TT Systems

[daisy]

Hi everyone.
A large industrial installation supplied with a T T System. The earth electrodes plus main earthing conductor measuring approx 10 ohms Ze. The mainswitch is a 800 amp Mccb with adjustable earth leakage.
My problem is this.
Because of the high Ze ( though not bad for TT.) the Mccb with earth leakage would ensure disconnection times at the mains position but what about say a lighting circuit at the far end of the plant.There could be two or three dis boards to get to this point and seeing as this lighting circuit would be above the max Zs do we have to install many Rcd"s with time delay,to achieve disconnection times for all circuits.

Thanks Daisy.

 

[ICE]

In Part 4 of the regulations the tables for max. Zs are for what sort of earthing systems? Hopefully in this installation there are further RCDs to discriminate with the main MCCB otherwise it would be very inconvenient and contrary to Regulation 314.1

 

[mrloy99]

Hi everyone.
A large industrial installation supplied with a T T System. The earth electrodes plus main earthing conductor measuring approx 10 ohms Ze. The mainswitch is a 800 amp Mccb with adjustable earth leakage.
My problem is this.
Because of the high Ze ( though not bad for TT.) the Mccb with earth leakage would ensure disconnection times at the mains position but what about say a lighting circuit at the far end of the plant.There could be two or three dis boards to get to this point and seeing as this lighting circuit would be above the max Zs do we have to install many Rcd"s with time delay,to achieve disconnection times for all circuits.

Thanks Daisy.

Hi Daisy
just a few pointers
1--your time delayed one would be closest to the origin.
2--you would have structural steelwork supplementry bonding efectively lowering
your ohmic values
3--have a look at table 41.5-----30ma Zs1553ohms
4--if each sub board is supplied TT they could be treated as "origin" ie; they could each have their
own rcd AND dedicated earth electrode,etc

 

[daisy]

Hi Everyone.
Thanks guys for your comments.Its confirmed to me that the installation does need more RCD"s, with discrimination.
I think TT in a large industrial installation sucks.
Thanks again Daisy.