Max zs readings for TT

[Ruairi Hansen]

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

 

[ferg]

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

So I record the theoretical value of 1667.

 

[Charlie_]

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

 

[Ruairi Hansen]

What do niceic want to see?

 

[Ian1981]

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]

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]

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]

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]

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]

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]

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_]

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

 

[Ruairi Hansen]

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]

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]

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

 

[davesparks]

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_]

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]

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]

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]

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

Really and no 1667?

 

[robd]

Really and no 1667?

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

 

[robd]

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]

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

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]

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]

An RCD isn't an overcurrent protective device.

It is when it is a BS 61009.

 

[Bobster]

It is when it is a BS 61009.

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

 

[Risteard]

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]

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]

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]

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.