Earth rod and bonding

[jonthesparks]

I've just done an EICR on an install earthed TT . The Ze on the rod was high 245 ohms , which in theory is fine as it is less than 1667 and the equation Ra X 0.03 > 50v is satisfied .
When the gas and water bonds are reconnected the Ze drops down to 4.4 ohms ,

Do I put another rod in ?
The property is in a built up area with pme available , do I get the Dno to fit pme ?

 

[telectrix]

it's the gas and water pipes bring down the Ze. rather than relying on that, a PME system would be advantageous.

 

[Andy78]

Yes an enquiry for pme would be worth it if the customer fancies the outlay. The measured ze would be worth a mention on the report, as would the less than desirable situation of RCDs providing fault protection where a viable and more reliable alternative exists.

 

[westward10]

The rod resistance shouldn't exceed 200 ohms. It is nearly always the case for it to drop with the bonding connected due to parallel paths. This resistance could even increase in the drier months so yes I would try to look at PME.

 

[madmaz]

Earthrod is unstable you can't rely on the fact that gas and water pipe bonding brings tears resistance down as metallic pipes may be replaced in future with plastic equivalents. Either install new rod or tnc-s enquiry

 

[davesparks]

The rod resistance shouldn't exceed 200 ohms. It is nearly always the case for it to drop with the bonding connected due to parallel paths. This resistance could even increase in the drier months so yes I would try to look at PME.

For what reason should it not exceed 200 ohms?

 

[Wilko]

For what reason should it not exceed 200 ohms?

Hi - Table 41.5 note 2 has an observation that >200 Ohms "may not be stable", but that's all I've seen.

 

[HandySparks]

For what reason should it not exceed 200 ohms?

Ha ha! I was going to say the same, davesparks, but couldn't be arsed to take the flack or argue the to$$.

 

[7029 dave]

If 200 ohms is stable, much better than the ra all over the place,with an earth to earth system which TT is, stability is key.