What input variations are you having to cope with?
You may well know the answer to this from past experience. If not, it must be ascertained by monitoring the supply over a period of time.

Where there is a good mains supply, a stabiliser covering input variations of –15% to +15% will usually be adequate, but variations of –25% to +25% or more can be encountered in a location with poor mains supply.

Choosing a stabiliser with the right input variation capability is important because if its input voltage range is exceeded, then its output will increase or decrease by the same amount by which it has gone "out of limit".

How about supply frequency?
Most supplies are 50 or 60 Hz. But we can meet other frequencies to order. Your normal mains supply frequency is unlikely to vary more than +/- 2%, and this is well within the capability of our stabilisers. But if the supply is from a local generator it is possible that the frequency could deviate beyond this range and suitable protection should be considered to protect both the load and the stabiliser. Our sales department will be able to advise you.

The Rating of your equipment
You need to check the rating of the equipment to be supplied, whether quoted in Amps or kVA, and whether single or three phase. We also need the nominal voltage line to neutral and line to line if three phase, plus the frequency and power factor.

This information will usually be shown on the rating plate attached to the equipment, otherwise you will need to consult the manufacturers of the equipment or take measurements. It is important to measure its true RMS value.

How stable a supply does your equipment need?
Most Watford Control stabilisers will provide an output voltage to within +/- 0.5% of the preset output voltage or better, but there is a cost factor to consider. If a lower accuracy, say to within +/- 5%, is adequate for your equipment needs, then the input voltage range is extended by –4% and +6% beyond the figures quoted for the higher accuracy. As the cost of the stabiliser is linked to the input voltage range it has to handle, then accepting a lower accuracy may make it possible to select a more economical unit.

Will transient noise create problems?
Transient noise is present to some extent in all mains supplies. High-energy voltage spikes and surges are introduced into distribution lines by the switching of equipment, especially heavy inductive loads, or by atmospheric electrical disturbance, such as induced lightning strikes.
All our stabilisers are fitted with voltage surge protection, which will "clip" surges or spikes to approximately three times the supply voltage. But will this be sufficient for your equipment? Some systems are little affected by transients, but electronic equipment especially if microprocessor based, can be damaged and make errors as circuits malfunction. In such cases merely clipping high voltage surges will not be adequate, and full transverse mode interference protection will be needed.

The wider pulse width of the "spike", the more difficult it is to attenuate. Our units are tested with a 50-microsecond spike generator, and they will consistently reduce a spike of 5000V to a harmless low frequency ripple.

How about "earth to supply interference"?
Common mode interference can also be a problem and most microprocessor-based equipment will require a supply with this earth to neutral noise factor limited to less than 0.25V peak. For this reason you may be considering installing a "clean line" from the mains distribution point, but this will not always give the required level of protection.

Line conditioning is the more positive solution, achieved by fitting an isolation transformer with the secondary neutral linked to earth. This eliminates the need for a dedicated line, and at the point of linking neutral to earth, common mode interference is nil.

Stabilisers with added interference suppression are often referred to as "Line Voltage Conditioners" or "Power Conditioners".

Circuit Protection There are other protective devices you will want to consider.

We recommend the specifying of an input or output Circuit breaker which, in addition to giving over current protection, will trip if the output / input deviates above or below a preset level.

You may also may want to consider fitting a bypass switch that will facilitate the inspection and maintenance by isolating the stabiliser and connecting your load directly to the mains supply. We can supply them either suitable for wall mounting or inbuilt to the stabiliser.