MTFCA Forum

I am not posting this saying not to put the main line fuse in, just some other ideas that might help. As has been said before by others and me, running on magneto and blowing the fuse, the car keeps running.

DYKE'S AUTOMOBILE AND GASOLINE ENGINE ENCYCL0PEDIA 1925
Fuse between generator and battery
If a complete open circuit should exist between fuse clip and fuse, and this fuse should be in series with the main charging circuit, between the generator and battery, this would have the same effect as disconnecting the battery from the generator. and if the generator regulation was means of a third brush, the result would be, at high speeds of the generator, that the generator would build up a high value and burn out the lamps and possibly the cut-out voltage winding and shunt-field windings, if the generator was run for a long period of time at speed. Thus the importance of of occasionally examining the fuse clips to see if they are clean and if they retain tight connection, must not be forgotten.
If a fuse should blown in the "main charging circuit" (1)between generator and battery, the same result would probably occur. For this reason the fuse is omitted in the main charging circuit on many of the electric systems using a third-brush regulated generator, unless there is a fuse in the "field-circuit" of the generator.

Shunt-Field Fuse
On almost all third-brush regulated generators, a fuse is placed in the "shunt-field" of the generator, in order to protect the generator in case an open circuit occurs in the "main charging circuit." (1)
In other words, if an open circuit (1) should occur in the "main charging circuit" when the generator was running at high speeds, the generator would build up a much higher output then normal. If a fuse is placed in the "shunt-field circuit," then the fuse will "blow" and open the "shunt-field circuit," thus preventing the generator from reaching and abnormal value-providing the "shunt-field fuse" is of proper capacity. If the fuse should be of a much higher value then required, it would not blow in time to prevent the generator building up, and the light would be burned out. Hence the importance of using the correct capacity of fuse in the generator "shunt-field winding," and a fuse of a large enough capacity, or none at all, in the "Main charging circuit." As already stated, a fuse is seldom placed in the main charging circuit from the generator to the battery (1) where a third-bush regulation system is used, and seldom in the ignition circuit.

(1) My note; when a fuse is placed on the main line, it would be in the the same as a fuse between the battery and generator on a Model T

Years ago I was riding in an A with a fuse between the battery and the rest of the electrical circuit. At some point the amp gauge started acting strange and then the engine began to run rough. The car made it home. Almost. When we looked to see what happened I first saw the ign points were fried and the point arm was very discolored and distorted. Then I noticed a fuse bracket on the starter. Fuse was blown. We determined when the fuse blew the voltage to the points spiked and burned them. After replacing the points and condenser the car would not start. The ammeter had opened due the uncontrolled voltage. I suggested that perhaps the fused circuit was not a good idea.

That is why my T’s run both safety electrical systems, a main fuse and a VR.