I can't say it's a test I've attempted - but many modern RCDs will have electronics inside that will need to draw power from the supply when the RCD is switched on - so some measurable resistance on one or other set of terminals doesn't surprise me. <1k feels a bit low, but often d.c. meter reading aren't representative of how the circuit will behave on mains a.c. (e.g. once capacitors are charged up), so I wouldn't be unduly worried if there are no other negative symptoms.

Some seem to power the electronics across the incoming terminals (so they're out of circuit from the load when the RCD is open) - handy for insulation testing downstream - others seem to use the outgoing terminals (which is helpful if you're testing the supply side - e.g. submains). And of course many designs allow connection either way around anyway.

   - Andy.

I can't say it's a test I've attempted - but many modern RCDs will have electronics inside that will need to draw power from the supply when the RCD is switched on - so some measurable resistance on one or other set of terminals doesn't surprise me. <1k feels a bit low, but often d.c. meter reading aren't representative of how the circuit will behave on mains a.c. (e.g. once capacitors are charged up), so I wouldn't be unduly worried if there are no other negative symptoms.

Some seem to power the electronics across the incoming terminals (so they're out of circuit from the load when the RCD is open) - handy for insulation testing downstream - others seem to use the outgoing terminals (which is helpful if you're testing the supply side - e.g. submains). And of course many designs allow connection either way around anyway.

   - Andy.