Max zs readings for TT

Ruairi Hansen · May 22, 2019

Hi just wondering what people are noting as there max permitted zs reading on test results sheet for TT systems? 200ohms or 1667ohms?

ferg · May 22, 2019

200 is just advisory as above 200 may not be stable.

So I record the theoretical value of 1667.

Charlie_ · May 22, 2019

I go by what the dno insist on and the only figure that is realistically acceptable <200

Ruairi Hansen · May 22, 2019

What do niceic want to see?

Ian1981 · May 22, 2019

Ruairi Hansen said:
Hi just wondering what people are noting as there max permitted zs reading on test results sheet for TT systems? 200ohms or 1667ohms?

Depends on the rcd used for fault protection ie 500mA,300,100,30?

Leesparkykent · May 22, 2019

Max Zs depending on mA of RCD. Ze/Ra then above 200Ohms is not considered stable by the regs and if you enquire the DNO normally state 21 Ohms for their electrode

Nigel · May 22, 2019

Max Zs permitted is the maximum value that the OCPD need to trip. It is not the DNO value or the 200 ohm stable value.

Deleted member 9648 · May 22, 2019

essex said:
Max Zs permitted is the maximum value that the OCPD need to trip. It is not the DNO value or the 200 ohm stable value.

There is no requirement for Zs to be of a value to operate the OCPD within the required time in the event of a fault to earth. The preferred means of earth fault protection on a TT is an RCD (411.5.2) and the maximum Zs value will be in accordance with table 41.5. (411.5.3).
The maximum value whether you like it or not is 1667 ohms for a 30ma RCD and that is the value to be recorded on a cert for a 30ma RCD.

Ruairi Hansen · May 22, 2019

I was thinking it had to be 1667 if 30mA 500 for 100mA etc as that is what it says on 41.5 but I was hearing different things.

Deleted member 9648 · May 22, 2019

Ruairi Hansen said:
I was thinking it had to be 1667 if 30mA 500 for 100mA etc as that is what it says on 41.5 but I was hearing different things.

You do hear a lot of different things on this, a lot of people simply don't accept what the BBB says. But it does say that so that's the values we have to use when compiling a cert.

Nigel · May 22, 2019

wirepuller said:
There is no requirement for Zs to be of a value to operate the OCPD within the required time in the event of a fault to earth. The preferred means of earth fault protection on a TT is an RCD (411.5.2) and the maximum Zs value will be in accordance with table 41.5. (411.5.3).
The maximum value whether you like it or not is 1667 ohms for a 30ma RCD and that is the value to be recorded on a cert for a 30ma RCD.

Yeah that is what I said ‍

Charlie_ · May 22, 2019

Just realised that this thread was about Zs not Ze or Ra..
My comment about the DNO figure of <200 was Ze..

Ruairi Hansen · May 22, 2019

Charlie_ said:
Just realised that this thread was about Zs not Ze or Ra..
My comment about the DNO figure of <200 was Ze..

I was thinking thay

Deleted member 9648 · May 23, 2019

essex said:
Yeah that is what I said ‍

No you didn't.
By the way, have you found that reg number for the other thread yet?

robd · May 23, 2019

If it helps the NICEIC online certification gives you 200 as the autofill value to select on their forms

davesparks · May 23, 2019

Charlie_ said:
I go by what the dno insist on and the only figure that is realistically acceptable <200

Which DNO insists on less than 200?
I've never heard of this before and considering the DNO have nothing to do with a consumers own earth electrode I don't see how it's any of their business getting involved?

If you enquire a DNO will give you a figure of 21ohms max for their side of things.

Charlie_ · May 23, 2019

davesparks said:
Which DNO insists on less than 200?
I've never heard of this before and considering the DNO have nothing to do with a consumers own earth electrode I don't see how it's any of their business getting involved?

If you enquire a DNO will give you a figure of 21ohms max for their side of things.

The Manx dno

Ian1981 · May 23, 2019

Like Dave said it’s not their earthing connection, it’s upto the installer and the installer alone to determine the suitability of the electrode they install

robd · May 23, 2019

I agree with the others saying winding a clamp on to it may have damaged their cable, they will want to take a look at that.

Ruairi Hansen · May 23, 2019

robd said:
If it helps the NICEIC online certification gives you 200 as the autofill value to select on their forms

Really and no 1667?

robd · May 23, 2019

Ruairi Hansen said:
Really and no 1667?

Yes pretty sure, got one in progress I'll take a picture of it later

robd · May 23, 2019

Charlie_ said:
Just realised that this thread was about Zs not Ze or Ra..
My comment about the DNO figure of <200 was Ze..

Me too sorry Ruairi I was thinking of Ze as well

Nigel · May 23, 2019

wirepuller said:
There is no requirement for Zs to be of a value to operate the OCPD within the required time in the event of a fault to earth. The preferred means of earth fault protection on a TT is an RCD (411.5.2) and the maximum Zs value will be in accordance with table 41.5. (411.5.3).
The maximum value whether you like it or not is 1667 ohms for a 30ma RCD and that is the value to be recorded on a cert for a 30ma RCD.

In this insta

wirepuller said:
No you didn't.
By the way, have you found that reg number for the other thread yet?

No I haven't.

In a TT an RCD would be used for fault protection so would therefor be the OCPD. As such the maximum Zs for the rated RCD would come into effect.

As I said - this is what I said.....

Risteard · May 23, 2019

essex said:
In this insta

No I haven't.

In a TT an RCD would be used for fault protection so would therefor be the OCPD. As such the maximum Zs for the rated RCD would come into effect.

As I said - this is what I said.....

An RCD isn't an overcurrent protective device.

Nigel · May 23, 2019

Risteard said:
An RCD isn't an overcurrent protective device.

It is when it is a BS 61009.

Bobster · May 23, 2019

essex said:
It is when it is a BS 61009.

That's an RCBO. Different animal so not really a fair comparison.

Risteard · May 23, 2019

essex said:
It is when it is a BS 61009.

Of course, but that's not the RCD element. (And BS EN 61009 to be pernickety.)

Nigel · May 23, 2019

Risteard said:
Of course, but that's not the RCD element. (And BS EN 61009 to be pernickety.)

I just voted for the Brexit Party so removed the EN.

Risteard · May 23, 2019

essex said:
I just voted for the Brexit Party so removed the EN.

You would also need to use a different number then as 61009 is still the harmonised Standard. And I voted to leave the EU in the referendum too.

Reuben Scicluna · Jun 1, 2019

Ruairi Hansen said:
Hi just wondering what people are noting as there max permitted zs reading on test results sheet for TT systems? 200ohms or 1667ohms?

In Malta we have an upper limit of 50Ω (RA). TT is by and large the most commonly found earthing system here. It is that low because there is a high probability of it increasing as most of the ground is some form of limestone.