A 1A RCD is pretty deaf, but quite sensible for something big and 3 phase.

I presume the load is not something known to inject DC into the L-E loop as part of normal operation ?

How many RCDs of this type have you tested with this meter ? Normally I'd say 'try a known good one' but that may not be so easy, as it would be for 30mA  say.

So, while a 5A test is out, can your meter extend to a test at higher current say 2A?, and also does it matter which combination of phase and earth are selected for the test ? I'm wondering if swapping the phases would reveal that really one phase is slightly less sensitive.  For that matter are the supply voltages for all 3 phases present and correct ?

A 1A RCD is pretty deaf, but quite sensible for something big and 3 phase.

I presume the load is not something known to inject DC into the L-E loop as part of normal operation ?

How many RCDs of this type have you tested with this meter ? Normally I'd say 'try a known good one' but that may not be so easy, as it would be for 30mA  say.

So, while a 5A test is out, can your meter extend to a test at higher current say 2A?, and also does it matter which combination of phase and earth are selected for the test ? I'm wondering if swapping the phases would reveal that really one phase is slightly less sensitive.  For that matter are the supply voltages for all 3 phases present and correct ?