Made 2 Measure: Loop Testing of RCBO/RCD Circuits

Made 2 Measure: Loop Testing of RCBO/RCD Circuits

The technical team at Megger answer this month’s reader query.


I’ve been working in a college recently where supplies to the lab benches are protected by RCBOs with a 10ma earth leakage trip function. Loop testing these circuits was a nightmare! Is there any way of loop testing circuits protected by a 10ma RCBO or RCD without it tripping?


Until recently the answer was ‘no’. Most loop testers use a test current of around 15mA, even when a non-trip test is selected. This will trip a 10mA RCBO or RCD every time. But now Megger has developed a new technique for loop testing, which is used in the MFT1741 multifunction installation tester. With the MFT1741 you can perform three-wire loop tests on circuits protected by 10mA RCDs and RCBOs with very little likelihood of tripping. And that’s not all the MFT1741 has to offer; it also supports two-wire loop testing for circuits protected by 30mA devices and it has a built-in confidence meter that reduces testing time for circuits with low levels of electrical noise, as well as clearly showing when you’ve got an accurate and dependable result on noisy circuits. And, of course, the MFT1741 offers all of the other facilities you’ll need for installation testing.

For more information about the MFT1741 from Megger visit:

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