Chicagoland MG Club: Driveline August 2017
Technical Stuff
Confused about Fuses?

KEN KYLE, VICE PRESIDENT of the Positive Earth Drivers Club in Central New Jersey

For years Iíve heard people say that British (Lucas) and American fuses (the tubular glass ones used through about 1981) are not interchangeable. This has always struck me as odd. Lucas and American fuses are the same diameter, 1/4 inch, and almost the same length (15/32 inches for Lucas, 11/4 inches for American AGC-type fuses) so they are in fact physically interchangeable. And amps are amps, right? Well...yes and no. Itís true that the ampere is the international unit of electrical current, so an amp in Coventry is the same as an amp in Detroit. However, itís the way the fuses are rated that makes them different. British fuse ratings indicate the amperage at which the fuse will immediately blow. So, if your LBC ownerís manual says to use a 35amp fuse in a given circuit, that means the manufacturer wanted you to use a fuse that will blow pretty much instantly as soon as it sees a current flow of 35 amps or more in order to protect your carís wiring from damage. American fuses, on the other hand, are rated to indicate the maximum current flow they can withstand without blowing for a specified period of time, usually 60 minutes. As a general rule of thumb, it takes about twice the rated amperage of an American fuse to make it blow instantaneously, so a 35amp American fuse would be roughly equivalent to a 70amp Lucas fuse, if they made them that large (which they donít).

Now consider what could happen if you replaced a 35amp Lucas fuse in your LBC with a 35amp American one. Say your car experiences a short or an overload of some kind in the affected circuit, and the current flow shoots up to 50 or 60 amps. That sturdy American fuse will hang in there for a time while your wiring harness goes *poof!* and the smoke escapes from it. Iím sure Iím not the first person to think of it, but this scenario could explain at least some of the escaping smoke incidents weíve all witnessed or heard about in LBCs.

So, can you use American fuses in your LBC, or do you have to chase down the proper British ones? The answer is yes, you can use American fuses if you carefully follow a couple of important guidelines:

 

First and foremost, make sure the American fuse you select has a current rating of no more than half that of the specified British fuse you are replacing. For example, if your manual says to use a 35amp Lucas fuse, substitute an American fuse rated at no more than 17 amps.

 

Second, use an SFE-type fuse if possible. SFE fuses are faster acting than similar looking AGC-type fuses so they provide a bit more protection. The only problem with using SFE fuses is that their length varies with their amperage rating (which was done purposely to prevent the wrong fuses from being installed in American fuse boxes), so most sizes will be too long or short for your Lucas fuse box as all Lucas fuses are the same length regardless of amperage rating. In those cases, you can use AGC-type fuses of the appropriate amperage rating, since all AGC fuses are the same length. AGC fuses are also fast acting, just not as fast as the SFE type, and come in 32volt and 250volt ratings, either of which is suitable.


Lucas style paper filled fuses are readily available from most of the parts suppliers that we deal with to maintain our older British rides. As with many other parts they may no longer be made in England however. Several sources we checked with knew this but said despite that these new fuses function correctly. If you are looking for a source, fuses of all amperages are available from Moss Motors 800-667-7872, Engel Imports 800-253-4080, and The Roadster Factory 800-234-1104.

If you prefer the real British Lucas paper filled fuses they are easy to source as well, new old stock (NOS) on eBay. Some sellers test and guarantee these items, others are sold as is. In conclusion, back to the point of Ken Kyles' article. It could be dangerous to "over fuse" your LBC, something many of us have been unwittingly doing for years.



Editorís Note: The following article by Ken Kyle appeared in the October 2016 issue of The Terminal Post, the newsletter of the Positive Earth Drivers Clu of central New Jersey. This could be very important safety information for many of us with LBCs who ...had no idea!! With the generous consideration of Mr. Kyle here is his fine article in its entirety.

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