Very High Zs and RCD not tripping

[yellowvanman]

Did an EICR today, Ze was 500ohms!

Went through the whole process though.

There is an up front time delayed 100mA RCD and a 30ma RCD protecting the sockets - 16th Edition board.

100mA trips ok at x 1

30mA trips ok at x1 but not at x5. It does trip at 100mA.

Could it be that the very high Zs causes the 30mA RCD failing to trip at x5?

Obscure make of CU (and RCD) so if this is the cause of the RCD failing then hopefully I won't have to change the RCD if the Zs was more appropriate.

 

[Engineer54]

Are time delay Rcd's allowed on a incoming supply ? I didn't think they were

So where would you incorporate the 100/300mA time delayed RCD (S type) then??

 

[yellowvanman]

I'd still replace the existing Rod with a new 2 X 5/8'' rod position giving the installation a local N-E connection (via the MET).

Is that 1 rod double length or 2 rods sufficiently spaced apart?

I don't hold much hope for the rods though its chalky ground - all installed 6 years ago and the certifcate then claimed a 120ohm Ze!

 

[GLENNSPARK]

Is that 1 rod double length or 2 rods sufficiently spaced apart?

I don't hold much hope for the rods though its chalky ground - all installed 6 years ago and the certifcate then claimed a 120ohm Ze!

you want 3 X 5/8" rods...2 couplers and a striking top....
look for a site about a meter away from any structure (so as not to be just driving it into building rubble and footing)....then dig a pit to ensure are not going to intercept any services (pipes, supply cables & subs etc)....once you are happy theres nothing there that could be damaged...start driving the first rod in....onec you get it nearly all the way use one of the couplers to connect the second rod onto the first....and carry on driving it in....once its there get an Ra/Ze on it....

 

[Deleted member 9648]

you want 3 X 5/8" rods...2 couplers and a striking top....
look for a site about a meter away from any structure (so as not to be just driving it into building rubble and footing)....then dig a pit to ensure are not going to intercept any services (pipes, supply cables & subs etc)....once you are happy theres nothing there that could be damaged...start driving the first rod in....onec you get it nearly all the way use one of the couplers to connect the second rod onto the first....and carry on driving it in....once its there get an Ra/Ze on it....

Better warn the Aussies first.....

 

[GLENNSPARK]

if its good return the third rod to the wholesalers undamaged together with the spare coupler....as long as its unused they should accept it back....if the Ra/Ze is still high then use the third coupler + rod....
depth is key with TT..

 

[GLENNSPARK]

Is that 1 rod double length or 2 rods sufficiently spaced apart?

I don't hold much hope for the rods though its chalky ground - all installed 6 years ago and the certifcate then claimed a 120ohm Ze!

and the conditions at the time of testing were probably the worst case.....worst case for testing that is (wet)...

 

[guest119]

Makita or Kango with a point bit and nose it into the recess in the drive bolt. Happy days.

 

[tony mc]

Did an EICR today, Ze was 500ohms!

Went through the whole process though.

There is an up front time delayed 100mA RCD and a 30ma RCD protecting the sockets - 16th Edition board.

100mA trips ok at x 1

30mA trips ok at x1 but not at x5. It does trip at 100mA.

Could it be that the very high Zs causes the 30mA RCD failing to trip at x5?

Obscure make of CU (and RCD) so if this is the cause of the RCD failing then hopefully I won't have to change the RCD if the Zs was more appropriate.

IMO the 30 mA RCD is not tripping when you are doing the 5 x test as the meter is seeing the touch voltage rise above 50 volts because your Ra is so high. 50/0.150 = 333 ohms

 

[Engineer54]

Is that 1 rod double length or 2 rods sufficiently spaced apart?

I don't hold much hope for the rods though its chalky ground - all installed 6 years ago and the certifcate then claimed a 120ohm Ze!

That would mean, as you put it, ''Double Length'' and should be considered as the minimum requirement for any Domestic TT rod installation.

Which reinforces my claim, that single 1.2m rods will never provide you with the all important stability that a TT system needs. The deeper you go with your earth rod(s), the better it will be be for both the stability and the Ra level of your system. Personally, i haven't used earth rods on any of my projects anything other than 3m X 3/4'' and often, with at least 2 coupled together. ...lol!!