Maximum permitted Zs Values recorded on Certificates

Discussion in 'Electricians' Talk' started by unphased, Nov 8, 2007.

  1. unphased

    unphased Screwfix Select


    Just wanted to get this straight as I was challenged on my responses on another thread. Just to repeat, the value we are supposed to record on our certificates is the corrected value from the one given in tables 41B1, 41B2 and 41D. If you look in the regs at the tables there is a Note at the bottom of each table which states:
    The circuit loop impedances given in the table should not be exceeded when the conductors are at their normal operating temperature. If the conductors are at a different temperature when tested, the reading should be adjusted accordingly. Now, we cannot know whether the conductors ARE at their normal operating temperature. Thats why rule of thumb was invented (75% officially, 80% adopted by NICEIC and soon to be officially recognised as 80% in 17th edition) to err on the side of safety when recording maximum permissible values. Its why the tables of Maximum Permitted Values for measured values are issued and its these values that should be recorded on the certificates. Rule-of-thumb is acceptable as an alternative to correcting the tabulated values in the regs. Its what we are taught in C&G 2381 and 2391 so those who are being told otherwise are being mislead and those who think otherwise didn't listen! :)

  2. JP.

    JP. Screwfix Select

    Hmmmm would think the average internal temp of a dwelling would fluctuate between what 60 to 70 degs F..

    What temp that is on the table (or where it falls within the table) I don't know..but there must be a normal measurements taken at 65 deg F etc which would allow for a percentage tolerance either side of this.
  3. unphased

    unphased Screwfix Select

    JP, believe me I am correct on this. The regs values are uncorrected and we use 80% for contingency. To be honest its another one of these ridiculous situations where we have sparks doing two different things. I get fed up of it really. The regs are so riddled with ambiguity it needs somebody who can write proper English to edit the damned thing and remove it. 17th edition will also be open to ambiguous situations (protecting wires by RCD in safe zones etc). I think this is a no win statement. :(

  4. JP.

    JP. Screwfix Select

    Right Unphased..interesting M8..will look at that when I find my book..

    It would be good for you lads to have corrected tables etc..would save a lot of messing about with perhaps taking ambient temp etc before doing the Z's tests etc and involving correcyion factors..

    Have a good evening Unphased..:)
  5. sysparks

    sysparks New Member

    I have a sheet given to me by my last firm with the corrected values on. Makes it much easier.
  6. stumpy987

    stumpy987 New Member

    i spent years entering the 80% values which are inside the niceic pads of certs, which were check at our yearly inspections, year after year.
    until we had a complaint against us due to a dodgy builder, the cert got sent to niceic technical department and they came an also inspected work, the only things wrong with work and cert was
    80% values recorded instead of maximum from BS7671 and 0.4 sec disconnection time stated for a lighting circuit suppling a bathroom. they made me re-issue cert with maximum values
    I personally think no one really nows which one to use
  7. bounceman

    bounceman New Member

    Niceic also pulled us about putting the 80% values
    Max values must be recorded as far as Niceic are concerned.
    I feel the 80% value should be used but we have to do as we a told
  8. bounceman

    bounceman New Member

    0.4 sec disconnection time stated for a lighting circuit suppling a bathroom
    Isnt that correct?
  9. stumpy987

    stumpy987 New Member

    thats what i thought
    they didn't
  10. stumpy987

    stumpy987 New Member

    the annual inspector never questioned my certs
    it was the technical department
    then new inspector agreed
  11. Pompey Sparks

    Pompey Sparks New Member

    'The circuit loop impedances given in the table should not be exceeded when the conductors are at their normal operating temperature. If the conductors are at a different temperature when tested, the reading should be adjusted accordingly.'

    I use the full (uncorrected) value from BS7671 as the column heading on the NIC sheet is 'Maximum Zs permitted by BS7671' not 'Maximum Zs permitted by BS7671 when testing at lower than normal operating temperature' which would also be hard to fit into the box!

  12. sparky Si-Fi

    sparky Si-Fi Well-Known Member

    Have always entered Max Zs values on certs no probs whatsoever from area engineer
  13. The Trician

    The Trician New Member

    Max UNCORRECTED Values should be entered under the 'Max Values permitted by BS7671. When I was doing PIRs for a LA, their NIC area bloke pulled em on what we were recording on the sheets.

