Type A rcd . EICR coding ? etc

The wording of 531.3.3 Note 1;

"NOTE 1: For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA." (and the other notes) could be read that either just the smooth component of the residual current could be up to 6mA or that the combined smooth and pulsating components will trip a 30mA RCD when the total peak residual DC current exceeds 6mA.

If the former, then i assume the RCD will trip when a DC pulse peak exceeds 30mA in a 30mA RCD. What would happen if the smooth DC component is 7mA? If the latter then why 6mA? It seems very low. Perhaps the type A is blinded? 

Yes - not clear there. The requirement is that the usual test with pulsating current is repeated, but with an additional smooth dc current that is not included in the measured current; the test must be passed in spite of the presence of this smooth dc. This is in IEC61008(2013), 9.21.1.4 "Verification of the correct operation in case of residual pulsating direct currents superimposed by smooth direct current of 0,006 A". 


The test with 6 mA dc (smooth) is only used together with a pulsating dc residual current. This is the case in which the core could already be taken towards saturation by the smooth dc, and then a 'pulsating dc' fault could happen in the same direction, causing little change in magnetization and so not tripping the RCD even with much more pulsating current.  I.e. it's blinded to this pulsating dc. (Meanwhile, type AC is permitted congenital blindness to the pulsating dc.) My earlier rather long description of RCD cores and trip coils might make this clearer. In a type-A RCD that's near the boundary of what is permitted, a 7 mA smooth dc could potentially make it fail to trip even with a large pulsating residual current in the same direction. But a balanced ac residual current would still trip it at not too very different a level from the usual, as it would push the magnetization out of saturation in some half-cycles.


I've not seen a rationale for the choice of 6 mA. A standard number, around the level that's been thought to be a reasonable limit for what smooth dc might leak out in a circuit except in special cases? Note that the smooth dc needed to saturate an RCD core can be less than the pure ac, as the ac induces current in the core's secondary (feeding the tripping coil), and this current opposes the core's field, so the overall current magnetizing the core is less than one would think from the residual current. Smooth dc doesn't have this opposing current in the secondary. 

The wording of 531.3.3 Note 1;

"NOTE 1: For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA." (and the other notes) could be read that either just the smooth component of the residual current could be up to 6mA or that the combined smooth and pulsating components will trip a 30mA RCD when the total peak residual DC current exceeds 6mA.

If the former, then i assume the RCD will trip when a DC pulse peak exceeds 30mA in a 30mA RCD. What would happen if the smooth DC component is 7mA? If the latter then why 6mA? It seems very low. Perhaps the type A is blinded? 

Yes - not clear there. The requirement is that the usual test with pulsating current is repeated, but with an additional smooth dc current that is not included in the measured current; the test must be passed in spite of the presence of this smooth dc. This is in IEC61008(2013), 9.21.1.4 "Verification of the correct operation in case of residual pulsating direct currents superimposed by smooth direct current of 0,006 A". 


The test with 6 mA dc (smooth) is only used together with a pulsating dc residual current. This is the case in which the core could already be taken towards saturation by the smooth dc, and then a 'pulsating dc' fault could happen in the same direction, causing little change in magnetization and so not tripping the RCD even with much more pulsating current.  I.e. it's blinded to this pulsating dc. (Meanwhile, type AC is permitted congenital blindness to the pulsating dc.) My earlier rather long description of RCD cores and trip coils might make this clearer. In a type-A RCD that's near the boundary of what is permitted, a 7 mA smooth dc could potentially make it fail to trip even with a large pulsating residual current in the same direction. But a balanced ac residual current would still trip it at not too very different a level from the usual, as it would push the magnetization out of saturation in some half-cycles.


I've not seen a rationale for the choice of 6 mA. A standard number, around the level that's been thought to be a reasonable limit for what smooth dc might leak out in a circuit except in special cases? Note that the smooth dc needed to saturate an RCD core can be less than the pure ac, as the ac induces current in the core's secondary (feeding the tripping coil), and this current opposes the core's field, so the overall current magnetizing the core is less than one would think from the residual current. Smooth dc doesn't have this opposing current in the secondary.