Zs measurement

[xraytek]

If Zs is equal to Ze + (R1+R2) and your max Ze must not exceed 200ohm then the R1+R2 reading would be rediculous! example: Max Zs 1667 - Max Ze 200 = an R1+R2 reading of 1467ohms

Can some help me out here am i right or not?

Yeah you are right- its a ridiculous figure!

Its purely theoretical in that a Zs of 1666 with 30mA flowing to earth will cause exposed conductive parts to rise in voltage to 49.98V.

Zs of 1668 with 30mA RCD (Volts= Current x Impedance) gives a voltage of 50.4 on exposed conductive parts so is a non conformance to BS7671.

In reality you will never be able to have a circuit with a Zs anywhere near figure quoted as you will fail on voltage drop- so really the figure is bull.

 

[Ridgey88]

So really it's the max zs for a 30ma rcd not for a tt system or anything else??
Looking at the deic and mwc I can't see when or where you would enter a max zs of 1667ohms unless it asked for the max zs for a 30ma rcd in which case this figure just a figure your meant to know but unlikely to ever record it. Is that right ?? Or is it the max permissable zs for any circuit on a TT system?

 

[xraytek]

So really it's the max zs for a 30ma rcd not for a tt system or anything else??
Looking at the deic and mwc I can't see when or where you would enter a max zs of 1667ohms unless it asked for the max zs for a 30ma rcd in which case this figure just a figure your meant to know but unlikely to ever record it. Is that right ?? Or is it the max permissable zs for any circuit on a TT system?

Not all certs ask for this info but NICEIC certificates do.
Both the minor works certificate, and test results page of the EIC have a box marked 'Maximum Zs permitted by BS7671'.

If the circuit in question is protected by a 30mA RCD then the figure is 1667 regardless of MCB rating.

With the introduction of 30mA RCD's on all circuits in domestic dwellings the 1667 figure is pretty much the only figure that will get used now.

I have heard some people say that they only quote the MCB Zs figure in this box, but my understanding is that it should be 1667 and would be interested to hear views on this.

 

[Spudnik]

TT yes, Max Zs permitted is 1667, but on the rest i also use the MCB figures, whether an RCD is fitted or not.

Dont forget that RCD's are not always required, dependent on installation methods used etc.

 

[Ridgey88]

I agree with Jason, for a TT the primary protection from earth faults is the RCD so the max ZS should be 1667ohm irrelevant of the MCB rating, but for TN systems where the RCD is only used for additional protection then the Max Zs for the MCBs should be applied.

 

[V]

in the equation
zs=0.8+(7.41x2)/1000x30x1.20 what is the 2 for i know its R2 but the the question i have there is no figure for R2 ?

a 30m radial cct supplied at 230v from source ze=0.8 wired in 2.5 sq live and cpc has resistance of 7.41m ohms at 20 c
protected by 20a fuse to bs88-6 zs max =3.04 ohms

if any one knows please let me know

cheers

 

[Mark.W]

in the equation
zs=0.8+(7.41x2)/1000x30x1.20 what is the 2 for i know its R2 but the the question i have there is no figure for R2 ?

a 30m radial cct supplied at 230v from source ze=0.8 wired in 2.5 sq live and cpc has resistance of 7.41m ohms at 20 c
protected by 20a fuse to bs88-6 zs max =3.04 ohms

if any one knows please let me know

cheers

Looks like the wiring is in singles therefore your live and CPC are both 2.5mm2.
So if your live and CPC are both 2.5mm2 then R1 + R2 = 7.41 + 7.41 or 7.41 x 2!

On a 20A fuse to BS88-6 max Zs for 0.4s disconnection is 1.77 Ohms and for 5s disconnection 2.91 Ohms, so I dont know where the 3.04Ohms comes from?!

 

[V]

you told me what i needed to know about 7.41x2

the 3.04ohms is the zs max

when worked out it shows zs is less than zs max 1.334<3.04, bs 7671 is satisfied

once again cheers

 

[Mark.W]

Where does it state 3.04 ohms as I had a look in BRB and it says a 20A BS88-6, max Zs for 0.4s = 1.77 ohms and max Zs for 5s = 2.91 ohms

 

[V]

example given for refresher maths on 2391 course

cct is protection is a 20 a fuse to bs 88-6 (zs max+3.04ohms)

may be it was given just to recall how to do zs by summation method and the figures are just made up

i was more thinking about the R2 figure just being times by 2

 

[ezzzekiel]

The 21 Ohm value is the dno connection to the general mass of earth at the star point, whereas >200 ohms to be classed as stable is from the electricians point.

 

[Mark.W]

Hi Vince

I was just testing your knowledge as you stated you were doing the 2391!

Best of Luck!

 

[V]

yer right

get the beers in