[ Technical: FAQs ]
On some RCBOs, the means to monitor the balance of currents in the out-going phase and neutral is not wholly integral to the device. The out-going phase and neutral conductors form part of the ‘sensing coil’ of the RCBO when they are connected to the device. This means that if the test probes are connected directly to the RCBO output terminals, they are connected before the functional part of the RCD and, therefore the device won’t trip on test. With this type of RCBO, the RCD function must always be tested at the end of the circuit. Never test at the device terminals and simply assume that the RCD function didn’t operate for the reasons discussed – you might be unlucky enough to have a genuinely faulty RCBO.
I do a lot of PAT testing, and I’m about ready to toss my PAT tester through the window! Whenever I switch it off to move around a site, I have to wait ages for it to reboot when I turn it back on again. Then, when I’ve got it going, it just shuts itself down if I do a lot of tests. What’s the solution?
Portable appliance tester
Buying a new PAT tester may seem
extravagant, but it will quickly pay for itself through the time it saves
Choosing an instrument with the right CAT rating minimises the risks from transients
Unfortunately, it looks like you will need to buy a new tester to overcome the problems you
mention. When you do, check out the available testers like the Megger’s latest PAT300 and PAT400 ranges. These have an internal back up battery that retains the instrument’s status for a few minutes even when it’s switched off. That means it doesn’t have to reboot from scratch whenever you move between locations, which will save you a lot of time. Your shutdown problem is caused by your tester overheating when you do tests one after the other – it has to shutdown to protect itself from damage. To avoid this, when you buy your new tester, be sure to choose one that’s rated for continuous use, even when you’re using it for bond testing at its maximum current setting. Once again, the Megger PAT300 and PAT400 testers meet this requirement. Buying a new PAT tester may seem extravagant but, as you do a lot of PAT testing, it will quickly pay for itself through the time it saves.
I’m going to buy some new test kit soon, and I’ve been told that I should look at the CAT ratings of the instruments. What is a CAT rating, and is it important? Yes, the CAT rating is important – it tells you how big a transient the instrument can withstand. If you’re testing and there’s a transient (spike) on the circuit, caused for example by nearby lightning, this could start an arc in the instrument, which will conduct current from the mains. That would damage the instrument and might even cause it to explode.
Choosing an instrument with the right CAT rating minimises the risks from transients. Where the supply enters the building, only CAT IV instruments should be used. After the supply has passed through the consumer unit, CAT III or CAT IV instruments are OK, and at the final circuit outlets, CAT II, CAT III or CAT IV instruments can be used. CAT I instruments aren’t suitable for contractors. Also make sure the voltage quoted as part of the CAT rating – for example, CAT III 600 V – is higher than the maximum RMS phase-to-earth voltage of the systems you’ll use the instrument on.
60 ECA Today September 2012
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