Supply Voltage

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Electricity Supply Voltage Requirements and Problems
(Australia with specific references to Victoria)


What Should the Voltage Be?

The Victorian Electricity Act (1993) stipulates that a local electricity distribution company must maintain a voltage level at the Point of Supply to the Customer’s Electricity Installation at the nominal voltage (e.g. 230 Volts);

Nominal Voltage Voltage Range for Time Periods
  Steady State Less than 1 minute Less than 10 seconds
< 1,000 Volts +10%
–  6%
Phase to Earth: +50%, – 100%
Phase to Phase: +20%, – 100%
 1,000 to 22,000 Volts ±6%
(+ 10% in rural areas)
± 10% Phase to Earth: +80%, – 100%
Phase to Phase: +20%, – 100%
66,000 Volts  ±10% ±15% Phase to Earth: +50%, – 100%
Phase to Phase: +20%, – 100%

Except for customers taking supply at high voltage (eg. 6,000 Volts, 11,000 Volts) the nominal voltage will be 230 Volts (phase to neutral).

Note that although the nominal voltage changed from 240 to 230 volts to align with European practice, the percentage tolerances were amended so that the upper and lower limits did not change significantly.

For three-phase power supplies and equipment this is often referred to by the phase to phase voltage (415 Volts). For these 240 Volts supplies, the stipulation is that the steady state voltage must be within the range 216 Volts to 253 Volts.  It was formerly 240V  -10%, + 6%, i.e. 226 to 254 V.

The legislation further states that if the Distribution Company fails to maintain these supply conditions, “it must, within 20 Business Days of that failure being established, notify the Customer of what steps are to be taken to remedy that failure”.

This of course first requires that the failure be established and brought to the attention of the Distribution Company

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What are the Effects of Voltage Problems?


The most common voltage problems which cause customers to contact us are under-voltages which temporarily prevent the customer’s equipment from functioning, or from functioning properly.

Typical symptoms include:

  • Relays do not hold in properly (e.g. motor contactors, even relays on older older telephone switchboards)
  • Motors slow down, possibly causing sensitive manufacturing processes to shut down
  • Circuit breakers (for individual circuits or the whole site) tripping due to over-current which has been caused by equipment drawing its usual power at the lower voltage
  • Safety control systems automatically shutting down production machinery

These may not be the most disastrous voltage fluctuations,but they are the most common.


Severe over-voltages can of course damage or destroy equipment.


Voltage fluctuations (short duration over-voltages and/or under-voltages) can also:

  • Interfere with the operation of some equipment
  • Shorten the life of incandescent lights (including tungsten halogen flood lights, low voltage lamps, and standard light “bulbs”)

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We have problems, but is voltage the cause?

There are a few tell-tale indications that supply voltage problems are the culprit for equipment malfunctioning. These comments refer to under-voltages, as these are the most common cause of the problems which customers contact us about.

If the voltage of the electricity supply to your business is going to experience problems, it will probably be when demand for electricity is at its highest in your area. The time that this occurs at will depend on the area (rural/city, industrial/residential), the weather, and the time of week, etc. As a guide, the very worst times will be on weekdays, 2 p.m. to 5 p.m. on very hot days, and 6 a.m. to 10 a.m. on very cold days.

If equipment runs fine on the night shift, but not first thing in the morning, this would be an indication that voltage could be a cause. It may be worth running equipment out of hours, as an experiment to see if this improves it’s reliability.

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How Do I Monitor Voltage?

A qualified electrician can measure the voltage and tell you the instantaneous voltage. This may or may not “catch” the problem, if there is one. If the reading from this quick measurement is below 226 Volts, it shows that there is a problem, but monitoring and recording will be required to document the problem.

To establish that the voltage is outside the requirements you will need to be able to show that the supply was below 226 Volts for more than one minute.

To establish that the under-voltage is caused by the supply voltage and not by a voltage drop on your premises, you will need to monitor at or very close to the point of supply.

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Actions if Voltage is Causing Problems

If the problem is caused by an excessive power drop in your premises wiring, you will need to rectify this at your expense, by improving the wiring and/or reducing the maximum electrical demand and hence the voltage drop.

If the problem is caused by the supply voltages being outside the allowed limits, you will need to write to your electricity distribution company. This is the company which operates the distribution network in your area, and may be different from the company which you purchase your electricity from, but your account manager should still be your first call.

If the problem is caused by voltages which are different from the nominal voltage, but still within the allowable limits, you will need to devise engineering solutions to the problem, such as making the equipment less susceptible to voltage level or by supporting the voltage with equipment such as dedicated regulated power supply for that equipment

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