dynamo fuse or not?

[andrew34ruby]

It has been said many times that the dynamo is not safe for more than maybe 8 or 10 amps. So when building a new loom I fitted a fuse of ten amps to protect the dynamo, in the output from the D terminal. But this week I read that the dynamo should not be run with nothing connected to D.

How would I know if the fuse blew? And would it then damage the dynamo?

[Bob Culver]

Fuses do not protect against the slight constant mild overload which cooks dynamos. The steady blow current is probably 20 amps or so. The various Seven models sported dynamo fuses in almost every combination possible.The later fuses seem to only protect the summer resistor.
The situation with the field connected but no output connection is the one to avoid. There is nothing to limit the voltage which soars and applied to the field can cook it. (The way to switch off a dynamo is to disconnect the field. (impractical on the DEL where internally connected) More certain still and contrary to all instinct directly earth both the wires.

[AustinWood]

So long as the current from the dynamo is adjusted correctly the dynamo is safe. I cannot supply more than the set current.
A fuse is not needed.
I have added two fuses. One for directly fed accessories and one for items fed via the ignition.

[andrew34ruby]

Thanks for the comments. Accepted, as Jim Austin Wood says, a fuse is not needed. But if I wanted the extra protection of a fuse to the D terminal it must be one that blows with a steady current of 10.5 amps, no more.

And if the D fuse blows, the field current could rise and melt the field windings. So another fuse to the F terminal seems a good idea.

To fuse D, and not F, might be best avoided.