Can Someone Clear Up Max Zs Values For Me

[Barx]

Everywhere I seem to look I see another set of values for Max Zs's.

Here is a copy of the value I tend to write in on test sheets (this is what i was leant if it's wrong).

click

Seems though I do mostly domestic, I'll use some common values that i use

60898 type B's

40a - 1.15

32a - 1.44

16a - 2.87

6a - 7.67 (well, says 7.66 in the link)

These are what I thought were 100% values.

Then I look in the OSG and I'm sure the values are different (so 80% values?). Not got my book by my side and cannot be assed to go get it.

off the top of my head:

40a - 0.96?

32a - 1.20

16a - 2.40

6a - 6.40

But then I look in the NICEIC testsheet book and get different again, Napit, different again.

Then I look at a the MaxZs table in the downloads section and see different again.

Max (100%) values of

40a - 1.20

32a - 1.50

16a - 3.00

6a - 8.00

Note, as I type this, I'm getting deja vu. Have I asked this before?

So, lets say you was creating a reference of 100% values and 80% values, What values do I use?

 

[Andy™]

7671 contains the 100% values

OSG is 80%

 

[Barx]

7671 contains the 100% values

OSG is 80%

So, what are the values in the cover of test sheet books? randoms

 

[Andy™]

no idea

 

[Nicky Tesla]

doh! :innocent

 

[betty swollocks]

I tend to use the osg 80% values

I find that true readings don't come even close to those values in a reasonable maintained installation, unless you have large or extended circuits

However I have found that the cheap socket range such as volex found in skrewfix etc can give sky high readings sometimes close or even about the permitted/expected values

 

[binky]

80% values usually. This sort of shite annoys the crap out of me, some bright spark dumbs down the numbers to suite those not fully conversant with BS7671 and how to use it, and causes bleeding confusion as a result. basically the numbers on certs should be good for any install, if you are over these then the idea is you need to use your own brain to work out if circuit is good. Personally i think they should just teach one set of numbers and how to use them properly. Failure to understand basic principles contained therein should render person concerned incompetent. Rant over. :^O

 

[kerching]

I agree 100% with Binkys rant....however if I was stressed out then I would blow my top at 80% of his rant and never actually reach 100% as my limit would have been reached!

 

[binky]

clearly showing you do not fully understand and are an incompetent ranter :slap

 

[kerching]

clearly showing you do not fully understand and are an incompetent ranter :slap

I consider myself to be more of a 'Ranters Apprentice' than a fully blown [taking into account network impedance, ELFI, U, Temp coeff, Splanshniks Constant and not forgetting the Savile/Glitter paradox] Ranter. My time will come, Oh yes, my time will come

 

[boltonsparky]

I do some testing with an NICEIC guy using his forms, we go off the values in the table in the book which are slightly lower than the OSG but even in the dodgiest of installations we've never even come close to the values, the circuits usually test way lower than the max permissible or if there is a fault it's usually way over (and I mean way over), never had an issue where I've had to question the NIC values,

Having said that though I've no idea where they get their values from :^O

 

[binky]

on most domestic properties you are unlikely to see even the lower values, but just occassionally something crops up. I just hate the dumbing down and confusion this causes, like all showers have to be wired in 10mm FTE - claptrap

 

[binky]

if I remember rightly paper forms do state values are 80% on cover, but it's been a long time since I used paper forms.

 

[SPECIAL LOCATION]

Everywhere I seem to look I see another set of values for Max Zs's.

Here is a copy of the value I tend to write in on test sheets (this is what i was leant if it's wrong).

click

Seems though I do mostly domestic, I'll use some common values that i use

60898 type B's

40a - 1.15

32a - 1.44

16a - 2.87

6a - 7.67 (well, says 7.66 in the link)

These are what I thought were 100% values.

Then I look in the OSG and I'm sure the values are different (so 80% values?). Not got my book by my side and cannot be assed to go get it.

off the top of my head:

40a - 0.96?

32a - 1.20

16a - 2.40

6a - 6.40

But then I look in the NICEIC testsheet book and get different again, Napit, different again.

Then I look at a the MaxZs table in the downloads section and see different again.

Max (100%) values of

40a - 1.20

32a - 1.50

16a - 3.00

6a - 8.00

Note, as I type this, I'm getting deja vu. Have I asked this before?

So, lets say you was creating a reference of 100% values and 80% values, What values do I use?

The figures in BS7671 big green book are the 100% max permissible Zs when the conductor is at its normal operating temperature. e

.g. conductor could be carrying its full load current..

Appendix 14 gives guidance as to expected values when testing 'cold' conductors.

The On Site guide gives adjusted figures for an ambient temperature of 100C..

Whereas the NICEIC tables are using an ambient temperature of 200C..

There is also the rule-of-thumb 80% which may be different to both of the above!

Guinness

 

[Barx]

I understand that in normal day to day, circuits done come near, only time I have is in when I changed a large CU in a nursing home. The main ring circuit for the communal hallways was JUST over the max by 0.02. I passed this onto the contractors who we were subbying to and I believe they 'adjusted' my reading so it was just below. lol

The reason I ask is I am making a reference tool and would like it to be acurate / useful as possible. I guess the only way would be to have many value types. 100% / 80% / NICEIC / NAPIT / etc for any are different. I must admit whenever anyone gives me a proper NIC book to use, I use the values in the cover.

So, on to the next question, seems though I don't have NIC / NAPIT / figures, can anybody take a snap of their inside page and send it me

Or point me to links of the NIC, etc values.

Would be very appreciated.

 

[Sharpend]

I don't get this, surely the only figures that are required are those from the IET Bs7671or associated books, figures from Nic, Napit and the likes are irrelevant? If I were to print a set of figures with an ambient temp of 15 degrees would you need a copy of them as well?

My point is if each and every body used a slightly different temp would it be necessary to have a list from all of them? Surely the figures in the regs books are the only ones that you need, if you feel the ambient temperature is going to be different can we not calculate this ourselves?

Sorry just trying to minimise unnecessary paperwork.

 

[Technician]

Page 184 of the OSG gives the correction factors for

conductor resistance at maximum operating temperature.

When measuring at 20 Cel a factor of 1.2 could be applied

to ensure that (R1 + R2) reads would not contribute to a

high Zs value at the maximum operating temperature.

If you applied the ambient temperature multipliers given on

the previous page you COULD do your correction from 15

Cel.

HTH.

 

[Sharpend]

So you have confirmed that it is not necessary to have the multitude of Zs figures that are about.

 

[Technician]

I understand where you are coming from Sharp, but I put

that bit in to demonstrate that using the OSG it was possible

to complete any correction you wanted to make from almost

any temperature to the higher one which must be considered

when taking the maximum rated temperature of the conductor

into consideration as a factor that would determine the impact

upon Zs under circuit conditions where a line to neutral or

line to protective conductor fault does NOT exist.

HTH.

 

[binky]

So you have confirmed that it is not necessary to have the multitude of Zs figures that are about.

hence my small rant earlier - I hate the dumbing down, teach one set of figures and how to apply them correctly, then maybe we would reduce misunderstandings, and maybe gain a bit more respect for the technical firkin job sparkying is!!!!