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Topic Title: Earthing rod resistance
Topic Summary: What evidence to support statement that above 100 ohms could be unstable?
Created On: 04 May 2013 03:55 PM
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 04 May 2013 03:55 PM
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Thripster

Posts: 561
Joined: 22 August 2006

NIC states that Ze to rods > 100 may be unstable? What evidence is there to support this? Given that 1533 ohms odd would allow 5 x 30mA at quick disconnect time where is the detailed analysis which shows that you need <100 (or <200) which takes into account soil types and resistivity around the British Isles that justifies this statement?

Regards
 04 May 2013 05:21 PM
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perspicacious

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BS 7430

Regards

BOD
 04 May 2013 05:58 PM
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daveparry1

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I think the book says that >200 ohms might become unstable, and the max. Zs with a 30m/a rcd is 1667 ohms,

Dave.
 04 May 2013 06:46 PM
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Thripster

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Thanks guys. I'll need to look up that BS number BOD - I'm ignorant of it. Dave, I understand the 1667 ohms max ie: 50v/0.03=1667 ohms - I got my maths wrong for the rest of it - I meant that 50v/0.15=333 ohms being 5 x 30mA and therefore enhanced trip time - if 333 ohms ok for enhanced trip time, why does the BGB say unstable over 200 ohms and NIC stipulate 100 ohms? If I read the standard,I'll probably be lot wiser.

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 04 May 2013 08:33 PM
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Thripster

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That is quite a read BOD. Other than for generating sets, I could not find a statement saying that values above 200 ohms might be unstable - do I need to get involved in the calculations in depth (so to speak)?

Thank you
 04 May 2013 09:16 PM
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mikejumper

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Originally posted by: Thripster
...why does the BGB say unstable over 200 ohms and NIC stipulate 100 ohms?

BS7671 seems to suggest that the max 200 Ohms figure only applies to 30mA and 100mA RCD's.
For 300mA and 500mA the max Zs's given are 167 Ohms and 100 Ohms respectively.

The NIC may simply have set the figure at 100 Ohms to cover all RCD's in the range 30mA to 500mA rather than giving various recommended maximums.
 04 May 2013 11:30 PM
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John Peckham

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Mike

I think that the NICEIC may have taken their maximum figure of 100 ohms from BS7430.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 05 May 2013 07:28 AM
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Thripster

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Hello John, Great web site but you may want to check 'discus' on home page and assess whether it is now correct to say that 'many insurance companies ask for a NICEIC report.......' (or words to that effect).

Regards
 05 May 2013 10:34 AM
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Christof

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I think stability need to take into account the resistance of the soil around the electrode, what may be a stable reading for one type of soil wont be the case for other's.
 05 May 2013 12:01 PM
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Thripster

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Yes, understand that it depends upon soil type, moisture etc. Just wondered how the categoric statement of >200 may be unstable came about and how you reconcile that against the fact that the measurement is probably made once at install and then not again until the next inspection (maybe 10 years for domestic) - in the intervening time, soil moisture and other factors (joint to rod integrity) could have deteriorated a lot.

Regards
 05 May 2013 06:59 PM
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broadgage

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I dont believe that an earth rod resistance of 100 ohms is any more liable to instability than a lower resistance, it could however have more serious results.

If an initial earth rod resistance of 25 ohms later becomes 75 ohms due to dry weather, that should not matter much in most cases.

If however an initial resistance of 100 ohms later became 300 ohms, then that could result in slow tripping of the less sensitive RCDs.
 05 May 2013 08:01 PM
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perspicacious

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"Just wondered how the categoric statement of >200 may be unstable came about and how you reconcile that against the fact that the measurement is probably made once at install and then not again until the next inspection (maybe 10 years for domestic) - in the intervening time, soil moisture and other factors (joint to rod integrity) could have deteriorated a lot."

542.2.4 is taken into account by the installer in deciding what value is acceptable..............

Would you be happy signing off say 150 Ohms when the ground is very wet or would you achieve a lower value based on your engineering expertise that would be less than say 200 Ohms when the ground returns to being parched?

Regards

BOD
 05 May 2013 10:14 PM
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statter

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IMHO this is a really good question. With 7671 listing values well in excess of this there does seem to be some inconsistency. When ever I have returned and remeausred Ze I have always found it to be lower when more than one rod has been used. I have always put this down to the soil settling back around the rod. The rod couplings are quite a bit wider than the rods and this means that the hole that the rods and couplers make is generally quite a bit bigger than the rod so initial contact is poor. I can the argument for making an allowance for drying out but there should be some science for this somewhere?
 06 May 2013 01:08 AM
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Phillron

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Why the concern ?
Domestic installations and many industrial installations having a TT system are never ever going to be low enough to not rely on an rcd, the maximum figure that the Rcd will operate will be well above the 100 ohms recomended,figure

Plucked out of the air comes to mind
 06 May 2013 07:28 AM
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Thripster

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Know what you mean Phil (though I think you have the sense the wrong way).

