Earth rod to reduce Zs?

[RF Lighting]

I was working on a circuit today which was fed in 2.5mm² SWA cable buried aruond the perimiter of the building with a circuit length of approximatly 150 meters. The protection device was a 16A type 'C' MCB. The Zs value for the circuit was 3.14Ω which is above the permitted value for the type and rating for the MCB, so we had to replace the MCB for a 10A type 'B' MCB. My question is: Had the circuit current or circuit Zs been higher making it not possible to fit the 10A type 'B' would it have been permissable to install an earth rod at the end of the circuit to reduce the circuit Zs or does this cause problems by introducing an additional potential into the installation and also possibly having two Zs values on the same circuit? This is just to cure my curiosity (again)

Thanks

Rob

 

[cozycats]

post this on the iet website, you're more likely to get an educated answer there. Without knowing more details I would suggest running a 2.5 earth cable from the end if a radial or mid point if a ring back to the met.

 

[plugwash]

adding an earth rod at the end of the circuit would not be a particularlly good idea.

It would have negligable effect on the Zs and if it did reduce the Zs it would also be likely to reduce the quality of the equipotential zone. Also if its PME you could get high current down the circuits CPC even during non-fault conditions (if you do add a rod to a PME system which is actually quite a good idea make sure you use cable at least as thick as the main bonds).

 

[RF Lighting]

post this on the iet website, you're more likely to get an educated answer there. Without knowing more details I would suggest running a 2.5 earth cable from the end if a radial or mid point if a ring back to the met.

It was a radial circuit, and normally I would have run in an additional earth of whatever size needed but with this particular circuit being buried for 150 meters and the flood being mounted on a concrete pad stone at ground level I just wondered out of interest. (I was bored and my mind went a-wandering). Also the earthing system was TN-S but would it be allowed / not allowed / better / worse with different earthing arrangements?

Cozycats do you have a link to the IET website? Or did you mean IEE website?

 

[cozycats]

Sounds like you have a problem but the iee is now the iet. But remember that apparently the nic write the rules now!!!. I would think that if tt-ing it gets the readings down it ought to be acceptable, but who am I???

 

[Spark123]

150m of 2.5mm cable requiring a 16A/10A breaker will have quite a massive volt drop, 43.2v@16A, 27v@10A, 16.2v@6A. What is the design current of the circuit?

 

[RF Lighting]

It is running 630W of SON floodlights (3no. 150W floods and 3no. 70W floods) plus the ballast power (can't remember this. Is it an additional 10W per fitting?) =690W / 230V = 3A So it is not a massive current.
I have 2.5mm² at 18mV/A/m = 8.1V if I have done my sums right and the ballst power factor is right. But its very late so they might be wrong!

 

[Spark123]

It is running 630W of SON floodlights (3no. 150W floods and 3no. 70W floods) plus the ballast power (can't remember this. Is it an additional 10W per fitting?) =690W / 230V = 3A So it is not a massive current.
I have 2.5mm² at 18mV/A/m = 8.1V if I have done my sums right and the ballst power factor is right. But its very late so they might be wrong!

For discharge lighting iirc you need to multiply by lamp wattage by 1.8 and divide by supply voltage.

 

[Spark123]

I'd wait for a 2nd opinion on this but I seem to remember being told it is OK to use an RCD where the efli is high, limiting touch voltage to 50v (or 25v in some special locs). This however will not take into account (L-N) short circuit current + duration nor volt drop.