Re: separate earthing for generator: Tony's right. In many places "earth" comes from the Power Utility Company as well as possibly dirt-rods/mats. In a power failure, the utility ground is not trustworthy. The line from the street may be broken. Or it may have to be cut to perform repairs. The generator needs an earth which will function even when utility power breaks.
Whether this be a dedicated rod/mat or the normal building ground depends a LOT on local regulations and power source type TN (TN-C TN-S TN-C-S) TT or IT, the required or orderly sequencing of PEN MET and PME terminals and service jumpers.
Residual-Current breakers may reduce (hardly eliminate) shock hazard, but that's not the same as grounding.
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> building has its own lighting protection and comes down to various rods in the ground so if the building is struck by lighting it travels down shortest route and disperses into the general mass of the ground why do we then link the lighting protection to the main earth bar surely were giving it a fault path back into the building
Follow the trails.
Power electricity flows only from brown to blue and back. Ultimately all the way back to the generator.
Lightning flows cloud to earth (or earth to cloud, same as far as we care).
So it is really two different paths. Like a highway and a railroad. We don't want the cars to end up in the railway yard, nor locomotives in the shopping center, even though both are "on earth".
If we had cheap perfect insulation, we could keep the two flows apart.
But lightning strikes can be a million volts. We can't insulate for that. And utility power systems run both near dirt and to top of buildings and towers. And they cover vast distances. When there is lightning around, odds are that a good percent of strikes will go "through" power wires on their way from cloud to earth.
A lightning bolt can be VERY high current, many-many thousand Amps. Even where it enters earth, dirt resistance means the voltage at the surface will be elevated from "normal" voltage.
Power utility insulation breaks down. At 10,000V you have sparks jumping out of wall sockets and jumping through brown blue and green plastic. At 100,000V the huge distribution transformers will break-down. Somewhere before that, arc-horns and other protectors will be trying to divert the potential.
It's a real mess. And if Thor is going to get you, he WILL. But he often near-misses.
There is no good answer. Except to tear-out all our puny wires, and the risk of lightning is low and the benefits of power are large, so we won't do that.
The power company "must" bond their lines and gear to local dirt throughout the system, so that lighting strikes have a short direct path to earth, with massive redundancy so that if one or a dozen dirt-rods are blown from the earth, others further down the street will take the overflow so that the entire system does not domino and repairs will be limited.
Imagine a hit on your lightning-rod going to its dirt-rod. The high energy "lifts" the earth potential many thousand volts. Meanwhile your power dirt-rod a few feet over is still at "normal" potential. Electricity is sure to flow, and violently.
Now bond the two rods together. Your electric wiring is "lifted" part-way toward the lighting-induced dirt potential, reducing the tendency to flow, and it flows in a designed conductor instead of blasting dirt and jumping through walls. Yes, this means your house wiring potential is now thousands of volts higher than the dirt far from your house or the power company wire along the street. There will be surge and possible damage. But the inside of your house is (probably!!) safer than if lightning and power dirt-rods were not bonded.
Other things can happen. Lightning is not easily predicted. The exact course and energy can't be known even at the moment, much less decades before when house and power and rods are installed. And different places vary. I'm on dry rock. I can't get a good dirt-bond. I might be safer with oil-lamps and no power line. (But oil-lamps caused many tragic fires....) Power companies treat dirt-bonding in different ways, and also treat customers in different ways (some expect to "give" earth to customers, others prefer customers find their own path to earth). Even when lighting and power are bonded, the bond conductors, rod spacing, resistance goals, and other details vary with local experience and conditions.
So best left to experts who are insured or can get you better insurance. However when they drive separate rods and then tie them together, it is pretty reasonable and widely accepted.
