Under which possible conditions is measuring with an multimeter a danger to life ?

Question

My knowledge of electrics is very limited, I am planning to buy a Multimeter, but I am confused when it comes to safety. I do not really know what is a danger to life when measuring a circuit with a multimeter. Is it the voltage, current, resistor, capacity, frequenz a combination of some of this or other factors. Could you please explain common conditions in working with an multimeter and when it is safe and when not.

Answer 1

Some general points- of course you should always follow the meter and test probe manufacturer's recommendations.

There is danger from electrocution and from release of high energy (shrapnel, molten metal, vapor) in case of a short. Arc flash is a particular type of high energy fault that can cause fatal injuries, A lesser danger of injury (death is less likely) is from high high current causing burn injuries, which can occur even at low voltage. Some cheap meters have no fuse at all in the high current range and putting the probes across a hefty battery or high current supply will melt or blow the test leads off. Others have a fuse but with woefully inadequate interrupting capacity.

In all cases using a multimeter with at least the minimum safety category for the situation will reduce the risk. A good summary of the categories is in this Fluke publication- ABCs of Multimeter Safety.

I would recommend CAT III for most mains measurement situations.

Here is an analysis of an incident that killed two people as a result of using cheap multimeter.

Answer 2

I'll skip the fun stuff, like measuring the voltage of a lightning bolt by standing in the middle of a thunderstorm with one DMM probe stuck in the ground while holding the other above your head.

There are two basic areas to be careful of.

First, measuring high voltage (basically, 110 volts or greater). You've got to be careful that you don't inadvertently complete a circuit. This means being careful not to let your fingers touch the metal probe ends, or try it while wet, or get careless about what you touch. A useful technique, if you are worried, is to keep one hand in your pocket. That is, you clip the DMM ground probe to a handy ground point, then use one hand to make the probe connection to the point of interest, while keeping the other hand in your pocket. The very worst thing you can do is to make contact with two voltage sources with both hands - the current path from one hand to another will include your heart, and this is technically known as A Bad Thing. Another thing to consider is to make sure that your contact points are mechanically stable. Do not, under any circumstances, try to use a DMM on a point which is not held firmly in place, such as the end of a wire which is hanging freely.

For voltages of, let's say, 24 volts or less, the above concerns are simply not much of a problem as long as you're not doing it while immersed in water. With dry hands, 24 volts may give you tingle, but nothing more, and lower voltages, such as 5 volts simply won't produce any sensation at all. Under these conditions, you need to worry more about accidentally short-circuiting two adjacent conductors with the metal end of the probe. This can happen, for instance, if your hand slips, or if you have the probe touching one point and turn away (for instance to look at the DMM face), and change the angle of the probe so that it makes contact with two points and short circuits them. This is most likely to just damage the circuit, but if the voltage being shorted can produce currents of many amps you may get spatters of hot metal being thrown off.

Answer 3

One risk is electric shocks.

It's difficult to pin down exactly what point a supply becomes hazardous because it depends on many factors but the general rule of thumb is that supplies below 50V or so are safe enough under normal conditions.

Sticking probes into bits of live metal carries a risk that you will end up touching live metal. How serious that risk is depends on the type of probe you are using, what you are using it to probe and how careful you are.

If one probe is touching an object that is live at a dangerous voltage then you should treat the other probe as also being live.

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The other side of the safety equation is energy related risks. Something goes wrong during measurement that results in very high current flow. Maybe a spike in the supply flashed over the meter. Maybe the user plugged the test leads into the wrong sockets.

Whatever the cause of the high current event the level of danger depends very much on the type of supply. Industrial and commercial distribution tends to be especially bad for this. In the worst case the energy released as an arc flash can be enough to kill.

Test gear is rated into "categories" depending on the environment in which it will be used. Another answer linked a document from fluke titled "the ABCs of mulitimeter safety which convers this pretty well" but basically you want at least CAT II for working on mains supplies inside portable equippment, CAT III for work on most wiring inside buldings and CAT IV for outdoor work and work on the main service intake.

Be careful of CE marks, they can be self-certified and there is no central tracking. So unscrupulous vendors can and will slap them on virtually anything. I would avoid no-name meters for working on anything hazardous regardless of what measurement category the lables on them claim.