thank god i found this post. exactly same thing happened to me. 20 rcbo's in DB and every one failed. Even went next door to check tester was working. Why does I'm pretty sure that when I have installed these before they have not had that information in the box but they do mention not to insulation test through RCD.
RCD testing a RCBO at the device terminals (MEM devices)
With an RCD, the sense coil and associated components are within the mechanism and fabric of the RCD and the conductors attached to the device have no bearing on operation.
An RCD can be tested either at the output terminals or at the end of a final circuit.
This is not the case with some RCBO devices. It has been found on certain installations attaching an RCD tester to the terminals on the RCBO has proved to be hopeless, the RCBO refuses to trip, however a test at the end of the attached circuit proves positive.
Following a consultation with a major MCB, RCD and RCBO manufacturer, it was found that some manufacturer's RCBO’s are constructed in such a way that when the output circuit conductors are connected, the small section of cable that sits within the 'tunnel' through which the conductor runs to the RCBO terminal forms the core of the sense coil.
Testing must be made at a convenient socket or at the end of the circuit using suitable test leads.
NB. The above mentioned devices are manufactured to BS standards and should not be considered as defective products.
When an RCBO does not trip it should not be assumed that the device is one of the above mentioned RCBO’s, leaving vital tests ignored.
The presence of such a device should be verified and appropriate tests made to BS7671.
