A possible solution to your problem is to verify the actual disconnection time for your short circuit at that level of impedance and ensure that it complies with regulation 434.5.2.

What problem does  a higher L-N loop actually impedance give you ? (LN loop is not normally expressed as a Zs value by the way  that is reserved for L-E loop - just LN PSSC.)

How long do the regs require your ADS to break an L-N fault ? - I suggest that you re-check the letter of the rules, as because there is no shock hazard from an LN fault, so there is no need to disconnect anything especially promptly as there would be for  fault between  L-CPC - the only consideration is cable heating, so even if it breaks in a few seconds or even tens of seconds, it is quite likely fine.

What is the rating of the breaker and if it is one of thre NSX models  with a programmable multiplier, what is it set to, and then, what is the thinnest cable it is supplying that is not also protected by something of a lower rating ?

Regards Mike

Reg 434.5.2: A fault occurring at any point in a circuit shall be interrupted within a time such that the fault current does not cause the permitted limiting temperature of any conductors or cables to be exceeded. The adiabatic equation is then shown for time.

There is no shock hazard from an L-N fault, so the disconnection time is not based on the need to prevent electric shock, but rather on the need to prevent damage to the conductors or equipment due to overheating. However, the disconnection time should still be as short as possible to minimize the risk of fire or injury.

That gives me a problem with the MCCB not tripping in the event of a short circuit.

Won't it still trip  via the thermal element?

Although if the loop impedance is that high I'd start worry about voltage drop under full load conditions...

   - Andy,

Well if the loop really is 1,84 ohms the PSSC is ~130 A so 20 odd volt  drop for a 13A kettle. Which sounds about right for a connection box with 16a socket at the remote end of a big campsite.... I know its not compliant, but it is not greatly dangerous. There is a reason caravan kettles take an age to boil after all.

Equally if that is the reading taken in the incoming switchroom, something is probably broken, or the reading is suspect.

Mike

Which sounds about right for a connection box with 16a socket at the remote end of a big campsite.

Provided there's only one hookup - if it's fed by an MCCB, I'm imagining it's feeding quite a few, so the load would be somewhat higher.

   - Andy,