Discuss Max zs Values in the on-suite guide in the Electrical Forum area at ElectriciansForums.net

Reaction score
0
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Hi all, Just wondering if anyone can shed some light on the max zs values in the OSG, they don’t seem to follow the 1.25 divisor stated in the note on page 126

For example BS 60898 circuit breaker, max zs at 70 degrees is 7.28, following the formula this should be 5.82 at 10 degrees which is what is in the niceic handbook, however looking at the tables in the OSG it is actually 5.87 which uses 1.24.

This does make sense as the simple way to work it out would 60 degree rise in temperature at simplified coefficient of 0.004 per degree which would equal 1.24, however this contradicts the formula on page 132 note 1 which is {1+0.004(ambient temp - 20)} / {1+0.004(10-20)} = 1.25

(Ambient temp being 70 degrees)

Just curious as to why the tabulated values in the OSG seemed to be calculated the simple way and not followed either the formula stated above or the 1.25 divisor stated in the first note page 126

They’re seems to be two different methods which both makes sense but both have different outcomes which is confusing me a bit.

What’s your opinions? Thanks in advance everyone.
 

timhoward

-
Esteemed
Arms
Reaction score
3,034
Just curious as to why the tabulated values in the OSG seemed to be calculated the simple way and not followed either the formula stated above or the 1.25 divisor stated in the first note page 126
To be honest my OSG only leaves the bookcase to show the assessor that I have one (daft rule!), and we both have a laugh about it's mint condition. I don't have a very high opinion about the premise behind it, it's structure, it's quality, or it's usefulness in the field.

When out and about I use the Napit Zs chart for the ones I don't know by heart (probably similar to your NICEIC pocket book) and/or the excellent ProCerts app, as I've verified both of these against the regs and trust them.
 

pc1966

Esteemed
Arms
Reaction score
6,812
If I had to calculate it I would use, for example using 6A B-curve MCB:
Zs = 230 * 0.95 * 0.8 / (6 * 5) = 5.82666... = 5.83 Ohm
Some are truncating to 5.82 ohms (e..g NAPIT guide) instead of rounding to 5.83 as above. The OSG value of 5.87 is 0.74% higher, but not quite obvious how it is calculated but maybe someone used something with more significant digits than 0.004 for copper temperature coefficient?

Still, it is under 1% so probably smaller than the measurement accuracy for DC resistance anyway, and far less than the accuracy I suspect Zs/Ze measurements can be made to in most cases.
 

pc1966

Esteemed
Arms
Reaction score
6,812
Wasn't there an errata for OSG that corrected some values? thought that OSG is your 80% Zs values (don't think Cmin is applied?)
My copy of the OSG has "Reprinted (with minor corrections) 2018" below the seventh edition version saying that is based on current 18th BBB.

The OSG values are very close to calculated values for Umin and 80% for cold-tested circuits (as above example). Certainly close enough for measurement accuracy!

What catches folk out often is the BBB values are the "working temperature" Zs limit so need corrected for whatever that is to whatever you measure at. But in the vast majority of cases the variability in hot working temp and in cold measure temp is not worth worrying about, so the OSG/NAPIT tables are good enough.
 

pc1966

Esteemed
Arms
Reaction score
6,812
Actually, that 5.87 in the OSG might be a typo from 5.827 from 5.826666... computed. I did the rest per OSG list of current value and the above calculation:
A​
3​
5​
6​
10​
15​
16​
20​
25​
Zs​
11.65​
6.99​
5.83​
3.50​
2.33​
2.19​
1.75​
1.40​
A​
30​
32​
40​
45​
50​
63​
100​
Zs​
1.17​
1.09​
0.87​
0.78​
0.70​
0.55​
0.35​

Most are within least-digit rounding/truncation of the OSG values, but the 6A B stands out as odd and probably that sort of typo/error in translation.
 
Last edited:

pc1966

Esteemed
Arms
Reaction score
6,812
Here is the same comparison for C-curve:
A​
3​
5​
6​
10​
15​
16​
20​
25​
Zs​
5.83​
3.50​
2.91​
1.75​
1.17​
1.09​
0.87​
0.70​
A​
30​
32​
40​
45​
50​
63​
100​
Zs​
0.58​
0.55​
0.44​
0.39​
0.35​
0.28​
0.17​
 

pc1966

Esteemed
Arms
Reaction score
6,812
And again for D-curve 0.4s
In practice less less time as based on (In * 20) to hit the upper tolerance for the "instant" magnetic trip, so likely 10ms or less:
A​
6​
10​
16​
20​
25​
32​
Zs​
1.46​
0.87​
0.55​
0.44​
0.35​
0.27​
 
Last edited:
OP
O
Reaction score
0
Thanks everyone for the replies, I’ve attached a photo with some notes if anyone wants to take a look.
 

Attachments

  • Max zs Values in the on-suite guide 7BB06442-EE0F-4FA4-B6B8-3B42C993DD3D - EletriciansForums.net
    7BB06442-EE0F-4FA4-B6B8-3B42C993DD3D.jpeg
    227.2 KB · Views: 9

newfutile

-
Arms
Reaction score
119
the onsite guide has the 6 amp value with a typo ,5.82666 is the correct value for a B6 ,the onsite guide takes 80% for temp correction the 95% for Cmin ,its at ambient temperature which i think in this case in 20C , though i have seen 10, 20 and 30 listed as ambient temperature in BS7671
 

Reply to Max zs Values in the on-suite guide in the Electrical Forum area at ElectriciansForums.net

Similar Threads

Hello Ladies and Gents. Apologies in advanced, i doubt my self a little too much. I have ripped the old fused bored of the wall, pulled the...
Replies
4
Views
486
I am looking at a scenario of temperature compensation in a text book (Practical Guide to Inspection, Testing and Certification of Electrical...
Replies
6
Views
967
On a couple of recent EICRs I've run into FCUs on a RFC, then 1.5 or 1mm to several lighting points and would be interested how people would note...
Replies
6
Views
663
Hi, as an aspiring sparky I'm needing help with the temperature adjustment formula for Zs. I'm using GN3 (17th). I understand that the values...
Replies
3
Views
897
D
Hi all, first post on the forum and I hope you's can help me with a few questions I have. I have been asked to test a caravan site with the...
Replies
12
Views
3K

New Posts (Please Reply)

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
Top