High ELP reading

[IMH]

Hi iam testing an istrallation on a TT system, the ELP readings at the board are 223 ohms, when i conduct an rcd test on a secondard distribution board the rcd 30mA fails to operate. The earthing system is in place,IE water gas are bonded and there are four earth rods installed any sujestions??

 

[MarkieSparkie]

Hi, welcome to the forum. What is the Ra of your earth electrode(s) ... 223 Ohms? If so you need to go much deeper with the rods, either extend the existing ones (if possible) or sink new ones.
Stability of a single rod earth electrode often causes confusion. The depth of a vertical rod is the decisive factor for stability as the effects of drying and freezing effect the upper part of the rod most significantly. Therefore a 3m rod with a R[SUB]A[/SUB]=300 Ohms will be more stable than a 1.5m rod with a R[SUB]A[/SUB]=100 Ohms. Although BS7671 says <200 Ohms for stability this is not a guarantee, depth is the key as stated above, you should aim to get your Ra below 100 Ohms and the lower the better.

 

[Deleted member 9648]

If the RCD fails to trip you need to look at other causes...a 30ma RCD will operate at much higher earth loop impedances than 223 ohms,in fact it will meet the requirements up to 1667 ohms. Of course you need to improve the present Ra, but this will not be the cause of the RCD not operating.

 

[IMH]

Hi thanks for the advice:icon4:

Hi, welcome to the forum. What is the Ra of your earth electrode(s) ... 223 Ohms? If so you need to go much deeper with the rods, either extend the existing ones (if possible) or sink new ones.
Stability of a single rod earth electrode often causes confusion. The depth of a vertical rod is the decisive factor for stability as the effects of drying and freezing effect the upper part of the rod most significantly. Therefore a 3m rod with a R[SUB]A[/SUB]=300 Ohms will be more stable than a 1.5m rod with a R[SUB]A[/SUB]=100 Ohms. Although BS7671 says <200 Ohms for stability this is not a guarantee, depth is the key as stated above, you should aim to get your Ra below 100 Ohms and the lower the better.