IT system then? (impedance earth)
- Andy.

Andy

To retain the advantage of the small fault current (damage and EMC), an impedance-earthed
TT (r » 12 W/Id = 20 A) emerges with a single earth connection. This system requires the use of
a surge limiter if the MV zero sequence current exceeds » 80 A - DDRs are used in the same
way (time discrimination)

Hybrid TNS/TT.

Regards

Sorry, I'm still struggling to picture this - are you saying it's like IT but with the consumer's earth directly connected to the substation's earth? (unlike IT where the consumer has an independent electrode) - "IN-S" I suppose?
- Andy.

Thanks for the paper Parsley!

I think I see what Schneider are on about now (wanting the low earth fault currents of TT, but with the EMC and MV (HV?) fault resilience of TN-S solidly earthed consumers.

I still think it would be classified as IT though - source earthed through an impedance, consumer's exposed-conductive-parts directly connected to earth (not directly to supply N). The only difference in the layout is that the source and consumer share an earth electrode - which seems to make little difference as you could have a separate electrode and reduce the impedance by the electrode's resistance for a similar overall effect.

Interesting all the same!

I can see how it might gradually replace an entirely TT system, but upgrading an existing TN distribution system would be a nightmare.

- Andy.

I agree upgrading an existing distribution system would be impossible, maybe an installation fed from it's own TX would benefit though.

Regards