Earthing Portable Generators

I trust you have answered similar questions many times... Almost like, but better and clearer than a text book.

Pls explain why you propose its better that the lightning conductor system is external to the building? or are you considering other types different from faraday cage e.g radioactive systems, installed on another structure close to the building?

Hello shayre

Presumably your "Radioactive Systems" are the ones which release a stream of upwards moving ionised air, hoping that the lightning strike follows the ionised path downwards.

While these do work to some degree, there is really nothing better than vertical height, and a solid pointed electrode, if you intend to "create the lower part of the lightning strike path".

The reason for the external conductor system, as installed on large buildings it does become a Faraday Cage, is self-evident.

A lighting strike initially travels downwards quite slowly, as an "exploratory feeler", sometimes only a few inches or feet per second from the charged cloud towards the general mass of Planet Earth, inducing at the time a charge into the area of ground nearest to the cloud but of the opposite polarity, the path taken is somewhat exploratory, as may be seen from the path soon afterwards.

When the pre-strike "exploratory feeler" arrives at an object at ground potential, a current flows to fully ionise the exploratory path, and the picture at that point may sometimes be seen by a very high-speed camera, and also by those with good eyesight.

So, the "exploratory feeder" path is now ionised along the full length, and the very large discharge current flows, perhaps 20,000 Amperes at a potential of 20,000,000 Volts between cloud and ground - this is what we see easily with the naked eye, and call it the lightning strike.

The heat generated by the main discharge superheats the air, and the shock wave echoes from reflective surfaces: trees, buildings, clouds etc - which echoes further away too, all intermixing the various echo sounds, until we hear it as thunder.

Because the speed of the main strike is extremely fast, the surface ("skin effect") of high-frequency electricity takes place and the current does not like to "turn corners" in the lightning conductor, because of electromagnetic field resistance (like poles repel), thus the path needs to be straight, without corners or bends.

Each of the joints should be both welded and bolted, in case the heat of the passing current melts a weld, in which case a large electrical arc would develop across the opened joint - the bolts are insurance, keeping the joint clamped.

If you have ever been very close to a lightning strike, and survived, you would be aware that even if you had your eyes closely shut, you could see a very bright "Electric blue colour" inside your head.

The "Electric blue colour" is direct stimulation of brain neurons by electromagnetic radiation from the current pulse, and leaves no lasting effect, providing one was not too close to the actual strike.

I have twice been within a couple of metres of lightning strikes, survived of course to tell of the event which is quite an amazing experience, and you never hear the thunder, (the rolling sound mixture of various echoes), instead hearing the characteristic sound described as "tearing calico" - the same sound as made by a long electric arc, at the same time as experiencing the "Electric blue colour" effect, this is perceived whether the eyes are open or shut.

Internal conductor versus external conductor system to ground - General:

Internal earthing conductor allows the electromagnetic pulse to travel outwards, impinging on everything in its path, including brain neurons, electronic equipment, semiconductor junctions and also induces voltages which may injure people or damage hardware or software.

External earthing conductors = Faraday Cage, because of both the high-speed skin effect in the conductor, and if the electomagnetic pulse travels inwards, each is repelled by other electromagnetic pulses travelling down the other earthing conductors spaced around the building exterior - (like poles repel) thus the current pulse is harmlessly discharged without harm to people or equipment.

If you are ever out in a level field during a thunderstorm, and feel your hair standing on end, drop to the ground and roll away very quickly - this may allow you to survive a near lightning strike without injury - In this, you have perhaps less than a second to act before the main current pulse travels through you.

I trust you now see why external downwards earthing conductors are much safer for persons and property, than an internal lightning conductor.

Kind Regards....

Thanks for the indepth analysis, Sparkstsation. But I still feel that the 5m x 10m mesh (Faraday Cage) roof mesh for high structures will give adequate protection without the risk of side flashing. Can you give an example of how and where to possibly install the LPS externally for this type of situation (assuming building is the highest structure within the immediate surroundings).

Cheers.

These genertors do need to be attached to an earth ground to ensure proper operation of the built-in circuit breakers. They can be bonded to the power ground rod or to any other low resistance ground source. Even at the top of a building there should be some tempory power source with a conductor to the power ground rod at the base of the building.

Refer to the manufacturer's specifiction.

Is it a big generator or a small one that you carry around to drive hand held power-tools?

Hello Isti80

From the Topic Post: <"....single phase, 220V AC, 4KVA....">

As you see, it is a portable unit, moved from floor to floor during construction.

Such an alternator unit is used for temporary lighting and floodlighting, along with power tool use.

Kind Regards....

I think no eathing is required. Let the system neutral floats. (Considering that's a single phase portable DG). If its a 3 phase(which I suppose is not the case here) earthing is important for safety of tool operators.

I believe these generators are considered as an isolated un-earthed output in the same way that the isolation transformers are used on site. the outputs of these sets cannot be connected to frame to produce a neutral as in the event of no earth being connected (or becoming disconnected) the frame would obviously take on a potential dependant on the leakage of it's loads.

Chas

Since it's a portable generator (I would assume a small one) there's no need for earthing as the internal grounding for safety would still be present. You may sometimes do not even have it on proper ground.

In the US for areas under the jurisdiction of the National Electrical Code article 250.34 applies and states "The frame of a portable generator shall not be required to be connected to a grounding electrode as defined in 250.52 for a system supplied by the generator under the following conditions:

(1)The generator supplies only equipment mounted on the generator, cord-and-plug connected equipment through receptacles mounted on the generator, or both, and

(2) The normally non current carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are connected to the generator frame

I tried googling "iso standards, portable generators" but came up with nothing more than the NEC code for USA as quoted by Goodho. Can anyone quote the EN or ISO standard for the earthing on portables. Ultimatly these codes/ standards are what count as, in the case of a serious incidend, it is the absolution needed in a court.

Chas

NFPA 111 - Standard on stored electrical energy emergency and stand-by power systems 2005 - doc # 11105

NFPA 110

NFPA 70B

Hi Bwire

those standards are not applicable as they refer to the fixed installations of emergency gen sets.

Chas

Actually as others have mentioned, there is no need to earth the generator, the only way a worker can get a shock is if he gets both live and neutral or the earth connection, touching his body at the same time.

Just standing on the ground and touching live will not do it, in fact anyone who does this will not get a shock or feel anything at all.

I am assuming that we are talking about 110 volts and not higher....for higher voltages I would suggest an elcb to be fitted - just in case!