The blower fan on my electric furnace will not shut off without tripping the main breaker switch. It is an older furnace that had not been used all too often as I generally use a wood/coal stove for heating, but had been operating properly when I have used it. The last time it was used , the thermostat was set for 70 degrees but when I noticed, it was at 75 and the furnace was still running. The only way I could shut it off was by the main breaker

I'm a little confused, you stated the blower keeps running but also said the temperature is increasing so I assume the heat is also staying on. You could have several things that will cause this. Remove the wire off the R or W terminal on your thermostat, if the furnace goes off, you most likely have a bad thermostat. If after removing the wire on W the furnace continues to heat, you probably have a bad relay or defective circuit board. When the heating elements are "on" the blower should never go off. Please respond back if you have anymore questions or updated information.

Serial n0. 129130768 in front of the blower is a switch that has 3different settings can you tell what it is for