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