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Posted By Topic: offgrid MEN board connected to inverter or genset

Apr 01 2019 16:31


I am installing two new MEN systems in two small seperate offgrid homes, one is to be fed with a 10A plug directly into existing inveter (does this inverter system need to be compliant? I don't want anything to do with it as I am only doing the MEN board, earth, and associated circuits), the other has a 10A inlet for a small generator.

Do both these installations require a double pole main switch, I am thinking to use a C10 2P for each if needed, having heard about centre winding*? generators starting fires, will that avoid such a thing, what is the fault condition that causes a fire?

Is there anything else I should be aware of, hoping to only get the inspector out once.

Thank you for reading.

Apr 01 2019 16:41

The fun that starts fires is that a typical building site genny isn't a 240V generator but a 120-0-120 generator, so the earth pin is the cnetre tap, and the "live" and "neutral" pins are each 120V away from the earth pin. Of course, what you actually have there is a and earth pin and two live pins.

Anyway, plug an MEN board into that. One is a live to live connection, that side is fine. The other is a live generator pin (that is, live with respect to earth) which goes to the installation neutral, which has a bond to installation earth, so there is a straight short across half the generator. Which often case the generator has no electrical overload protection, so if the generator doesn't stall then the smoke sometimes escapes.

A circuit with two lives either side of earth should have double pole breakers really. But it's so odd it's seldom seen in the normal world.

Modern inverter generators usually have floating earths so don't suffer from this malaise, it's usually building site simple generators with centre tapped earths.


Apr 01 2019 16:58

Hi Dbuckley,
Thank you for your explanation on the issue with centre tapped generators.

So a double pole MCB might protect against the generator shorting, but not resolve the actual problem?
The generator to be used is a very small Honda, around 2.2kW.


Apr 01 2019 17:02

Being residential, you're required to comply with Part 2 of "3000" [ESR 59]; and as a Part 2 Standalone power system must also comply with "4509.1" [ESR 60].

If each of these installations has the main earthing system that "4509" requires; then there will be only one un-earthed conductor so only that conductor is required to be switch4ed ["3000"; ].

The only bit you need an inspector for is the main earthing system. Installing a MES is "mains work". So's installing mains, but for installations that are solely powered by self-generation there are no "mains" (as defined).

With an inlet allowing the genset / inverter source to be unplugged, you need to provide overload protection for these supply fittings / conductors; to avoid someone plugging in an over-sized generating source.

Can't prevent an idiot from plugging in a centre-tap genset; after all they're the cheap ones. Best you can do is warn your customer; and also install a sign at the inlet warning that any generation to be used should have an isolated output.

There are no requirements in ESRs for an inverter used to supply off-grid to comply with any particular Standards.


Apr 01 2019 17:45

Apr 01 2019 16:58

Your comment 1 (part only)
The generator to be used is a very small Honda, around 2.2kW.

My comment 1
Run the generating set put a 1 bar heater on the output and measure that voltage on the Active neutral socket pins of the output with respect to the generator frame and the earth pin of the output socket outlet to see if the generator set output winding is not connected to earth.

I would expect the generating set you quote is vey likely to be earthfree

If earth free don't use AS/NZS 4509 it was not written for use with a generating set, (AS/NZS 4509.1 was wriiten for an inverter output only).

Figure 7.4 of AS/NZS 3000:2007 gives the correct arrangement to use but just omitt the wiring shown as L2 and L3.

If you wish to retain the connection to the generator set by a plug-in connection use Figure 7.4 omitting the Normal supply wiring connections and the changeover device.

The figure changes are as follows; connect the appliance inlet active pin via a 2 pole MCB of the same rating as the appliance inlet; the neutral pin of the appliance inlet to the neutral busbar via the neutrl pole of the MCB and the earth pin of the appliance inlet to the earth busbar.

The earth electrode and MEN connection for the installation is required.


Apr 02 2019 07:27

A correction to my above post

The first figure reference should read Figure 7.6 and NOT 7.4.

Apr 02 2019 08:24

Pluto's statement that "4509.1" was not written for generating sets is not supported by the facts. Generating sets are specifically included in the definition of "standalone power system" clause 1.4.16; and there's an entire section of the Standard dealing with them.

Worse; his instruction not to follow the Standard is contrary to law; as ESR 60 requires the Standard to be followed.

I do have reservations about the Standard's requirement for a main earthing system when supply is from an isolated source; but since ESRs require the Standard to be followed, there's no alternative.

The suggestion to follow Fig 7.4 of "3000: 2007" is equally flawed; as that Fig is for an alternative (standby) supply (as are all the other generator Figs in that edition); and as for omitting L2 & L3, they don't exist.


Apr 02 2019 10:31

Hi AlecK –
I’m curious about your comment yesterday that -
“for installations that are solely powered by self-generation there are no "mains" (as defined)”

I was not aware of that distinction and thought I’d have a look at the definition of “mains” in the ESR and found the following –
“means those fittings forming part of an installation that are used for the supply of electricity to the MEN switchboard of the installation that is closest to the point of supply”

The ESR talk about the point of supply but does not specify what that supply needs to look like. This leads me to wonder what is the basis to exclude stand alone power systems?

I would be grateful for any guidance so that I can learn from this ….


Apr 02 2019 11:01

Look in the Electricity Act definitions.

Apr 02 2019 12:05

Definition of "electrical installation" is in the Act. It was modified some years ago because existing definition relied on there being a "point of supply", and for a standalone installation there isn't one. Revised definition covers both cases.

With no PoS, there can't be a swbd closest to it, so there can't be "mains". The conductors that connect the energy source to the MSB don't really have a proper name; but regardless of what they may be called they are still part of the installation. Just that by not fitting within the definition of "mains", work on them can't be "mains work"; so can't be classed as high risk PEW.

PoS is also defined in the Act, with default being where the "mains" cross over the boundary; but can be and often is elsewhere,eg in a shared meterbox, or at the nth-floor swbd of a high-rise (in which case the "rising mains" isn't "mains" at all, or even part of any installation; but instead privately-owned "works").

Another complicating factor is that both definitions of "electrical installation" refer to "property"; so the ownership model for the underlying land or part-of building (cross-lease; shared title, etc) may change how the rules apply.

This whole area needs (very) careful reading, leaving all preconceptions at the door.


Apr 02 2019 13:56

Thanks AlecK - your explanation makes it all so much clearer