Technical Advice for Alternative Power Supplies

This is the simplest – and cheapest – option.

It’s just a matter of locating a genset, putting it in a suitable, well-ventilated place (NEVER inside a building that wasn’t designed as a generator house); and using extension cords to get a supply to essential appliances.

There’s some guidance in this in Appendix B of AS/NZS 3010: 2017; and more in Section 3 of AS/NZS 3002. “3002″; which covers Shows & carnivals.  While an emergency supply to a house or business is not a ‘show” or a “carnival”; this Standard is also untended and used for scout jamborees, army camps, and civil defence shelters. So, despite the title; it’s exactly what we want for emergency work after a natural disaster.

Specifically, Section 3, which covers “event wiring”; which can be assembled by unskilled persons. This is not PEW; so, doesn’t need to be installed by licenced electrical workers – and there’s no certification needed.

Simply check that the equipment you’re using is safe – e.g. current test tag -; then apply common sense.

Worksafe have published a guidance document (“technical bulletin”; or TB for short) providing for short-term connection of a genset to an installation in an emergency.

Available here and can be downloaded as a PDF.

What counts as an “emergency”?

The TB doesn’t specify. But what caused Worksafe to issue it is the expectation that it will take a long time to get distribution networks restored in the east coast of the North Island after the devastation caused by Cyclone Gabrielle. So, we can assume that it requires a similar level of disruption by natural disaster before we can use it.

Basically, the guidance in the TB covers

1. disconnection & making safe of the normal supply mains at line side of main switch.
2. modification of the installation, and the generator if required, to make them compatible and provide a safe supply
3. connection of the generator, in place of the normal mains; to provide a short-term source of supply.
4. certification requirements
5. reinstatement requirements

It’s important to realise that following the TB’s guidance will result in an installation that does NOT comply with the Electricity (Safety) Regulations. It should be functional; and it should be safe. But it will not be compliant. And there’s nothing in writing – not even an email – to say people who follow it won’t be prosecuted. However, they would certainly look very foolish in the court of public opinion if they prosecuted someone who followed this official document – provided it was done for the intended purpose of emergency supply following Cyclone Gabrielle or similar emergency. And provide it was followed exactly

An outline of the TB’s guidance (as at 8/3/2023) is below. There are significant safety issues with this sort of work; so, DON’T rely on this outline alone. Go to the website and download the official document. It’s very important that this TB is followed to the letter, with absolutely no deviations.

The problem is that most small gensets available are intended for direct supply to equipment; and are NOT suitable for connection to an MEN installation.  So, in order to get a short-term supply, either the genset needs to be altered, or the installation does; or both. Worksafe have opted for altering the installation in every case; and the guidance gets around this by allowing for use of;

1. an RCD-protected genset; or
2. a centre-tapped genset in conjunction with an RCD; or
3. an isolated-output genset, modified to provide a polarised output.

In all cases:

• the incoming mains – A(s) & N – must be disconnected at main switch (line side)   and the N-bar; & terminated safely;
• a (compliant) genset is connected to the main switch;
• load must be reduced so that the max demand does not exceed the rated capacity of the genset;
• the installation’s MEN link must be removed – unless adopting the option of setting up a “standalone” installation (see below) (any others, eg at DBs,  must also be removed; though that’s not stated);
• the fact that these alterations have been made must be clearly labelled at the MSB;  and
• the work must be certified, including stating that the work was done in accordance with the Worksafe guidance.
• no inspection required, even though it’s ‘mains work”; because they’ve declared this work to be classified as “general PEW”.
• “larger” connections require design by an Inspector or Electrical Engineer.

Alternatively; the TB specifically allows us to set the installation up as a stand-alone installation in accordance with AS/NZS 4509.1 .  In which case we will NOT be removing the installation’s MEN link, nor making any N-E connections within the genset; just disconnecting the normal mains and connecting a generator that has an isolated output. This is probably the best option for a multi-phase installation.

The guidance in the TB is intended only for generators with rated output 15 A or greater. If you only have one with rated output less than 15 A, or that only has 10 A sockets for connection; you can’t use it under this TB.

 And if it’s a “larger” connection; you must get a specific design by an Inspector or Electrical Engineer. “Larger” has not been defined; but all multi-phase installations are deemed to be “larger”. For single-phase; it would be wise to treat any connection intended to carry 32 A or more as “larger”.

 This design work may not be a “certified design” (as per ESR 58); because it may not be specific to one site. But what we’re doing would normally be what’s called a “Part 1 solution”; so it certainly makes sense to get another brain involved in working out how to do it .

