What sort of protection should electricians consider for testers?

A small electrical fault can become a very big problem on a high energy unfused supply. If we lived in a perfect world, the 'Category' rating system on electrical test instruments, developed and administered by the International Standards Organisation (ISO), would not matter. But in the real world, every single electrical mains supply has transients of all different types superimposed on it, and that's what category ratings are all about - helping electrical professionals better understand the dangers of transients by categorising them.

A distant lightning strike, for example, can produce a transient of several kV on the mains supply. The transient may only last for a few tens of microseconds, and in itself, is likely to do little damage. The problem is that it may initiate an arc; and this arc then presents a low impedance path for current from the mains supply.

Often, that supply can deliver 1000 A or more until the circuit breaker or other protective device operates. In that time, the amount of energy liberated at the site of the arc is easily enough to start a fire or even cause an explosion. If the arc is within a test instrument that happened to be in use when the transient occurred, there is a very high probability that the person using the instrument at the time will be injured or worse still, killed!

There is a simple solution - from a test instrument manufacturer's perspective, design the test instruments with internal clearances that are large enough to prevent transients from establishing an arc in the first place, along with appropriate protection devices. Guidance to this is given in the IEC 61010 standard in order to comply with category ratings defined in the IEC 60664 standard. And from a user's perspective, it's a matter of understanding the category rating system and choosing the most suitable electrical test instrument.

In practice, transients are damped quite quickly as they pass through a typical building distribution system. Their energy is highest at the point where the supply enters the building, and only instruments with a CAT IV rating are suitable for use at this point.

Once the supply has passed through the distribution board, the transients have lower energy levels, and CAT III instruments are safe to use. At a socket outlet, the energy levels are lower still.

For performing tests within appliances, such as televisions and photocopiers, CAT I instruments can be used although major suppliers rarely offer instruments of this type.

Many instruments in use today are rated CAT II, with the better ones CAT III. These should be fine within their limitations, but they're simply not designed - and not safe - for use by anyone whose work involves testing on mains circuits before they enter a building, or at the point of entry. They should use a CAT IV instrument.

There is simply no case for making compromises.

Instruments such as the Megger MIT 400 series insulation resistance testers are suitable for CAT IV 600 V applications, while the Megger LT300 loop tester is rated suitable for CAT IV 300 V applications.

At first consideration, the CAT IV rating may seem unnecessary for the MIT 400 tester, since insulation testers are used on dead circuits. The tester, however, also offers useful facilities for measuring voltage, so the CAT rating is every bit as important as it is for a multimeter.

In live circuit applications, a CAT IV test instrument is essential for loop testing carried out at or before the incoming circuit cut out. CAT IV rated test instruments are also particularly important for electrical utilities, as all of the installations in their domain are CAT IV environments. That's why French electrical utility EdF and many other utility companies around the world insist that all of their contractors use CAT IV test instruments when working on their installations.