Generators and Earthing

Perhaps I should add that it may be impossible to NOT ground a portable generator.
If the generator has an exposed metal frame then simply placing it on the ground will "ground" it. Perhaps not very well, but it is grounded nevertheless. If the soil, concrete or whatever is wet then it may create a very effective grounding.
Even if the gennie has rubber feet, contact between the metal chassis and "ground" may be effected due to unevenness etc allowing direct contact.

Accordingly, there is no point in prohibiting "grounding" or earth electrodes if it can easily be created inadvertently. It can do no harm and may even be beneficial in some circumstances but is not prescribed as being necessary.

The other issue to consider which renders grounding next to useless is small portable inverter generators are like 12/240V inverters, both legs are hot, you don't have an active and a neutral.

thanks Alllan (with 3 L's ??)

Adding something else in, I believe it was also said that you do not tie the Neutral to the Earth back at the source as you do in House wiring (MAN) where the Earth and Neutral are tied together back at the main distribution point. Or maybe it was that you do not tie them together in the Caravan (load) wiring as that would be done externally if you were on a powered van site.

As I see it you would have to tie the Neutral of the Generator to the Frame and all exposed conductive components and the Earth of the power outlets and then Earth Stake it to get a true MAN system and then an RCD would work properly AS LONG AS you had an effective Earth system.

HOWEVER, I think you probably hit the nail on the head when you said "with the usual dryness of the soil I was never confidant that the stake was of much value" assuming it is moisture in the soil that makes our earthing system work. In many cases the earth stake would be ineffective.

The paper referenced by Notso in this post at a brief glance appears to suggest you at least tie all exposed conductive stuff together which would be the frame of the caravan or whatever and the gen set components so at least all that is at equipotential, although it seems to be more on Welding applications, but the same reasoning would apply to anything. At least that way you cannot get yourself between 2 conductive items that might be at different potential and become a conductor yourself.

If the system was left "floating" then it would seem the only danger to you is if you get yourself between the "active" and the "neutral" themselves (or call them L1 and L2 if you like) - if you were to come in contact with one or the other on their own you would not be affected as there is no path to Earth (like a bird sitting on a single power line). So then in the case of faulty wiring or a faulty appliance if you came into contact with the "active" it would be "safe".

I note paulnsw just posted about not having an "active and neutral" as such - well yes that applies to any floating system that has no external reference - I am only using active and neutral to differentiate the wires.

Ken

Re; "thanks Alllan (with 3 L's ??)'
Yes Ken, I carry a spare. LOL

Ken, The ideal electrical supply system is "Fully Isolated" where there is no reference between the live conductors and "ground" or the frame. This is how portable generators and inverters are constructed. In the case of a single phase system there is therefore no "neutral" so the convention is to designate the conductors as L1 and L2.
In the case of a reticulated system as we have in towns it cannot be assured that the system could be maintained as "Fully Isolated" due to unintentional (or otherwise) connection between the live conductors and ground. This could be as simple as some leakage due to moisture in an installation or connected appliance. Accordingly it was adopted that one line be earthed and designated as the "Neutral" as it would be at ground potential. For system reliability and reduced impedance of protective conductors, the MEN (Multiple Earthed Neutral) system was introduced where the neutral was connected to the earthing system at each installation. In addition an earth electrode is connected at each installation in order to maintain minimal potential difference between local ground and the local neutral/earthing system.

As a portable generator is limited to a small local installation then there is a better chance of keeping the system isolated so earthing the neutral is less desirable. Nonetheless it is always possible for faults to develop in either the gennie, the cabling, or connected appliances so a protective earthing system is required to ensure equipotential of all exposed metal. It is possible to postulate all manner of faults which can reduce the integrity of the system but normally it will require two such faults simultaneously to present a hazardous situation. Unfortunately the first of such faults could pre-exist without becoming apparent until the advent of the second fault which may be bodily contact with a conductor.
In such a system, fully isolated with equipotential bonding, there is no reference to ground and an earth electrode will provide no real benefit. At the same time it will not compromise the system.

Certainly the incorporation of RCD protection is of benefit but it needs to be properly installed. A simple plug-in type will not provide full protection. This subject is complex and I do not wish to go into it here.

