The safe use of alternative power supplies by Pieter Coetzee, Explotac

End-users who use portable generators to interface with fixed electrical installations must adhere to the regulations of the OHS Act and SANS 10142-1.

Many end-users cause illegal electrical installations when they resort to alternative power supplies during utility outages. SANS 10142-1 clause 7:12 prescribes the minimum safety requirements for the use of alternative power sources. This standard makes it very clear that alternative supplies include but are not limited to LV generating sets and photovoltaic (PV) installations.

The requirements are of a very technical nature and must be clearly understood and applied. All electrical installations must also comply with the Electrical Installation Regulations. Failure to comply with the regulations and standards may lead to prosecution or invalidate insurance claims.

Power generation

There are several different methods to regulate the output voltage on generators.

Brushless generator

Brushless generators are popular in industry because of their inexpensive design, but they have the least reliable voltage control (see Fig. 1). These generators cannot react to changes in load and produce either brown-outs or surges, both of which can cause damage to equipment.

Fig. 1: Brushless exciter.

Fig. 1: Brushless exciter.

Automatic voltage regulator (AVR)

Many generators feature AVRs, which are designed to control voltage consistently (see Fig. 2). The AVR maintains a more constant output voltage regardless of the load. This means no spikes or brownouts.


Inverters produce the cleanest power of all. These units are ideal for sensitive electronics such as computers.

Fig. 2: Static exciter.

Fig. 2: Static exciter.

Safe work practices

  • Maintain and operate the generator in accordance with the manufacturer’s operating manual.
  • Never connect a portable generator or any other alternative supply (hydro or wind generators, photovoltaic systems etc.) directly to an existing electrical system unless the alternative supply has its own automatic or manual changeover switch.
  • Connecting any unauthorised power source directly into any point of electrical wiring without a changeover switch is illegal and dangerous because of so-called “back feeding” which, by definition, means supplying power back into the grid when the grid is down. This must be avoided at all cost as the voltage fed back into the lines poses an electrocution hazard for utility workers and others who may not know that the line is charged.
    Connection of any alternative power supply must comply with SANS 10142-1 Annexure S Emergency power installation configuration.
  • Never use a plug-to-plug cable to connect the generator to a socket outlet. Many generators are supplied with plug-to-plug cables but their use is strictly forbidden as it provides no earth leakage protection.
    Should the utility power be restored while the main circuit breaker is not switched off, the generator may be damaged or the entire system may be burnt, causing fire hazard.
    While the main supply is not available, the generator will send power back into the main cable. This may overload the generator and shock electricians working on repairs on other power lines.
  • Always use the portable generator in a well-ventilated area due to the threat of carbon monoxide (CO) poisoning from the engine exhaust. Generators should never be run in garages, under carports or near open windows. A typical 5,5 kW home generator can produce as much CO as do six idling cars.
  • Always refuel the generator when it is not running and never store the fuel near the generator.
  • Use a registered person in accordance with the Electrical Machinery Regulations to do the electrical installation. The registered person can be an installation electrician (IE) or master installation electrician (MIE) registered as a contractor with the Department of Labour. The registered person must issue a Certificate of Compliance upon completion of the installation.
  • Always ensure that the generator is started first and that the power is switched on while the generator is running.

V-O-V generators

SANS 10142-1 states in clause that a 230 V generator with a V-O-V earth connection (centre tap on winding which is earthed) shall not be connected to a fixed electrical installation. Such a generator may only be used as a free-standing unit to provide power to specific appliances (see Fig. 3).

Fig. 3: V-O-V winding.

Fig. 3: V-O-V winding.

This problem can be overcome in a number of ways:

  • Never use the earth of a generator with a V-O-V winding. A number of these generators imported into South Africa but they may not be connected to fixed installations if they have V-O-V configurations. However, on some of these, the earth connection can be removed from the centre tap. They can then be earthed correctly by means of an earth electrode. Note that opening the generator and removing the earth may invalidate the guarantee.
    Fig. 4 shows no earth connected to the centre tap of the generator.
  • Use an earth electrode and connect the generator earth point to the earth electrode with a proper earth cable.
  • Use the same earth as your supply earth from the portable generator to the changeover switch and distribution board.
  • Also connect the earth of the earth rod to the supplier’s earth (same potential).
  • Bridge out the neutral and the earth at the generator to have the neutral as close as possible to 0 V, in accordance with SANS 10292.
Fig. 4: Generator connection points with no earth conductor between connection point and body.

Fig. 4: Generator connection points with no earth conductor between connection point and body.

