Questions and answers:
1. The system I have is 24 VDC using either an older Trace square wave or the new sinewave to convert to 3kW 240VAC (it was the sinewave that blew!).
This is the ONLY electricity source I have at my property. The house is very remote and the nearest electricity supply is over 5km and £30,000 away. As well as the diesel generator I am also building a dam and tiny hydroelectric generator, as I have a natural stream and a well for water.
I'm sure there is a reason for such a low voltage inverter system, although typically I would have expected higher DC voltages. The good news is that theoretically these low voltage systems SHOULD be less susceptible to damage. Having said that, I would expect the sinewave to blow simply because the larger number of components makes it less fault tolerant.
This sounds a fascinating installation, but I'm intrigued how you synchronise all these power sources? Do you endeavour to maintain good power quality, or just buy ruggedised equipment and replace or repair this as necessary?
2. Given the likely problem with installing ground rods, the majority of experts suggest I install copper lattice earth mats. Do you think that these would be a more suitable alternative to rods?
This is definitely a better way to go than rods into solid stone and with no reasonably easily accessible water. This is a similar approach to the Earth Farm that we advocate, connected in a ‘net’. I would recommend a reseller for this sort of gross copper purchasing.
3. Do you have any suggestion on how many and what size and depth to bury these?
I suggest that you refer back to the target impedance that you need. Based on application I would guess in the region of 10 square meters of mat with points at every meter.
The France Telecom box had melted!
4. I cannot say for certain what or how the sinewave blew. However, I know is that I returned here last October to find the France Telecom box on the wall outside had melted. All my phones were fused/blown plus the sinewave. The old square wave, solar panels (in the garden) and the controllers were just fine since they were insulated from the walls by a plastic panel mounted on wooden batons!
Indeed the France Telecom engineer who came to mend it said I was one of 5/6 houses in the area all with the same problem!! Although they were all bonded with very heavy wire I did not allow for passage through/along the walls. Also, the actual phone line was indeed burned out but the current was probably borne by the quite heavy ‘support wires’ they use here!
Even though you are in France, many UK domestic BT supplies still use support wires, but the jointing is done differently as the contract is written so that BT pays for termination damage. In France I believe you take out insurance to cover these costs? I would agree that actual lightning strikes are much less common than surges/transients induced by coupling effects. However for other people with a supply in common with the general public a still more frequent cause of transients is in the network itself.
From a professional perspective, we differentiate between surges (slow rise time ‘humps’ in the supply) which are best absorbed/suppressed and transients (fast rise time ‘spikes’ in the supply) which are best dealt with by diverting the leading edge (highly damaging and difficult to suppress) and absorbing the after effect.
Lightning tends to induce transients and frequently produces them in bursts. We advocate a combined approach of diversion and suppression in the aforementioned zoning system, where large devices are fitted to major distribution and smaller devices throughout.
5. It's been suggested that I need to get the resistance down to 0.4 ohm, although this is VERY low. The US NEC suggests a minimum of 20 ohms, but a paper delivered at the 1995 Conference on Power Quality in Omaha said that 5 ohm should be quite sufficient. Indeed their exhaustive measurements showed that you would need to drive the rods as deep as 50 feet to get under 1 ohm!! Even 5 ohms is 20 feet! Unfortunately I do not have equivalent measurements for ground plates or lattices? Any ideas where I could look?
I'm sure that these figures are correct but I would point out that every different nation uses a slightly different principle to define earth. This, and differing measurement techniques, can lead to some very confusing anomalies. In America for example, 20 ohms is considered fine, and 5 ohms is good enough for everything.
To quote directly from the IEE 16th edition the UK ‘bible’: - Amps (A)
Recommended earth fault loop impedance Z(s)
For A = 6 Z(s) = 8.89 ohms
For A = 50 Z(s) = 0.63 ohms
These figures are for faults back to ‘earth’ but by inspection you can see that if the earth impedance is 20 ohms its hardly likely to divert the energy.
Part of the problem lies in defining it. Do you wish to fit a lightning protection earthing system to divert lightning into specific earth (rods or nets) or are you trying to fit transient/surge protection to a building which you don't need to protect from lightning?
The issue is that, while for lightning diversion a solid ring of copper around your building planted into earth at each of four corners and probably with elevated spikes on top would seem like a good solution, if your principal problems are from coupling energy this approach is as likely to make the problem worse as better.
Secondly it is virtually useless to fit anything other than energy absorbers (converting to heat) to a system which is poorly grounded, and trying to absorb a lightning strike is like catching Niagara Falls in a paper cup.
Diverting a direct strike away from sensitive equipment :
6. I understand that it is better to join the ground/casings of all the items in a single unbroken loop using 25 mm wide 3mm thick copper conductor tape, as multiple cables often mean that one of the items is poorly earthed. But, with a continuous loop attached carefully to each item with proper clamps, the overall resistance to the ground plates is significantly reduced. Each end of the loop would be terminated at each end of a set of plates buried about 20 feet apart.
These are all good principles for diverting a direct strike away from sensitive equipment. This approach is taken directly from the one used in aerial cell sites, where the aerial is patently a good attractor of direct strikes and the attached equipment is of high value low density (and typically encased in a metal cabinet which is the thing the earth loop is attached to). In this system it doesn't matter if the earth loop behaves like a huge aerial, except that radio equipment may not function correctly in the vicinity.
7. Would you suggest that the plates should be buried in Bentonite?
Bentonite or any good grounding cement is a good approach for connecting earth rods/nets to solid rock. Again, set yourself a target impedance and fit earthing points until you reach it, or it becomes not cost effective to continue. Obviously the target impedance is 0 ohms and the target cost is £0 but these things define the earth more than the other way around.
8. Most experts maintain that the negative side of the battery set must also be grounded. One comment is that 'The battery bank on most systems acts as a fairly good surge arrestor if you have good connections. If the battery bank is not grounded, damage can be much more severe – the 'flashover' may then leap around all over trying to find a path to ground.'
I'm adamant that you should not connect your inverter battery to earth, unless you have an excellent earth. 'Flashover' will only occur if your inverter is hit by energy it can't divert safely to earth, and while flashover failures are expensive ones(usually leading to replacement of the DC end of the inverter) grounding one side of the battery is only a protection if the earth is a good one. If the earth is poor it makes a flashover MORE likely to occur.
Additionally, while batteries are plausible arrestors (they just get hot) there is a significant problem in your scenario. Your batteries are large and low voltage and so exhibit very low impedance possibly presenting the best discharge route for any energy. Would you rather replace your batteries every time you have a direct strike or spend the money on a decent earth in the first place?
9. In addition, I have a large pair of armoured cables bringing 240VAC and 24 VDC from the inverters/batteries across 20m of land and into the house supply! They are buried 700 mm deep in concrete and both the 240 VAC earth wire AND armoured sheath are connected to the common ground point. However, with 'ground differential' might this cause more problems?
You have two options as I see it. Connect one end of the sheaths to a reasonable earth and consider the other end potentially 'live' or earth both ends. Neither is ideal but I prefer the second (giving you a multi-ground-neutral effect as is common in distribution networks). However, I have to ask - why do you take 24VDC 20m into the house?
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