Advance Managing Director Answers

Nathan Briggs, Managing Director of Power Conditioning Specialists Advance

Tackles a Typical Lightning and Earthing Issue

The Problem

I am a UK citizen and have an alternative power system at my house in an

area of France susceptible to lightning. This consists of a wind generator,

4 solar panels, two inverters, various controllers and a 1200 amp/hour

battery set in a small "energy centre".

I have already been 'hit' by a remote strike which travelled up the phone

cable, around the walls of the building and took out my new sine wave

inverter!! Although all items were earthed and bonded to a single point it

was clearly insufficient. All items in the system are close together. The

fall-back diesel generator in also inside the same building.

I have read almost ALL the literature I can find on the Web on

bonding/earthing systems and the effects of lightning. My biggest problem

lies in that - with the equipment available to me - I cannot sink a 3/8"

earthing rod any more than 1.5 metres into the ground around the house since

it is on a solid rock mountainside.

Therefore, I would be glad of your advice on various issues that arise.

Nathan Briggs' Response

Firstly, you don't mention either the DC voltage or the duration of backup

or the power level. I have assumed 120Vdc, 1 hour (nominal) 100A inverters.

What was the route from phone line to inverter? Can you demonstrate that

this was the path by damage, or is this an assumption? Experience tells us

it is much more likely a ground strike raising the earth potential of your

system and thereby causing the damage. Telephone wires are thin and unlikely

to carry seriously damaging currents.

Specific Questions

1. Should I use earthing rods of larger diameter? (3/8" or 9.5mm seems

to be standard in UK and France)

A larger diameter will not help, as surface area is the critical factor.

However, into solid rock it is unlikely 1.5 metres would be sufficient. The

aim of the earth rod is to achieve a low impedance to earth of approximately

0.4 ohm. A test drilling and rod measurement would answer the question of

how deep you need to go, but this seems like a lot of hard work for little

return.

2. Since I cannot drive them into the ground more than 1.5 metres

should I use several 'grouped' together?

Yes this "Earth Net" or "Earth Farm" approach is advocated by several

experts for overcoming high impedances to earth.

3. If so, how many should I use?

This is almost impossible to answer, as there are so many variables. I'd

suggest that you put in as many as it takes to get your target impedance

value.

4. How should they be grouped? How far apart? How should they be

joined?

If you put them apart there is a potential for current to flow between them,

so whatever links them together must be of a low impedance. They should be

grouped in a star configuration for small earth farms or in a net

configuration for larger ones.

5. How should I connect the various items of equipment together? One

expert report that I read suggested copper "strapping" rather than heavy

duty copper wire.

To be honest, large diameter copper wire can cause several problems when

conducting high current impulse energy, stranded or braided "strapping" is

better but more expensive. It is common practise to use large single strand

in external rods/nets and braids where appropriate indoors. In addition you

have the "Aerial" affect where a large loop of copper can act like the loop

aerial on a TV and pick up high frequency.

6. What size strapping or wire?

Back to your target impedance, there is no point getting a 0.4ohm Earth and

then connecting such thin copper to it that you reduce its effectiveness, on

the other hand cost is the limiting factor upwards.

7. Where should the connections be? I'm aware that they should be to

the earth point and any frame, but should they be to anywhere else?

I'd recommend that you avoid Loops, and create Stars to your earth point.

You need to make connections wherever energy can be created either by fault

or by coupling effect.

8. Are the connection points themselves a particular problem? Do you

have any suggestions?

Again a low impedance connection is the aim, as two clean flat surfaces

together are better than a clamp on a screw thread. In addition, each

connection point should increase the gauge of conductor so that the

preferred path for energy is the desired direction.

9. Do I run a single cable from each item back to the rods or can I

run a "ring main" around them?

I'd recommend that you avoid a loop, and create a star instead.

10. What size cables/strapping would you recommend from each item?

These should be the appropriate size for the likely "energy" associated with

a fault, and should increase in size towards your earth point.

11. A report that I read suggested that the NEGATIVE side of the battery

set (2 parallel racks of 12 x 2v [600 amp/hour] cells) MUST also be

connected to the common earth. Is this correct?

Absolutely not, as this depends entirely on your inverter. If your inverter

operates with one side of the battery earthed it should already be

connected, although this is down to how good your earth is. If your earth is

bad, the connection allows energy into the inverter front end, which is

typically susceptible to damage. Conversely, if your earth is good, then

this is probably a good idea.

12. What is the additional effect of connecting the batteries into the

earthing system using, for example, more or heavier rods?

The only reason for increasing earthing size by connecting batteries is if

they are likely to induce an earth fault, which should not apply in your

case.

13. Several suppliers have suggested I should include several

lightning/surge arrestors? Is this a good idea?

This is a very big subject. In basic terms, "lightning" cannot be arrested -

the best that anyone can hope to do is divert the energy where it will do

the least harm. Electricity Sub-stations do this by using large spark

"arrestors" which disconnect explosively so dissipating energy. Large

factories and new building installations tend to feature earthing which

prevents the energy from entering their wiring network by diverting it to

earth rods. Typically, this is augmented by "surge suppressors" which can

absorb some energy and "surge diverters" which can divert the excess back

into earthing system.

However, there is no point diverting energy to earth if your earth is so

poor it will end dissipating anywhere you have connected to earth. Moreover,

a surge suppressor is expensive and maybe a one shot device for serious

amounts of energy.

Having said that, a clearly defined approach, identifying good earthing,

diversion and suppression is what most people look for in these

circumstances. Often, however, at least one of these three items will have

little effect. Frequently, therefore, the customer needs to contact an

expert when one approach has failed.

14. If I use surge suppression, how many devices will I need? Where do I

put them? How do I earth them?

You need to adopt a zoned approach, with larger TVSS near incomers/outgoing

supplies and large pieces of equipment, and smaller devices on smaller

systems. Each unit comes with wiring instructions and recommendations

including size of earth cable. As before, the earth cable should run back to

the star point.

15. How do I eliminate the effects of lightning through and around the

walls of the building?

You can't completely eradicate these effects. The best you can expect is to

deal with the effect and limit the damage, unless you wish to line your

walls with steel and bury them deep in the ground! However

diverter/suppressors fitted for transmitted energy will be just as effective

for coupling energy.

16. Do you have any other suggestions that could help?

There are three additional suggestions that might help: -

a. Consider earthing alternatives. A recent Scottish customer of ours runs a

bare cable into the sea, because he was on solid rock.

b. Consider inverter technologies that are less susceptible to damage.

c. Identify the REAL cause of the damage and spend money to fix just that

problem. The biggest problem with earthing and transient suppression is

spending money at the wrong sort of solution.

For further details on the full range of Advance Electronics' transient

suppression technology, contact 01978 821 000, or visit www.aelgroup.co.uk

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