TR2 & TR3 Forum

domestic fuses are rated different than english,& i can’t remember how it goes.

Here's an explanation and conversion as supplied by Andrew Uprichard on another post:

Fuses:
A fuse works on heat; basically it "blows" when the fuse metal melts. So, for a small overload, it may take a long time for the metal to get hot enough to melt. American fuses are rated by the nominal current that they will carry "forever" (with surrounding air at 25C). But for some reason, Lucas decided to rate their fuses by the current that would make them blow "instantly". In fact, the original fuses usually contained a bit of paper inside the glass with the "continuous" rating marked on it, which was half of the "blow" rating on the outside. So, for example, a "35 amp" fuse would have the legend "17.5 amps continuous" inside. There are other reasons for differences between other UK and US fuses (the IEC rating method results in values even lower than the US method); but that is the big one that affects old British car owners. Here's a conversion chart that was once recommended by Bussman (an American fuse maker):
Bussman Conversion Chart
English Type American Standard Replacement
50 amp AGC 30
35 amp AGC 25
30 amp AGC 20
25 amp AGC 15
20 amp AGC 10
10 amp AGC 7 1/2
5 amp AGC 3

Thanks Andrew. Unfortunately for me, I still can't find what actual size fuses that I should use.

Steve

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68 TR250
57 TR3
Both regularly driven
49 Triumph 2000 Roadster project

i would get some 10-15 & 20 domestic fuses & go from there, if a 10 blows,go to 15

The short answer is -- yes, you can use a 30 amp or 35 amp AGC/SFE fuse or a lower rated fuse -- your choice. Using a lower rated fuse might result in spurious blow if it is in a circuit where continuous current flow is near the rated value.

Service Manual states:
50 amp fuse for horns, 35 amp fuse for items which can only operate with the ignition on. -- direction indicators, windscreen wipers, brake light, fuel gauge and heater. Those ratings are Lucas fuse ratings.

Difference between Lucas ratings and AGC/SFE ratings --
A 50 amp rated Lucas fuse is rated for up to 25 amps continuous current -- it will tolerate 25 amps continuous flow, it will blow at 60 amps instantaneous current.

All AGC/SFE fuses are rated at 35 VDC (unless stated otherwise). Using them in a 12VDC circuit means they will blow at a current rating a bit more than what their stated rating is. This is because Ohms Law Volts = current x resistance. The resistance value is set for the 35V condition. Using it at 12V means it will take about 35/12 = 2 to 3 times more current to cause it to blow as compared to if it were in a 35V circuit.

An AGC/SFE (American style) fuse rated for 25 amps will blow when instantaneous current exceeds 25 amps (subject to the above stated voltage difference).

A 35 amp rated Lucas fuse will tolerate 17.5 amps continuous current and will blow at 40 amps instantaneous current.

An AGC/SFE fuse rated for 20 amps will blow when current exceeds 20 amps (subject to the above stated voltage difference)

Roy



Edited 2 time(s). Last edit at 2021-09-29 09:38 PM by 60TR3-Red.

Roy this statement is incorrect. All AGC/SFE fuses are rated at 35 VDC (unless stated otherwise). Using them in a 12VDC circuit means they will blow at a current rating a bit more than what their stated rating is. This is because Ohms Law Volts = current x resistance. The resistance value is set for the 35V condition. Using it at 12V means it will take about 35/12 = 2 to 3 times more current to cause it to blow as compared to if it were in a 35V circuit.

You even stated in a previous message about bulbs that they blow due to higher current and that it is not the voltage. You were correct when you wrote that, so why would you think a fuse would be any different? A fuse has a fixed resistance which is very small within it's current rating (smaller amperage fuses have higher resistance). That value is fixed for all intents and purposes but probably does rise a wee bit as the fuse heats up. However, it is I squared R and time that determines the current at which the fuse blows, not the applied voltage. That is why fuses are rated in amps, not watts. Fred

Fred

You're correct. I misstated my statement. The current flow through the fuse depends on all the resistances in the circuit and the fuse will blow when the current reaches the amp rating regardless of the voltage. My writing conflated the fuse blow current with what current flows through a fixed resistance (that value of t&at current is determined by the resistance). A short circuit essentially has zero resistance so the current value will rise until something in the circuit opens the circuit.

