In a perfect world, category ratings would not matter. But in the real world every mains supply has transients superimposed on it, and that’s what category ratings are all about.
A distant lightning strike, for example, can produce a transient of several KV on the supply. The transient may only last for a few tens of microseconds and, in itself, it 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 1,000 Amps or more until the 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 high probability that you the user will be injured or worse!
The solution is simple – design the 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 IEC61010 in order to comply with category ratings defined in IEC60664.
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 television and photocopiers, CATI instruments can be used although major suppliers rarely offer instruments of this type.
Many instruments in use 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.
Fortunately, there is no need to, as Megger’s MIT 400 series insulation resistance testers are suitable for CAT IV 600 V applications.
At first, the CAT IV rating may seem unnecessary for the MIT 400, since insulation testers are used on dead circuits. The MIT 400, however, also offers useful facilities for measuring voltage, so the CAT rating is every bit as important as it is for a multimeter.
CAT IV rated instruments are also particularly important for electrical utilities, as all of their installations are all in CAT IV environments, which is why many utility companies now insist that all of their contractors use CAT IV instruments.