# UKMax zs Values in the on-suite guide

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

#### OllieB21

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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.

#### timhoward

##### -
Esteemed
Arms
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
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.

#### Lister1987

##### -
Trainee
Esteemed
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?)

#### pc1966

Esteemed
Arms
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.

OP
O

#### OllieB21

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?)
Hi, thanks for the reply, you may be correct as some of the values also differ from the 18th edition GN3 I have in front of me.

#### pc1966

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Arms
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.

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#### pc1966

Esteemed
Arms
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
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​

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OP
O

#### OllieB21

Thanks everyone for the replies, I’ve attached a photo with some notes if anyone wants to take a look.

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#### newfutile

##### -
Arms
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

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