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Topic Title: niceic 2392 dvd
Topic Summary: tncs Ze of 0.69
Created On: 06 May 2010 01:32 PM
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 06 May 2010 01:32 PM
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inamess

Posts: 62
Joined: 19 August 2009

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
 06 May 2010 01:37 PM
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Avatar for OMS.
OMS

Posts: 18919
Joined: 23 March 2004

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
 06 May 2010 01:54 PM
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M4tty

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Joined: 06 May 2010

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
 06 May 2010 02:05 PM
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OMS

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Thats what i was taught anyway


LoL -

OMS

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Failure is always an option
 06 May 2010 02:19 PM
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M4tty

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was it right or wrong oms please
 06 May 2010 02:46 PM
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OMS

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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
 06 May 2010 02:55 PM
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M4tty

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Thanks alot oms its appreciated
 06 May 2010 03:52 PM
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AJJewsbury

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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"

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

- Andy.
 06 May 2010 07:15 PM
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CMElectrical

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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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Carl.
 06 May 2010 07:34 PM
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OMS

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There is no maximum limit of loop impedance for a PME supply defined in either the current edition of BS7671 'Requirements for Electrical Installations' or Electricity Association Engineering Recommendation P23/1.

The values of loop impedance in Electricity Association Engineering P23/1 are indicative values for the majority of installations whether they use PME or other forms of earthing. They are not maximum values permitted for provision of PME.


In order to prevent Distribution Network Operators being inundated with this type of enquiry, the Electricity Association produced Engineering Recommendation P23/1.

P23/1 provides a typical maximum value of LV network loop impedance (eg 0.35 ohm for supplies up to 100 amps) with the caveat:

'Higher values could apply to consumers supplied from small capacity pole transformers and/or long lengths of low voltage overhead lines.'


OMS

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 07 May 2010 12:11 AM
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pmenetwork

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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.
 07 May 2010 10:26 AM
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OMS

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Originally posted by: pmenetwork

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

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 07 May 2010 12:28 PM
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jcm256

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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
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