 Note that it’s the rating of the connection that counts; not the maximum output of the generator. So if you have a large genset available, but are inly using it to supply a small load; you don’t need a special design. You may even be able to use a single large, even multi-phase, genset to supply several installations.

The key issue here is to ensure that there can no NO back-feed from the emergency generator onto the normal supply mains.

If the normal supply wasn’t off, or unreliable; we wouldn’t be trying to make an emergency supply. So, we can assume that the mains are not energised. But safety first, so

1. Turn all main switch(es) OFF.
2. Test to confirm mains are de-energised.
3. Treat the mains as live at all times (unless the supply fuses have been removed); because normal supply could be restored at any time.
4. Carefully label each active conductor; to ensure correct phase rotation when normal supply has been restored.
5. Even more carefully label the neutral; to ensure correct polarity when normal supply has been restored.
6. Disconnect each mains conductor in turn, and cap each with an insulated terminal.Insulation tape is not enough

The TB stipulates disconnection of mains from (line side of) main switch. This is correct for cases where the metering is upstream of the main switch as is often the case for domestic installations.

However, we don’t want the energy from the emergency generator to be recorded by the meters; so if the metering is on the load side of the main switch; find a suitable alternative position. For example, the first terminals supplied from the meters. If possible; avoid breaking any seals for metering / load control.

Larger installations may have several main switches, including separate switches for each “safety service” (eg lifts, fire pumps, etc). We need to carefully consider which part(s) of the installation are “essential” enough to require the emergency supply. And it’s important that buildings are not occupied unless there is power for all “safety services”.

The TB also specifically requires disconnection of the mains neutral. This is unusual; and contrary to specific rules that normally apply.

In normal circumstances; the MEN system relies on the integrity of the mains N, in conjunction with the MEN link, to carry fault currents. It also relies on the earthing systems of all the connected installations, acting in parallel, to maintain the distribution N at close to zero volts relative to mass of earth. Strictly speaking, it’s actually a PEN conductor, with the “PE” part indicating that it’s performing a protective earthing function. Which is why AS/NZS 3000 prohibits switches or circuit breakers from operating in it [clause 2.3(b)] unless it’s linked with devices that switches the actives at the same time. It’s also why clause 7.3 prohibits a source selection (changeover) device from operating in the incoming (mains) neutral.

But what’s wrong for normal conditions is actually right for these unusual conditions. When there’s no supply from the grid; it doesn’t much matter if the voltage on the out-of-service distribution PEN floats up a bit. And when we’re not taking supply from grid; the mains (PE)N is not part of the fault current path. So, it does no harm to disconnect it from the installation’s MEN; and doing so means that even if the connection of the emergency generator to the installation is botched; we can’t energise the network via the disused mains.

The MEN link is a mandatory part of the “main earthing system” for any installation set up for supply from the MEN distribution system. And when we set ip an alternative supply system, we don’t touch it. So why remove it in this case?

 We need to march the earthing system of the installation to the earthing system of whatever type of generator will be used for the emergency supply. In normal circumstances, the only type of genset that is permitted for connection to an installation is one with an isolated output.

In order to safely use any other type; the installation’s MEN must be removed.

Additionally if there are any other N-E links within the installation, they also need to be removed.

If there’s no removable link – for example an older installation with a single busbar for both neutrals & earths; you can’t set up an emergency supply under this TB unless you first upgrade. The best way to achieve this is to add a neutral bar. Doing it this way will mean that there is less risk of removing any protective earthing conductors.

Lastly install a prominent and clearly-worded label advising that the MEN has been removed; including details of any other M-E connections that may have been removed.

RCD-protected genset

With an RCD-protected genset; the installation’s MEN would be in parallel with the N-E link within the generator. That means some of the return load current will flow in the protective earth conductor; and the RCD will trip. Which is no use to anyone. And far better to have the protection of the RCD than not; so the best option is to remove the MEN.

Centre-tapped

With a centre-tapped genset, it’s far worse. These generators have a winding that puts 115 V between N & E of the output (see below). The MEN acts as a short-circuit for this source which at best means the overcurrent protection that should be on the output will trip. Unfortunately, many if these sets don’t have any protection on this “neutral” side, only on the “active” side. In which case the short circuit is likely to burn out the winding and may well cause a fire.

So again, we remove the MEN. But now we have 115 V between N & E throughout the installation – with no, or only limited, protection. Very dangerous. Which is why the TB requires us to install an RCD (30 mA) between genset and where it’s connected to the installation. If someone receives a harmful shock, the RCD will operate to remove the current quickly, and so save their life.