    Daft I know, but that's the way it is.
    Quite why the uncorrected value is insisted upon, when the reality is different, god only knows.

    I have always questioned the wisdom of measuring/recording Ze & Zs, since both vary so much through time, which in my view renders them next to useless.

    To verify that an earth loop path is present, and that it is at a level either equal to, or below the corrected value at hte time and place of measurement should be enough IMHO.

  14. Lokkars Daisy

    Lokkars Daisy New Member

    The uncorrected values are the ones to enter ,but I wonder what YOU enter unphased when using say a 100mA rcd for protection on a TT system ?
  15. the arbitrator

    the arbitrator New Member


    Look in GN3 it gives the requirements in order. Zs should be checked against;

    1. GN3 & OSG appdx 2 values (ROUGHLY 80% values, they compare against the niceic values)
    2. Designer values (if you can get them)
    3. The Max values in 41B1, 41B12 and 41D
    4. The 75% value.

    Option 1 is the most preferred route, option 4 the least.

    In the 2391, as you do not have the regs, you use the 75% rule. – There is no 80% rule, the appdx 2 values happen to be approx 80% so the NICEIC use that as their rule.

    The NICEIC forms require the max value to be entered, and although I’m no mathematician, the last time I checked 80% was some 20% short of maximum.

    Where a reading is greater than the 80%(ish) value you must calculate it (option 3). See GN3 for the process, give up and make it an RCD/RCBO protected circuit and you’ve complied provided you meet 413-02-16.

    As the 17th draft (which wasn't signed off today) refers to the BS for earthing the max value for an RCD will be 100 british standard ohms!
  16. Lokkars Daisy

    Lokkars Daisy New Member

    Using again the TT board with the 100mA rcd . then all circuits protected against indirect contact by the 100mA rcd would have in the Zs box Zs = 50/100mA
    = 500 ohms
  17. snipps

    snipps New Member

    The question about these values is a valid on, especially when considering what to put on PIR etc.

    Firstly the 75% or 80% percentage question. If I remember rightly the values in the osg are 80% of those found in the regs. The values from the osg under go a further correction for operation temperature.

    The reason for the 80%(osg) or indeed a 75 percentage buffer is that long way of working out whether you come within the maximum permitted value involves a number of factors including cable type. This is why the corrected values in the osg say underneath the table that the values are calculated given a k value of 115 (general purpose pvc) from section 54 c of the regs. So if a different value of k (resitivity, temperature coefficient and heat capacity) is used ie a different type of cable the value of zs is given. The value in the regs are the highest they can be under any circumstances or calculation.

    To some degree when you do a pir or new install you state on most versions of the paperwork what the wiring type and method is. So I always use the adjusted values from the osg because those are adjusted for the sort of cable we normally use (domestic t&E.)

    we could go done the road of seperating out the r1 and r2 measurement correcting for temperature and then correcting for cable type and then finally adding to ZE. Seems alot of effort!
  18. jimmythemap

    jimmythemap New Member

    put down exactly what the original question is

    it doesn't say temp corrected does it

    MAX on RCD's (TT) as well 1666, 500
    (at 50V touch voltage) etc

    had this with a Senior Enginner at NIC and that was his answer MAX = 100% MAX not 80%
  19. sinewave

    sinewave Screwfix Select

    Amen! ;)

    Let that be an end to it!

    That includes YOU Handy!
  20. unphased

    unphased Screwfix Select

    Well. S'pose its only right I should have the last word. It would appear I was wrong. I have been filling out test certs incorrectly for ages. From now on will do the following.

    I record the max. values as stated in tables in BS7671.
    When testing I compare the test readings with the 80% values. They should be well inside under normal conditions.
    For TT systems I record the 50V/Ideltan of the RCD on circuit
    so 50/0.1 = 500ohms for a 100mA RCD
    50/0.03 = 1667 ohms for a 30mA RCD

    Thank you for correcting me. :)


    [Edited by: admin2]

Share This Page