E-amp fuse

Charlie Richardson talked me through some troubleshooting steps this morning on the phone. The house was freezing! Figured out that I had blown the secondary fuse on the furnace circuit board (when those wires had touched with the power still on). Looked like a special kind of fuse.
"Does it have any markings on the fuse?", Charlie asked.
"It says 'E'," I said.
Hmmm, what was this?
Until I saw a note on the circuit diagram that said 3-amp automotive blade type fuse.
Sure enough, when I flipped it over, suddently it said '3'.
I bought the fuse, plugged it in and we were back in business.
I'd like to say I turned the power off before I took out the old tstat and rewired the new one, but I don't want to lie. (Besides, I had spare E-amp fuses :-)