Claude Lyons - Specialists in AC Voltage Stabilisers

Series MSX Voltage Stabiliser



Electrical equipment has to be designed to accept a range of input voltages about its nominal design centre, but if the actual supply is higher than necessary for proper operation, the equipment will consume more power than needed, and in many cases life is reduced - both increasing the cost of ownership.

However, the stability of the UK supply mains has always been reasonably good, but is likely to degrade as competition and decreasing profitability forces generators to take capacity off-line and indeed to decommission power stations.

European Voltage Harmonisation

The United Kingdom for many years had a standardised supply voltage of 240V ±6% (415V for three-phase) whereas continental Europe had a nominal supply level of 220V (380V). From 1 January 1995 the nominal voltage across Europe has been 'harmonised' at 230V/400V.

This is not a real change, since the former 240V countries, including the UK, have in the first stage of voltage harmonisation a tolerance of 230V -6% to +10% (i.e. 216.2 - 253V) as compared with the 'old' limits of 240V ±6% (i.e. 225.6 - 254.5V). However, the former 220V countries (most of Continental Europe) have limits of 230V -10% to +6% (207 - 243.8V).

The Electricity Safety, Quality and Continuity Regulations 2002, which came into force on the 31st January 2003, replacing The Electricity Supply Regulations 1998, formally confirm the UK standardised supply voltage tolerances at 230V -6% to +10%. DECC, the Department for Energy and Climate Change has confirmed that this remains the UK position but is currently (April 2010) under review for a possible change to 230V ±10% in 2011.

Plans to harmonise the whole of Europe to 230V ±10% (i.e. 207 - 253V), which were due to be applied from 1st January 2003, were first postponed until 1st January 2008 and are now postponed indefinitely pending a consensus between the various parties. Despite this postponement, any equipment intended for use anywhere in Europe and carrying the 'CE' mark will have to be capable of working over this wide range.

Furthermore, the Institution of Electrical Engineers recommended in a 1996 report that for safety all electrical equipment needed to be tested across the range 230V +10% to -14%. This allowed for the lower and higher limits plus an allowance of 4% for voltage drops within the installation. Appendix 12 of the new IEE Wiring Regulations 17th Edition (BS 7671:2008), 'Voltage Drop in Consumers' Installations' now specifies a maximum value of voltage drop of 3% for lighting, 5% for all other uses, where the supply is directly from a public LV distribution system (larger drops, of 6% and 8%, are permissible with a private LV supply). Even taking only public supplies, the implication is that electrical equipment needs to operate satisfactorily over the very wide range of -15% to +10% or 185.5V to 253V!

More significantly, because the old UK limits of 225.6 - 254.5V lie almost entirely within the new limits of 216.2 - 253V, there has been no incentive at all for the suppliers of 415/240V nominal AC mains power to make a real change, just to 're-label' the supply nominal and tolerances. Indeed a July 2005 DTI note "Electrical Supply Tolerances and Electrical Appliance Safety" points out that "Though the electrical supply is quoted as being over a range, it is preferable for power distribution companies to keep supply voltages as high as possible to minimise current in the supply network and hence delaying the need to make improvements in the supply infrastructure to increase current handling capability."

Effects on Equipment

The main effects are that all electrical equipment is likely normally to be supplied at a voltage near to the upper limit involving both unnecessarily high current draw and thus electricity consumption and also operating at higher working temperature than necessary. However due to a misunderstanding of the effect of voltage harmonisation much imported equipment which would previously have been made specially for the UK 240V nominal supply is now designed either for the new 230V nominal or in many cases European manufacturers have assumed that equipment really designed for a 220V nominal can continue to be supplied into what is, after all, now a nominal 230V area. Since the real voltage continues to be 240V nominal, however re-labelled, much imported equipment is forced to operate in the UK on a higher voltage than it would in its country of origin - in the case of more recently designed equipment by about 10V but older designs in continued production receive some 20V more.

There is a further problem with equipment of North American origin. Many US and Canadian manufacturers continue to design their 'export models' for 220V but also fail to take adequate account of the supply frequency being 50 Hz as opposed to their 60 Hz. This is particularly seen in the design of power-supply transformers incorporated in such products.

