Lmax - Maximum Cable Length equation for TNS Vs IT system

Odd formula - are R and X per unit length, or the full value for the length under consderation.

Also the factor of 1k is a bit odd.  What are the other units ?

This is of the right form but you need to keep your wits about you  - I presume that Im relates to instant breaking and already includes the edge of tolerance ,so for example your ‘Im’ for a 10 amp B-type MCB might be 5 times the nominal rating, 50 amps.

Is U the supply voltage, after allowing for the voltage drop of the supply side upstream of the cable in question ?  A PME supply may manage ⅓ of an ohm of Zdb, but an IT system may not depending how it is derived. You may need to add in the effect of impedances outside the building, as these are still in the fault loop.

And do you need instant trip, or is fault protection provided elsewhere ? You may be fine with a higher fault impendance.

Mike

Odd formula - are R and X per unit length, or the full value for the length under consderation.

Also the factor of 1k is a bit odd.  What are the other units ?

This is of the right form but you need to keep your wits about you  - I presume that Im relates to instant breaking and already includes the edge of tolerance ,so for example your ‘Im’ for a 10 amp B-type MCB might be 5 times the nominal rating, 50 amps.

Is U the supply voltage, after allowing for the voltage drop of the supply side upstream of the cable in question ?  A PME supply may manage ⅓ of an ohm of Zdb, but an IT system may not depending how it is derived. You may need to add in the effect of impedances outside the building, as these are still in the fault loop.

And do you need instant trip, or is fault protection provided elsewhere ? You may be fine with a higher fault impendance.

Mike