Zs measurement

[eagleeye]

For some reason i keep getting confused about the maxium Zs permitted in the 7671 regs. I have just finished a new rewire on a TT system. The Ze was 14.5 ohms so therefore i assume the Rn is 14.5 ohms. But i am getting confused with what maxium i am allowed on the Zs because when tested at each circuit i got 14.5 ohms. Is this correct or have i made a mistake somewhere ? I fitted the board with RCBO's as well so i had to link out the netural from the RCBO itself to stop it trippingon test. Is that correct ?

 

[stumas]

For some reason i keep getting confused about the maxium Zs permitted in the 7671 regs. I have just finished a new rewire on a TT system. The Ze was 14.5 ohms so therefore i assume the Rn is 14.5 ohms. But i am getting confused with what maxium i am allowed on the Zs because when tested at each circuit i got 14.5 ohms. Is this correct or have i made a mistake somewhere ? I fitted the board with RCBO's as well so i had to link out the netural from the RCBO itself to stop it trippingon test. Is that correct ?

maximum ze for tt=21 ohms

zs=ze(r1+r2) = your14.5 ohms + (r1+r2)

maximum if all circuits are 30ma protected is 1667 ohms

 

[eagleeye]

[thankyou for your reply. where did you find the maxium Ze ? and whats the difference between table 41.3 and 41.5 of regs ? reason i ask is because 41.3 mentions type b circuit breakers and rcbo's.

thanks for your help here.

 

[stumas]

[thankyou for your reply. where did you find the maxium Ze ? and whats the difference between table 41.3 and 41.5 of regs ? reason i ask is because 41.3 mentions type b circuit breakers and rcbo's.

thanks for your help here.

maximum ze p.11 on-site-guide
table41.5 covers 411.5.3 p.50 new regs book touch voltage and diconection times of 40 ms (x5) or200ms(1x)

table41.3 diconection times of 0.4s and 5s not milliseconds hope this helps

 

[Ridgey88]

Stumas

Where do you get a maximum Ze of 21ohms for a TT?? Dont you mean a maximum of 200ohms! and the Niceic Reccomend no more than 100ohms. this is for a TT earthing arrangement.

 

[roukel01]

I thought max Ze on a TT system was 200ohms or less and that Maximum Earth Electrode Resistance was 21 ohms?

you can find this just under table 41.5 in the regs book

Table 41.5 describes the maximum Zs in order for an RCD to operate

i.e for a 30mA RCD to operate Zs should be 1667 or less (using 50V touch Voltage 50V/30mA = 1667 ohms)

So a Ze of 14.5 ohms on a TT system is fine. The reason you might also be getting a Zs reading of 14.5 ohms at the sockets is because you have all the bonding connected and so you get parallel paths bringing the reading down a little.

I hope this helps

 

[Ridgey88]

Appologies i must have miss read this. yes max Ze <200ohm different to Max Ra earth electrode resistance, i think thats correct.

please correct me if im wrong on the Ra!

 

[Ridgey88]

Sorry mate, insulation resistance, how do you figure that?? Max Ze on TT is no more than 200ohms

not insulation resistance.

 

[stumas]

Stumas

Where do you get a maximum Ze of 21ohms for a TT?? Dont you mean a maximum of 200ohms! and the Niceic Reccomend no more than 100ohms. this is for a TT earthing arrangement.

ctrode

this ifo is on p.11 on site guide (excluding consumers electrode)

your correct a minnimum of 200ohms is perfered

but he has no problems with everthing coverd by 30 ma as he is allowed 1667ohms not 14.5

 

[Ridgey88]

Cheers Stumas,

i thought so.

so max ze 1667ohms because of rcd and tough voltage i.e 50v
no rcd 200ohms, funny as i always thought even withan rcd you should not have more than the stated 200 ohms?? funny ey! common misconception i think.

as for Ra i am correct in thinking that the max earth electrode resistance is 21ohms correct??

Stumas what do you mean consumers electrode?

still wondering where insukation resistance came from??

 

[peteludman]

Cheers Stumas,

i thought so.

so max ze 1667ohms because of rcd and tough voltage i.e 50v
no rcd 200ohms, funny as i always thought even withan rcd you should not have more than the stated 200 ohms?? funny ey! common misconception i think.

as for Ra i am correct in thinking that the max earth electrode resistance is 21ohms correct??

Stumas what do you mean consumers electrode?

still wondering where insukation resistance came from??

sorry its late thats my why honest

 

[stumas]

Cheers Stumas,

i thought so.

so max ze 1667ohms because of rcd and tough voltage i.e 50v
no rcd 200ohms, funny as i always thought even withan rcd you should not have more than the stated 200 ohms?? funny ey! common misconception i think.

as for Ra i am correct in thinking that the max earth electrode resistance is 21ohms correct??

Stumas what do you mean consumers electrode?

still wondering where insukation resistance came from??

max ze 1667 only on 30ma 100ma=s500 300ma=167 500=100ohmes
consumers electrode =earth stake

 

[Ridgey88]

Thanks Stumas, i understand the touch voltage equations, thats not the issue.

When you say consumers electrode, you said excludeing consumers electrode, how so? would it only apply to an electrode for a garage supply then?

Cheers

 

[xraytek]

max ze 1667 only on 30ma 100ma=s500 300ma=167 500=100ohmes
consumers electrode =earth stake

Dont you mean Zs?

With regards to the 21 Ohm figure for TT, I was told by my course tutor that it was the figure you would if designing an installation that will be later connected to a TT supply. Alternaltively if you were designing an installation that will later be connected to a TN-S supply then your calculations will be based on a Ze of 0.8. So if all your Zs calculations work out with this figure then all is well.

In the case of TT Ze of 21 Ohms then an RCD will always be required as there is no MCB that will disconnect within 0.4 seconds with this figure.

With a 30mA RCD present the max Zs for circuits will be 1667 Ohms, of which the Ze must be <200 Ohms to prevent an unstable rod (according to note 2 of table 41.5).

 

[Ridgey88]

Dont you mean Zs?

With regards to the 21 Ohm figure for TT, I was told by my course tutor that it was the figure you would if designing an installation that will be later connected to a TT supply. Alternaltively if you were designing an installation that will later be connected to a TN-S supply then your calculations will be based on a Ze of 0.8. So if all your Zs calculations work out with this figure then all is well.

In the case of TT Ze of 21 Ohms then an RCD will always be required as there is no MCB that will disconnect within 0.4 seconds with this figure.

With a 30mA RCD present the max Zs for circuits will be 1667 Ohms, of which the Ze must be <200 Ohms to prevent an unstable rod (according to note 2 of table 41.5).

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?

Ok i get the 21ohms reading now. its the maximum resistance of the distributers earth electrode at the supply transformer. so irrelevant to us really, unless were designers.