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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

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

So I record the theoretical value of 1667.
 

Leesparkykent

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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
 
N

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.
 
D

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.
 
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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.
 
D

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.
 
N

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 ‍♂
 

robd

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

davesparks

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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_

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Arms
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

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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

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Arms
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.
 
N

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

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Arms
Esteemed
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.
 
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.
 
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