I have both TNS and IT system and While sizing the cables, Can I use the below equation for both the TNS and IT systems, in arriving at the maximum cable length for the end of fault current.
I have both TNS and IT system and While sizing the cables, Can I use the below equation for both the TNS and IT systems, in arriving at the maximum cable length for the end of fault current.
Looks like this formula is only for voltage drop. There are other factors such as thermal effects (adiabatic) and meeting the relevant disconnection times for protection against electric shock which may shorten the maximum length.
In this formula, looks like Rc and Xc are provided in ohms per km or milliohms per metre (not a problem, just pointing this out).
Looks like you are using a voltage factor Cmin of 0.8 at the utilisation (far) end of the cable, which assumes a maximum voltage drop of 20 % - assuming U is the nominal voltage (the peak being U x Cmax) it exceeds the usual limits of product standards, and IEC and CENELEC standard voltages … but that may well be OK for your application but not in general.
Instead of Im, BS 7671 would probably use In (nominal rating of the device) or Ib (design current) if this is lower than Im.
Not wanting to restrict the discussion to the UK (where PEN conductors are largely unused in private installations), I also don't believe the formula is valid for single phase circuits where the metallic sheath or armour of a cable is used as a PEN conductor.
I have both TNS and IT system and While sizing the cables, Can I use the below equation for both the TNS and IT systems, in arriving at the maximum cable length for the end of fault current.
Looks like this formula is only for voltage drop. There are other factors such as thermal effects (adiabatic) and meeting the relevant disconnection times for protection against electric shock which may shorten the maximum length.
In this formula, looks like Rc and Xc are provided in ohms per km or milliohms per metre (not a problem, just pointing this out).
Looks like you are using a voltage factor Cmin of 0.8 at the utilisation (far) end of the cable, which assumes a maximum voltage drop of 20 % - assuming U is the nominal voltage (the peak being U x Cmax) it exceeds the usual limits of product standards, and IEC and CENELEC standard voltages … but that may well be OK for your application but not in general.
Instead of Im, BS 7671 would probably use In (nominal rating of the device) or Ib (design current) if this is lower than Im.
Not wanting to restrict the discussion to the UK (where PEN conductors are largely unused in private installations), I also don't believe the formula is valid for single phase circuits where the metallic sheath or armour of a cable is used as a PEN conductor.
