Type A rcd . EICR coding ? etc

so we need to consider other DC connected measuring methods


Yes - we can be confident that any method that should detect milliamps of imbalance against hundreds or thousands of amps of load current will rely on the magnetic field caused by a balance of currents. The usual method to detect this field, e.g. in a type B RCD, is a magnetic current-balance core that has a circuit stimulating it with weak ac and using the relation of current to induced voltage to determine the dc level based on the different magnetic behaviour at different dc levels. 

If there is a fault of negligible impedance just downstream of a rectifier, which is close to the supply of a piece of equipment, wouldn't ADS operate?


In a system where overcurrent protection is enough for ADS, i.e. probably a TN* system, then yes; or else the diode in question might explode first. But, in a TT system with perfectly plausible earthing resistance, e.g. 50 ohm or 300 ohm, a direct L-PE fault through a diode could continue to supply a moderate but easily lethal fault current, and the entire installation's earthing would be near a half-wave 230 V potential. TT systems are surely a minority in the UK now (statistics anyone?) but that's where the biggest concerns about RCDs lie - ADS by TN* methods has very nice features for reliability regardless of waveform, and for limitation of voltage. A high resistance half-wave fault (e.g. condensation) would not give such a problem, as typical TT earthing could still hold the potential well down. 

This begs the question, "What is additional protection for?"


I don't know if there's an official view of which cases are aimed at in the standards. The additional protection provided by a good RCD could give a person a very good chance of being ok even if (1) making direct contact with a live part in a broken flex, plug, etc; (2) making indirect contact via exposed conductive part whose protective conductor has broken; or (3) fiddling around inside an appliance; or (4) exposed to a fault like a bad quality usb charger where the output has become connected to the mains by condensation or other contaminant or a faulted component. 

      In case 1, contacting a cut neutral coming back from an appliance with single-diode rectifier could produce a current that type AC wouldn't respond to, but a bridge-type rectifier in the appliance would give bidirectional current on the ac side, which would be ok with type AC.  The single-diode case is unusual, although it is found in many hairdryers and electric blankets to give the half-heat setting. So additional protection in case 1 is generally ok with type AC: it seems like a very small subset of all case-1 situations where this type would not work. 

      All the other cases, 2--4, involve currents coming from inside loads, so potentially coming from the 'dc side' of rectification, where half-wave currents can arise. Internal faults from dc-side parts to frames are not something I can say I've seen: good equipment makes this very difficult, with  classic heater elements in cookers still being the main cause of appliance faults that I see making significant leakage. I agree with Dave that most of the earth faults that occur are probably plain ac.  My bigger worry is bad equipment, such as a dodgy-imported USB charger with near-zero clearance of input and output parts and with unsuitable capacitors bridging these; or this 'camping lamp' here (notice the diode connection). 

      For myself I wouldn't touch type AC, as I don't like to compromise on protecting from several plausible types of danger for little or no reduction in cost. But if advising others I'd not be confident to justify the parts and labour cost for changing existing devices just to protect against unlikely combinations of events that appear to pose far less risk to life than most people's car use, recreational activities, etc, particularly if the people are sensible with what electrical products they use and buy.  For new installations, no question... 

It sounds as though an IET official view should be provided on the right code for a legacy type-AC in different situations, in order to avoid widely different codes being given by different people. 

so we need to consider other DC connected measuring methods


Yes - we can be confident that any method that should detect milliamps of imbalance against hundreds or thousands of amps of load current will rely on the magnetic field caused by a balance of currents. The usual method to detect this field, e.g. in a type B RCD, is a magnetic current-balance core that has a circuit stimulating it with weak ac and using the relation of current to induced voltage to determine the dc level based on the different magnetic behaviour at different dc levels. 

If there is a fault of negligible impedance just downstream of a rectifier, which is close to the supply of a piece of equipment, wouldn't ADS operate?


In a system where overcurrent protection is enough for ADS, i.e. probably a TN* system, then yes; or else the diode in question might explode first. But, in a TT system with perfectly plausible earthing resistance, e.g. 50 ohm or 300 ohm, a direct L-PE fault through a diode could continue to supply a moderate but easily lethal fault current, and the entire installation's earthing would be near a half-wave 230 V potential. TT systems are surely a minority in the UK now (statistics anyone?) but that's where the biggest concerns about RCDs lie - ADS by TN* methods has very nice features for reliability regardless of waveform, and for limitation of voltage. A high resistance half-wave fault (e.g. condensation) would not give such a problem, as typical TT earthing could still hold the potential well down. 

This begs the question, "What is additional protection for?"


I don't know if there's an official view of which cases are aimed at in the standards. The additional protection provided by a good RCD could give a person a very good chance of being ok even if (1) making direct contact with a live part in a broken flex, plug, etc; (2) making indirect contact via exposed conductive part whose protective conductor has broken; or (3) fiddling around inside an appliance; or (4) exposed to a fault like a bad quality usb charger where the output has become connected to the mains by condensation or other contaminant or a faulted component. 

      In case 1, contacting a cut neutral coming back from an appliance with single-diode rectifier could produce a current that type AC wouldn't respond to, but a bridge-type rectifier in the appliance would give bidirectional current on the ac side, which would be ok with type AC.  The single-diode case is unusual, although it is found in many hairdryers and electric blankets to give the half-heat setting. So additional protection in case 1 is generally ok with type AC: it seems like a very small subset of all case-1 situations where this type would not work. 

      All the other cases, 2--4, involve currents coming from inside loads, so potentially coming from the 'dc side' of rectification, where half-wave currents can arise. Internal faults from dc-side parts to frames are not something I can say I've seen: good equipment makes this very difficult, with  classic heater elements in cookers still being the main cause of appliance faults that I see making significant leakage. I agree with Dave that most of the earth faults that occur are probably plain ac.  My bigger worry is bad equipment, such as a dodgy-imported USB charger with near-zero clearance of input and output parts and with unsuitable capacitors bridging these; or this 'camping lamp' here (notice the diode connection). 

      For myself I wouldn't touch type AC, as I don't like to compromise on protecting from several plausible types of danger for little or no reduction in cost. But if advising others I'd not be confident to justify the parts and labour cost for changing existing devices just to protect against unlikely combinations of events that appear to pose far less risk to life than most people's car use, recreational activities, etc, particularly if the people are sensible with what electrical products they use and buy.  For new installations, no question... 

It sounds as though an IET official view should be provided on the right code for a legacy type-AC in different situations, in order to avoid widely different codes being given by different people.