Zs values?

[bs7671]

Hi there I have another question for you guys.What is the maximum Zs values for different circuit breakers i.e type B,C,D also where would we find them and why the difference?

 

[Guest123]

BS 7671, page 48 onwards.

 

[bs7671]

Cheers lenny,hey are you that guy from the hit american show the simpsons,I didnt know you lived in Wales.How is karl?

 

[telectrix]

the difference is due to the different characteristics of the breakers. the Zs max. is the max. you can allow for the particular breaker to disconnect in the permitted time

 

[bs7671]

I want to find a table or something so I can see the Ka ratings as per different device type.Or at least someone to explain it in simpleton terms

 

[bs7671]

I found this but its from Australia so its not valid here Frequently asked questions Hager B&R

 

[andyb]

Hi there I have another question for you guys.What is the maximum Zs values for different circuit breakers i.e type B,C,D also where would we find them and why the difference?

Look in your namesake.

 

[IQ Electrical]

Look in your namesake.

Good point!

 

[Spinksy]

Any one answer this new circuit Ze=0.06 R1 + R2=0.21 ZS=0.51 why is zs high ? using megger 1553 on no trip

 

[IQ Electrical]

It's the 'on no trip' bit that you need to question.

Most no-trip readings are much higher than conventional Zs readings using a full-current test.

There's also a chance that the RCD is distorting your reading.

 

[Spinksy]

Yeah it was but has not happened before ! and the meter is fairly new it must be reading the Rcd !

 

[tomxhug]

I want to find a table or something so I can see the Ka ratings as per different device type.Or at least someone to explain it in simpleton terms

on site guide and GN3 have tables with the standard kA ratings of all protective devices manufactures will give more details

 

[telectrix]

0.06 seems low to me for Ze. if i am doing Zs on no trip, i usually do the Ze on no trip . no doubt someone will correct me on this, but i do it this way so as to make the readings same parameters as regards the test equipment.

 

[jam2020]

why are osg tables 80% of bs7671?

 

[J@mes]

to allow for changing temperature conditions I think!

 

[JUD]

why are osg tables 80% of bs7671?

When you do the Zs test the cable temperature will be around 20°C. The tables in the regs are the max Zs values when the cable temperature is 70°C.

As resistance increases as cables get hotter (0.4% per °C) you need to adjust the max Zs value for the lower temperature of the cable.

e.g The difference between 20°C and 70°C is 50 so the calculation would be 50 x 0.4% = 20% so the corrected max Zs values for 20°C would be 20% less than the ones for 70°C.

Hope that makes sense

 

[jam2020]

on the eic you would still enter the max zs allowed though?

 

[JUD]

on the eic you would still enter the max zs allowed though?

correct

 

[malcolmsanford]

on the eic you would still enter the max zs allowed though?

Unless the circuit is RCD protected then you would use 1667 for either TT or TN

 

[telectrix]

malcolm, could you clarify your last post please. say for a 32AMCB, the max. allowable value of Zs is 1.16, what you are saying is, if the circuit is RCD protected you enter 1667, not 1.16?