max Zs, Which Value

[telectrix]

Max. Zs values for Mcbs given in table 41.3 BRB. Correction factor of 0.8 applied in OSG. This correction i believe is to allow for temperature variation, but in my experience, cables rarely go anywhere near 70deg. esp. in a domestic situation ( some exceptions with high current appliances like cookers and showers), so which value do you put on the test results. ( e.g. 1.44 or 1.15 for a 32A B type 60898)??????????

 

[Amp David]

Max values with no factor applied IMO

 

[telectrix]

those are my thoughts as well.

 

[morph]

do you mean max zs value?

you enter value from big red book with out the factor applied , but compare to value with faactor applied

 

[SirKit Breaker]

I use the figure quoted in the chart in the OSG.

 

[baldsparkies]

1.15 would go on my cert, or is that 1667. cus of that thingy with a test button. (Here we go again)

 

[telectrix]

lol. my point though was: why correct values for a temperature that is rarely, if ever , attained?

 

[Guest123]

It asks for max values permitted in BS 7671 not the OSG or anywhere else.

 

[telectrix]

thanks for that clarification, Lenny.

 

[ian.settle1]

You should use the BS7671 values as when you design the installation these are the values you use not 80% as when designing the installation temp variation etc should be taken into account at design stage, unless it is a Bobit job

 

[oldtimer]

Well I have seen a big company who do this for a living and they say the big red book but as you say the OSG says 80% of the big red book

Hold on have we not had this conversation before ?

 

[Guest123]

Hold on have we not had this conversation before ?

Yes.....

 

[telectrix]

before my time i think. read similar threads, but no definitive answer on which value to enter on test results. i agree with using the higher value as in brb. just wanted some other thoughts. came across a ring with Zs of 1.25 last week. looks better on the sheet if using 1.44 as max. instead of 1.15.

 

[Amp David]

It asks for Maximum Zs permitted by BS7671. Not 80% of maximum permitted by BS7671.

Yes we've had this convrsation before, many a time, but the same can be said for other topics like 'its my first assessment what do I need to do' or 'I need to run a feed to a shed' blah blah, but everyone seems to take the time in reply to and answering these questions, so what the difference here?

 

[malcolmsanford]

Max. Zs values for Mcbs given in table 41.3 BRB. Correction factor of 0.8 applied in OSG. This correction i believe is to allow for temperature variation, but in my experience, cables rarely go anywhere near 70deg. esp. in a domestic situation ( some exceptions with high current appliances like cookers and showers), so which value do you put on the test results. ( e.g. 1.44 or 1.15 for a 32A B type 60898)??????????

When you talk about the 70c we talk about the temperature of the conductor and when you get a load on that conductor the temperature will rise quite a bit, even on a small load. You just have to work with large cables to know that when you de-energise a cable it is very malable and easy to work with. I do laugh when someone grabs hold of a cable through the double insulation of a T+E and say oh it's warm but nowhere near 70c.

You should use the BS7671 values as when you design the installation these are the values you use not 80% as when designing the installation temp variation etc should be taken into account at design stage, unless it is a Bobit job

Spot on Ian. When desiging a circuit you don't take into consideration the 80% rule but you still need to take conductor temperature rise into account.

For instance if you hade a simple lighting circuit of 1.5/1.0mm T+E which at a length of 18 mteres and propose a 6amp type b MCB BS 60898

Your calculation should be Zs = Ze + (R1 + R2) x Length X 1.2 / 1000. That 1.2 is the correction factor for the heat rise of the conductor.

So it would be on design Zs = 0.35 (TNC-S by enquiry )+ 30.20 ( Resitance value of conductors per metre 12.10 per 1.5mm + 18.10 per 1mm) x 18 x 1.2 /1000 = 0.66 ohms

In the BRB the maximum Zs value for that protection device is 7.67ohms so your circuit design is ok. then though you deal with Vold Drop, installation methods, but that is another thing.