Electricity is a silent killer. You can’t see, hear or smell it, but it will bite if given the chance. While those working in offices and labs are in generally safe environments largely shielded from hazardous electricity, engineers, electricians and linemen are often directly exposed. They are at greater risk from electrical hazards and should get the best made device they can.
If you intend to measure mains circuits, safety should be paramount in choosing the right meter. Just because a multimeter touts a high category rating and looks tough from the outside, and can measure nominal mains current without issue, doesn’t mean it can protect you in the event of a voltage spike.
That that cheapo has an accuracy and response time on par with an expensive brand is immaterial. The top of the range brands don’t necessarily charge more than the others because they can, they are built and tested to a high standard and are priced accordingly.
Electric shock: it doesn’t take much of a shock to cause a fatality. A 30mA current is enough to cause respiratory paralysis alone, with around 150mA over five seconds or so being enough to stop the heart in an average size adult. Shocks in the amperage range at high voltage, say 5A @ 480V, can easily kill within milliseconds. As this type of potential energy is frequently measured by multimeters, good protection should be a given.
There are three main criteria to consider in handheld multimeter safety; voltage rating, transient over-voltage protection, and thus the category rating.
North America generates domestic voltages in the 110V range, with European and Asian countries 220-240V. A multimeter therefore needs to be able to withstand these amounts if you intend to test mains systems. But the kicker is you also have to take into account transient voltages, or spikes, which can easily be many times the supply voltage.
Transient over-voltages are more likely to happen today than at any other time on account of complex electrical systems incorporated into distribution supplies and industry. These spikes can occur within low-voltage power circuits through the switching of plant equipment, motor inductance, load variations and the tripping of circuit breakers, for instance. Lightening is the other main culprit. Transient voltages from these factors can hit 8kV (8,000V) almost instantly.
As you can see, you would need adequate protection within a multimeter to make allowances for these very real occurrences. Without it, you could end up with an injury or worse through exploding devices. The higher the powers you come into contact with, the higher protection you will need. This is where CAT ratings come in.
Multimeter Category Ratings
There are four categories as laid out by the IEC (International Electro-technical Commission). Three of these pertain to mains circuitry – Category II, Category III, and Category IV. Higher category (CAT) numbers equate to greater danger from transient voltages. As for category I, it relates to equipment not directly connected to mains circuitry and is not covered here.
CAT-II: single phase circuits, for equipment connected to socket outlets, such as appliances and power tools. For testing the socket itself, you would want a CATIII meter.
CAT-III: up to three phase systems and including single phase circuitry of installations; that is, beyond the socket and cord. Includes distribution boards, three phase outlets and commercial lighting.
CAT-IV: where spike levels can be high, such as at a building’s source power; includes measurements of supply meters, main panel boards and outdoor conductors.
|Voltage Rating||Category II Peak||Category III Peak||Category IV Peak|
|100V||800V||1500V (1.5kV)||2500V (2.5kV)|
Multimeters should be marked with the appropriate category rating – CAT-II, CAT-III, CAT-IV. It is usually marked across the common/voltage input jacks. With this, you can discern the transient voltage it can accommodate, though not in the case of some cheap meters.
Many meters nowadays are marked with two CAT ratings, denoting the ability to withstand same over-voltage transients at different measured voltages and different energies, such as CAT-III 1000V and CAT-IV 600V – the latter being lower voltage but higher energy.
Be aware of cheaper multimeters that show a high CAT rating, and avoid meters that don’t show any category rating like the plague.
That a multimeter is fused does not mean it is safe. Amperage inputs should be fused with a special type of fuse, but it should also have basic input protection components, as should the voltage/resistance input. Unfortunately, not all have this level of protection.
Fuses: offer a basic level of protection and are typically not quick enough to protect a multimeter from transient exposure without the addition of further protection. They protect from excessive current, with High Rupture Capacity (HRC) type fuses providing the surest protection. These are high quality, fast-blow, sand-filled fuses that prevent arc flash from infiltrating the insides of the meter should there be a catastrophic failure. Traditional glass fuses do not offer this level of protection and are found on lower quality meters. Manufacturers can unfortunately get away with this, or even omit fuses altogether, so you need to be aware.
11A HRC Fuse
Along with HRC fuses, CATII-rated multimeters and up should also have a number of other components built in to protect from voltage spikes. These include power resistors, positive temperature coefficient (PTC) resistors (known as thermistors), zener diodes, and metal oxide varistors (MOVs). Brand, as much as price, may dictate whether or not these essential electronic components have been incorporated.
Mechanical protection is also of prime importance, to include a sturdy case and rubber holster that protects against drops and splashes. Both multimeter and test probe leads should be double-insulated and have recessed ‘banana’ input jack sockets.
Probes: good test leads are often overlooked but are equally important, since they are the point of contact with live conductors. They should have finger guards and shrouded jack plugs, and should be certified to the same category as the meter, if not higher. A meter with an input configuration alert system is also useful; warning the user should they have the leads plugged in incorrectly. Testing current while plugged into voltage is one of the most common ways to destroy a meter.
The Bottom Line
Along with good mechanical properties, input protection is the most important factor in selecting a safe multimeter. Strive to choose one with a category rating higher than the circuits you intend to measure, which would have the properties to protect against high voltage spikes.
As a pointer, check the voltage rating and over-voltage category. Look to a CAT-III meter if testing mains supplies (includes socket outlets) and a CAT-IV multimeter for industry or measuring near a mains source. High energy, HRC fuses should be installed on both amp and milliamp inputs. Independently certified meters, such as UL-marked, are safest.
More about multimeter safety…