hi guys just watched the niceic dvd on inspection & test and in it they measured a Ze of 0.69 and tony cable said that for a tncs system this was ok as they were "in the sticks" what happened to a max of 0.35 ohms that i've been using for the last 20 years
cheers guys

Since when has a limit on Ze determined what earthing arrangement exists.

The value of 0.35 ohm simply comes of an engineering guidance note - it's a probable upper value for a domestic supply(TN-C-S), there is no "max" about it.

Regards

OMS

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Failure is always an option

i thought 0.35 ohms was the limit on tncs or you have to call supplier saying theres a problem? Thats what i was taught anyway

LoL -

OMS

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Failure is always an option

was it right or wrong oms please

Well, not wrong per se - but not correct either.

There is no upper limit to a TN-C-S supply (or to any other earthing arrangement).

Values exceeding 0.35 ohm in a small installation might suggest there is a developing problem but equally it might just be that you are a long way from a transformer in a lightly serviced section of the network.

All you need to ensure is that Ze is sufficiently low to operate your selected protective devices and that if it is actually a PME supply that you have sized the bonding correctly.

The stability of Ze is probably more important than the specific value

Regards

OMS

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Failure is always an option

Thanks alot oms its appreciated

Although it might imply a maximum supply of about 53.3A if the supply voltage limits (+10%/-6%) are to be kept within.

- Andy.

When I spoke to WPD quite a while ago, they said that for an overhead supply for TN-C-S they would accept values up to 0.80 Ohms.

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Regards
Carl.

OMS

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Failure is always an option

Why would you go to the niceic, or the "wiring regs" for advice on
a distributors network?
The external loop impedances are:
100 amp=0.38
80 amp=0.56
60 amp=0.72
Trust me.

???????? - how can you give definitive values for a network, it would appear that you are simply applying ESQCR maximum voltage tolerance of +10%,-6% to derive Ze on a TN-C-S system (incorrectly it would appear - I suspect the latter figures should be 0.46 ohm for the 80A service and 0.61 for the 60A service.)

Regards

OMS

-------------------------
Failure is always an option

Pmenetwork,
Are those Ze values you quoted only theoretical values.
Take one example a housing site recently supplied by aerial bundled cables between houses + one quarter mile to sub station, (PME). Ze on last house = 0.9 ohms. This example may be the same all over the country where long runs of overheads are installed, to comply with your readings a transformer on a pole would need to be plonked in every ones back garden (exaggerated but some validly). On my list of disconnection times of 0.4 & 5 seconds Ze for 100A BS 88 is 0.42 ohms.
Maximum Ze & Zs Values
BS 88-2.2 & BS 88-6 gG Fuses
5A : n/a
6A : 0.4s = 8.52, 5s = 13.5
10A : 0.4s = 5.11, 5s = 7.42
15A : n/a
16A : 0.4s = 2.70, 5s = 4.18
20A : 0.4s = 1.77, 5s = 2.91
25A : 0.4s = 1.44, 5s = 2.3
30A : n/a
32A : 0.4s = 1.04, 5s = 1.84
40A : 0.4s = n/a, 5s = 1.35
45A : n/a
50A : 0.4s = n/a, 5s = 1.04
60A : n/a
63A : 0.4s = n/a , 5s = 0.82
80A : 0.4s = n/a , 5s = 0.57
100A : 0.4s = n/a , 5s = 0.42
125A : 0.4s = n/a , 5s = 0.33
160A : 0.4s = n/a , 5s = 0.25
200A : 0.4s = n/a , 5s = 0.19
Regards
jcm