# CB blew when trying to measure the current using multi meter. What went wrong?

I set my multimeter like this.

Plug the black lead to the short tab of NEMA5-15 receptacle first and the red lead to the long tab.

The moment I put the red lead in, there was a big spark. The circuit breaker was blown. And both of my lead is burned pretty bad like this

What did I do wrong? Can someone explain it to me? Is it because I plugged the black lead to hot line of the NEMA5-15?

Before doing like what I described, I had done another run with the lead mistakenly plugged in the meter like this

Everything seemed fine to me. And the meter displayed ~3.5A Then I realized I plugged the lead in wrong post so I swap the red lead and accident happened.

Can someone please explain this phenomenon to me?

• You need to take a step back from mains electricity for a while until you know enough to be safe. Then you need to ask yourself what you're trying to measure the current through, and possibly also educate yourself about the difference between voltage and current and how each are measured. Commented Jan 10, 2019 at 12:16

Current is aways measured in series with something.

An ampere meter "looks" like a short circuit. It works by measuring the voltage across a (very) small value resistor. Like, far less than 1ohm.

Assume your meter is using a 1 ohm current sense resistor, then at 120VAC it would draw 120A and "look" like a short circuit to your circuit breaker (which is rated for probably around 15A.)

If you are lucky, you only blew a fuse in your meter and damaged the cables. You may also have destroyed the meter completely.

The sparks may also have damaged the contacts in the outlet.

The way you had it connected the first time was for measuring voltage.

With the selector set to measure current but the cables connected for measuring voltage, all you got was non-sense - 3.5A is unrealistic for short circuit current from an outlet, and 3.5V wouldn't make much sense on a 120VAC outlet.

• The sparks may also have damaged the contacts in the outlet. is important. This socket might be a fire hazard now. Commented Jan 10, 2019 at 14:18

So, according to what you have set on the dmm, you want to measure current.

To do that you need to put the meter in « series » with the circuit, which means removing the lead from its long tab and connecting hhe leads of the dmm to the tab and the, now removed, lead.

What you did before created a short-circuit and would, of course, blow the CB.