TN-S and continuity calculations

[kermit]

Hi, I am in need of a helping hand calculating continuity on a lighting circuit , its a TN-S type, ze is at 0.43 , reading of furthest point on lighting is 1.05 and the cables are over 25 years old so red and black in colour, I know 0.8 is the max for tn-s but for zs is 1.48 to high ?

[sxsparky]

You have to check it with what mcb, fuse, trip time as you dont want cable to burn out before the breaker goes.

[spinlondon]

Hi, I am in need of a helping hand calculating continuity on a lighting circuit , its a TN-S type, ze is at 0.43 , reading of furthest point on lighting is 1.05 and the cables are over 25 years old so red and black in colour, I know 0.8 is the max for tn-s but for zs is 1.48 to high ?

The 0.8 max refers to the Ze, which you are stating is 0.43.
When you say 'reading of furthest point on lighting is 1.05'. Are you refering to the R1+R2, or the Zs?
If it's the R1+R2, then you would as you have done add it to the Ze to calculate the Zs.
If it is the measured Zs, then you do not add it to the Ze.
Compare the Zs to the maximum values as detailed in BS7671 for whichever protective device is in use, remembering to fisrt multiply the values in BS7671 by 0.8 before comparison.

[kermit]

Hi,
bs7671 table 41.2 gives a max 8.52 for 6 amperes, my R1+R2 is 1.05 without any other calulation, Ze is 0.43 , could you refresh what calculations need to be carried out to acertain weather or not the cable for the lights is good or needs to be ripped out ?

[spinlondon]

The calculated Zs is Ze+R1+R2, which in your instance is 1.48Ω.
If the CPD is a gG BS88 fuse, then the maximum permissable Zs is as you state 8.52Ω.
When making the comparison, you would first multipy 8.52 by 0.8 which would produce a value of 6.6Ω.
If the measured or calculated Zs is below this value, the cables comply, which in this instance they do.
If you were to change the CPD to a 6A MCB, the maximum permitted value fo Zs would be 7.67Ω.
You would adjust this value by multiplying by 0.8 producing a value of 6.136Ω.
Again the cables would comply.
A table of adjusted values can be found in the OSG, or GN3.

[kermit]

Hi Spinlondon,

Thank you for your imput , which has given me the confidence in 2 things , the meter i use and calculation how it all ties in,

kermit