A 30 amp fuse rated for 35 volts had a resistance of about 1.17 amps. 35 volts = 30 Amos x 1.17 ohms

If that fuse is the only load in a 12 volt circuit, the current would be about 10.25 Amos. 12 volts = 10.25 Amos x 1.17 ohms.

If a short circuit develops downstream of the fuse, it would blow when the current through the fuse reaches 30 amps.

getting a little confused, but a dead short is zero resistance, causing the fuse to blow?

This is my solution to protecting the red circuit on the early cars. I ganged all of the red wires in a common 6 way and then put the inline fuse between that and the switch as suggested. Thanks for everyone's help!

Steve

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68 TR250
57 TR3
Both regularly driven
49 Triumph 2000 Roadster project

So the postings have gone a bit off track.

But one last time, here's what happens when a short happens downstream of the fuse.

Let's say downstream of the fuse are 4 marker lamps. They are on the one circuit. That means the bulbs downstream of the fuse are 4 resistances in parallel. Maker lights are connected in parallel so if one bulb goes out, the others keep working unlike old style Christmas lights connected in series where if one lamp goes out, it opens the circuit and all the lights go out.

A typical 12 volt marker light has about 6 ohms resistance when operating. Whatever the cold resistance is, when the filament heats up within milliseconds to its glowing operating temperature, its resistance when glowing is higher. Anyway, it has about 6 ohms resistance when hot.. With 12 volts applied,, according to ohms law it will draw about 2 amps.

Ok, now 4 lamps in parallel have an equivalent resistance calculated as follows

1/Req = 1/R1 + 1/R2 + 1/R3 + 1/R4.

So the equivalent resistance of the 4 bulbs is

1/Req = 1/6 +1/6 +1/6 +1/6
1/Req = 4x 0.167 = 0.667
Req = 1/0.667 = about 1.5 ohms

Each bulb is drawing about 2 amps. The fuse is seeing the sum = 4x2 amps = 8 amps.

Now let's say one leg gets a short. We'll call that 0.01 ohms (almost zero resistance) to demonstrate the point.

Now, we have
1/Req = 3 x 0.167 + 1/0.01
1/Req = 0.5 + 100 = 100.5
Req = 0.001 ohms
Now, using Ohms law
12 volts = amps x 0.001 ohms
Amps = 12 / 0.001 = 12,000 Amps

So of course the fuse blows instantly when the current reaches 30 Amos protecting the circuit from the hundreds of amps stored on the battery.

Roy

I tried bringing it back to topic.....

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68 TR250
57 TR3
Both regularly driven
49 Triumph 2000 Roadster project

So the answer is that the fuse should be placed so that it protects as much of the wiring as possible which usually means near the source. Two days ago I was fitting one of my 6V CDIs to a friend's 1949 International pickup (6V, positive ground). The truck has 3 fuses for the entire truck and all 3 are metal inline fuse holders. The only thing preventing a short from the fuse holder is a thin piece of insulating paper. A fire waiting to happen especially with the crumbling original wiring harness. The horn had its own fuse which was 3 inches away from the starter lug where it gets power from the battery. We removed that fuse holder that was nearly impossible to access and put a fuse up near the horn instead after running new wire. So the horn circuit now has some unprotected wire on the supply side but at least the fuse can be removed quickly if the old wire going up the steering column grounds out the column making the horn come on when it shouldn't. I strongly suggested that he get some good quality bakelite fuse holders and ditch the other two metal cased ones as well. Fred

anybody remember fusible links?

Another option is a polyfuse.

doesn’t look like something that will handle a lot of amps?

Yes fusible links work too. The problem with them is the usually are usually in hard to reach places.

My modern car had one for the OBD--2 connector. It burned out so the connector would not work. The problem was it was not shown in any wiring diagram I could find so I could not figure out why the OBD-2 link was not working. Eventually I lucked out and saw an internet discussion. The fusible link wire was buried in a multi wire harness up under the dash in the vicinity of the brake pedal switch. Once I learned what the problem was, I jumped the burned out link with an online fuse.

A better choice would be a circuit breaker. At least then you can reset it. If it trips again you know to look for a fault and you don't have to keep buying replacements fuses.