TT network

[grumpyjohn01]

Have recently been on an industrial job where a 3 phase 200A supply is distributed as a TT network between DB's in 1 large building and subbed out to others via 5 core cables, ie The cpc is continuous throughout.There are stakes at the origin and at all db's outside the eq zone.There is an S type rcd at the origin and another at the main panel that appear to discriminate.No further Rcd's other than RCBO's in local boards to give 30ma protection to sockets etc that require additional protection.It was wired to the 17th.Am aware of the need for each cpc to be able to carry all fault currents but what do people think of this set up? overall Ze with everything connected is as low as 5 ohms. comments appreciated. could work out a schematic if anyone is interested.

 

[Engineer54]

That sounds like a typical industrial TT system to me John. Some S type RCDs have adjustable elements and others are time set. I'm surprised that the 5 ohm Ra isn't lower, if all of the several stakes have been linked out...

 

[grumpyjohn01]

Thanks for that, my experience of TT in an industrial setting is limited so advice is appreciated.
Another query about the same install then....
My MFT (Dilog) on performing an Earth loop gives 2 results: Earth fault loop impedance & Line loop impedance . In this case clearly The line loop is much lower..question is which value to record as Zs when measuring for final circuits? my colleague has Megger MFT and this only gives the lower reading.Amtech doesnt like this as it exceeds the auto filled max Zs for the device.In this case should one record the max Zs for the nearest upstream RCD ie 1667ohms or whatever and the earth fault loop for the overcurrent device max Zs and the lower figure from the tester? If the former is the case how would my colleague get on not having the true earth loop from his Megger?