Confused readings testing TT Ze with fluke 1653

[Paul Willoughby]

Afternoon

I have a TT earthing system with a 63Amp (30ma)RCD Garage unit.

my ze reading earth disconnected is 169 ohms - happy with that falls within range.

when i do a 3 lead test on my fluke for pfc gives me 1 A reading - also happy with this
ohms reading with 3 test lead is 194

when i do zs i get a reading of 195ohms, my R1+R2 Reading is 0.4, it seems that my ze reading should be more inline with the 3 lead pfc test reading.

Am i missing something here, any experienced help would be appreciated.

PAUL

 

[telectrix]

well, i would not be happy with 169ohms. that would give a pefc of 1.36A. the bonding to the water/gas must be giving you this 1kA. get a decent rod installed before eng54 comes on and slaps your legs.

 

[chopper76]

Some cheap rcds can give high readings through them on zs,very often it can only be done by calculation

 

[Engineer54]

Now, what's the betting that the DNO earthing system has been left at the house??


As for being happy with either of those values, ...dear oh dear!!

 

[telectrix]

i win the bet. i said you'd be on this thread in less than an hour. LOL.

 

[Paul Willoughby]

Have dropped another rod in and now have Ze of 115ohms and PFC of 2.1 A

Is it worth me changing the RCD or should I just calculate ZS, the RCD passes RCD test ok.

 

[Engineer54]

Have dropped another rod in and now have Ze of 115ohms and PFC of 2.1 A

Is it worth me changing the RCD or should I just calculate ZS, the RCD passes RCD test ok.

I wouldn't be too happy with that value either ..lol!! Oh and it's NOT Ze, it's Ra when talking about a TT systems earth electrode!!

Do you have a DNO supplied TN-S/TNC-S earthing point at the house, or is the house on a TT system as well??

 

[telectrix]

so now you have discovered that the 3 ft long twiggies are useless as earth rods. go out and buy 5/8" screw together rods. 2 or 3 of these getting depth of 7ft. to 11 ft. might just give you a decent Ra/Ze.

 

[monkeyelectric]

At least 2 or possibly 3 5/8th rods coupled together, they fit lovely inside a kango 900's chuck and can be hammered into pretty much any surface in seconds, i've just done a tt today, 3 rods coupled gave a healthy 2 ohms Perhaps i hit the water main

 

[Paul Willoughby]

I wouldn't be too happy with that value either ..lol!! Oh and it's NOT Ze, it's Ra when talking about a TT systems earth electrode!!

Do you have a DNO supplied TN-S/TNC-S earthing point at the house, or is the house on a TT system as well??

DNO supplied TNCS at the house, swa run to garage TT as earthing for garage, Ze / Za on certificate is zs at db, I have joined to 4ft 5/8th rods together and still only get 115ohms, will this value not suffice? ground is very hard and dry, I have even tried running the hose on it.

 

[telectrix]

so why on earth ( no pun intended) have you not used the TNC-S that is a better earth than any rod?

 

[tony mc]

Why not export the earth to the garage ?

Do you have any extraneous parts in the garage that need bonding as this would need to be a minimum of 10 mm csa!

 

[Paul Willoughby]

No extraneous parts in garage.

Reason for not exporting earth is: One of the potential faults with exporting an earth from a TN-C-S (PME) head end is that consideration needs to be given if the suppliers neutral were ever disconnected but the live left unaffected. This would leave the neutral and the earth floating. Given it would be connected to the live side of the supply via all your appliances, and there would be no current flowing, the potential of the neutral and earth (and by implication the metal cases of all your appliances) would tend to rise to 240V. This obviously raises a serious shock risk.

or have i got this all wrong, phoned Elecsa and they recommended using earth rods for anything over 10meters from the house and getting the lowest earth reading possible under recommended values.

Information overload here.

 

[SPARTYKUS]

Paul,

surely the chance of that happening applies to about 40 million homes in the uk therefore they should all be TT'd or converted back to TNS....

 

[stuartcourt]

No extraneous parts in garage.

Reason for not exporting earth is: One of the potential faults with exporting an earth from a TN-C-S (PME) head end is that consideration needs to be given if the suppliers neutral were ever disconnected but the live left unaffected. This would leave the neutral and the earth floating. Given it would be connected to the live side of the supply via all your appliances, and there would be no current flowing, the potential of the neutral and earth (and by implication the metal cases of all your appliances) would tend to rise to 240V. This obviously raises a serious shock risk.

or have i got this all wrong, phoned Elecsa and they recommended using earth rods for anything over 10meters from the house and getting the lowest earth reading possible under recommended values.

Information overload here.

If this was such a high risk they would make us earth rod as back up also, export the earth

 

[telectrix]

solution. use the PME buyt leave the rods in just in case . sssimpleess.

 

[brucelee]

use the TN-C-S then your Ze will be below 0.35 ohms or should be lol so the PFC will be a lot better and not jsut relying on a RCD to hopefullly work and disconnect in the required time

I used a 6mm 3 core swa to supply my bruvs shed a run of about 30 metres and all Zs comply so the RCD is only there for additonal protection this job was used for my assessment I wanted to use 10mm to lower Vd as it was close in the end with final circuits for the lights but within the 3% max and the 5% for power for the sockets and 4mm swa 3 core which then feeds a double socket and FCU on decking all comply with Zs, and Vd
I am with ELCSA and was given was awarded an A and congratulated on my design

 

[Engineer54]

Now, what's the betting that the DNO earthing system has been left at the house??

I just knew that he had left a perfectly good earth at the house and was struggling to get a decent Ra level with a rod!!

Forgive the Pun, but your just making a rod for your own back. Leave the rod in the ground and it's connection to your garage CU and use that PME earth connection at the garage CU. Forget all the scaremongery you've been told it's rubbish. There is nothing wrong with extending a PME earth out to a garage, where you will have a very respectful min Ze of 0.35 ohms or thereabouts, rather than a limp 115 ohm Ra!!

 

[Paul Willoughby]

Took advice and used PME earth at garage with 3rd core and by SWA Gland to earthing terminal, zs at db reading of 0.45 ohms.

Just confirming this is acceptable as this reading would now be Ze of the TNCS supply + R1+R2 of the SWA.

Yes?

 

[spark 68]

Now Zdb at your sub-board, which is Ze (at origin) + (R1+R2) of your sub-main (armoured cable).

You only have one Ze, this is at the origin

 

[Paul Willoughby]

Now Zdb at your sub-board, which is Ze (at origin) + (R1+R2) of your sub-main (armoured cable).

You only have one Ze, this is at the origin

thanks for confirming, finally got it now...