# Wrong Multimeter connection / floating ground

What does happen if you connect a multimeter without COMmon ground?

Like in this question (Why does a permanent, non-moving magnet, stuck to a long piece of metal generate a voltage?) How would a multimeter react if you connect the leads to the current and the volt measure inputs.

1. Just "in the air", so not connected to anything
2. if measuring DC/AC voltage

(second one beeing probably more destructive)

Is there a fuse to prevent disaster? I'm especially interested in what components are responsible for the output (the chip, the circuit board, etc.)

• Are you asking what would happen if you plug, say, the red into the V socket and the black into the 10A socket? The answer to Q1 you can check for yourself without risk of damage. Hit the edit link to clarify. Commented Apr 12, 2020 at 13:15
• You're right, I should probably add to the question that I'm more interested in what and WHY it happens. For example if the circuit board influences the measurement. Commented Apr 12, 2020 at 13:40
• Multimeters are not all the same, so you need to be more clear about what you are asking. Commented Apr 12, 2020 at 13:49

simulate this circuit – Schematic created using CircuitLab

Figure 1. A simplified representation of a digital multimeter schematic. In almost all modern multimeters the voltage range will be auto-ranging and protect itself against over-voltage. (This one doesn't.).

If you plug the "black" lead into the 10 A socket (instead of the COM) while measuring voltage with the red in the red VΩ socket you will introduce a small series resistance which will very slightly affect the voltage measurement.

Form Figure 1 we can see that an extra 10 mΩ to 100 Ω would be introduced between the 10A or mA socket to ground where the V reference is. Compared with the 1 to 10 MΩ input impedance of a digital meter the error would be insignificant for most purposes.

The answer to Q1 you can check for yourself without risk of damage.

Is there a fuse to prevent disaster?

Usually. Most multimeters I've seen have at least one fuse, usually 10A. The only times I've seen them blow is during misuse.

Picture of a fused multimeter:

Notice the text 'FUSED' near the bottom?

Easiest method to get one to trip is by putting it on Amps, connect the leads to 10A and COM and try to measure any significant voltage. You will effectively short the voltage source over the fuse, blowing it up.

Keep in mind those fuses tend to be a bit more expensive than your average glass fuse. Oh, and never try to by-pass the fuse with aluminium foil. It works, until you do exactly as described above and weld the aluminium...

Some multimeters check that the correct sockets are being used for the selected range, and if not they beep and/or flash lights. So that is one possible outcome.