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.

 

[Octopus]

What is the earthing arrangement?

 

[yellowvanman]

What is the earthing arrangement?

TT - earth rod

 

[Octopus]

When you do the 5x test do you find the 100mA trips, in which case you need to reset it quickly to get the 30mA to trip!

 

[Deleted member 9648]

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.

A 30ma RCD should trip at much higher Zs readings than 500 ohms....so no.
Test at RCD with nothing connected to the load side if you are not already.

 

[yellowvanman]

When you do the 5x test do you find the 100mA trips, in which case you need to reset it quickly to get the 30mA to trip!

No 100mA doesn't trip because its time delayed and my tester (Fluke) doesn't apply the fault for long enough!

 

[yellowvanman]

A 30ma RCD should trip at much higher Zs readings than 500 ohms....so no.
Test at RCD with nothing connected to the load side if you are not already.

But my thinking is that for a 30mA to trip you use 50V/30mA = 1667ohms.

When in fact I'm doing 50V/150mA = 333 ohms and my Zs is higher than that!

 

[Octopus]

C1 or C2 then!

 

[guest119]

Have you tried it against the incoming neutral? Just to see?

 

[yellowvanman]

Have you tried it against the incoming neutral? Just to see?

Sorry don't understand what you'e getting at.

If you're implying that I could have PME, then thats possible. There is a label on the meter board that specifically says no PME, but the overheads have been renewed recently and a solution could be to call SSE in to check if PME is now possible. New overheads means there is more earthing along the route, from what I was told it used to be every 4th pole now its every other pole.

- - - Updated - - -

C1 or C2 then!

Not sure what Cat I'm giving it yet, but definately unsatisfactory!

 

[telectrix]

the RCD should trip on test irrespective of the Ze, even if there's no earth at all. you are confusing the 1667 ohms with fault protection. i would suggest replacing the RCD and , as you say, investigating earthing.

 

[Richard Burns]

If you have a Ze of 500 ohms then the fault current would be 230/500 = 0.46 A = 460 mA so there is enough current flow to allow the 150mA current you are applying to test at x5.

If the RCD trips at 100mA and not at 150mA this is strange, I assume you tried a couple of times so it was not just a poor connection at the probe.

If you tested the RCD: load Line to supply neutral, this should have the same effect as testing to earth and would ensure the problem is not the Ze.

Either way it sounds initially like a shot RCD, sorry.

 

[GLENNSPARK]

Sorry don't understand what you'e getting at.

If you're implying that I could have PME, then thats possible. There is a label on the meter board that specifically says no PME, but the overheads have been renewed recently and a solution could be to call SSE in to check if PME is now possible. New overheads means there is more earthing along the route, from what I was told it used to be every 4th pole now its every other pole.

- - - Updated - - -



Not sure what Cat I'm giving it yet, but definately unsatisfactory!

its a 2....dangerous under fault condidions

 

[Engineer54]

With a Ra of 500 ohms, it's at least a code 2, before you even get to the RCD side of things!! At least this TT system has a 100mA S type back-up facility!! So at least 2 code 2's before you get past the CU!! lol!!

Think it would be prudent to enquire about the possibility of a PME connection. 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). But at the moment, your only conducting a EICR. As far as i am aware, it's still every 3rd pole that an overhead supply has a N-E connection made on a PME'd O/H system.

 

[cobrauk]

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

 

[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!!