Hopefully flameport can clarify but, as you say, on the face of it, it looks as if what was being tested with was an L-N imbalanced DC current ,
That test rig was made to demonstrate a particular DC detection device made by Blakley (the large white item in the centre), and uses a 4 pole type AC RCD with 3 poles used, L, N and the third one with a variable DC current through it.
How type AC RCDs react when exposed to DC is undefined, they are not designed or tested for that, and the standard for them doesn't have anything on DC either. That particular one in that rig will not work at all with 250mA of DC, but that's only one example of one specific RCD from one manufacturer.
To detect smooth DC, a type B RCD or equivalent is required, which is what the white Blackley device is. Smooth DC on an AC circuit is only likely if there is a fault with some DC item such as an electric vehicle or solar installation, or in the case of that demonstration rig, a railway with DC traction current returning through the AC installation. That's why EV charge points contain DC current detection or a type B RCD.
Type A RCDs will detect pulsed DC, which is far more likely in normal use from items which contain rectifiers / diodes. How type AC RCDs react to that is also undefined.
The 6mA referred to is just the maximum amount of smooth DC that a type A is stated to work as intended, above that it's behaviour is undefined. Generally unrelated to normal use on an installation with pulsed DC loads, as the smooth DC would as above typically only be from a fault or some other source external to the installation and a type B would be used instead if that was a possibility.
For normal installations, pulsed DC in normal operation is common, and type A is intended to be the minimum requirement.
Some installations will require a different type, such as type B in situations where faults or external influences could result in smooth DC or type F for certain types of equipment.
Exactly, and it's the grave difficulty I have had in trying to ascertain "the extent of the problem" that I have been moaning about for a long time.
No information is available, because no one bothered to even consider it until fairly recently.
However type AC RCDs are not fit for purpose, and haven't been for a very long time. They were designed for AC loads, something which is rare in a modern installation.
Germany banned the use of type AC RCDs almost 40 years ago. Many other countries have done the same.
Meanwhile in the UK, BS7671 has been more concerned with not having socket outlets in bathrooms, perpetuating the use of 70+ year old circuit designs and other minutiae.