fuses' in the volts and sparks trade.
I thoroughly recommend them. Being a luddite at heart, safety devices with no moving parts have a certain elegance.

If you want to BSS your boat, fit a second 'master master' switch very close to the battery.
Then the only nonsense fat cable you may need is a return from battery to engine. Not a bad thing to have anyway.

A master switch very close to the battery is quite sensible, it means you are more likely to isolate the battery before removing it.
Ideally the code would allow a fuse, sized for the downstream cable, instead, but if every boat had to be analysed from first principles, the cost of inspection would go through the roof.
It's cheaper to fit that isolator than to take up the examiners time with explanations of why your unique system is safe.