Zs values?

[Deleted member 38070]

b=5x c=10x d=20x
zs = nominal voltage/Ia (current causing automatic disconnection) x0.8
examples of making disconnection times with various mcb types
eg b32
b x 5=160 230/160=1.43 x 0.8 = zs max of 1.15ohms
eg c20
c x 10=200 230/200=1.15 x0.8 = zs max of 0.92ohms
eg d16
d x 16=320 230/320=0.71 x 0.8 = zs max of 0.57ohms

c and d type breakers are used for circuits with high inrush currents

ive gotta get out more!

 

[Smnspark]

think its mem so old but hey ill put a new one in then i can ring them up cheers

 

[telectrix]

somewhere around the 0.12 area IMO.

 

[SimpleSimon]

b=5x c=10x d=20x
zs = nominal voltage/Ia (current causing automatic disconnection) x0.8
examples of making disconnection times with various mcb types
eg b32
b x 5=160 230/160=1.43 x 0.8 = zs max of 1.15ohms
eg c20
c x 10=200 230/200=1.15 x0.8 = zs max of 0.92ohms
eg d16
d x 16=320 230/320=0.71 x 0.8 = zs max of 0.57ohms

c and d type breakers are used for circuits with high inrush currents

ive gotta get out more!

That's a great formula but would it still apply to bs88 fuses?

 

[Deleted member 38070]

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

 

[JUD]

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

What gives you that idea??

Zs = Uo ÷ Ia is basic ohms law (R = V ÷ I).

Zs = Earth fault loop impedance -- Uo = Nominal voltage -- Ia = Current causing operation of the protective device in the required time

The 0.8 is a rule of thumb. The max Zs values in BS 7671 are at a cable operating temperature of 70°C but the cable won't be at that temperature when you take a Zs measurement, so you multiply the values in the BRB by 0.8 to account for the reduced temperature of the cable.

Ia is taken from the time/current characteristics graphs/tables in Appendix 3 of the BRB.

That's a great formula but would it still apply to bs88 fuses?

So yes, it does apply to BS 88 fuses. It's just easier to work out for 60898 and 61009 breakers because it's just 5 x In (type B), 10 x In (type C) and 20 x In (type D).

 

[Geoffsd]

Soooo - Smiffy is right then.

So yes, it does apply to BS 88 fuses.

Obviously, Ohm's law applies to everything but -

It's just easier to work out for 60898 and 61009 breakers because it's just 5 x In (type B), 10 x In (type C) and 20 x In (type D).

...there is no one value which applies throughout the amperage range of BS88s to calculate the maximum Zs for each.

 

[JUD]

Soooo - Smiffy is right then.
Obviously, Ohm's law applies to everything but -...there is no one value which applies throughout the amperage range of BS88s to calculate the maximum Zs for each.

No. What Smiffy said is that Zs = (Uo ÷ Ia) x 0.8 cannot be used for anything but 60898 and 61009 breakers.

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

Uo ÷ Ia is how max Zs values are calculated no matter what the protective device.

Have a look at the time/current graph for BS 88 fuses on page 247 of the BRB. Let's say you have a 32A fuse.

For a disconnection time of 0.4s you need a fault current of 220A. Zs = Uo ÷ Ia (230V ÷ 220A = 1.04Ω). Now compare this with the value in Table 41.2 on page 48.

You then multiply this value by 0.8 to account for the reduced temperature of the cable -- 1.04 x 0.8 = 0.83Ω. This is your maximum measured value.

 

[Geoffsd]

Yes, I know but smiffy's thread is about the fact that you don't need the graphs for 60898s because the relationship between Ia and In is linear,

i.e. 5, 10, 20 for B, C, D respectively.

This is not the case with BS88s and, indeed, other fuses.

 

[JUD]

Yes, I know but smiffy's thread is about the fact that you don't need the graphs for 60898s because the relationship between Ia and In is linear,

i.e. 5, 10, 20 for B, C, D respectively.

This is not the case with BS88s and, indeed, other fuses.

He didn't say anything about not needing the graphs.

What he said in post #75 was the formula for calculating max Zs "only works for B C D mcbs 60898 rcbos 61009". This simply isn't true.