tabulated values of Ze and Zs

[Viners001]

right I get a bit confused with working out the Tabulated values (I am training and have exams tomorrow for Initial verification) on the Ze and Zs anyone on here what to explain how this works on the earth loop impedance, please can someone explain what I am overlooking ?thanks, Jules I understand he Equation Zs =Ze+ (R1+R2 ) but I not getting the answer I want I must be overlooking something

 

[GMES]

It might be worth joining the Trainee forum if you're on a current course.

 

[Richard Burns]

I am not quite sure what you are referring to on working out the tabulated values as the tabulated values do not need working out as they are in the table.
To calculate the Zs required without using the tables you use the formula Zs max = Uo * Cmin/ Ia.
U0 is 230 Vac for single or three phase general supplies.
Ia is the current to operate the protective device in the time required (e.g 0.4 s for a final circuit <32 A (63 A with sockets)operating at the above U0).
Cmin is given as 0.95, to account for drops in the supply voltage.

So for a B32 circuit breaker to BSEN60898:
Type B operates by magnetic trip (i.e. almost instantly) at between 3 and 5 times rated current. For 32A the worst case would be 160A (5*32).
Zs = (230 *0.95)/160 = 1.37Ω as given in BS7671.

The tables for the maximum values of Zs permitted for the protective device in use:
The tables in BS7671 are giving the maximum resistance permitted when the live conductors are at their normal operating temperature (generally 70°C for most situations).
The tables in the on site guide are corrected for the fact that when actually measuring resistance the conductors are more likely to be at ambient temperature, so the values are given for 10°C which is applying a correction of 0.8 times the BS7671 tabulated value because the resistance will rise as the conductors heat up.

This means that when you are measuring Ze and R1+R2 in practice and you want to compare your results with the values given in BS7671 you should either multiply your result by 1.25 or multiply the BS7671 values by 0.8 to see if the measurement is compliant.

e.g. You have measured a Ze of 0.3Ω and an R1+R2 of 1.0Ω and your are using a 32A type B circuit breaker.
Your calculation Zs= Ze+(R1+R2) gives 1.3Ω
If you compare 1.3Ω to the BS7671 tabulated limit of 1.37Ω you may think this is compliant.
However if you multiply your result by 1.25 to give you the expected resistance when the conductors are at 70°C you get a Zs of 1.625Ω which is non compliant.
If you look in the on site guide the tabulated (corrected) value is 1.1Ω which is lower than your measured 1.3Ω so the circuit is non compliant.

 

[gazdkw82]

Not sure what you are asking. Are you asking what the difference is between Zs and Ze?

Only things you should need to know for initial verification is, what they both mean, how to calculate and measire both and how to compare results with the values. Also need to know the difference between tabulated values and the rule of thumb (x0.8)