Very Hi ZS Reading on TT system? any advice

[srl-8]

Hi guys,

i have changed a fuse board today in a outbuilding/large shed (it has a kitchen and shower and lounge etc.. a )

It is a TT system with earth rod.

My ZE reading was 187 ohms - now the regs state I am ok upto 200, so I assumed this was ok.

All the R1 + R2 and insulation resistance came back normal.

When I carried out the ZS tests, the reading were very high, for example:

Shower circuit on a 40a MCB - 2.6 omhs !
Kitchen Ring on a 32a mcb - 2.7 omhs !

same for lounge and bedroom rings.

I have checked to regs and these figures are over the maximum allowed.


Any advice ???

 

[timbobelfast]

And was the ground wet? Ie in favourable conditions?

If so then you could try coupling two rods together if they are 5/8s.

 

[telectrix]

max. Zs is 1667 ohms if 30mA RCD fitted to all circuits.

 

[IQ Electrical]

I checked the date first but it's not April 1st.

What Zs readings would you expect on such a system with an Ra value of 187 Ohms?

Your bonding will have reduced Zs already but RCD protection will give you much higher max Zs figures depending on RCD ratings (up to 1667 Ohms for 30 mA)

You really need to brush up on the reg's before you undertake such work, clients are paying for your competency and skills...

 

[srl-8]

teletrix. just to confirm, you are saying the my readings were acceptable ? I understood that (on page 103 of osg) my maximum ZS for a type b 40a Breaker is .093 ohms. ?? The reading i got was 2.6 ohms ??

where are you getting the 1667 from ?

thanks

 

[jaresquire]

oh dear..

 

[Des 56]

My ZE reading was 187 ohms

So far so good,its after that you are losing the plot

Clue for research purposes only
Its a TT system with a 30 m/amp rcd

 

[srl-8]

Ok, so on page 50 on the regs, is states that the max ZS is 1667 for TT, so does page 103 of the OSG not contridict this ?

Sorry, but i am a bit confused......

 

[GLENNSPARK]

i think its time the o/p took a look at 411.5.....

 

[Richard]

Sorry and everything, but oh my god! You quite obviously don't really understand what your doing! If you had 180ohm odd as a Ze, then theres no way in hell your going to get 2.6 ohm Zs!! Zs will never be lower than Ze

 

[JONOXNAM]

Hi srl-8 If you cannot get get your Zs down to meet disconnection you can employ an RCD but remember touch voltage 1667 x 30mA = <50V (Touch voltage).

Also remember disconnection in TT is less than a TN system i.e. 0.2s vs 0.4s - 411.3.2.2

Plus are the figures correct - remember Zs = Ze + (r1+r2) so Zs cannot be lower than Ze...

 

[telectrix]

Sorry and everything, but oh my god! You quite obviously don't really understand what your doing! If you had 180ohm odd as a Ze, then theres no way in hell your going to get 2.6 ohm Zs!! Zs will never be lower than Ze

yes it will, due to parallel paths via the bonding of gas and water. lead pipe is a bloody good earth.

 

[Octopus]

Sorry and everything, but oh my god! You quite obviously don't really understand what your doing! If you had 180ohm odd as a Ze, then theres no way in hell your going to get 2.6 ohm Zs!! Zs will never be lower than Ze

Sorry Richard, but I have to disagree with you. From my experience a ZS on a TT system is more often than not lower than the ZE. This is due to parallel paths through water and gas services.

 

[Richard]

yes it will, due to parallel paths via the bonding of gas and water. lead pipe is a bloody good earth.

Indeed, the readings obtained don't suprise me atall!

 

[Richard]

To be fair, the OP hasn't mentioned anything about bonding. But yes, i'd aggree that more often than not, a TT system with a lead water pipe will he significantly lower Zs's than the Ze.