TR6C wiring problem (continuation)

[Bob Kellock]

Making those changes didn't cure the problem of the seat shorting to the battery but, at least, it only resulted in burning out an easily replacable cable rather than an expensive loom and allowed me to find the cause of the problem.

Bob

[Rex]

Some worthwhile mods there, but have you considered both reducing the fuse rating from 35A (which is much too big) and fitting a fuse in the other battery line, ie one each in both pos and neg wires?

The factory looms were cheap and primitive, but there's no need for wires to be burning in 2009.

[L.A.B.]

but have you considered both reducing the fuse rating from 35A (which is much too big)

That depends how you interpret 35A?

As the old British way of rating auto fuses was to use the BLOW RATING.
So a "35A fuse" as specified for many British bikes actually means 35 Amp blow, which is the equivalent to 17A-17.5A CONTINUOUS. (some of those 1-1/4" glass fuses are marked with both ratings) but also note that the 25mm glass fuses as used on many Japanese motorcycles are continuously rated.



So 35A only applies to the old 1-1/4" British glass fuses as all modern fuse ratings are listed by their continuous rating, and the nearest equivalent would normally be either 15A or 20A such as the modern blade type fuses commonly used these days.

    

[L.A.B.]

Where there has been a problem with the seat pan shorting the battery terminals, always fit a battery that has recessed terminals, and one that has terminal screws/bolts that fit horizontally to the battery posts. Also, modern equivalent batteries can be slightly taller than the Lucas originals, so an extra rubber battery mat can be placed on top of the battery and secured by the battery strap.

And always (on Triumph twins) fit the battery with its terminals facing towards the rear of the machine.

[Rex]

Not disputing what you say re fuses, but to put it at it's most simplistic, if wires are burning and the fuse ain't blowing, the rating is too high!

Sadly the old auto fuses (as in the pic) always were the most insensitive fuses made; nothing more than a nod to "it's got a fuse" thinking. Presumably they're now knocked up in a Taiwan back street, and the sensitivity/accuracy is anyone's guess. The rupture current could be 20A or it could be 60A, who knows?

Adding up all the load on a TR6C's system, and it's nowhere near even the quoted 17.5A, so my point remains the same, ie add more fuses and use suitably rated fuses too.

[L.A.B.]

Not disputing what you say re fuses, but to put it at it's most simplistic, if wires are burning and the fuse ain't blowing, the rating is too high!

From what Bob actually said originally, the seat was shorting the negative battery terminal to earth, therefore the fuse would not have been protecting the positive wire, which was the one that burnt out.

Also a 35A blow fuse shouldn't result any damage being done to the wiring if it a dead short did occur in the system (I certainly know this from experience!) although I do agree it makes more sense to fit fuses to protect individual circuits.

Adding up all the load on a TR6C's system, and it's nowhere near even the quoted 17.5A,

A typical late 60's Triumph would have:

50W Headlamp main beam
6W Pilot/Park (stays on with headlamp as far as I know?)
5W Tail light
21W Brake light
3W Speedo light
3W Tacho light (if fitted?)
2W Ignition warning light
2W High beam warning light  
25W+ (approx) Ignition system
60W+? Horn
_________
= 177W

Theoretically, all those circuits could be in operation simultaneously, so would draw around 14.7A of current 177W/12V = 14.75A).
And with the normal extra (20%?) safety margin added it would become 17.7A continuous.

So a 17A continuous (35A blow) fuse would be about the right amp rating I think?  




  
 

[Bob Kellock]

After the meltdown I switched to a 10A quickblow fuse which was fine but I didn't try the horn with it. Now that all is well I'm running it  with a 16A type T (slow blow) which I guess is not far from the Lucas 17/35.
With the seat touching the battery neg,  the fuse doesn't come into it. The circuit is battery neg, seat metal & frame, earth wire in loom from Zener mounting to rectifier stud and, then, from rectifier stud to batter pos.
If there'd been a single 30A  wire directly from the stud to the frame that would probably survived; OTOH if it fused then the earth wire in the loom would have rapidly followed.
I've now removed all terminations to the (red) earth wires in the loom - circuit diagram is attached.

Bob

[Rex]

From what Bob actually said originally, the seat was shorting the negative battery terminal to earth, therefore the fuse would not have been protecting the positive wire, which was the one that burnt out.

All the more reason to have multiple fuses, ie one for each battery pole.

A typical late 60's Triumph would have:

50W Headlamp main beam
6W Pilot/Park (stays on with headlamp as far as I know?)
5W Tail light
21W Brake light
3W Speedo light
3W Tacho light (if fitted?)
2W Ignition warning light
2W High beam warning light  
25W+ (approx) Ignition system
60W+? Horn
_________
= 177W
Theoretically, all those circuits could be in operation simultaneously, so would draw around 14.7A of current 177W/12V = 14.75A).
And with the normal extra (20%?) safety margin added it would become 17.7A continuous.
So a 17A continuous (35A blow) fuse would be about the right amp rating I think?  

You forgot the diversity factor. While theoretically possible to have everything drawing current at once, in reality it's extremely unlikely, so, just as in domestic and industrial systems, the fuse can have a lower rating.
The total steady load for a night ride is reckoned to be about 90W so a continously rated fuse of 7/8A would be sufficient. It wouldn't need a 20% safety margin either, as the rupture current (by design) is much higher, as you've already mentioned.