When voltage varies

22 October 2012
The nominal low-voltage level in Australia is now 230V. Chris Halliday of the Institute of Electrical Inspectors outlines the levels that electrical appliances and equipment may receive, and the implications for electricians when testing.

The nominal low-voltage level in Australia is now 230V. Chris Halliday of the Institute of Electrical Inspectors outlines the levels that electrical appliances and equipment may receive, and the implications for electricians when testing.

The nominal voltage in Australia was set at 240V in the 1920s. However, a change began in 1980 with the International Electrotechnical Committee (IEC) deciding to rationalise the 220V, 230V and 240V nominal voltage levels around the world to a consistent 230V.

This rationalisation was ostensibly made to improve the economics of making appliances by allowing manufacturers to produce a range of items with a rated voltage of 230V. In 2000, Standards Australia issued a system Standard, AS60038, with 230V as the nominal voltage with a +10% to –6% variation at the point of supply.

AS/NZS3000 (Wiring Rules) allows an additional 5% voltage drop in electrical installations (Clause 3.6.2) that electricians will be well used to calculating. The Wiring Rules recognised the change in nominal voltage level in the 2007 edition at Clause 1.6.2.

The Australian Capital Territory, Queensland and Western Australia have not adopted the new 230V Standard at this stage, but this is not a big concern as the old 240V range of plus or minus 6% fits within the new Standard. Other states have some additional peculiarities, and these should be understood if disputes occur.

It must be remembered that voltage levels always vary as loads cut in and out, so an allowable range of voltage is important.

The new Standard

A new power quality Standard, AS61000.3.100, has recently been released that details requirements additional to the existing systems Standard.

The new Standard stipulates a nominal 230V, and the allowable voltage to the customer’s point of supply is, as mentioned, +10% to –6%. However, the preferred operating range is +6% to –2%.

The idea of this preferred operating range is to provide a reasonable voltage for any remaining 240V appliances. A statistical approach has been taken, and the 50 percentile value of voltage is in the preferred operating zone (see accompanying histogram).

The histogram shows the logged voltage along the ‘x’ axis with the logged results included in bins based on the voltage level. Each logged value is counted and the total count displayed on the ‘y’ axis. The bins are divided into operating zones as shown.

To complicate matters, the limits are not hard and fast but based on voltage levels sampled at high speeds and averaged over 10 minutes in accordance with AS/NZS61000.4.30. A statistical approach has also been taken with these limits, the upper limit being based on a 99 percentile value and the lower limit on a 1 percentile.

This means that under normal conditions the average voltage levels over 10 minutes can go above the upper 10% limit for 1% of the time and below the lower –6% limit for 1% of the time.

These limits may be exceeded when the network is not in its normal configuration, such as may occur in storms or bad weather when parts of the network have tripped off and supplies have been rerouted.

The upper operating zone is particularly important given the installation of solar systems on many residential and commercial premises across Australia. Exporting energy from these systems can cause a voltage rise in the installation due to the export current (I) and the impedance of the system (Z), that is, Vrise = I x Z.

An upgrade of the network and/or installation wiring will be called for if the voltage is likely to rise above the upper operating zone at the installation, or because of limits that may be set in statebased service rules or relevant Standards.

There has been much discussion in New South Wales on this issue, and the NSW Service and Installation Rules may be the fi rst to limit voltage rise in the installation service wire to 1%.

Utilisation voltage range

The utilisation voltage range is particularly important for appliances and electricians. This is the full range of voltage an appliance should receive, and an electrician should measure it in an installation under normal conditions.

This range takes into account the voltage range allowed at the point of supply and the additional 5% voltage drop in the installation. This equates to a total range of 253V to 205V.

The Wiring Rules specify that a 7% voltage drop may be satisfactory where the point of supply is at the terminals of a substation (Clause 3.6.2), but the additional 2% voltage drop is sacrificed in the LV system and therefore does not increase the utilisation range.

Most appliances and equipment items coming into the country are likely to be stamped as 240V (equipment Standards have been slow to keep up) but may in fact be 230V rated, as they have been designed to IEC Standards that are 230V based.

Practical application of limits

The foregoing discussion may be complicated for those that have not examined the issues in detail, but there is a simple application of limits.

An electrician measuring voltages in an installation should expect levels to be within the utilisation range of 253V to 205V. However, voltage can be above this range under normal conditions for 1% of the time or below for 1% of the time.

If voltage levels are regularly exceeding this range, then the network company or a consultant should be contacted for advice.

I was recently discussing electrical issues with a computer sales and repair company representative. His supply varies regularly from 244V to 239V, triggering the alarm on the uninterruptible power supply (UPS).

A range such as this is acceptable and within the preferred operating range of the Standard. A UPS should not react when voltage goes down to 239V, so the alarm settings need adjusting.

Voltage alarm limits probably should be set to at least the utilisation voltage range, or even wider, to allow for the 1 percentile allowance – say 200V to 258V. Most computerised and electronic equipment should be capable of handling this range of voltage, and possibly more.

It will pay to check the specifi cations of equipment being connected to the UPS before adjusting the alarm settings.

Testing

All testing should be done in accordance with safe work procedures and legislated requirements. Safety should always be your priority.

A true RMS meter should be used to measure voltages. Older style averaging meters will be inherently inaccurate, as voltage waveforms are non-sinusoidal due to harmonic loads.

Meters and power-quality loggers should be calibrated regularly to ensure accuracy, or any advice given will be questionable.

I am often asked how often meters should be calibrated, but this should be discussed with your calibration company. Its advice should be based on how your particular make and model of meter holds its calibration, and the conditions under which it is likely to be used. The typical calibration period is 12 months.

A multimeter can be used for measuring voltage levels in an installation, but this will provide only a spot indication of voltage. A powerquality logger will generally be needed to sort out voltage problems or to check compliance with the new 230V Standard.

The problem being experienced will determine the installation location of the logging instrument, and it is always best to install the logger at the piece of equipment experiencing problems.

A Class A power-quality logger will be required to ensure compliance with AS 61000.3.100 if the supply is suspect, and it will need to be installed for at least one week. Class B instruments are great for locating faults but may not be adequate should a dispute arise.

Conclusion

Australia has a 230V Standard that has not been universally adopted across the country. State-based requirements may need to be checked.

Voltage levels in installations will generally be within the utilisation range of 253V to 205V and may go above or below these limits for short periods.

Although this range is quite large, 230V rated equipment should be capable of coping with it. Some assistance is built into AS61000.3.100 for old 240V appliances and equipment, with the 50 percentile value preferably lying within the range of 244V to 225V.

Those who compile specifi cations for equipment purchases should seek equipment that can cope with at least the utilisation range mentioned above, and it would pay to note that the range is based on 1 and 99 percentile values under normal conditions. Reference to AS61000.3.100 and AS/NZS3000 is recommended in specifications.

Compliance with the new 230V power quality Standard can be checked only with a Class A power-quality logger over a log period of at least one week.

Test instruments should be true RMS and regularly calibrated.

Voltage drop calculations should now be made using 230V instead of the 240V as provided by Wiring Rules clauses such as 1.6 and 3.6, and not withstanding such clauses as 1.6.4 and 1.7. Check these clauses if you are not fully conversant with them.

The voltage levels that customers and their equipment receive don’t constitute an easy issue – but then probably never did.

Electrical Consulting and Training
chris@elect.com.au