Ze

[martysparky]

Question

How does the Ze value affect requirements for shock protection?

 

[johnboy6083]

well the lower the ZE reading, ie the earth fault loop then the higher fault current that will flow, and the quicker the disconnection time.

 

[martysparky]

I have to answer this question with reference to 7671 and, well, can't find it.

 

[Guest123]

Well Ze is proportional to fault current i.e the lower the Ze the higher the fault current which may impact on your breaker selection with regard to maximum withsatnd current (6KA, 10KA) etc. Also as more current will flow disconnection will be achieved quicker.

Oppositely, the higher the Ze, the higher the Zs which may again impact your breaker selection with regard to disconnection times (slower, or not at all) and possible additional protection via RCD or larger cable if at the design stage.

 

[telectrix]

as lenny says, it's all down to good old ohms law.

 

[Des 56]

Try 411.5.2
TT system, with reference to Notes 1 and Notes 2

Ze being the major factor in a circuit Zs
The requirements being for overcurrent device if its low enough
If its too high it can be an rcd

 

[GrannySmiff]

the higher the ZE the lower your PFC will be therefore this will increase the voltage beyond the safe touch voltage of 50v as the amount of time it will take to disconect will be longer.

 

[martysparky]

Cheers guys that simplified things.

Muchio gracias

 

[martysparky]

Ok, TT system with a circuit over 32A, thus, requiring a 1 second disconnection time. Is there a table for max Zs or is it escaping me? or have I lost the plot?

 

[Ponty Massive]

Where does the 1 second come from?

 

[martysparky]

Reg:411.3.2.4 p47

 

[Ponty Massive]

I'm not 100% sure on this. Using 1667 as your max Zs (per table 41.5) will give you a touch voltage of 50 or 50v divide by 1667 = 30mA.
Note 2 states exceeding 200 ohms may not be stable so will make your fault current way above the 30mA required to trip the RCD.

As far as a table to show reg# 411.3.2.4 then I don't think there is one? In theory, provided your Zs is below 1667 ohms you should always have your disconnect time?

Perhaps this allows for a time delay RCD up to 1 sec for sub-main distribution? Not a great reg for plumbing in a 10Kw shower though.

Hopefully someone here could point us in the right direction

 

[trebor]

I have to answer this question with reference to 7671 and, well, can't find it.

Just to answer the above the reason you cant find it is because its not specifically mentioned apart from the symbols on page 36, and referred to as earth fault loop impedance in Note 3 under 132.2 on page 16, as this is under the supply characteristics it is in fact Ze

As far as BS7671 is concerned Zs is the is the top dog and Ze is automatically included in this as you know Ze + R1+R2 = Zs

Ze is covered by the ESQCR as it is DNO`s responsibility

 

[malcolmsanford]

Ze is covered by the ESQCR as it is DNO`s responsibility[/QUOTE]

Not if it's TT

 

[trebor]

Ze is covered by the ESQCR as it is DNO`s responsibility

Not if it's TT[/QUOTE]

Oh yes it is, if its TT the esqcr require the source electrode to be 20 ohms or less which is equivalent to Ze on TN systems

 

[malcolmsanford]

The 21 ohms is the DNO earth at their supply transformer. No matter what system of earthing you have in your house the supply transformer will have an earth electrode and that is the DNO responsibilty

If you have TNS or a TNC_S then the DNO have the responsibility to maintain that earth.

If you have a TT then it's the consumers responsibilty to maintain that not the DNO so they don't have the responsibilty of your Ze at a TT. All they have to ensure is that the Electrode at the transformer is there at that 21 ohms. and it is your responsibilty to get the earth path back to it.

 

[martysparky]

The Ze in a TT changes with the weather and is unreliable so we don't really know it; isn't that why its RCD'd

 

[trebor]

The 21 ohms is the DNO earth at their supply transformer. No matter what system of earthing you have in your house the supply transformer will have an earth electrode and that is the DNO responsibilty

If you have TNS or a TNC_S then the DNO have the responsibility to maintain that earth.

If you have a TT then it's the consumers responsibilty to maintain that not the DNO so they don't have the responsibilty of your Ze at a TT. All they have to ensure is that the Electrode at the transformer is there at that 21 ohms. and it is your responsibilty to get the earth path back to it.

I would agree that it is the consumers responsibility to maintain a low resistance electrode 200ohms or less at the installation, however if the DNO did not maintain a source electrode of 20 or 21 ohms max then the earth fault path would be incomplete for TT, I know that in addition the source electrode is there for referencing the neutral of the transformer for the TN systems and voltage purposes, but this does not preclude it from a TT system, by enquiry to the DNO their standard quote for TT will be that the source electrode is 20 or 21 ohms which is an external impedance ie Ze which in turn is a requirement of the ESQCR

 

[martysparky]

The Ze in a TT changes with the weather and is unreliable so we don't really know it; isn't that why its RCD'd

To overcome this we should install and earth electrode 2.4m deep opposing a 1.2m deep electrode. as this will be more stable than the 1.2m deep one even with a higher resistance.