Testing the RCD

All RCDs Approved for use in NZ are designed to operate on 230 V supply. With a centre-tapped supply source; there’s only 115 V to earth. So in the event of an earth fault, only half the current will flow as would normally be expected. This means that external RCD testers, designed to create a fault of approx 30 mA to flow in the earth conductor (Active – Earth fault); will actually only cause approx 15 mA to flow – and of course the RCD won’t trip. However the integral test circuit works between Active and Neutral; and since this is still 230 V this test will work.

Isolated output

This type is perfectly OK for connection to a normal MEN installation. This is what is used for every alternative supply. However, when creating a set of instructions for emergency generators, they all have to be treated the same. Especially since many of these will not be “permanently” connected; instead being just plugged into. Which means there’s a good chance that generators may be swapped from site to site; including by people who have no training, qualifications, or experience

So the TB’s instructions:

• “do not use an isolated supply (separated supply system) to connect to an existing MEN installation”; and
• “Make sure there’s a connection between the generator earth and the neutral pole of the generator. If there isn’t a connection, install one” are really only for consistency.

While we’re at the MSB removing the MEN; the TB requires us to check the main earthing system. Including:

1. check that there actually is an earth electrode
2. check that the main earthing conductor is actually connected to the earth electrode;
3. check that the main earthing conductor is actually connected to the earth bar
4. NOT to the N-bar, as used to be common.  If it’s connected to the N-bar, it can’t do its job while the MEN isn’t in place. So if that’s where it is, move it.

One other thing to watch is if the installation has an inverter. Mostly these are grid-connect units for “mains parallel” operation; and as such are required to have “anti-islanding” to prevent back-feed onto the grid when it goes down. DON”T alter this set-up.

A multi-mode inverter is capable of also operating in “island” mode; but to take advantage safely we have to take this output through a changeover device same as any other “alternative supply” and use an extra pole of the device to change the mode of the inverter.

But regardless of what type of inverter; don’t set up an inverter and a genset to run in parallel.

It may seem logical to disconnect the mains, and use a genset to simulate mains supply so the inverter will operate; but the two forms of source don’t work together. AS/NZS 3010 requires generators providing alternative supply to be isolated from any inverter by the source selection device (changeover switch).

If setting up an emergency generator for an installation with an inverter; arrange the generating set to supply it’s own separated-off part of the installation only.

The TB doesn’t specifically cover setting up an emergency supply from an inverter; but logically the same principles should apply. And the declaration that emergency supply work is “general” rather than high risk PEW can be interpreted as covering emergency supply from an inverter. So if the inverter that’s there is a multi-mode type; it should be OK to disconnect and safely terminate the mains before changing modes. Issue a CoC for the change. Change it back, with another CoC; before reinstating the mains supply.

It is one of the TB’s “general safety principles” that “the maximum demand of the electrical installation must not exceed the rating of the generator”.

This could require a lot of disconnection / isolation of circuits, which would late all need to be re-instated. Luckily there’s a far simpler solution – maximum demand by limitation- which is how we normally do it for mains, typically using the rating of the supply fuse as the load-limiting device.

So all we need to do is use an over-current device to limit the current that can be drawn. This could be the overload protection within the genset, or a separate device within the installation; eg upstream of the main switch(es).

Two CoCs will be needed; one for the establishment of the emergency supply; and another when the normal supply rom the grid is restored.

Setting up the emergency supply

Before certifying this work; make sure you’ve done a FULL set of tests of your work. DON’T allow urgency to lead you unto skipping this essential part of the work. And preferably keep a copy of the test results.

Disconnection of mains, disconnection of MEN link, and (where required) moving the MEC connection from N-bar to E-bar are all “mains work”; and would normally be classified as “high risk” PEW requiring inspection. However the TB has declared “ the prescribed electrical work carried out to install an emergency generated power supply”  to be “general” PEW. So no inspection required. And this should also apply to installing a N-bar so that the installation has separate N & E bars.

 The CoC and ESC (either separate or combined) for this work must include a statement that the TB has been followed.

• as always, give a detailed description of the work done
• cross out the bit on the CoC that states that either Part 1 or Part 2 was complied with
• write in that the work was done in compliance with the technical Bulletin
• In the declaration that the work was done ‘lawfully and safely”; consider crossing out the word “lawfully”.

Note:  The TB does NOT suspend the requirement set by the Act to comply with ESRs; nor does it suspend the ESRs requirement to comply with AS/NZS 3000. The TB is official guidance; but it’s not “law” in any way. So declaring that you’ve followed the law, when you’ve actually done something different, is making a false declaration.

The TB guidance allows use of three types of genset

1. an RCD-protected genset; or
2. a centre-tapped genset in conjunction with an RCD; or
3. an isolated-output genset, modified to provide a polarised output.

