Zs too high

[babyshanks]

I have a Ze reading of 0.35 on a TNCS system and I've had the DNO out to check and they say its ok.

From my main board I have 2x 10mm sub mains feeding another board and my Zs on the cables is ~0.45 ohms. Max allowed according to BS7671 is 0.35 ohms. What can I do in this instance as the high Zs is caused by the high Ze which the DNO has okayed.

Thanks for the input.

 

[Risteard]

I have a Ze reading of 0.35 on a TNCS system and I've had the DNO out to check and they say its ok.

From my main board I have 2x 10mm sub mains feeding another board and my Zs on the cables is ~0.45 ohms. Max allowed according to BS7671 is 0.35 ohms. What can I do in this instance as the high Zs is caused by the high Ze which the DNO has okayed.

Thanks for the input.

Where does BS 7671 stipulate any such limit? (And incidentally, you have already stated that the DNO have met this imagined "limit".)

 

[Bluetit]

Submains have a Zdb, not a Ze. You mention the submains are fed from the main CU, so as long as your ~0.45 ohm Zdb is suitably low that the protective device feeding the submain will trip within the time (assuming 5s for distribution circuits on this TNC-S) and the adiabatic says the 10mm will survive under that fault current for the time, then to my understanding all is well.

 

[DefyG]

Max allowed according to BS7671 is 0.35 ohms.

For what OCD? There may be other means of over current protection that allow a Zs of0.45 (HRC Fuse?)