What do DMM Category Safety Ratings Mean, Cat III, Cat IV

When buying digital multimeters one of the confusing specifications may be the category safety ratings, Cat III, Cat IV - what does they mean?

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Many digital multimeters have ranges that extend up to 1kV or so, but for work using these very high voltages, how safe really are they?

Although many people do not use the high voltage ranges of digital multimeters very often, it is helpful and reassuring to know that they have been designed and tested to withstand the voltages and the associated transients that might occur.

For professional multimeters the markings CAT III or even CAT IV may be seen. These indicate that the meter has been designed and tested withstand higher voltages, but even so there are a few points that may still need some explanation.

The need for multimeter safety

We all use seatbelts when riding in an automobile, and we all wear helmets when riding bikes and motorbikes. We all expect every safety precaution to be taken when we are flying.

But why do so many people neglect the need for safety when using a multimeter. A huge number of people use what are relatively low voltage devices, say below 1kV and then suddenly the unit blows.

Although the unit specification may indicate that it can withstand 1kV, when measuring AC lines there are often transient spikes that exceed this by a good degree.

Having a meter that is tested and certified to a certain category enables the user to be sure of its safety and pedigree.

The real issue with multimeters is not just the steady state voltage, but a combination of the steady state voltage coupled with the transient voltages that might be experienced.

Although this transients are more likely to be encountered on AC lines, the same can occur on many high voltages used within electronic circuits under some circumstances.

It's well worth bearing this in mind because even though we may deal with only low voltages for most of the time, there will be the odd occasions when AC lines may be measured and it is easy to forget, or tempting to ignore the possibilities of transient spikes.

What are the CAT levels?

The IEC, International Electrotechnical Commission which sets many standards for electrical equipment states that multimeter manufacturers must follow the standards for safety testing of their products.

There are four categories that are used and these are based upon the fact that dangerous high energy transients that may come from sources like lightning strikes will be attenuated as they pass through the distribution system.

Therefore a multimeter designed to withstand a CAT III environment will be able to withstand much higher energy spikes than one designed to CAT II.

There are four category levels that defined and these depend upon the type of environment in which the multimeter is used.

Summary of Multimeter Categories
Category	Basic Summary	Details
CAT I	Electronic Devices	Anything from a small circuit board to larger device with high-voltage but low-energy
CAT II	Single-Phase AC connectorised loads	Appliances or portable tools
CAT III	Three-Phase Distribution including single phase commercial lighting	Some large building lighting systems and polyphase motors, panel devices, short branch circuits, etc
CAT IV	Three-Phase Utility Connections or Outdoor Conductors	Electrical meters, outdoor connections, any low-voltage and high-energy connection

Voltages with the category

In addition to the basic categories used to define the withstand requirements for a multimeter, there are also voltage ratings that apply.

For example a meter may be quoted as having a rating of CAT III 600V or CAT III 1000V.

Multimeter IEC 1010 Category Voltage Requirements
Category	Working Voltage (V DC or AC RMS to ground)	Peak Impulse Transient (V)	Test source impedance (Ω)
CAT I	600	2500	30
CAT I	1000	4000	30
CAT II	600	4000	12
CAT II	10000	6000	12
CAT III	600	6000	2
CAT III	1000	8000	2
CAT IV	600	8000	2

As would be expected the higher the voltage the greater the withstand capability. However things can be a little confusing, for example a CAT III 600V meter has a higher withstand capability than a CAT II 1000V meter.

First of all, the CAT II 1000V meter is designed to withstand a a steady state 1kV,, whereas the CAT III 600V meter is only designed for 600V.

But, it is necessary to look at two of the other aspects of the standard. The first os the peak impulse transient, which happens to be 6000 V for both, but the source resistance fort he CAT III meter test is only 2 Ω against the CAT II meter for which it is 20Ω This makes the test for the CAT III meter more stringent.

IEC 1010

Although the basic Categories are defined in IEC 1010, there are a few other standards that also apply tot he multimeter design.

In addition to the maximum voltages and the transient voltages that apply, the standard also specifies the minimum clearance between the internal components and PCB nodes, etc.

The higher the category, the greater the required clearance must be. This is very important when working on high voltages as it helps ensure that there is no internal breakdown when high voltage levels are being tested.

If the meter has not been designed and certified to IEC 1010, then there is no knowing what the internal clearance might be and there could be a risk of damage, and possibly even an electric shock in very extreme cases.

The multimeter categories may seem like an unnecessary complication, but when asking others in particular to use test equipment, then it is necessary to ensure that it is safe and fit for purpose. And also would you want to use a multimeter on high voltages that might fail?