Live to Neutral Zs

Question

I have a situation on a caravan site were the sub distribution boards are fed from the main DB via a SHNEIDER nsx MCCB which as a Zs of 0.45 ohms. Zs earth is taken car eof via 300mA rcds, but tlive to neutral Zs on the sub board is is 1.84 ohms(TNC-S). That gives me a problem with the MCCB not tripping in the event of a short circuit. Any suggestions?

mapj1 · Accepted Answer

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

AMK · Answer

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.

AJJewsbury · Answer

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,

mapj1 · Answer

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

Live to Neutral Zs

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