Earth Rod Reading?

[TPES]

Evening all...

I think ive asked this before but just want to check...

If i take Ze at the board this is the same reading as the earth elctrode resistance. so to take me "Electrode Resistance" this is all i do? and put the same value in both boxes on the cert (being Ze & earth rod)

I have taking my Ze, and it is coming up as "120" which is withing the rod resistance of 200.

But when i take my R1+ R2 and add these together for My Zs the figure is above the Zs limits and fails.

Example.. 40a, Type B, 60898. Max Zs = 1.15
Ze = 120 add R1+R2 0.10... Total = 130 Fail

Am i doing somethig wrong or do i just need to get a better resistance on the e/rod ?

 

[piz]

Hello I think you put the reading as max Zs down on cert as 200ohms then add R1+R2 =120.10ohms

 

[TPES]

Hello I think you put the reading as max Zs down on cert as 200ohms then add R1+R2 =120.10ohms

Mmmmm, That dont seem quite right to me

I've just remembered.....

All the Max Zs limits in the regs book as displayed are not the limits are they... seems daft to me..
But i have to multiply the limit in the table by 0.8 (correction factor) dont i ?

 

[Guest123]

Chaps.

On a TT system max Zs is 1667ohms providing that 30mA RCD is installed.

Your rod reading is fine.

And no you dont multiply by 0.8 as it clearly asks for max Zs permitted by BS7671, not on site guide.

 

[piz]

Hi Lenny

p50 of the regs says 1667 yeah your right so when i called nic they say max zs permitted by bs7671 is 200ohms
for the cicuit being tested how come

 

[TPES]

Chaps.

On a TT system max Zs is 1667ohms providing that 30mA RCD is installed.

Your rod reading is fine.

And no you dont multiply by 0.8 as it clearly asks for max Zs permitted by BS7671, not on site guide.

Im confused

So on the cert, under circuit details where it says "Max zs permited by bs7671" i would put in 1.15 is this correct?

Then if my R1+R2 is 0.10 and my Ze is 120 added together i get 130,More than what i wrote in the max permited box of 1.15

 

[Guest123]

The 200ohm limit is the max for earth electrode resistance as anything above this regarded as unstable.

The 1667 limit is the max for a 30mA RCD to operate within the safe touch voltage of 50V. Although you should never come across anything this high and thats where the 200ohms comes in.

Im confused

So on the cert, under circuit details where it says "Max zs permited by bs7671" i would put in 1.15 is this correct?

No, 1667.

 

[TPES]

The 200ohm limit is the max for earth electrode resistance as anything above this regarded as unstable.

The 1667 limit is the max for a 30mA RCD to operate within the safe touch voltage of 50V. Although you should never come across anything this high and thats where the 200ohms comes in.



No, 1667.

So because its a TT supply, and only because of this i would write "1667" in every column under max zs permited by bs7671 no matter what there circuit is or what size breaker is on it?

 

[Guest123]

Correctamundo.

Provided there is 30mA RCD present, which there should be.

 

[piz]

your G8 Lenny and make sense for me and lots of others
the disconnection time should be 0.2 on tt is that right?? my nic software says 0.4 or 5 secs dont give me a choice of 0.2

 

[Spudnik]

0.2 since the 17th for TT

 

[TPES]

Oh right excellent... Ive not had much of experiance with TT systems, And when i speak to other sparks there not to sure either..

Also how come then that the regs state the Maz Zs table for TN systems being, 16=2.87, 20=2.30, 32=1.44, 40=1.15 these are different to what is in the NIC test cert book front page. NIC have multiplied all Max Zs from the regs and X by 0.8. which is right?

 

[Guest123]

0.2s.

Oh right excellent... Ive not had loads of experiance with TT systems, And when i speak to other sparks there not to sure either..

Also how come then that the regs state the Maz Zs table for TN systems being, 16=2.87, 20=2.30, 32=1.44, 40=1.15 these are different to what is in the NIC test cert book front page. NIC have times all Max Zs from the regs and X by what the regs says. which is right?

If you read the inside cover of the pad it says something like these values are 80% of the values of BS7671 i.e already corrected.

 

[piz]

Thanks len
but new software still only got 0.4 secs and 0.5 secs have to over type for 0.2
not good when spending so much money should I point out this is easy cert software

 

[Spudnik]

As lenny said, these are the corrected values and your readings should ideally be under those.