Type A rcd . EICR coding ? etc

Comments on a few recent posts:


Testing of type-AC RCDs.  I've done some of this in a variety of ways last year, as a side issue to an assignment about testing type-A with smooth dc.  (Where I am, there's no type AC anyway. The AC interest was because of my UK connections.)  

 1:  I saw, as I've mentioned here earlier, that some AC-branded RCDs basically behave as type A. So don't be too confident from testing a single unit, e.g. about what levels and waveforms cause a trip. A really classic type AC with hard magnetic core may be different: identify them by failure to trip on one polarity of half-wave rectified residual current, even at many times the rated tripping level.  When standards about installations and appliances specific RCD types, they appear to focus on how an standards-comformant RCD must behave, rather than how a typical one actually does. 

 2:  I'd recommend against MFTs if you've an oscilloscope available. Or, at least, use the oscilloscope too. Otherwise, you're guessing at what the MFT responds to, or what currents it applies at different times. If you want a simple variable resistor for leakage currents, a wire dipping in a saucer of water gives quite good control. 


When this manufacturer says it doesn't any longer require type A, I assume it means there are others (such as F, B, etc, etc) that would be ok, not that AC is suitable. 


Yes, it gets complicated between standards and manufacturers, and meat-in-the-sandwich as was mentioned some months ago.  If I remember right, the opposite happened with PV inverters some 10 years ago, after the competition to have high efficiency had caused them to drop the isolating transformer that older models had. Then there was fuss about needing type B RCDs in case of internal faults.  Given the availability and cost of these RCDs, there was a definite benefit for inverter manufacturers who could state that (rather like some EV chargers) they already had suitable protection built in to prevent DC into the AC side or at least to detect low levels and disconnect. (I'm only going by distant memory .. should look this up better if I had time.)  The special feature there is that an electrical-specialised person probably chooses the components (inverter etc) to suit the customer's project and give overall lowest cost, so they see the advantage of paying x more for the inverter to save 2x on the RCD.  With boilers, washing machines etc the item is typically chosen by others, and the electrician gets, too late, the job of fulfilling the manufacturer's requirements or the wiring-regs requirements in view of what leakage currents the manufacturer says could arise.

Comments on a few recent posts:


Testing of type-AC RCDs.  I've done some of this in a variety of ways last year, as a side issue to an assignment about testing type-A with smooth dc.  (Where I am, there's no type AC anyway. The AC interest was because of my UK connections.)  

 1:  I saw, as I've mentioned here earlier, that some AC-branded RCDs basically behave as type A. So don't be too confident from testing a single unit, e.g. about what levels and waveforms cause a trip. A really classic type AC with hard magnetic core may be different: identify them by failure to trip on one polarity of half-wave rectified residual current, even at many times the rated tripping level.  When standards about installations and appliances specific RCD types, they appear to focus on how an standards-comformant RCD must behave, rather than how a typical one actually does. 

 2:  I'd recommend against MFTs if you've an oscilloscope available. Or, at least, use the oscilloscope too. Otherwise, you're guessing at what the MFT responds to, or what currents it applies at different times. If you want a simple variable resistor for leakage currents, a wire dipping in a saucer of water gives quite good control. 


When this manufacturer says it doesn't any longer require type A, I assume it means there are others (such as F, B, etc, etc) that would be ok, not that AC is suitable. 


Yes, it gets complicated between standards and manufacturers, and meat-in-the-sandwich as was mentioned some months ago.  If I remember right, the opposite happened with PV inverters some 10 years ago, after the competition to have high efficiency had caused them to drop the isolating transformer that older models had. Then there was fuss about needing type B RCDs in case of internal faults.  Given the availability and cost of these RCDs, there was a definite benefit for inverter manufacturers who could state that (rather like some EV chargers) they already had suitable protection built in to prevent DC into the AC side or at least to detect low levels and disconnect. (I'm only going by distant memory .. should look this up better if I had time.)  The special feature there is that an electrical-specialised person probably chooses the components (inverter etc) to suit the customer's project and give overall lowest cost, so they see the advantage of paying x more for the inverter to save 2x on the RCD.  With boilers, washing machines etc the item is typically chosen by others, and the electrician gets, too late, the job of fulfilling the manufacturer's requirements or the wiring-regs requirements in view of what leakage currents the manufacturer says could arise.