Zs values?

[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?

 

[IQ Electrical]

That's exactly right, many people incorrectly assume this is just for TT systems.

 

[telectrix]

so, one advantage of the 17th is, with everything now on RCD ( well almost), we don't have to remember all those max. Zsvalues for different MCBs. every cloud has a silver spoon!

 

[malcolmsanford]

RCD/RCBO circuits will have the Zs part of the form filled in as 1667 as that is the "Zs" as such for the circuit.

On domestic the chances are that as the board that is fitted will be a dual board then you also have overload protection by MCB and that circuit will have your Zs, and you will enter the Zs results. A lot of argument is now happening that even on dual boards you can fill the Zs for each circuit as 1667 as all circuits are RCD protected. IMO in a few years once we are all use to this we will all fill the schedule of resutls with RCD protection as 1667. I have to admit until recently I use to fill the Zs part in for each circuit as measured but I will start doing it the 1667 method.

Obviously on PIR's for older installtions, and installtions not needing RCD protection you will still do Zs as normal.

 

[telectrix]

even so, still put in the measured Zs in it's column, but max. as 1667.

 

[malcolmsanford]

even so, still put in the measured Zs in it's column, but max. as 1667.

Yes 1667 everything that as RCD protection

 

[malcolmsanford]

That's exactly right, many people incorrectly assume this is just for TT systems.

I was told recently by a NIC man that TT is 1667 and TN is 1.6 Kohm !!! After I stopped laughing at him he told me that as he was an assessor that was what should be written

 

[IQ Electrical]

Well that's your answer Malcolm, the Lord hath spoken!

And on the seventh day, the NICEIC wrote the regulations!

 

[andyb]

1667 is for TT systems.
On a TNS system the mcb or rcbo is your method of complying with 411.3.2.2 & you use table 41.3
The rcd is for additional protection so 1667 does not come into it.

Table 41.5 is for TT systems.

 

[jam2020]

errrr whats the diffinitive answer here chaps??

 

[Sintra]

1667 is for TT systems.
On a TNS system the mcb or rcbo is your method of complying with 411.3.2.2 & you use table 41.3
The rcd is for additional protection so 1667 does not come into it.

Table 41.5 is for TT systems.

If the maximum Zs value for a circuit in a TN system cannot be met, the circuit may be protected by a 30ma RCD. 531.3.1