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hi guys,

Seen on the circuit details schedule in a DB on our site, someone had inserted Zs values for the circuits as follows.

16a C type breaker as 1.44 ohms
10a C type breaker as 2.30 ohms
80a C type breaker as 0.28 ohms

I believe he’s calculated Zs by doing (in the case of 16amp c type)
230/(16x10) = 1.437 and rounded up.

Instead of 218.5/(16x10) x 0.8 = 1.092

Who is wrong ?
 
You are correct. A factor of 0.95 needs to be applied for Cmin (or a voltage of 218.5V as you have presented it).
 
hi guys,

Seen on the circuit details schedule in a DB on our site, someone had inserted Zs values for the circuits as follows.

16a C type breaker as 1.44 ohms
10a C type breaker as 2.30 ohms
80a C type breaker as 0.28 ohms

I believe he’s calculated Zs by doing (in the case of 16amp c type)
230/(16x10) = 1.437 and rounded up.

Instead of 218.5/(16x10) x 0.8 = 1.092

Who is wrong ?
Both are wrong if the schedule was completed since amendment 3 come out. You insert the maximum Zs permitted by BS7671. The values in BS7671 are 100% values so for example a 16A type C you would insert 1.37. If the schedule was completed before amendment 3 then for the same circuit breaker you would of inserted 1.44 on the schedule.
 
hi guys,

Seen on the circuit details schedule in a DB on our site, someone had inserted Zs values for the circuits as follows.

16a C type breaker as 1.44 ohms
10a C type breaker as 2.30 ohms
80a C type breaker as 0.28 ohms

I believe he’s calculated Zs by doing (in the case of 16amp c type)
230/(16x10) = 1.437 and rounded up.

Instead of 218.5/(16x10) x 0.8 = 1.092

Who is wrong ?
Is that the Max tabulated Zs allowable?
 
Both are wrong if the schedule was completed since amendment 3 come out. You insert the maximum Zs permitted by BS7671. The values in BS7671 are 100% values so for example a 16A type C you would insert 1.37. If the schedule was completed before amendment 3 then for the same circuit breaker you would of inserted 1.44 on the schedule.

I believe this may have been carried out prior to amendment 3s release...

So the tester would have been correct, I thought I was in the wrong as the guy was NICEIC and therefore should have known what he was doing.

During my training I was always taught to calculate the Zs, particularly when testing, is the correct practice to just insert the maximum tabulated ?

I think it’s in table 4.1 something off the top of my head.
 
Both are wrong if the schedule was completed since amendment 3 come out. You insert the maximum Zs permitted by BS7671. The values in BS7671 are 100% values so for example a 16A type C you would insert 1.37. If the schedule was completed before amendment 3 then for the same circuit breaker you would of inserted 1.44 on the schedule.
Oops... I missed the fact that he had applied a 0.8 factor! (I really should read posts more carefully!) Obviously this is only for verification of results and should not be recorded as the maximum value. The "design" values should be used.
 
I believe this may have been carried out prior to amendment 3s release...

So the tester would have been correct, I thought I was in the wrong as the guy was NICEIC and therefore should have known what he was doing.

During my training I was always taught to calculate the Zs, particularly when testing, is the correct practice to just insert the maximum tabulated ?

I think it’s in table 4.1 something off the top of my head.
You insert the values in bs7671 which are the 100% values. You use the 80% rule of thumb when testing to check if the circuits meet disconnection times when in operation.
The 80% rule of thumb is to account for conductor operating temperature at 70 degree C. When testing the conductors will be near room temp. The resistance increases with temp by a factor of 0.004 per degree. If the room temp is 20 degree C which the 100% values are based on. There is a 50 degree C difference between room temp and conductor operating temp when fully loaded. 50x0.004=0.2 which equates to 20%. Reducing the 100% values by 20% is the same as multiplying by 80%. You should be verifying that the Zs meets the 80% values other wise you may not be achieving the required disconnection times due to conductor operating temperature and consequently ADS.
 
Last edited:
Is that the Max tabulated Zs allowable?

According to amendment 3, no it wasn’t...
You insert the values in bs7671 which are the 100% values. You use the 80% rule of thumb when testing to check if the circuits meet disconnection times when in operation.
The 80% rule of thumb is to account for conductor operating temperature at 70 degree C. When testing the conductors will be near room temp. The resistance increases with temp by a factor of 0.004 per degree. If the room temp is 20 degree C which the 100% values are based on. There is a 50 degree C difference between room temp and conductor operating temp when fully loaded. 50x0.004=0.2 which equates to 20%. Reducing the 100% values by 20% is the same as multiplying by 80%. You should be verifying that the Zs meets the 80% values other wise you may not be achieving the required disconnection times due to conductor operating temperature and consequently ADS.

Thanks very much for the concise response !!

Greatly appreciated.

I knew why we used the 0.8 multiplier but, just can’t understand why the tabulated value would differ from the calculated max.

I suppose the calculated is just a good guide as to wheather you will meet required disconnection times without referencing bs7671.

Is it purely for verification when testing or is there a scenario where calculated would be used on certificates ?
 
The value in BS7671 is the maximum that will still allow the OCPD to operate within the specified time limits.
The value does not take account of any operating factors, because the book does not know what factors would apply.
When we measure the Zs the circuit is not in operation, we have to make allowance for the operating factors.
 

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