A small fault becomes a big problem on Category IV high energy unfused supplies!
In a perfect world, category rating 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 problems 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 1,000A 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 CATIV rating are suitable for use at this point.
Once the supply has passed through the distribution board, the transients have lower energy levels, and CATIII 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 today are rated CATII, with the better ones CATIII. 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 CATIV instrument.
There is simply no case for making compromises.
Fortunately, there is no need to, as Megger offers a line of insulation resistance testers that are suitable for CATIV 600V applications.
At first, the CATIV rating may seem unnecessary for that type of tester, since insulation testers are used on dead circuits. This type of 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 CATIV instrument is also essential for any loop testing carried out at or before the incoming cutout. CATIV-rated instruments are also particularly important for electrical utilities, as all of their installations are all in CATIV environments, which is why EdF in France and other major utilities in the U.S. now insist that all of their contractors use CATIV instruments.
In Summary
For safety reasons as well as additional capability, the insulation tester you choose should be rated to category IV, 600 volts to properly protect your employees in their everyday testing activities. As stated, there is simply no case for making compromises.
About the Author:
Jeff Jowett is Senior Applications Engineer, Megger – Dallas, TX. He can be contacted at jeff.jowett@megger.com.