The effects vary according to the type of equipment. Over-heating can occur as incorporated voltage regulators have to work harder. Supply transformers which are designed with minimally sized iron cores can easily go into saturation, especially toroidal designs. This can cause circuit breakers to trip on the excessive inrush current. Incandescent lamps have a non-linear characteristic which means only a small over-voltage can reduce their life expectancy markedly.

In addition, power consumption is unnecessarily high. A resistive load subjected to a 5% overvoltage will consume 10.25% more power. Subjected to a 10% increase in voltage the additional power consumption will be 21%.

It is therefore always worth checking whether equipment will operate at a reduced voltage, with likely beneficial effects both on equipment lifetime and the cost of power consumed. In some cases, if the supply voltage tends to vary this strategy may not be possible as the voltage sags may take the supply below the minimum operating voltage of the equipment. In such cases, stabilising or regulating the supply voltage will enable the equipment to work correctly and will simultaneously allow the supply voltage to be chosen to optimise equipment life and also energy consumption.


(a) Energy Saving Voltage Regulators (Claude Lyons PowerSave™)

Claude Lyons Limited have been involved in electrical voltage and power control for more than 75 years culminating in their world-renowned series TS® automatic voltage regulators which have for many years provided industry, academia, healthcare, entertainment and hospitality, broadcasting and government with closely regulated power, ensuring optimum performance and improved reliability.

Originally designed to cater for conditions where the supply voltage varied both above and below a nominal value (equal "buck" and "boost"), as the realisation that operating at a reduced and regulated voltage would provide both energy saving and equipment life prolongation, models were introduced firstly which would regulate at a lower output voltage, providing more "buck" than "boost", and finally, for situations where the incoming supply is always higher than the desired optimum operating voltage for the load equipment, "all buck" regulators.

Because automatic voltage regulators and stabilisers self-adapt, they overcome the problems caused by the electricity generator reducing the supply voltage to limit demand at peak times.
Phase balancing

Claude Lyons three-phase energy saving regulators and automatic voltage stabilisers correct each phase voltage independently, thus providing a balanced supply to the load even if there is imbalance on the incoming supply. This further improves energy performance and also the reliability and lifetime of loads such as motors.

A development of the established TS series voltage stabiliser, Claude Lyons ESS-A-1 (single phase) and ESSA-2 (three phase) PowerSave™ Energy Saving Voltage Regulators provide even higher throughput for a given frame size by being configured for voltage reduction only. Standard models provide a controlled and regulated reduction from 0 to 12˝% (i.e. a 30 volt reduction range from a nominal 240V input). For lower nominal supply voltages and where the required reduction is less, models are available providing higher current ratings.

(b) Voltage Stabilisers

If a voltage stabiliser is fitted, the output voltage setting (usually 240V or more recently 230V in the UK) can be set down to 220V. Even the fact that recently supplied stabilisers will usually be factory preset to 230V rather than 240V will be a help in itself.

An important point is that the input correction range of a voltage stabiliser is relative to the set output voltage. For example, the very widely used Claude Lyons Series TS voltage stabiliser has an output voltage settable anywhere between 200V and 250V, and standard models have three tappings, providing input correction range, relative to the set output voltage, of -17˝% to +7˝% ('tap A'), ±12˝% ('tap B'), and -7˝% to +17˝% ('tap C').

Thus by changing to tap C instead of tap A, essentially the same input range is provided with the stabilised output at 220V - providing the advantages of reduced energy consumption and longer equipment life at no additional cost.

Stabilisers are particularly useful in situations where the input voltage could fall below the required stabilised output level, since they provide 'boost' as well as 'buck'. Where the input is expected always to be above the required nominal output, the "buck only" ESS (see a above) is to be preferred.

(c) Voltage reducing transformers ("energy saving transformers").

An energy saving transformer (sometimes called a "voltage optimiser") is essentially a single or three phase autotransformer with tappings providing lower output than the input. Typical transformers may provide tappings at 440/250, 415/240, 400/230, 380/220V for example, or percentage reductions of -2˝%, -5%, -7˝% and -10%, and it is easy to select a suitable reduction.

