There are two ways a multimeter can kill you, both of which are usually caused by
human error:
Failing to protect you from a high voltage it is measuring
When using a multimeter to measure high
voltage, it is vital (literally) to ensure that:
- The multimeter is rated to measure voltages in the range you expect to be measuring (mains voltage, for example, is around either 220v or 110v)
- The multimeter probes are rated to measure voltages in the range you expect
- There are no uninsulated parts anywhere between the tip of the probe and the multimeter.
Safety features on modern probes include wide insulated 'collars' on the probes to ensure your fingers don't slip and touch the
business end,
interlocking insulation on the plugs to ensure that there is no exposed metal where the probe plugs in, and fully moulded fittings to lessen the chance of the wire working loose. If your multimeter doesn't have these features, consider replacing it before working with high voltages.
Failing to warn you of a high voltage it is measuring
When measuring the voltage of a
dc source, a digital multimeter will show a number corresponding to a variable reference voltage generated inside the meter. If the source is truely dc, the reference voltage can be accurately compared to the input voltage, and will 'home in' on it, until the number displayed is equal to the input voltage. If the input source is
ac, the refererence voltage will keep varying to try to match the input, and the multimeter will show either the
average voltage, which is zero, or some
random number. Assuming a wire is safe on the basis of this number could be a mistake
1. In ac mode, the input voltage is
rectified, and the resulting dc voltage is measured. Calibrated properly, this gives a good approximation of the ac voltage.
The other easy mistake to make is to use the wrong voltage range. If the range is set to high (say 5000v), the voltage measured may be too small (compared to the maximum voltage) to register on the display. Likewise, if the range is set too low, any voltage higher than the range will overflow it, and not be displayed. Depending on the multimeter, it may display nothing, 1, or 'high'.
More
stupid mistakes include not setting the meter to measure voltage, using a multimeter with a blown
fuse, and using the wrong terminal for the live probe (many multimeters use different terminals depending on whether they are measuring current or voltage. It's also common to have different terminals for high and low voltages.
Be sure to read, understand, and follow all the instructions that come with your multimeter. Knowing how to use your multimeter safely will greatly reduce the risk of injury.)
Multimeter safety checklist
- Is your multimeter rated to deal with the voltage you intend to measure?
- Are your probes insulated all the way from the tip to the multimeter?
- Is the casing of the multimeter undamaged?
- Does your multimeter work? (Try measuring a low voltage like a battery first.)
- Is your multimeter set to measure voltage?
- Is your multimeter set to measure AC or DC?
- Is your multimeter set to measure an appropriate voltage range?
- Is your live probe plugged into the appropriate terminal?
1 - I did this fitting a light socket, and was fortunate enough to fall off the chair I was standing on, breaking the circuit. It was not a pleasant experience.