Megger CBT4 - RCD tester

The instruments are

connected through a normal mains

socket outlet or directly via the RCDs

terminals. Neons indicate if there are

no open circuits in the installation and

if the phase connections are correct.

A rotary switch selects the RCD rating

from one of six values available and a

membrane push-button is pressed to

execute a test. A known fixed current,

determined by the type of test select-

ed, flows to earth in the installation the

RCD is protecting, for a given length of

time. The types of test available are:-

(i) a ‘no trip’ test carried out at half

the rating selected, the result of

which should be that the device

under test does not trip.

(ii) a ‘trip’ test carried out at the rating

selected, the result of which

should be that the device trips

within the time specified for that

particular design of breaker

.

(iii) a test carried out at 150 mA and

designed for testing devices (up to

30 mA rating) fitted to protect

against direct contact with the

installation. The result should be

that the device trips within 40 ms.

(iv) a test carried out at five times the

rating selected, the result of which

should be that the device trips

within 40 ms