Blew a fuse looking for 24v terminal..

I was looking inside the furnace to find where to hook up the disconnected blue wire for my new thermostat. The circuit board has a connection labeled as Com/24U which I assumed was the 24v I was looking for. The Com terminal has a wire connected that appears to lead off towards the A/C unit outside. I pulled out my digital multimeter, set it to DC, and touched a probe to the Com terminal and the other probe to the R terminal and I got an immediate spark. The 3 amp fuse to the left of the Com terminal blew (thankfully I suppose) and I guess I found voltage. My question though is why did I get the spark and blow a fuse? Was the R terminal the wrong one to test with the Com to find voltage? My new thermostat came with a very small light bulb that says to touch it to the R and C terminals inside the thermostat housing to verify voltage on the common wire but I'm afraid I'm going to end up blowing another fuse. Actually, I had to swap a 15a fuse in place of the 3a fuse because that's all Wal Mart had at this late moment so I really don't think I should be doing much more until I get a proper fuse in there anyway, I'm hoping Ace Hardware has the 3a ones in the morning. But I'm just wondering if any of you experts who are probably doing a palm of hand to face motion while reading this can tell me where I went wrong here? I'm honestly a little hesitant to proceed with installing this thermostat after this occurrence.

 

Connect your blue wire to C at the furnace and stat, it is common.

That 15 amp fuse will not protect the transformer. You can now damage the transformer if you short R to C or ground.

Radioshack should have a 3 amp blade type automotive fuse.

 

Ok.....first things first....you need to know how to use your meter.
In this project we are working on here.....the controls use 24 volts AC.

You probably had your meter set to DC amps or (and this is real important) you had the meter leads in the wrong ports. For measuring voltage and continuity.....you'll want to use the COM for the black probe and VOLTS/RES/CONT for red probe.

Most meters have an additional port that says something like amps. That port and black are a dead short to what you are testing. That's how it's supposed to work. Obviously in this case if you were using that port in error (or had the meter set to AMPS) you would have put a direct short on to what you were measuring.....which is what happened in your case.

Also.....when measuring AC like we are here in the furnace, there is no polarity. It doesn't matter which lead goes to which terminal.

Polarity only becomes an issue when measuring DC circuits.

Those blade type fuses (ATC) are the same ones used in automobiles. You can find them at almost any auto parts store. ATC-3 is what you are looking for.