I've definitely seen RCDs with longer trip times at 5IΔn than at IΔn, so IΔn is not always the "worst case" either!

In which case, can we have any confidence that a 1x (or 5x) test will give realistic assurance that the RCD will open in the required time when subject to a real earth fault - which could be many amps in magnitude?

Most RCD testers do not test at 2IΔn, and therefore the only option is 5IΔn.

Cue for a new range of "18th Ed" RCD testers with a 2x option?


Perhaps more practically we should only be thinking in terms of 'sampling' the RCDs performance at a few points and if they're within spec then say it's reasonable to assume that the RCD is functioning as designed and so will operate within specification throughout its entire range. Perhaps single test (1x perhaps) would be sufficient. I gather that some manufacturers suggested that operating the T button and observing an 'immediate' trip could be sufficient.

These require RCDs for Additional Protection - in this case this would invoke the 5IΔn test. To add all these requirements together, we have a maximum loop impedance for a 30 mA RCD of 230*.95/0.15 Ω (where additional protection is needed), i.e. 1457 Ω and not 1667 Ω

Humm - not sure of the logic there - additional protection is usually aimed at situations of direct contact (nails through cables, flexes cut by lawnmowers, touching the contacts of lampholders) - where the c.p.c. and hence Zs aren't usually of relevance (likewise any calculations involving Uo or Cmin). In a functioning installation we're only really worried about 150mA going through the victim's body, where it goes after that isn't a concern. Certainly for a successful test we'd need a lowish loop impedance, but that requirement doesn't seem to be obviously written into BS 7671.


Would you think it acceptable to perform a 5x test by placing the tester's earth probe on the supply N rather than PE? That would avoid problems due to a high Zs but still test the RCD's fundamental operation?


    - Andy.

I've definitely seen RCDs with longer trip times at 5IΔn than at IΔn, so IΔn is not always the "worst case" either!

In which case, can we have any confidence that a 1x (or 5x) test will give realistic assurance that the RCD will open in the required time when subject to a real earth fault - which could be many amps in magnitude?

Most RCD testers do not test at 2IΔn, and therefore the only option is 5IΔn.

Cue for a new range of "18th Ed" RCD testers with a 2x option?


Perhaps more practically we should only be thinking in terms of 'sampling' the RCDs performance at a few points and if they're within spec then say it's reasonable to assume that the RCD is functioning as designed and so will operate within specification throughout its entire range. Perhaps single test (1x perhaps) would be sufficient. I gather that some manufacturers suggested that operating the T button and observing an 'immediate' trip could be sufficient.

These require RCDs for Additional Protection - in this case this would invoke the 5IΔn test. To add all these requirements together, we have a maximum loop impedance for a 30 mA RCD of 230*.95/0.15 Ω (where additional protection is needed), i.e. 1457 Ω and not 1667 Ω

Humm - not sure of the logic there - additional protection is usually aimed at situations of direct contact (nails through cables, flexes cut by lawnmowers, touching the contacts of lampholders) - where the c.p.c. and hence Zs aren't usually of relevance (likewise any calculations involving Uo or Cmin). In a functioning installation we're only really worried about 150mA going through the victim's body, where it goes after that isn't a concern. Certainly for a successful test we'd need a lowish loop impedance, but that requirement doesn't seem to be obviously written into BS 7671.


Would you think it acceptable to perform a 5x test by placing the tester's earth probe on the supply N rather than PE? That would avoid problems due to a high Zs but still test the RCD's fundamental operation?


    - Andy.