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A Typical Lightning and Earthing Issue
The Problem
I have an alternative power system at my house in an area 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 center”. I have already been ‘hit’ by a remote strike which traveled 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 insufficient. All items in the system are close together. The fall-back diesel generator is 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 very far into the ground around the house since it is on a solid rock mountainside. Therefore, I would be glad for your advice on various issues.
Response
First, 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 the phone line to an 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. This post by:San Antonio Residential Electricians
Specific Questions
1. Should I use earthing rods of larger diameter?
A larger diameter will not help, as the surface area is critical. However, in the solid rock, it is unlikely 1.5 meters would be sufficient. The earth rod aims to achieve a low impedance to the earth of approximately 0.4 ohm. A test drilling and rod measurement would answer 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 meters should I use several ‘grouped’ together?
Yes, this “Earth Net” or “Earth Farm” approach is advocated by several experts for overcoming high impediments 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 low impedance. They should be grouped in a star configuration for small earth farms or a network configuration for larger ones.
5. How should I connect the various items of equipment? One expert report 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 practice to use large single strands in external rods/nets and braids where appropriate indoors. In addition, you have the “Aerial” effect 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, the 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 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 by fault or coupling effect.
8. Are the connection points themselves a particular problem? Do you have any suggestions?
Again a low impedance connection is an 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 the 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 for 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?
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, this is probably a good idea if your earth is good.
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 the 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 include several lightning/surge arrestors. Is this a good idea?
This is a vast 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 Substations do this by using large spark “arrestors” that disconnect explosively, dissipating energy. Large factories and new building installations tend to feature earthing, preventing 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 the earthing system. However, there is no point in diverting energy to earth if your earth is so poor, it will dissipate anywhere you have connected to the earth. Moreover, a surge suppressor is expensive and may be 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. However, at least one of these three items will often have little effect. The customer frequently 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, large pieces of equipment, and smaller devices on smaller systems. Each unit comes with wiring instructions and recommendations , including the earth cable size. 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?
Three additional suggestions might help: – a. Consider earthing alternatives. Our recent customer ran 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 that problem. The biggest problem with earthing and transient suppression is spending money on the wrong solution.
