Max Zs 80% values.

[Barx]

Did the '80 percentile' values change from 16th to 17th?

I was learnt a set of 80% values a few years ago (I was given a chart with 100% and 80% on it) and have never really qustioned it until now (When reading the values for my app. But it appears these values dont match 17th OSG. My values DO though match This Guys and I can't find my 16th OSG to confirm. Also niether generally match the calculated value... :C

Here is an example:

60898 MCB 6A Type B -

BS7671 value = 7.67 (100%)

80% values -

My value from past memory (and same on link above) = 6.40

17th OSG = 6.18

Calculated value = 6.14

Example 2:

60898 MCB 32A Type B -

BS7871 Value = 1.44 (100%)

80% values -

My value from past memory (and same in link above) = 1.20

17th OSG = 1.16

Calculated value = 1.15

Is it just me or is nothing in this game simple black and white. Which values would (do) you use? I think for the app I will use (And state) the OSG values. But it would have been alot easier if the calculations actually worked out as then I would only have to store 1 value and calculate the other on the fly.

 

[Doc Hudson]

The 16th edition max 80% Zs values were based on calculations using 240v nominal voltage, the 17th edition are to 230v nominal voltage.

Doc H.

 

[MajorOak]

You have a value of Zs and 80% of that is 80% independent of voltage. The OSG uses a more complicated formula based on max temp of the cables etc. The 80% rule is just a quick and dirty method and good enough for most situations.

I am up in Yorkshire and don't have my books. From memory its something like Zs=Ze+((R1+R2)*somefactor)

somefactor is a de-rating the cable, somefactor is lower than one bringing Zs down which reduces Zs, bringing it near the 80% value.

 

[Barx]

ahhhh, the 'somefactor' lol. similar to a fudge factor but more official. lol

Cheers for the replies, clears it up somewhat. Will use OSG values and now need to update all the values i had put into my database as I had used the link above for reference. doh headbang

 

[Barx]

You'll also find a set of 80% figures from memory in GN3

Yeah....never seen that doc. I'm sure it will only complicate matters further. Lol

Sent from my Desire Z

 

[MajorOak]

It's pretty good reading and the basis of the 2391 exam

 

[Barx]

And how does one obtain said document?

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Ahhh, just checked amazon. Another