All,
Back to the question?
From my reading of Regs page 167,
544.1.1 Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional
area not less than half the cross-sectional area required for the earthing conductor of the installation and not less
than 6 mm[SUP]2[/SUP]. The cross-sectional area need not exceed 25 mm[SUP]2[/SUP] if the bonding conductor is of copper or a
cross-sectional area affording equivalent conductance in other metals.
Except for highway power supplies and street furniture, where PME conditions apply the main protective bonding
conductor shall be selected in accordance with the neutral conductor of the supply and Table 54.8.
[IMHO: I think that this sentence would be clearer if it began 'Where PME conditions ... '.]
...
Therefore, Table 54.8 applies only to PME, the minimum cross-section of a protective bonding conductor for a TN-S system is 'half the cross-sectional area required for the earthing conductor'.
Interpreting Regs page 160:
543.1.1 The cross-sectional area of every protective conductor, other than a protective bonding conductor, shall be:
(i) calculated in accordance with Regulation 543.1.3, or
(ii) selected in accordance with Regulation 543.1.4.
Calculation in accordance with Regulation 543.1.3 is necessary if the choice of cross-sectional area of the line conductors
has been determined by considerations of short-circuit current and if the earth fault current is expected to be less
than the short-circuit current.
...
In this example there is a significant difference in fault currents so the regulations state that 'Calculation in accordance with Regulation 543.1.3 is necessary'.
If the other option in 543.1.1, 'selection', is used then according to Table 54.7 on page 162 in the Regs the earth conductor must be the next larger standard cross-sectional area up from the line conductor cross-sectional area, S, divided by 2. S / 2 is 47.5 mm[SUP]2[/SUP] and this accords with the installed earth conductor cross-sectional area of 50 mm[SUP]2[/SUP]. Using 'selection' for the earthing conductor and Reg 544.1.1 on page 167 results in a minimum cross-sectional area for the protective bonding conductor of 50 / 2 or 25 mm[SUP]2[/SUP]. According to 'selection' the protective bonding conductor at 16 mm[SUP]2[/SUP] is undersized.
Returning then to determine the required size of the earthing conductor using calculation; on page 160 of the Regs:
543.1.3 The cross-sectional area, where calculated, shall be not less than the value determined by the
following formula or shall be obtained by reference to BS7454.
S = (I[SUP]2[/SUP] t)[SUP]0.5[/SUP] / k
...
I, the current flowing under earth fault conditions, is 1780 Amperes. The constant k is given in the Tables on page 161 of the Regs; assuming Table 54.2 is valid and that the earth conductor is copper insulated with 70 deg c rated thermoplastic insulation, k = 143. What though is t? One person has got this far and through a wrong assumption fell at the last but one hurdle! This requires a further regulation reference, Regs page 54:
411.3.2.3 In a TN system, a disconnection time not exceeding 5 s is permitted for a distribution circuit and for a circuit not covered by Regulation 411.3.2.2.
At this point in the system it is a 'distribution circuit', not a 'final circuit not exceeding 32 A.' to which 'Table 41.1 - Maximum disconnection times' refers.
Using I = 1780 A, t = 5 secs and k = 143; the minimum earthing conductor cross-sectional area required is 27.8 mm[SUP]2[/SUP]. Therefore applying Reg 544.1.1, the minimum protective bonding conductor cross-sectional area required is 13.9 mm[SUP]2[/SUP]. The installation would therefore be satisfactory with an earthing conductor of 35 mm[SUP]2[/SUP] and the current protective bonding conductor cross-sectional area of 16 mm[SUP]2[/SUP] is correct.
As this is my first posting and I only completed my C&G 17[SUP]th[/SUP] Ed course recently I am open to challenges ... or a slap on the back and a pint of good quality cider like Scrumpy Jack will do!
Yours Aye
GB
well done chap! good work, consider this a big slap on the back!
like telectrix says the value should be gained from the curve on the chart
FIG 3A3(a)
200A fuse which comes in at slightly over 1sec (1700A - 1 sec)
which is amazingly 12.44mm!
if the PEFC would of been smaller (eg 1200A) the duration of time to blow the fuse would of been longer, hence a larger earthing conductor like in your 5 second calculation
which means that a 16mm bonding conductor is ok!
Rattlehead85,
i wasnt trying to be argumentative at all but this is why i asked the first question
i was not trying to 'get out of upgrading the bonding'
if i would of started jumping up and down about it being undersized and they then got a second opinion, i would of looked either very stupid or like i was trying to rip them off
either way i could of lost their future business
so its important to check
on another note; i wonder how much unnesscesary copper is wasted on 16mm main earths, 10mm bonds 25mm main conductors on 60A TNS domestic supplies
but it just looks right dont it lol!
