Fuse 35amp melted holder, why did it not blow?

auldblue said:

Well , I was kind off thinking along those lines but I bought them in an auto parts store and the pack just said 35amp glass car fuse .
Jg Cheers guys , I think I will just treat the fuses I have as continuous rating till I get my new ones in , thank you.

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Bit of a giveaway here- I think the BLUE paper means 35 amp continuous which is a big NO NO.

Deets55 said:

It has always been my understanding that wire size dictates fuse size. The fuse is only there to protect the wire. When a circuit is designed, load is calculated, which determines the wire size, then the fuse matches the wire. Case in point, your (USA) house may have 220 volt and 200 amp service. The main breakers protect the panel box, each sub circuit has a calculated load with wires to match and then a breaker to protect the wire. i.e. 15 amp circuit gets a 14 gauge wire, etc. I am guessing Norton (and probably other companies) went with the slow blow fuse to compensate for surges. With only one main fuse it probably would not take much to pop a fuse when riding with lights on and then hitting the brake and horn at the same time, or something similar. Pete

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FYI the horn does not go through the fuse box on most British bikes so the horn is irreverent.
The fuse should by rights be of a amp rating of the highest rating of the respective load- the manufactures got lazy and started to put 35 Amp fuses (Not continuous) in nearly all the fuses on some models both bikes and cars.
I strip them out and change them for one more suitable to the load.
On one bike i had, the regulator went and the alternator pumped out 36 volts ….. it cooked the battery, fried the light switches and didn’t even blow the main fuse, so that was expensive.

Stephen Hill said:

Not mentioned so far is that fuse holders also have an amperage rating. There are a lot of junky light plastic fuse holders that are not be adequate for a 25 or 35 amp fuse. A higher amp fuse holder will have heavier wire, better connections, and a higher temperature plastic such as bakelite. Here is one example: http://www.wiringproducts.com/inline-he ... 10-ga.html
This could be an explanation as to why the holder melted but the fuse did not blow. Just a thought.

Stephen Hill

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On examination of the problem ( at the roadside ,in a downpour) a passing motorcyclist went to town and brought a new fuse holder( cheap and nasty but glad to get it ).When it was fitted , with a new fuse similar to the one in the photo(had it with me), smoke was being emitted from the reg/rec, when tested with a multi meter ( another passing motorcyclist, many thanks to both men) it was found to be the fault and when disconected readings returned to normal, no heat ,no smoke, bike ridden on total loss. No further problems were encountered.
It was not the condition of the fuse holder or the fuse or its connections that caused the problem if it had been then as Concours stated earlier why did it not return to normal service until after the reg/rec was disconected .
I only noticed the problem when the bike stopped running and the fuse was the first thing I looked at. Got some 15 amp fuses yesterday fron fat Tam at the Auto shop and am good to go. It was my own fault for assuming that if a fuse is marked for blow and not continuous rating , but I did catch the ferry and had a great couple of days in Armoy.

Jg

Bernhard said:

FYI the horn does not go through the fuse box on most British bikes so the horn is irreverent.
The fuse should by rights be of a amp rating of the highest rating of the respective load- the manufactures got lazy and started to put 35 Amp fuses (Not continuous) in nearly all the fuses on some models both bikes and cars.
I strip them out and change them for one more suitable to the load.
On one bike i had, the regulator went and the alternator pumped out 36 volts ….. it cooked the battery, fried the light switches and didn’t even blow the main fuse, so that was expensive.

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Bernard,

I was using the horn as an example of how you can get a surge thru the wiring, and why a slow blow fuse might have be used. I follow your posts so I know you have more experience with this than I do, so I don't doubt what you are saying. I only had two British bikes in my life, one was a long time ago and the other is a MKIII, which I own now. If I recall correctly my wiring diagram shows the main fuse powers the ignition switch and everything else, including the horn runs through it. I have since run seperate wiring and a relay for the horn to take the load off of the switch.
I don't know of anyway to catch a run-away alternator, almost every one I have seen goes directly to the battery, and then feeds the car/bike system, and usually its a diode that would go bad producing less voltage. Unless it is a mechanical regulator, (or a bad Zener). I have seen those go bad and play havoc with the electrical system. Thanks for the correction.

Pete

For the american norton owners
FWIW:
According to the Norton factory schematics, the horn IS in the circuit : typical 3.5A
The whole bike while running, runs off the alternator.
If at slow RPM the battery supplements or if stopped the battery does the whole load through the fuse including the horn.

I use 15amp fast blow MAX ABC15, AGC15, 314015,
ceramic or glass about 50 cents a piece through industrial supply network
You can even source a nice fuse holder

Here is your resource if you care to be educated...I had to be very familiar with this for my entire career/trade
If you dig enough, You can even find the blow time curves including the slow blow to see why your harness or holder will melt if you oversize your fuse

Buss fuses:
http://www.cooperindustries.com/content ... ramic.html

Littlefuse:
http://www.littelfuse.com/~/media/elect ... et.pdf.pdf

Bernhard said:

FYI the horn does not go through the fuse box on most British bikes so the horn is irreverent.

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The horn circuit certainly is on the fused side on all Commandos (except '68 - '70 which have an ammeter) and later British bikes.

Bernhard said:

On one bike i had, the regulator went and the alternator pumped out 36 volts ….. it cooked the battery, fried the light switches and didn’t even blow the main fuse, so that was expensive.

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Fuses blow due to Amps, not Volts.

How about one of these in place of the usual glass fuse holder? Maybe not OEM, but arguably better protected from the elements with its attached rubber cap.



I just found one at NAPA with a way-cool feature; it has an LED on its side that lights up when the fuse blows! https://www.napaonline.com/en/p/NW_785207 12ga wire, so it's good for 30 amps continuous, and only $4.50 USD.

Nathan

Nater_Potater said:

How about one of these in place of the usual glass fuse holder? Maybe not OEM, but arguably better protected from the elements with its attached rubber cap.

I just found one at NAPA with a way-cool feature; it has an LED on its side that lights up when the fuse blows! https://www.napaonline.com/en/p/NW_785207 12ga wire, so it's good for 30 amps continuous, and only $4.50 USD.

Nathan

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I have that NAPA one with the LED. It's pretty neat. I was working in the headlight shell and didn't even realize I popped the fuse, but the light caught my eye. Saved me some aggravation.

Pete

L.A.B. said:

Bernhard said:

On one bike i had, the regulator went and the alternator pumped out 36 volts ….. it cooked the battery, fried the light switches and didn’t even blow the main fuse, so that was expensive.

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Fuses blow due to Amps, not Volts.

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of the external system...

The fuse blows because it is a mini self destructing heater.
The heat comes from amps driven by the difference between the battery and drain or charging supply (bike) The volts is the microscopic voltage drop across the fuse it self being between the two giants battery & bike charging or load.
When the amps x volts (across the fuse) = watts
When the watts exceeds the rating, the alloy melts.
In Bernards case the battery does not act like a dead short and is slowly cooked into oblivion, yet the actual peak current never made it hot enough or long enough to blow.
That is why a fuse can work on 6V, 14.3V or 120VAC