In fact, the TB specifically forbids us from using an isolated output genset. Which may seem odd; because that’s the only type that’s suitable for supply to an MEN-type installation. But they TB also includes a requirement that, if there’s not a connection within the genset between N & E, we must make one. And if we follow that instruction, it will no longer be an isolated output.

It is likely that the guidance will be amended to require an RCD with this option, as for RCD-protected gensets.

How do we know what sort of genset output we have? Asking the supplier isn’t likely to help; most won’t understand the question, let alone know the answer. And it isn’t likely to be written on the tally-plate.  But we need to know; and not just so we can follow the TB’s guidance.

There can be serious issues if we get it wrong.

Generator exhausts are hot – so put it where it won’t set fire to anything, and people won’t get burnt.

They give off toxic exhaust fumes, so choose a well-ventilated location. Never in a building that will be occupied by people; especially for sleeping. Consider a carbon monoxide alarm

Connection methods

The TB allows us to either plug into a socket on the genset (if it has sockets); or to connect to terminals within the genset.

At the installation end, we’ll be connecting into the same terminals we removed the normal supply from.

Keep these conductors as short as practicable.

One method that must NEVER be used is connection by a flex with a plug at each end; one end plugged into the generator and the other into a socket in the installation. These are widely known as “suicide leads”; for good reason. Using them is very unsafe, and completely illegal

Earthing

There MUST be an earth conductor between the genset and the installation’s earthing system. But this can be the PEC of whatever cable we use for the load conductors.

There is NO need for the generator to have it’s own electrode or any other connection to the mass of earth.

Conductor size

The conductors used to connect the generator to the installation need to be heavy enough to carry the load that will be supplied.

Overload

And they need to have overcurrent protection – this can be either at the upstream end (at the genset) or downstream (in the installation). Most gensets will have some form of over-current protection built in. Bit we need to check it to ensure that it will protect the conductors we’re using to make this connection to the installation. And if it is rated too high; either upgrade the conductors or install a protective device.

Short-circuit

They also need short-circuit protection; unless installed in such a way as to minimise risk of short circuit. Short-circuit protection must be at the upstream (generator) end. Bearing in mind that small generators generally can’t provide enough current to meet trip-time requirements for short-circuit protection by over-current device. So it would be wise to provide very good protection against mechanical damage for these cables. NOT just a bit of flex lying on the ground.  Clause 2.6.7 of AS/NZS 3010:2017 provides guidance. 

If the part of the installation needing emergency supply is normally supplied from a DB; then you can connect the generator at the DB as if it was the main switchboard. Instead of removing supply fuses (as for mains); isolate & secure the submains at source, and disconnect them from the DB. While AS/NZS 3010:2017 has extra requirement for connecting an alternative supply generator at a DB; these requirements don’t apply when we’re working under the TB guidance.

The fastest way to check what type of generator you’re working with is to fire it up before connecting it and testing the output with a voltmeter. It’s best to do this with some load connected; to eliminate stray voltages that can occur under no-load conditions. Load also steadies the voltage, which is likely to be on the high side with no load.

RCD-protected

RCD-protected gensets are obvious. They have an RCD built into them as supplied from the factory. And to enable the RCD to operate as intended; they have a connection between neutral and “earth” made upstream of the RCD. This connection may be a simple link and may even be removable. Or for generators with an inverter output, it may be made within the inverter stage. The frame will also be connected to the “earth”.

Bearing in mind that it isn’t really “earth” at all, as it isn’t connected to the mass of earth. It’s actually an “equipotential bonding system”; but most people think of it as “earth” because it’s connected to the “earth” terminals of the output

The output, as measured with a voltmeter, will look like a normal (MEN) supply:

Centre-tapped “single phase”

Many, especially older, small portable gensets are this type. Basically the winding is in two halves, with a connection to frame / “earth” made at the mod-point. As a result, they are not actually a single-phase genset at all; the output is actually 2-phases with 180 degrees phase angle between them; and there’s no neutral. What looks like a neutral is and is connected to the “neutral” terminal of any 230V socket, is actually Phase 2.

The output, as measured with a voltmeter, will be:

Isolated output

With this type, there no connection between the winding and frame / earth. But it would be unwise to test this using an insulation resistance tester; as there may be suppressors or other electronic components that could be damaged by the test voltage.

The output, as measured with a voltmeter, looks the same as from an isolating transformer:

For 3-phase, there will be 400 v between phases; with each phase generated by a separate winding, and neutral being the star point where the 3 phase winding are connected together

Electrical Diagrams