I was playing around with an automotive battery charger over the weekend. Well, I suppose I was playing with the combination of the charger, a motorcycle battery, and my Fluke 87-5 multimeter.

Yeah, I can play around with almost anything and have a good old time.

The charger in question is a Schumacher SSC-1500A charger I picked up from Amazon last week. I got it because my long-term charger is really a maintenance charger rather than a full on charger. The Battery Tender I have outputs between 1 and 1.5A depending on which reference you believe. The new Schumacher puts out 15A max.

But enough about the charger.

Like I was saying I was having a ball measuring the current flowing through the circuit. I was getting something around 200uA after the battery was fully charged. I had an alligator clip from the Fluke clipped to the negative terminal of the battery and was alternately connecting and disconnecting the clamp from the charger (which also had an alligator clip on it). No problems.

Then I wanted to measure the voltage.

I clamped the black on the negative, and touched the red clip to the positive.

Then I knew I did it in the wrong order.

The procedure is simple:

  1. Disconnect
  2. Change modes
  3. Reconfigure if needed
  4. Reconnect

If I had followed the right process the Fluke would have incessantly beeped to warn me that the lead was still in the mA/uA jack. What I did was short out a very low ESR current source through my multimeter.

One of the good things about the Fluke is it’s very well protected against my own stupidity.

All I did was blow a $5 fuse. It’s $5 because it’s a special fuse.

Why is it special?

The front is pretty normal: it’s rated at 440mA. No worries.

If you look look at the safety rating of it though you’ll see it’s rated to 1000V with a rupture capacity of 10,000A! It’ll blow, but not blow up. That’s a good thing. It not only saves the meter, it can save the operator as well. There’s a reason they call them HRC fuses — High Rupture Capacity. The same fuse on the other side of the range — the 10A range — is an 11A fuse that has a rupture capacity of 20,000A at 1000V!

That’s a metric crap load of energy it can absorb without blowing up.

That’s why these meters get an honest CAT-III @ 1000V and CAT-IV @ 600V ratings.

It’s to save you. Really.

In my case, the energy in a 12V motorcycle battery wouldn’t be enough to hurt me unless I’m actually trying to be stupid. Can I think of ways to do it? Sure… but careless and outright stupid are a bit different in this case.