I suppose that if you were the designer of a power station supplying the grid then the requirements for/science behind earth rods may be a lot more stringent than, say, would be applicable in a domestic situation? Are you really saying that a domestic installer has to understand that science before they bang a rod in? If so, then the industry is seriously flawed. BOD - I understand the point you are mmaking but if we are looking at 30mA protection for a domestic situation, then aren't we going over the top? Suppose you installed a rod and measured 300 ohms when dry, should I be worried? Could there (and should there) be a split in the Regs by industry sector?

Edited to correct 800 to 300
Regards

Edited: 06 May 2013 at 07:53 AM by Thripster
 06 May 2013 07:50 AM
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perspicacious

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"Suppose you installed a rod and measured 800 ohms when dry, should I be worried?"

Lay a 4' rod on your lawn when the dew has gone today and measure the value you get. If that achieves 800 Ohms, would you be happy as your means of earthing?

Next, push the rod in 100 mm and take a value, then hammer in at 200 mm and take a value and repeat every 100 mm. Plot a graph of Ohms v depth and tell us what shape the line is

Regards

BOD
 06 May 2013 09:13 AM
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Thripster

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I will do that one day BOD. Humour me for a moment though - suppose that, for an electrician not involved in generation plant or distribution equipment and who is simply wanting to install a safe installation - if he banged in a 4' rod and the reading when dry was not more than 300 ohm then what has he/she got to worry about?

Regards
 06 May 2013 04:59 PM
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perspicacious

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" if he banged in a 4' rod and the reading when dry was not more than 300 ohm then what has he/she got to worry about?"

Changing gender part way through your scenario for one thing A nasty (man thing) accident with a hammer

Below is an extract from BS 7430:2011

Basically, the Note and 542.2.4 are in BS 7671 and considering the last words of 120.3, you could have difficulty in finding support from other industry recognised "print" as to the suitability of your 300 Ohm when the two main BSs (7671 and 7430) give 200 Ohm. I gather that 100 Ohm is suggested by an "advisory" body.

Regards

BOD

7.2.9 Additional protection against electric shock
NOTE 1 Some three-phase generators are supplied with a built-in RCD and the star point of the windings is internally bonded to the generator frame and the protective conductor terminal. Alternatively, it might be necessary to install a separate RCD and to make the appropriate connections externally.
To provide protection against electric shock when in contact with a live part and the general mass of earth a 30 mA RCD in accordance with BS 7671:2008+A1, Regulation 411.3.3, should be provided and the common reference point/main earthing terminal connected to true earth.
NOTE 2 Although, in the absence of a deliberate connection between the protective conductor and earth, fortuitous contact between metal cladding of equipment and earth can be such that any earth fault current likely to involve a risk of shock would be sufficient to cause the RCD to operate, it is better to ensure operation by making the earth connection wherever it is at all practicable.
In conformity to BS 7671:2008+A1, the resistance of the earth electrode should be less than 50/IΔn where IΔn is the rated operating current of the RCD.
NOTE 3 For a 30 mA RCD at 230 V, this would allow a resistance to earth of 1667 Ω.
However, a note to BS 7671:2008, Table 41.5 advises the resistance be as low as practicable and a value exceeding 200 Ω might not be stable.
 06 May 2013 06:09 PM
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Thripster

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Thanks Bod. Will not change gender half way through discourse again - promise!

I have read your extract both from you and from the standard. That reference comes under Section 7, entitled 'Generators'. It goes on:-

Commentary on Clause 7

Three main types of generating set are considered in this clause:

a) small sets having ratings below 10Kw that are not earthed and not operated in parallel with the elctricity supply;

b) sets having ratings usually in excess of 10Kw that are normally 3-phase and require earthing; and

c) small-scale embedded generators (up to 16A/phase) for operation in parallel with public low-voltage distribution networks.


7.2.9 specifically relates to generators - I had spotted that before and mentioned in my post above. Where does the standard make reference to the >200Ω possibly being unstable in conjunction with the public low-voltage distribution network? I didn't spot anything - or have I missed it? Or do you think that that comment means that all earth rods have to comply with 7.2.9 ?

Thanks again
 06 May 2013 06:57 PM
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perspicacious

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To me, an earth electrode whether it be for a stand alone generator, for a "mains supply" TT or part of a lightning protection system etc has to be of a suitable resistance depending on the application.

You could trawl through the lightning protection Standards (good luck) to find from memory, that a value of 10 Ohms is mentioned along with the need to test at different times of the year.

My understanding from all the publications I've read is that electrodes with resistance exceeding 200 Ohms should be considered unreliable and prone to instability.

Quite where this is sourced from, other than experience, I don't know.

Perhaps returning to my suggestion of planting an electrode to a depth that gives you 300 Ohms and leaving in situ for a year taking weekly test readings along with others at say 200, 100 and 50 Ohms could satisfy (if you have the patience) your questioning of why 542.2.4 and Note 2 of Table 41.5 are in BS 7671?

Mind you, you need to take this electrical advice in context of me on finding the LH main of my H4 bulb not working spending the best part of an hour today extracting the bulb expecting to see a damaged filament, but intact.... How many cars don't share a common fuse for both main beams I ask you? Quick look at the fusebox and I see that there are 8 fuses dedicated to the dip, main, drive and fog for each side. Replaced the 7.5 A and perfect. Hey ho, another half hour to refit.

Regards

BOD
IET » Wiring and the regulations » Earthing rod resistance

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