Type A R.C.D. 6mA tolerant.

Are we to understand that the small D.C. current needed to drive a typical L.E.D. with series resistor, say 10-15mA, is enough to disable an A type R.C.D?

But it is reasonably difficult to inject such a d.c. current into a LV a.c. circuit - you'd need to create a complete loop/circuit for the d.c. current that included the a.c. circuit (or at least some path through the RCD) - so a single fault from something with a separated source (like a LED driver?) is unlikely to do it. A d.c. fault to earth with a simultaneous N-PE fault elsewhere is sort of the minimum I could imagine at the moment - but then we're into two-faults-to-danger territory, which is generally acceptable anyway (like an earth fault + a broken c.p.c.).

A few situations are riskier though - e.g. EV chargers where there's d.c. control/signalling circuitry that's deliberately referenced to the c.p.c. or where there are large d.c. currents wandering around exposed- and extraneous-conductive-parts  (e.g. returning traction currents on the Southern Railway) where a single fault could possibly create a d.c. circuit around a.c. conductors - i.e. the sort of situation where type B RCDs get specified (or some alternative like a RDC-DD in conjunction with an A-type RCD).

    - Andy.

Are we to understand that the small D.C. current needed to drive a typical L.E.D. with series resistor, say 10-15mA, is enough to disable an A type R.C.D?

But it is reasonably difficult to inject such a d.c. current into a LV a.c. circuit - you'd need to create a complete loop/circuit for the d.c. current that included the a.c. circuit (or at least some path through the RCD) - so a single fault from something with a separated source (like a LED driver?) is unlikely to do it. A d.c. fault to earth with a simultaneous N-PE fault elsewhere is sort of the minimum I could imagine at the moment - but then we're into two-faults-to-danger territory, which is generally acceptable anyway (like an earth fault + a broken c.p.c.).

A few situations are riskier though - e.g. EV chargers where there's d.c. control/signalling circuitry that's deliberately referenced to the c.p.c. or where there are large d.c. currents wandering around exposed- and extraneous-conductive-parts  (e.g. returning traction currents on the Southern Railway) where a single fault could possibly create a d.c. circuit around a.c. conductors - i.e. the sort of situation where type B RCDs get specified (or some alternative like a RDC-DD in conjunction with an A-type RCD).

    - Andy.