Fuses and Fuse Box Inconsistencies

[Bainford]

I have a Federal Special with a (mostly) un-modified electrical system and the original four circuit fuse box. I am blowing fuses in one of the circuits, which has led me to query which fuses are supposed to be in the fuse box. I can't find this information in the shop manual or the circuit diagram. The Owner's Handbook says 35A fuses for all four circuits, but that sounds like far too big a fuse (the fuse box is a 35A box, and I wonder if this resulted in an error in the Handbook). Currently (and for at least the last 8 years) the box has had 17A fuses on three of the circuits and 35A on the other, and until recently the 17A fuses have held up, further suggesting that 35A for each circuit is too much. Additionally, the three 17A fuses are old school Lucas fuses, which could be original since it is unlikely (though certainly not impossible) a replacement fuse sourced in North America would be Lucas.

Curiously, and further complicating the matter, the four fused circuits on my car are not connected to the fuse box in the same order as they appear in the circuit diagram, so any listing of fuse sizes must be used carefully, as must the tracing of circuits.
As can be seen in the diagram, the input leads (colours) to the fuse box from L to R are; RG - N - Y - W, whereas my car is wired RG - W - Y - N.
No biggie, really, but one must be diligent.

Anyway, a lot of words just to ask, what fuses are supposed to be fitted?

[EuropaTC]

My (paper) manual for the Europa TC says 35 amp fuses on the wiring diagram. The text mentioning fuse boxes harks back to the S2 in that curious mixture of old and new that forms the TC manuals, and that also says 35amp. (but only 2 of them by the looks    )

I guess protecting equipment wasn't the prime concern and just blowing a fuse to stop a fire was the height of sophistication back then.....   

Brian

[RoddyMac]

I think fuses were discussed lately either here or on the FB page.  The Lucas fuse is a 35 amp blow with a constant of 17.  Here's a link to a write up:

http://www.mgtoronto.com/pdf/Tech/fuses.pdf

[jbcollier]

Yup, common British car problem.  British fuses are rated at instantaneous blow while SAE fuses are rated a max continuous current.  As Rod has pointed out, this means that it is very easy to fit a 35 continuous fuse (50+A blow) instead the proper 17 amp continuous fuse (35A blow).  Whatever caused the fuse to blow in the first place will probably not pop a 50A fuse so then the wiring melts.

Can't find the correct British 35A fuse?  Fit an SAE 15A fuse.

[4129R]

I would have thought the lights would be drawing the most power. So if the two headlights are 55w each, 110w is as near to 9 amps as makes no difference. 15 amps continuous would not blow anything if it were working properly.

The window glass motors would be next on my list, and then the windscreen wiper motor.

There is not much left that draws much power, that I can remember.

[Bainford]

I think fuses were discussed lately either here or on the FB page.  The Lucas fuse is a 35 amp blow with a constant of 17.  Here's a link to a write up:

http://www.mgtoronto.com/pdf/Tech/fuses.pdf

Roddy, HUGE thanks for posting that link! I never knew this, but it explains everything I was confused about.

This disparity in fuse rating systems is news to me, but extremely important to be aware of. I have forwarded the link to the guy who prepares the news letter for our local British car club. Most Brit car enthusiasts may be aware of this, but for those who aren't it could someday save their LBC.