# Maximum Ze on TT system

Discussion in 'Electricians' Talk' started by 63AMP, Mar 26, 2006.

1. ### 63AMPNew Member

Hi what is the maximum Ze permissable on a TT system I have a reading on an install of 155 ohms (tested with all other earths disconnected). While testing each circit with all main bonding reconnected the Zs readings come out at 3.60 - 4.99 ohms. Whilst realising these readings are above the maximum permissable Zs readings I have a split load consumer unit with 100mA time delay RCD and a 30mA RCD. Any comments would be appreciated.
2. ### sinewaveScrewfix Select

Anything upto 500 ohms is OK on a 100mA and 1666 ohms on a 30mA.

However any reading above 200 ohms is classed as 'unstable' by the IEE, so you need to be under this in general to be safe and compliant.
3. ### 63AMPNew Member

Spot on have NICEIC inspection tommorrow at this job, you start to doubt yourself and your work!!
4. ### dr watsonNew Member

hi max is 200 ohms however they like under 100 ohms this is to be the new max for tat soon i belive as came up in 2391 course last month.
5. ### gaz58New Member

DW
Did you put 200 ohms on the 2391 as required by BS7671 or use the formula answer of 1666ohm?

I put 200ohm but think it could have been either.

Gaz
6. ### dr watsonNew Member

hi 200ohms checked after with lecturer when doing practical said that was right.
7. ### squeakyNew Member

I qoute OSG, page 78

For TT systems, the value of the earth electrode resistance Ra in ohms multiplied by the operating current in amperes of the protective device i^n shall not exceed 50v e.g. if Ra=200ohms, then the maximum RCD operating current should nto exceed 250mA.
e.g.
30mAX1660ohms = 49.8v
8. ### TeesdaleSparkNew Member

I agree with what's been said regarding BS 7671, the Onsite Guide and Guidance Note 3. However the NICEIC book on Inspection and Testing says "BS 7430: Code of Practice for Earthing, suggests that a value of earth electrode resistance exceeding 100ohms may be unstable.
9. ### BandoNew Member

I got caught out on this on a NICEIC inspection. Had a rod measuring 180 ohms on the EIC. He took me back there and asked me to test it. Got a Ze of 600 Ohms It was a split board with 100mA TD. So I had an excessive touch current under fault conditions. I had to go back and drop a second rod at bottom of steps taking it less than 100 Ohms. They charged for the re-visit and made me re-test it.