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MG MGB Technical - 50 amp fuse

I recently installed three relays in my 1975 mgb. One for the low beam headlights, one for the high beam headlights, and the horn. All went well and after it was finished I deceided to check the fuses. The lowest fuse on the four fuse block (one side has brown wires the other has solid purple wires) had a 50 amp fuse in it. It read "50 amp uk". I have changed it to a 30 amp fuse, but could this 50 amp be right or did the dpo strike again. Could this have been in since new? thanks for your input. Karl
kra Karl

Karl - What you are seeing is British style fuse rating. They normally come with two ratings, such as 17/35 amps. What that means is that at 17 amps, the fuse will hold indefinitely, while at 35 amps the fuse will blow withing a few seconds. anything in between will blow the fuse quicker and quicker, depending on how close it gets to the maximum rating of the fuse. I would guess that the 50 amp fuse you have in the car has a second rating of somewhere between 23 and 25 amps. In American fuses, the 3AG fast acting fuse is the closest to the British fuse ratings. The 3AG fuses are rated at the lower amperage level where the fuse will not blow. This means that a 25 amp 3AG fuse will hold a 25 amp current for a minimum of 4 hours, but will blow in a maximum of 20 seconds if the current is 200% of rated current level. If the 50 amp British fuse is the correct one for your car, then you would want to use a 25 amp 3AG American fuse. Most MGs use a 17/35 amp British fuse for everything except the horn circuit, which uses a 25/50 amp British fuse. Cheers - Dave
David DuBois

50 amps is too high for these circuits, they should all (including the horn on the purple circuit for the MGB at least) be 17amp rated 35amp blow, as the fuse is there to protect the wiring, and larger fuses would only be used with heavier gauge wiring and smaller fuses with thinner wiring as is normal on modern cars. There is one exception and that is the in-line fuse for the sequential seat-belt system which is 500mA presumably to preotect the electronic module in this case. That's standard fuses. If you fuse the headlights there should be one per filament i.e. four fuses in all to avoid a short at a headlight plunging you into darkness. I'd recommend 15A, and if you fit a main supply fuse that should be double that at least.
Paul Hunt 2010

Paul and David Thanks for the advice. I found the fuses I needed at Radio Shack.
kra Karl

This thread was discussed between 24/04/2010 and 26/04/2010

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