Power supply
May 1st 2007

Managing your electricity supply is vitally important. Devices have to be able to cope with voltage sags, dips and interruptions among other factors, John Symonds, Managing Director of REO UK explains

It was more than 10 years ago that the European public low-voltage power supply network was harmonised to 230V between phase and neutral conductors. The electrical supply regulations in the UK (HD472) were amended at that time to reflect this change but also widened the low-voltage tolerances to 230V +10% -6% for 10 years.

This period was extended for another 5 years taking the transition period up until 1 January 2008. Consequently, the nominal voltage in many locations in the UK can still be at 240V and this can cause some inaccuracies with power consumption calculations and ratings.

For instance, a 1kW immersion heater designed to run at 230V will actually consume 1.088kW when connected to a 240V supply. Taking the simple light bulb as another example; these are designed to run for 1000 hours when the supply is 230V, however, when they are connected to a 240V supply their life is reduced drastically to about 550 hours, because they run hotter and brighter. More relevant to panel builders, the secondary voltage of a transformer with 230V primary could at times be higher than expected. Hence, it is manufacturer's local supply, it might not function correctly on the UK public supply (e.g. its output rate might be too low or too high), or it might be unable to operate at all. Although such situations are typically unusual in the UK, this issue could be a more important consideration for equipment that is to be exported to a country with a less well-controlled power supply. In such instances, the local supply should be checked and a UPS or voltage stabiliser fitted if power stability is doubtful.

Voltage sags, dips and short interruptions are commonplace, although there is a European power quality standard, EN50160:2000 – Voltage characteristics of electricity supplied by public distribution systems, that provides the limits and tolerances of these and other phenomena that can be expected.

Voltage sags or dips, defined as having duration of less than 1 minute, at a level of less than 85% of the nominal voltage can occur up to 1000 times per year, whereas short interruptions (less than 1% of nominal voltage) are permitted up to 50 times per year with durations of up to 3 minutes. Therefore, it is most important to test panels to ensure that software, safetyinterlocks and self-resetting equipment can operate correctly under these conditions.

Furthermore, the power quality standard allows for accidental, long interruptions (duration > 3mins) to occur up to 50 times per year, so panels that incorporate PLCs or other programmable devices should be tested to ensure correct start up is achieved after an unplanned shutdown caused by power loss. The appropriate test method for this is EN/IEC 61000-4-11.

It is equally important to ensure that heavy loads do not cause unacceptable dips in the supply when they are started, and a soft-start can be fitted to prevent this from happening. Most variable-frequency motor drives include this as a standard feature but in instances where variable speed is not so important (such as in the case of compressors or air-conditioning fans) then a simple electronic soft-start can be used to steadily increase the start-up voltage over the first few mains cycles after switching on.

Generally, one would not expect power frequency variation to be a problem but, if the power source is a stand-by generator, such as in the case of hospitals (which have to test their generators for half a day every few months), the situation could arise. Also phase-unbalance is something that most installers of electrical systems are aware of but problems of this nature are not uncommon. Therefore, these are two phenomena which have to be considered and appropriate immunity tests are recommended.

There are standards available for testing the immunity of equipment to most power supply variations and practical guides on these standards are available free from REO (UK), enquire below:

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