Some transformer designs provide output tappings at -5, 6, 7 and 8%. However 1% steps are unnecessarily fine, and are probably intended to provide an unrealistic impression of 'high resolution' adjustment.

These transformers are often called "voltage optimisers", a misnomer if there ever was one, since a fixed reduction can clearly only "optimise" a fixed incoming voltage, and the supply voltage often varies.

Three phase voltage reducing transformers can be wound so as to provide other features such as third-harmonic neutral current suppression and/or a degree of phase balancing ('static balancer', 'artificial star point' etc.)

It should be noted that energy saving transformers are only appropriate when the mains supply is stable. If the mains supply voltage were to drop, the effect of the transformer would be to reduce an already low voltage, possibly to the point where the load equipment fails to function.

Most publicity about energy saving transformers or 'voltage optimisers' fails to mention this crucial point. Increasing use of wind turbines implies increasing instability in the supply voltage. The forthcoming "energy gap" where commissioning of new-build nuclear generation or the hoped for carbon sequestration and storage (CSS) coal fired generators will not occur till after the forthcoming decommissioning of obsolescent nuclear and conventional coal generation plant now seems unavoidable - see and many references in the press.

The likely result of such an eventuality would be that to reduce consumption the electricity generators would reduce the voltage during the period of peak demand (sometimes called a "brownout"). The potential problem caused by a fixed-ratio energy saving transformer or "voltage optimiser" continuing to reduce the voltage further is completely avoided by fitting an energy saving voltage regulator or automatic voltage stabiliser or instead (see 'a' above). However where the mains supply is, and is likely to remain, stable, Claude Lyons PowerSave™ EST-1 (single phase) and EST-3 (three phase) Energy Saving Transformers (voltage optimisers) will provide an effective solution.

(d) Distribution transformer tappings

If you have the luxury of 'importing' your electricity at medium voltage (11 kV for example) and have your own site distribution transformer, then you can probably also reduce the voltage for energy saving and equipment life prolongation without cost. Distribution transformers are usually fitted with primary taps at, for example, +5%, +2˝%, 0%, -2˝% and -5% and it may simply be possible to arrange to use taps for a higher nominal MV supply voltage, thus reducing the LV output voltage (e.g. 415V to 400V).

As with voltage reducing transformers, changing the tappings on a distribution transformer is only effective if the supply is known to be stable and not liable to voltage reductions imposed by the generating company. In particular, the electricity distribution company could decide to change the taps on a local distribution transformer, making a permanent reduction in supply voltage.

(e) Automatic tap changers and tap-changing transformers

A disadvantage, inherent in all tap-changing regulating devices, and absent from the continuously-correcting voltage stabilisers described in (a) and (b) above, is the sudden step change in voltage as taps are changed. In some applications this is not a serious problem, but there will be a sudden and disconcerting instantaneous change in illumination, in the image size of computer and video screens (and sometimes flickering and momentary loss of synchronisation), and in some cases a potentially damaging inductive kick.


To summarise, a modest voltage reduction, in the region of 5-8%, will produce a valuable reduction in energy consumption and cost and extend the life of many types of electrical apparatus. Where a voltage stabiliser or distribution transformer is fitted, this can be achieved at no cost simply by adjusting the voltage setting. In other cases, an Energy Saving Voltage Regulator or Transformer will repay its capital cost within months. However a fixed-ratio voltage reduction may cause problems if the supply voltage reduces, whilst an energy saving voltage regulator or a voltage stabiliser will automatically compensate for this, and in many cases allow safely for an even larger reduction, typically up to 12˝%, and thus enhanced saving.

As the UK's leading manufacturer of automatic mains voltage stabilisers and energy saving voltage regulators from a few kVA to over 1 MVA, and incorporating the TEC range of single and three-phase power transformers, the Claude Lyons Group are well placed to provide such solutions.

For recommendations and information on specific applications please Contact Us.


The Claude Lyons Group
Brook Road, Waltham Cross, Herts, EN8 7LR, England
Tel: 01992 768888  Fax: 01992 788000

Site last updated: Wednesday, March 7th 2012