Zs values?

[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.

 

[Geoffsd]

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 - you are strictly correct but post #75 was in reply to another post and I, at least, think it quite obvious that he is referring to his discovery/realisation that the 5, 10, 20 only works with 60898s.

Also, as you can see he does refer to 'without looking it up' or as I put it 'needing the graphs'.

I just got the impression that you were 'raining on his parade' when he wasn't really wrong if read in conjunction with his previous post.


Edit -substituted I for you

 

[JUD]

Jud - you are strictly correct but post #75 was in reply to another post and I, at least, think it quite obvious that he is referring to his discovery/realisation that the 5, 10, 20 only works with 60898s.

Also, as you can see he does refer to 'without looking it up' or as I put it 'needing the graphs'.

I just got the impression that you were 'raining on his parade' when he wasn't really wrong if read in conjunction with his previous post.


Edit -substituted I for you

I didn't intend to rain on his parade. His previous post regarding 60898 and 61009 breakers was bang on the money.

I was replying soley to post #75, which is why I quoted it. SimpleSimon asked if the formula also applied to BS 88 fuses, to which Smiffy replied...

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. ;-)

 

[Geoffsd]

As I said - "you are strictly correct".

t f b. :seeya:

 

[Dan]

Just bumping some threads in the Electrical Wiring Theories and Regulations forum category here on our Electricians Forums. If this specific topic isn't current, just ignore it, it'll drop off the list in no time. However, if it's something you'd like to contribute to, feel free to reply and bring it back into current discussion.