TT max Zs

[jmtisme]

When filling in EIC for a TT system do I put 1667 ohms in the box for max allowable Zs. For every mcb that is protected with a 30ma RCD

 

[Sintra]

Yes for a 230V supply it is 1667 for each as stated.

 

[rhys4]

Can you help me with two questions. What is the max disconnection time permitted.? And max Zs permitted.?

 

[tomxhug]

chapter 41 of BS7671 has a table not sure i think its 41.1 for disconnection times also look at the 2 top regs of the next page fro Dis circuits and circuits not covered byt the table

 

[Guest123]

Can you help me with two questions. What is the max disconnection time permitted.? And max Zs permitted.?

If you are still referring to a TT system the disconnection times are 1s for distribution circuits and circuits not rquired to have additional protection via 30mA RCD & 0.2s for all others.

There are notes to these times so as Tom said best to read up on pages 46 & 47 of ol' red.

 

[academy]

don't want to hijack this thread but didnt see the point in starting another, I have looked at table 41.1, what would you put if the rcd was time delayed as well? The problem I have is for a TT system to some prefab office units, the rcd that protects them is one of the variable type used on building sites, from 100ma upwards and with a variable rcd with time from 0.25 to 1 second. (it is set at 300ma for 0.25 seconds) The cable from this feeds a tp&n board (with no additional rcd) which in turn feeds each bay (via a 60898) that has a 30ma rcd protecting all circuits. I am filling out the sheets for the circuits that feed each bay and need to put in the max zs. The overcurrent device is a type b 60898 50a which gives an 80% max zs of 0.73, the Zs measured due to TT was 46ohms. Do i put 153 (230-400v 300ma it is protecting a tp&n boar but the circuit itself is single phase) or 167 (120-230v 300ma). Ordo I go and boil my head cos I am making it harder than it needs to be!!!

 

[Guest123]

I would record the lowest value in this case 167ohms, for the highest setting of 300mA.

As you have a pretty good Ra of 46ohms the 300mA RCD will disconnect in the required time anyway should a fault occur upstream of the 30mA RCD's which will also clear any fault downstream before the volatge rises above 50V as the max permitted for them 1667ohms.