Ok, so I'm still a bit confused about the characteristics of the various types of RCDs, i.e. AC/A/F/B. I've concluded that's because there are two classes of effects: a type of residual current that will cause a trip, and a type of residual current that will blind the device against other types of current it would normally trip on. I find that online discussions and 531.3.3 don't clearly distinguish these cases. Can anyone provide, or give a link to, a clear description of what each type RCD is designed to trip on, and what can blind it? For example, might a smooth DC residual current >6ma blind a type A? And if so, is that completely blind, or just blind against pulsating DC (AC still works)? Etc.
In particular, 531.3.3 says that type A will trip for pulsing DC imposed on smooth DC up to 6mA. Which I guess is a promise that it won't be blinded by smooth DC as long as its <6mA (and no guarantee about AC not being blinded). Type F has a similar note for 10mA. Then type B has a note that it will trip for pulsing DC imposed on smooth DC up to 0.4 x Idn. This seems to imply that even type Bs can be blinded by a large enough smooth DC added to a pulsating DC.
And there seems to be no provision for coordination. E.g. a downstream type B won't protect an upstream of any type "less than" B against residual DC currents which could blind it. So that 300mA fire RCD is stuffed.
It all seems to be a bit of mess, with the only logically safe arrangement being for every RCD to be type B.
How easy it is it to get blinding currents - e.g. could a fault in a cheap AA battery charger do the job?
Well we'll have to rely upon A.D.S. in the form of M.C.Bs tripping off in the case of an earth fault, that and good bonding should make things safe if R.C.D.s can't be trusted to work reliably.
In particular, 531.3.3 says that type A will trip for pulsing DC imposed on smooth DC up to 6mA. Which I guess is a promise that it won't be blinded by smooth DC as long as its <6mA (and no guarantee about AC not being blinded). Type F has a similar note for 10mA. Then type B has a note that it will trip for pulsing DC imposed on smooth DC up to 0.4 x Idn. This seems to imply that even type Bs can be blinded by a large enough smooth DC added to a pulsating DC.
And there seems to be no provision for coordination. E.g. a downstream type B won't protect an upstream of any type "less than" B against residual DC currents which could blind it. So that 300mA fire RCD is stuffed.
It all seems to be a bit of mess, with the only logically safe arrangement being for every RCD to be type B.
How easy it is it to get blinding currents - e.g. could a fault in a cheap AA battery charger do the job?
Well we'll have to rely upon A.D.S. in the form of M.C.Bs tripping off in the case of an earth fault, that and good bonding should make things safe if R.C.D.s can't be trusted to work reliably.
