Max zs equations

[Apollo]

When I went to school (was a long time ago granted) 23/10 was 2.3.

No matter which way you do the calculation they come out the same....

To get 0.43 you would divide 10 by 23.....

perhaps the quick Qual course providers can do a new course on how to use a calculator properly!!!!

 

[IQ Electrical]

When I went to school (was a long time ago granted) 23/10 was 2.3.

No matter which way you do the calculation they come out the same....

To get 0.43 you would divide 10 by 23.....

perhaps the quick Qual course providers can do a new course on how to use a calculator properly!!!!

I must have missed the bit where qualifications were discussed.

Remember Apollo, you have members here ranging from those who have just left school right up to guys that have spent 40+ years in the industry, we all have to start somewhere.

Anyway, the good looking young chap in post 4 posted a nice, simple, clear guide to calculating maximum Zs values.

 

[Apollo]

I must have missed the bit where qualifications were discussed.

Remember Apollo, you have members here ranging from those who have just left school right up to guys that have spent 40+ years in the industry, we all have to start somewhere.

Anyway, the good looking young chap in post 4 posted a nice, simple, clear guide to calculating maximum Zs values.

mmmmmm I was taking the preverbial!!!!!!

You keep telling yourself that last bit, the post was spot on though....

 

[mrmisior]

Thanks for correction.
For C10 10A maxZs = 2.3. My measured Zs (26 light points, 3 floors) is within 1.70 - 2.19 range.
On higher side but acceptable, right?

 

[IQ Electrical]

Thanks for correction.
For C10 10A maxZs = 2.3. My measured Zs (26 light points, 3 floors) is within 1.70 - 2.19 range.
On higher side but acceptable, right?

The corrected maximum value for comparison with measured values is 1.84 Ohms.

Your measured maximum value should be equal to or below that value.

Why do you say your Zs value is 'within the range of' it should be the highest measured value at the circuit extremity.

 

[JONOXNAM]

Hi if you check table 41.3 in BS7671 you will see IQ Elecrtrical is correct, the values are in the far right column and are provided so you can work out any value not given in the table:

e.g. for a type C 60898 the equation is 23/In
if we take a value given in the table of In=6A then 23/6 = 3.83Ω

or for the B32 you mention a type B 60898 the equation is 46/In then In=32A so 46/32 = 1.4375Ω or as they have rounded it up to 1.44Ω

If we take a value not given in the table as an example of when to use this = type B 45A then type B's use 46/In so 46/45 = 1.02Ω

Don't forget the values in 7671 are not adjusted for temperature so you'll have to do rule of thumb, alpha or F to adjust accordingly or get them to pass!


The regs do not supply a calculation for BS88 etc in table 41.4 as the value is not universal for fuse type and the regs tell you to refer to the appropriate British Standard to determine its value.

I however knocked together an excel graph to plot the curve for BS88's as I had a 60A which is not given.

I can post it up if anyone is interested?

Hope this helps, this was my first post.

Jon.

 

[IQ Electrical]

Post it up Jon and welcome to the forum!

 

[La Poste]

This is all new to me.
Excellent stuff.

:sifone:

 

[JONOXNAM]

Hi, here is the BS88 plot and one for comparison with type B 60898 the IEE give values for - you can see the BS88 moves around hence the IEE not wanting to publish any guidelines on its calculation, you can also see the values printed in BS7671 are rounded.

I would also comment that I have read several posts around where people are struggling to get their Zs value to comply with the regs value please don't forget that the rule of thumb 80% value in the OSG is not the only way as the use of the ambient temperature factors F and alpha can give you big differences as you can be out on one and not the other and the other way around depending on all the variables.

One Q how do you upload a file to a post, I could not see how to do it hence putting the images in directly.

Jon.

 

[La Poste]

"please don't forget that the rule of thumb 80% value in the OSG is not the only way as the use of the ambient temperature factors F and alpha can give you big differences Jon."

How would you apply ambient factors F and alpha in practice?
What is the difference between the F and the alpha?
Thanks.

 

[malcolmsanford]

Welcome to the forum Jon and nice work though to be honest personally I prefer to calculate the Zs = Uo/Ia and most of the time curve characteristics are in appendix 3 for BS 88-2.2 and 88-6.

Granted there are some sizes that are not covered, but I would use the manufacturers data for this.

 

[Hawkeye]

hi guys maybe im being silly here just had a read through your posts is the Uo always 230? like i mean even if you have a TP 60898 or would you divide the current by 400 in this case?

Cheers

 

[malcolmsanford]

Uo is the nominal line voltage to earth so in UK cases will always be 230

All the tables calculated in the BS 7671-2008 are to Uo so all Zs values will be at 230

There are no current calculations needed for Zs values and so no need to divide it or anything else for that matter