A Zs observation for all

[Mark2spark]

Hi guys.

I had my annual assessment renewal for Elecsa today (passed of course ), and something came up as an observation that I didn't previously know, so thought I would pass it on.

When you are filling out your test sheets, there is a column for Max Zs for that circuit.

For example i'll use a 32A type B 60898. I've been using Table 41.3 (page 49) from the big red book, and coming up with 1.44 ohms, which is the figure i've recorded.

I didn't realise there was a correction factor to this, and therefore it's best to use the Table 2D (page 103) from the on-site guide, which gives figure of 1.16 ohms for the same example, with ambient temperature added in.

There are also correction factor tables overleaf on page 104.

If you all already knew this, don't worry, i'm getting my coat :coat

 

[steptoe]

The NIC have these all printed inside the cover of the very pad. I could scan the table later and up load it, or would it be copyright?

 

[Zeespark]

No harm in phoning them to check Steps, that is if you can actually get through to them.

AndyGuinness

 

[m4tty]

If the circuit is covered by an RCD will that not be 1666 ohms ? (tin hat ready as im probably wrong lol)

 

[Mark2spark]

If the circuit is covered by an RCD will that not be 1666 ohms ? (tin hat ready as im probably wrong lol)

Why are you thinking that Matty? The way I see it, (i'm assuming your using the next two tables along, ie 41.5?) the Zs as quoted in those tables is effectively the PFC/PSCC? (and not a per individual circuit figure)

---------- Post Auto-Merged at 16:59 ---------- Previous post was made at 16:55 ----------

Oooh I'm probably also putting my foot in it lol, that section is to do with TT installations :_|

 

[M107]

But on your sheets doent it state "max Zs permitted by BS7671"?

Sooooooooooooo for your 32A 60898 that would indeed be 1.44 & not the corrected figure from the OSG.

 

[steptoe]

your circuit should be designed in such a way so that max Zs is never higher than that permitted under a fault condition, hence the corrected figure.

you could however design a circuit in such a way so that it does indeed exceed the max corrected Zs and still remain safe, I am sure, but for the vast majority of instances it is normally a case of using the provided design parameters and then you have cut out a heck of a lot of work for yourself.

Im fairly sure there is a reason in there somewhere as to why the 50v touch is no longer pushed at students, just use the design criteria already there for you and all will be honky dory.

perhaps some people were utilising it in the wrong manner and coming up with crazy max Zs figures??? I dunno :C just a thought.

 

[ajkend]

m4tty is correct but the test sheet wants the max Zs for the 32a 60898 cb

 

[nsbs]

But on your sheets doent it state "max Zs permitted by BS7671"?Sooooooooooooo for your 32A 60898 that would indeed be 1.44 & not the corrected figure from the OSG.

yep, thats wright. Max Zs is Max Zs, not the 80% rule.

 

[Manator]

I have had this debate a few times, whilst I always record the 80% values, they are not the maximum allowed. I continue to put the 80% values because in my eyes they are more realistic. Having said that providing you know the differences I seldom pick anyone up on either method. The only time maximum Zs will come to be a problem is when doing a PIR, because as we all know any designed circuit would never be designed so close to its maximum.

 

[NozSpark]

I've just had my office visit this evening and he didn't mention anything about me using 1.44 as my max Zs for a 60898 B32,,, we did discuss the 0.8 correction though!!

 

[rainydays]

So what do you do when you exceed the 80% value but you do not exceed the maximum Zs permitted by BS7671?

 

[ADS]

So what do you do when you exceed the 80% value but you do not exceed the maximum Zs permitted by BS7671?

You're measurement is compared with the 'corrected' figure - not the 'maximum allowed' figure - so your Zs (in this case) would be too high.

 

[rainydays]

So theoretically the corrected value is your max value if you cannot exceed this

 

[ADS]

So theoretically the corrected value is your max value if you cannot exceed this

No

Think of it the other way around.

The maximum permissable Zs is the figure given in the Regs - this is the figure that goes on the forms.

If you could measure the Zs of the circuit at full operating temperature, then you could compare your measurement to this figure.

As you can't/don't do this, you have to do one of two things.

You either increase your 'measurement' value by a percentage (roughly 20% say) - and compare it to the 'Regs' figure.

OR

You decrease the 'Regs' figure by a percentage and compare your measurement to that.(Again, roughly 20%, which gives us the 'rule of thumb' 80%)

Either way - the figure given in the 'Regs' is still the 'Maximum Permitted' Zs

 

[kme]

I DID have this disussion on my last AE visit....I had been putting the 80% figure; which I was told was wrong; I ought to put the figure from 7671; as this is what is actually asked for on the cert.

 

[ian932]

I DID have this disussion on my last AE visit....I had been putting the 80% figure; which I was told was wrong; I ought to put the figure from 7671; as this is what is actually asked for on the cert.

My last visit from the NIC AC assessor said the same as I'd also been recording the 80% figure, but the DI guy who did my 1st assessment years back said I was wrong using the 100% table and should use 80% rule as it was more relevant to the actual testing environment.

Maybe they should change the column to say 'Max Design Zs' or be bloody consistent about what they would like to chuffin see in the column.

If it was a vote I

 

[MajorOak]

I was told by my NAPIT guy that I should be putting 80% of the value BS7671 in the Max Zs column. He said the value in BS7671 is for an electrical designer who will apply the correction factors to the value when designing the installation. 80% is just a rule of thumb, the actual calculated value of ZsMax will be

ZsMax = Ze + (((1/Fa)*Fc)*(R1+R2))

where Fa is the ambient correction factor and Fc is the cable correction factor and Ze is the the Earth Fault Loop Impedance external to the origin.

Can you tell I am just in the process of doing my C&G2391 exam revision?

 

[S60TEM]

It's another one of those grey areas where consistency again fails miserably!!

Take for example a 32A BS:EN 60898 type B MCB

Easycert program automatically puts in the value of 1.44 ohms which is what the BRB states.

The onsite guides value is 1.16 ohms corrected (80% rule of thumb)

NAPIT document No NA/TDZD2 which is what your given for MAX Zs values for certification purposes is 1.15 ohms

Which ones right? I put in the max value of the BRB solely because thats what we work to, the MAX value.

 

[steptoe]

It is so inconsistent, have mine in 2weeks time, must ask again. Wonder if NIC will stop putting the 80% value on their cert pads?