General requirements
 SANS 10142-1:2012

7.12.2 Requirements for alternative sources of supply Where any form of alternative supply (emergency supply, UPS etc.), is connected to an electrical installation, a notice to this effect shall be displayed at the main switch of  the installation, and where such supply
    a) supplies power only to certain circuits in a distribution board, a power-on indicator (visible or audible) shall be provided on each such distribution board as well as a notice  indicating that the standby power main switch shall also be switched off in an emergency,
    b) only supplies a part of the electrical installation, the notice shall also be displayed on each distribution board in that part of the installation (see The means of excitation and commutation shall be appropriate for the intended use of the generating set and the safety and proper functioning of other sources of supply shall not be impaired by the generating plant. The prospective short-circuit current and prospective earth fault current shall be assessed for each source of supply or combination of sources, which can operate independently of other sources or combinations. The short-circuit rating of protective devices within the installation and, where appropriate, connected to the main supply, shall not be exceeded for any of the intended methods of operation of the sources. Where the alternative supply is intended to provide a supply to an installation that is not connected to the main supply, or to provide a supply as a switched alternative to the main supply, the capacity and operating characteristics of the alternative supply shall be such that danger or damage to equipment does not arise after the connection or disconnection of any intended load as a result of the deviation of the voltage or frequency from the standard range. Means shall be provided to automatically disconnect such parts of the installation, as may be necessary if the capacity of the alternative supply is exceeded. Where an alternative supply is provided to an installation or part of an installation as a switched alternative to the main supply, the change-over switching device shall disconnect the main supply before the alternative supply is switched in. The change-over switching device shall be interlocked in such a way that the main supply and the alternative supply cannot be connected to the installation or part of the installation at the same time. Except where otherwise permitted in this part of SANS 10142, where a socket-outlet is installed in a circuit on standby power, such circuit shall be protected by an earth leakage protection device with a rated earth leakage tripping current (rated residual current) I∆n not exceeding 30 mA. A 230 V generator with a V-O-V earth connection (centre tap on winding which is earthed), shall not be connected to a fixed electrical installation.
Note: Such a generator may be used as a free-standing unit to provide power to specific appliances (author’s emphasis).

Earth leakage protection

The main distribution board is the main point of entry for electricity into the premises before it is distributed throughout the installation. The distribution board contains the mains switch, individual circuit breakers and the earth leakage circuit breaker.

Circuit breakers are automatic protection devices fitted in the DB which switch off a circuit if there is a fault. When they switch off (“trip”), you must find and correct the fault before resetting the switch.

Fig. 5: Earth leakage and polarity tester.

Fig. 5: Earth leakage and polarity tester.

The earth leakage circuit breaker (ELCB)

An earth leakage device works by comparing the input current on the live side to the return current on the neutral side. A minute difference in current (as little as between 15 and 30 mA) means that current is leaking out somewhere, possibly through somebody’s body. These devices must be tested regularly – every three months or so.
The test button on the earth leakage protection provides a partial test, but you can also use a tester that plugs into the sockets for more accurate testing on all the plug points in the installation (see Fig. 5).

  • Ensure that your polarity is correct.
  • Ensure that you have an earth – the earth indicator on tester must indicate.
  • Test the earth leakage and confirm that it trips between 15 and 30 mA.

Earthing and earth leakage   

    SANS 10142-1:2012
 7.12.3 Earthing requirements and earth leakage protection Neutral bar earthing Protection in accordance with the requirements of 6.7 shall be provided for the electrical installation in such a manner as to ensure correct operation of the protection devices,     irrespective of the source of supply or combination of sources of supply. Operation of the protection devices shall not rely upon the connection to the earthed point of the main supply when the generator is operated as a switched alternative to the main supply.
Where there is no existing earth electrode installed in the electrical installation, a suitable earth electrode may be installed in accordance with SANS 10199. When installed, the electrode shall be bonded to the consumer’s earth terminal and to the earthing point on the generating set by a conductor of at least half the cross-section of that of the phase conductor, but not less than 6 mm2 copper, or equivalent. This also applies to a single phase supply (see also In an installation that is supplied from a combination of transformers and alternative supplies located near to each other, including an alternative supply (supplies), the neutral points of each of these items shall be connected to a single earthed neutral bar (see annex S). This earthed neutral bar shall be the only point at which the neutral of the installation is earthed. Any earth leakage device shall be positioned in such a way as to avoid incorrect operation due to the existence of any parallel neutral/earth path. Where alternative supplies are installed remotely from the installation, or from one another, and where it is not possible to make use of a single neutral bar which is earthed, the neutral of each unit shall be earthed at the unit and these points shall be bonded to the consumer’s earth terminal (see 6.12.4). The supply from each unit which supplies the installation or part of the installation, shall be switched by means of a switch that breaks all live conductors operating substantially together (see annex S), to disconnect the earthed neutral point from the installation neutral when the alternative supply is not connected (see also 6.1.6). Where only part of an installation is switched to the alternative supply in the same distribution board, the neutral bar shall be split as in Figures S.2 and S.3 in annex S (author’s emphasis).