Thanks Allan with a Spare L

Incorrectly I was using MAN meaning MEN in my comments - thanks.

That has truly satisfied my curiosity. From what I have read generally sniffing around this subject I would imagine there are a lot of incorrect installations out there and worse a lot of people that may believe they are protected with an RCD when they are not .......

So I guess the bottom line message is if you want to use a Gen set with RCD protection go get it done by someone who knows how to do it properly for your specific situation.


Ken

Alllan - FYI - I found this in an article on safework.sa.gov.au - and for those out there think that pushing the test button on an RCD means it is protecting them well that is not necessarily the case. I have seen that misconception floating around also.


Example of portable generator problem

At a major construction project 15 portable generators brought on site by subcontractors were tested by the project managers as a normal part of site safety procedures.

IN ALL CASES THE RCD PROTECTION DID NOT WORK because the
internal wiring connections within the generator unit, but outside the RCD,
were incorrect.

In this case the test button functioned normally because the test button only
tests the tripping mechanism of the RCD not the full wiring of the generator.

Note that the RCD itself was not faulty, the problem was the connections
between the RCD and the generator.

gbc, the real danger isnt the frame of the gen-set, but use of tools, extension leads etc. If the gen set frame is earthed, then the voltage it produces is referenced to the earth/ground. If someone hooks up an incorrectly wired lead then any tool connected to it that it not double insulated (which is rare these days) will be "alive" with reference to the ground, and someone holding it can get a shock.

An earthed gen set is safer than a non earthed one as not only is the gen set equipotential but it is also equipotential to the ground/earth. But its only the gen set that is safer.

The RCD works by measuring the current in the active and the neutral and summating them. If there is a residual (or leakage), i.e. one is greater than the other, the RCD trips. The leakage is usually used by the person getting the electric shock.

For an RCD to work on a Gen Set such as GBC is waiting for I can only assume they have tied one line to the Earth pin of the supply sockets so the whole system has an Earth reference (in other words it is not a floating system any more but a MAN system like house wiring).

All the RCD's I know about rely on an imbalance of current as Bonz indicates - as long as the same current is flowing in both lines (call them L1 and L2 rather than active and neutral) then the RCD will not trip.

For it to trip there has to be a leakage to a 3rd reference point (the Earth in a house system) such that if you get between the Active in a normal house and you are sufficiently in contact with the Earth (be it the metal case of an appliance or bare feet on the floor) you will cause an imbalance in the current in one line as some of the current is flowing through you to Earth and not going back the neutral line to the source. Hence the RCD will trip due to the imbalance. Think of it as a water pipe with a leak along the way - what you are pumping into the pipe is not what is coming out the other end as you are losing some water along the way to a 3rd point - the ground.

HOWEVER if you get yourself directly between the active and the neutral wires and you are isolated or well insulated from Earth, you will not trip the RCD as there is no "leakage" to Earth. All the current flowing will pass through your body from the active to the neutral and the amount of current in both lines will be the same - there is no imbalance as there is no "leakage" to Earth.

This second case is the danger in a floating system - you can put an RCD at the source (the Generator) but if you have no 3rd point of reference for current to leak to there will never be an imbalance of current and the RCD will not trip. At the same time though, in a floating system you are not at risk by coming into contact with either of the lines L1 or L2, only if you get between them somehow (and the RCD will still not trip and protect you). This could be due to say a faulty appliance where one line has come into contact with the metal casing (and and you manage to come into contact with the other line somehow - you will be directly between the 2 lines.

Yes

I am not sure about the theory of "Earth" and how much it relies on moisture as well - all my theory says most soil is not a good conductor and requires moisture to make it more conductive.

But it generally must have some conduction under all circumstances but if I was relying on it for an RCD to work I would rather be on wet ground than dry ground.

Ken

ob,

We are running 14 x 40Kva gensets & 3 x 20 Kva gensets on mine site and as you said all the powers to be, state they have to be earthed to an earth stake.
They are also tested & tagged every quarter
Wether it is right I don't know, I am a Heavy Duty Fitter not an electrician.

Cheers
Simon.