How to improve Zs reading on a TT system

[Devon sparks]

Hi new here

Can anyone suggest how I can improve my Zs readings on a TT system for my final cct's.

Already have split board with 2 x 30ma RCD's and type B breakers!

A Should I be changing to RCBO's?

B try adding more earth rods! dont fancy this as problematic...

C what can you suggest PLEASE..............................................................

The main Za reading is 181 ohms which is under 200ohms and I have had to install 3 eath earth rods to get this reading.

Many Tks Devon Sparks

 

[telectrix]

usually your main bonding will bring the readings down. as long as all circuits are on RCDs, then enter 1667ohms on your schedule of test results. .... sorted.

 

[Devon sparks]

Hi sorry
Should have added my final cct's reading are all around 134 ohms guess these are acceptable???

 

[Devon sparks]

Telectrix - Many Tks for your info......

Can you clear something up for me Looking a guide for Zs re circuit breakers and it says in the heading 0.1s to 5s and has reading - for a 32A Device on a type B mcb to BS EN 61009 @ 1.15

What's the best way to check the mcb will operate within said time???

Many Tks Again

 

[telectrix]

if your Zs is above the max.e.g. 1.15 for a 32A type B , as you say, the MCB will not trip within the specified time. trherefore, you are relying on the RCDs to trip (for a TT ststem this is 200mSec.). the RCD is then acting as fault protection instead of the MCB. to comply with disconnection times then, the max. Zs is 1667ohms.

 

[Guest112]

tt disconnection times for devices rated up to 32A 0.2 seconds and over 32A 1s your high readings are acceptable like tele said the max zs for rcd protected circuits (30ma) is 1667ohms which u think is 50v touch voltage divided by the 30ma

 

[spark 68]

Hi,

Longer earth rods are better than more shorter ones and if you are using more than one rod these should be spaced apart by at least the length of the rods.

I personally would try to get the Ze as low as possible (easier said than done, I know!), but as the other posters have said max Zs = 1667 ohms for a 30mA RCD.

 

[telectrix]

just remember that all circuits must be RCD protected. no exceptions.

 

[Devon sparks]

Hi Guys - Brilliant and Thanks very much for all your info
I will take on borad all you say,
N.B. Ground is very difficult as granite area, "real pain"....

So hopefully last question :

How can I best improve the mcb protection so I'm not relying on the RCDs to trip OR is it that is all that is needed for a TT ststem. I.E. the RCD TRIPS WITHIN 200mSec and the Zs is under 200ohms.

What should I put down on the test sht for Max Dis-con times permitted for each cct - 200ms???


Once again Thanks very much for the responce and Advice...

Great Forum well done

 

[telectrix]

What should I put down on the test sht for Max Dis-con times permitted for each cct - 200ms???

​NO!! the 200ohms is the advised max. for the Ra of the rod. the figure you enter for Zs is 1667ohms.

and the disconnection times are 0.2sec for circuits up to 32A and 1 sec. for circuits above 32A

edit. makes note to read post correctly before replying

 

[spark 68]

How can I best improve the mcb protection so I'm not relying on the RCDs to trip OR is it that is all that is needed for a TT ststem.

You have more or less answered your own question here, as Tel said above the RCD takes care of the disconnection times (for earth faults), unfortunately you have to rely solely on the RCD(s) for earth fault protection in this case due to the high Ze.

 

[telectrix]

sorry, mate. mis read your last post as 200ohms, not mSec. you are correct, the 200mSec. is for circuits up to 32A.

 

[Devon sparks]

Apologies : noted

Many thanks

 

[Devon sparks]

All the very best Thanks