Fig. 6: Changeover switch (63 A) installed next to a distribution board inside a house.

Fig. 6: Changeover switch (63 A) installed next to a distribution board inside a house.

LV earthing

Note: SANS 10292 contains many references to “touch voltage”, and the fact that touch voltage can be unsafe, but the subject has not been elaborated upon.

SANS 10292:2013
4.1 The earthing of the neutral of a low-voltage (LV) system should be as follows:
a) to provide a return conductive path for any earth fault current and earth leakage current;
b) to maintain the neutral of the LV system as close as possible to the earth potential;
c) to ensure that the medium-voltage (MV) protection operates in the event of a fault between  the medium-voltage and LV windings of a transformer; and
d) to reduce the prospective touch voltage as much as is reasonably practical
(Author’s emphasis).

Changeover switches

SANS 10142-1:2012 Edition 8 Precautions that comply with the requirements of 6.9.1 for disconnection shall be taken, so that the generator cannot operate in parallel with the main supply where the supply to the electrical installation is supplied by a supplier
(Author’s emphasis).

Fig. 7: Caravan plug.

Fig. 7: Caravan plug.

Note: Suitable precautions can include:

  • Electrical, mechanical or electromechanical interlocks between the operating mechanisms or control circuits of the changeover switching devices.
  • Systems of locks with single transferable keys.
  • Three-position break-before-make changeover switches.
  • Automatic changeover switching devices with suitable interlocks.
  • Other means that provide equivalent security of operation.

Two types of changeover switch exist:

Changeover switch next to DB/meter

The changeover switch must have a proper IP rating if installed outdoors, next to the meter/prepaid meter.

  • Isolate the main supply and test for any voltage to make sure it is safe.
  • Install a changeover switch (about 63 A) near the main distribution board, either indoors or outdoors (see Fig. 6).
  • Connect the main power supply to one end (marked line) of the changeover switch.
  • Connect the load side of the changeover switch back to the earth leakage.
  • Install a male 3-pin caravan or similar socket outdoors the house against a wall (see Fig. 7).

Connect the caravan plug with a 4 mm² Surfix cable to the changeover switch marked line or “Generator Live” (GL) and “Generator Neutral” (GN).

Note: It is important that the male plug be used as in Fig. 8 and not a socket outlet. The socket must be a female to prevent any person from touching live parts. The benefit of using the caravan application is that the same supply from your portable generator can also be used on your caravan or camping trailer. The generator lead can be connected directly to the changeover switch where a caravan socket outlet and plug are not used.

Fig. 8: Generator connected with male and female caravan plugs.

Fig. 8: Generator connected with male and female caravan plugs.

  • Connect the 3-pin power plug of the generator with a female caravan plug by means of 4 mm² flexible cable. The socket outlet of the generator is only rated 15 A
    (see Fig. 7).
  • Connect the generator 3-pin caravan power plug to socket previously installed (see Fig. 8).
  • Test the earth leakage with an earth leakage tester.

Changeover switch installed inside the distribution board

There is often not sufficient space to install the changeover switch as the old switchgear fills the space within the DB. Replace the old switchgear with miniature circuit breakers.

This type of changeover switch is not as costly and is much easier to install than switches next to the DB as all the wiring except the cable fed from the generator will be done inside the DB.
It is always good practice to install a 220 V buzzer or light in the DB to warn you when the main supply is back (see Fig. 9). This is also a requirement of clause (a) SANS 10142-1.

Fig. 9: Modular changeover switch installed inside the DB. The arrow indicates a 220 V buzzer light connected to the main supply.

Fig. 9: Modular changeover switch installed inside the DB. The arrow indicates a 220 V buzzer light connected to the main supply.

Note: Remember that this installation must be executed by a person registered with the DoL. The registered person must issue a CoC upon completion of the electrical installation.

Contact Pieter Coetzee, Explotac,

Tel 079 507-3423, This email address is being protected from spambots. You need JavaScript enabled to view it.