Portable generator - Grounding

My recently acquired Chinese portable 2KVA generator handbook says: "The generator must always be grounded". I also have a very heavy (hence the acquisition of the lightweight Chinese generator) vintage 3.5 KVA generator. This has the neutral and ground (earth on the 3 pin 230V outlet socket) bonded together. I suspect the Chinese generator is the same. I have never grounded this vintage generator and I am still alive to ask the following question!
How is effective grounding achieved, when the generators are used down my garden, powering hedge trimmer, strimmer and other garden and DIY tools?
I ask the question here since several contributors have remarked that they own / use portable generators, and maybe someone can enlighten me.
-- cerberus

I have a 950 watt Chinese Genny that I use on my Caravan and camper when rallying, that has a brass earthing rod about a metre in length which I shove into the ground for about 500mm. The grounding rod came with a 500 watt industrial Inverter I acquired, 12VDC to 240VAC which had been fitted to a Mobile workshop Landrover. A good friend who is a Industrial Electrician suggests I should not use the Genny unless it is grounded, so piece of mind saves the day.
Martin P
snipped-for-privacy@address.invalid wrote:
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On Sun, 05 Feb 2006 12:14:41 +0000, snipped-for-privacy@address.invalid wrote:
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No interface to your domestic mains wiring in the house, at all, ever?
With no connection to house wiring, I'd not be too worried about a real ground connection for the generator but ensure that the first thing the output of the generator encounters is a 30mA RCD and everything is fed from that RCD. That way if any current trys to find an alternative route back the RCD will trip.
If you do want to ground the generator a copper rod driven into the ground a couple of feet next to the generator and connected to the frame of that generator with at least 6mm^2 wire should be adequate. What you are doing with the power after that determines if you really ought to bond one of the generator phases(*) to ground creating the "normal" mains arrangement of "neutral" and "live" wires.
(*) These small alternators produce two phases 180 degrees apart and floating. It's what happens to these two phases and any ground connections after they leave the alternator that is important.

AFAIK there are 2 ways of running a generator. In order to get a shock, there has to be a complete electrical circuit via you. Since you are normally connected to the ground, that is the most likely current return path. A generator produces a voltage across two terminals. If neither of these are connected to ground, you cannot complete the circuit that way. This is earth isolation and is favoured for portable generators as it saves having to find a good earth. If the system is run like this, the only way to get a shock is to contact both of the conductors. The down side is that, as there is no voltage reference to earth, the power side can escalate to high voltage. Again, this is normally only a problem if the insulation brakes down providing an earth connection. Since there is normally no earth by design, it's best not to make such a connection. The other way of running a genny is to fully earth one of the power terminals. This mimics domestic wiring. Metal appliances are now earthed and protected but touching the live lead will give a shock. By definition, portable equipment is used in unusual environments where cables are likely to get trodden on etc. Here the earth isolation method is best as both cables have to be touched to make danger. I would always advise a RCD for safety but be warned that some domestic plug in types also look for earth connection. If they are used in an earth isolated system, they trip as soon as power is applied. Not a lot of use.
John

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Thanks John, Dave and Martin for your replies. My vintage generator has one 240V AC o/p connected directly to the frame, the earth pin on the o/p socket is also bonded to frame. (This is an ex recreational vehicle / ice cream van Onan genny). The wiring of the new lightweight Chinese genny is to be investigated. I will look for RCDs for both.
That leaves me with another question. In the event of a mains power failure I would be persuaded to use one or the other generator to provide emergency lighting and cooking facilities and run the CH pump and boiler. For every device I would use an extension lead from the generator and plug in the lights (table top lamp variety), an old table top standalone oven and cooker ring and the CH boiler, and I have no doubt the idiot's lantern. The boiler would be unplugged from the mains and plugged into the extension lead. The TV aerial sockets outer are unearthed.
This seems to me to be running in the earth isolation method except that the CH boiler is earthed because the incoming gas feed is connected to building earth and also by means of the copper pipes to the boiler and hence all the CH radiators.
In the above scenario, with the two choices of generator, one with o/p terminal bonded to frame, the other with both o/p terminals isolated from the frame (and ground), but with the connection to building ground through the CH gas pipe being fixed, what is recommended for grounding of either generator?
I hope this doesn't degenerate into a Part P discussion!
-- cerberus

On Sat, 11 Feb 2006 17:06:55 +0000, snipped-for-privacy@address.invalid wrote:
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This is the best option and pretty much covers all bases. John mentions some RCDs requiring an earth. This was the case years ago with voltage operated trips but these days I'd say that they are all current operated. That is a difference between the current flowing to the appliance and that coming back causes the device to trip.
I do have a 10mA plugin jobbie that is a pain to use, have more than about 10 yds of cable after it and the cable in rush current is enough to trip it.
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The exposed metal work of the boiler/heating system etc is bonded to building earth but the live and neutral shouldn't be. This is where it gets complicated and potentially dangerous under fault conditions.
If you get a live *or* neutral short to the "chassis" (and hence building earth) no overload protection will trip as there is no circuit but unexpected parts may now be at mains voltage relative to the chassis. If you have live/chassis fault, *all* the the neutral connections are at mains voltage relative to ground. Throw in a some single pole switches and stuff that is "off" still has the abilty to bite you badly up the now very much "live" neutral connection.
IMHO once you start connecting equipment that would normally have an earth connection you really need to emulate the proper mains supply with one generator phase bonded to the generator frame and to a decent local earth. This is to provide the proper fault overload protection. Shock prevention is best provided by a 30mA, current operated, RCD at the generator.

On Sat, 11 Feb 2006 22:24:58 +0000 (GMT), "Dave Liquorice"
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Thanks Dave.
-- cerberus

Dave Liquorice wrote:
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My modern 30mA plug in RCD needs an earth connection to stay in. If I plug it into my genny, it trips instantly even with nothing plugged in to it. Took me a while to figure out what was happening but it works OK on 'normal' mains supply.
John

On 13 Feb 2006 11:52:51 -0800, John wrote:
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I wonder why? Is that an isolated earth or an earth bonded to "neutral"? Perhaps the unit is checking for earth continuity as an additional "safety feature".
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Is that with the RCD set on then starting the genny or having the genny running, the RCD plugged into the live socket, then set to on?

Dave Liquorice wrote:
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I assume it's an additional safety feature. The RCD has to be reset when power is removed so I can't leave it 'on' and start the genny. My old BSA is earth isolated so runs without an earth. That's why I couldn't figure out why the RCD kept tripping even with no plug in the socket. Eventually I assumed it checked for earth continuity as well as current balance.
John