The technical team at Megger answer another common query.
I’m going to buy some new test kit and I’ve been told that I need to look at the CAT ratings of the instruments, even if they’re things like insulation testers and low-resistance ohmmeters that are used on dead circuits. Is this right and, if so, why?
Yes, it’s right. The CAT rating, as defined in the IEC61010 standard, tells you how big a transient the instrument can withstand. If you’re testing with an instrument that doesn’t have an appropriate CAT rating and there’s a transient (spike) on the circuit, this could start an arc within the instrument. The arc will conduct current from the mains which can be very dangerous. You need to consider CAT ratings even for “dead circuit” instruments because you might accidentally connect these to a live circuit. Instruments with a CAT IV rating can be used anywhere on an LV installation and those with a CAT III rating can be used anywhere after the consumer unit or main distribution board. We don’t recommend instruments with lower CAT ratings for general purpose use. Also take notice of the voltage included as part of the CAT rating, which must always be higher than maximum RMS phase-to-earth voltage of the systems you’ll be using the instrument on.
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