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Lightning Protection.

Former Community Member over 5 years ago

Good morning all.

Part of our electrical contracting services involve lightning protection testing and maintenance. On more than one occasion now we have come up against a situation where the resistance of 1 or more of a group of electrodes is greater than 10 Ohms, i.e

R1 = 8.51

R2 = 22.4

R3 = 13.82

R4 = 71.3

R5 = 42.8

R6 = 38.4

R5 = 112.4

The IEE electrical maintenance publication calculates maximum resistance to earth should not exceed the number of electrodes X 10. On the example above 70 Ohms would be the maximum indicating R4 & R5 would not comply. When Lightning protection companies commission lightning protection systems their certificates use the resistances in parallel method: R TOTAL = 1 divided by (1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5 + 1/R6 + 1/R7) In this case the total resistance of the system would be 3.28 Ohms.

Although the test complies with the overall resistance of the system the individual electrode resistance does not. The publication seems to contradict itself here. Please see attached highlighted sections. Improving the resistance of the earth is not practicable as the surrounding area in encased in concrete in most cases.

Should I be issuing a satisfactory certificate for the installation with comments on the individual electrode readings?

Kind Regards

Stephen Cherry

0 mapj1 over 5 years ago

I must admit I'd be wondering what was wrong with the second no 5. and no 4 Is not that hot either.

But there is no problem saying that it does not meet both the test conditions in the book.

1) is the effective parallel resistance less than 10 ohms ?

1/Rtotal = 1/ 8.51 + 1/22.4 + 1/13.82 + 1/ 71.3 + 1/42.8 1/ 38.4 +1/112.4

Rtotal = 3.8

(note that if all 7 electrodes had been 10 ohms each we would see 10/7 = 1.4 ohms)

so YES

and

2) but resistance if any one electrode < 7*10 = 70 ohms ?

NOT TRUE

What the rules are really saying is that Ideally the combination should be less than 10 ohms, and also that the upper limit for planting a new one would be a reading over 70.

you could, just, have 7 electrodes of 70 ohms each and pass both tests.

Given the idea is that the electrodes share equally, and we do not get one side of the building at a much higher voltage than the other during the flash, I'm not that convinced that even if this one only just met the requirement it would be that great. An additional/ replacement electrode is probably going to be needed. How does this compare to historical readings - is it degrading or has it always been this poor ?
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0 AJJewsbury over 5 years ago

The IEE electrical maintenance publication

How old is that publication? (it's been the IET rather than the IEE for a while now). Likewise I think the current standard for lightning systems is BS EN 62305 rather than BS 6651 (that the book refers to) - I'm just wondering if the resistance requirements are still current?

- Andy.
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0 Former Community Member over 5 years ago

Thanks for the response Mapj

Apologies the second R5 is a typo. Should be R7.

Unfortunately there is no previous certification for the installation.

We have had instances where large industrial buildings are built on ground made up of crushed hardcore and concrete which we have excavated and introduded additional electrodes or plates encased in Marconite with little or no improvement in soil resistivity.

From these results I am assuming we would need to install an additional 5 electrodes for the system to fully comply with the recommendations. i.e maximum electrode resistance of 120 Ohms.
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0 mapj1 over 5 years ago

Andy - the "new" (2008 onwards) standard does retain the 10 ohm limit, with a note it may not be possible on a rocky base, and keeps the old way of one drop, one electrode (calling it 'type A' earthing), but add an option to ring round linking all the electrodes near (above or below) ground level, to create a 'type B' earth, so in effect the electrodes are all paralelled up at the bottom of the drop lines, to some extent equalising the worst effects of any resistance imbalance. In such a case, the fact that an odd electrode is a bit weak is then a lot less serious.

Also the rolling sphere replaces cones, and a really complex risk assessment replaces a fairly complex one, but usually justifies retaining the existing system or only making small changes.

Steve, is converting to type B easier than sinking new electrodes ?
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0 Former Community Member over 5 years ago

Hi Andy
Its the second edition from 06.
I think the BS EN/IEC 62305 standard still requires 10 Ohms overall but without buying it I'm not sure about the individual measurement.
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0 Former Community Member over 5 years ago

Hi Mapj.

It probably would be eaisier with a type B system. There's a lot of concrete.
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0 AJJewsbury over 5 years ago

We have had instances where large industrial buildings are built on ground made up of crushed hardcore and concrete which we have excavated and introduded additional electrodes or plates encased in Marconite with little or no improvement in soil resistivity.

Sounds like you might have to go (a lot) deeper then to get a decent resistance. Making better contact with upper layers is likely to be defeated if a layer below is dry/airy enough to pose a high resistance to Earth proper.

- Andy.
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