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Answers to the questions we get asked most often

Here we attempt to answer the most frequently asked questions. If you require any more details then please get in touch and we will try to help you out.

What does kVA mean?

kVA – kiloVoltAmps, this is a measure of the power of a transformer in thousands of VA.

What’s the difference between VA and Watts?

In general, the number of watts is less than the VA and the latter has to be multiplied by a quantity termed the power factor to give the power in watts. The power factor is the product of the RMS values of the current and voltage, that is the phase difference between them.

What is a 3 phase transformer?

A transformer that uses all three lines of the non-domestic supply, usually in commercial premises to run machines.

What’s the difference between Line to Neutral and Line to Line?

The supply in the UK is set at 415V Line to Line with three phases, this voltage is, as it states, measured from one live phase to another live phase. When it reaches the average home only one phase is used, this is nominally 240V, when measured on the three phase supply this is the Line to Neutral voltage.

What does EN61558 on a transformer mean?

It refers to a British and European standard for the construction of a transformer, it superceeds the previous specification which was known as EN60742. It specifies various safety and construction standards that have to be met. There are similar specifications for different types of transformers, ie medical or autotransformers.

What are the temperature classifications all about?

National standards, like BS7452 above, give values for the permitted increase in temperature that a transformer can achieve when working at full power.

  • Class A = 75 degree rise
  • Class B = 95 degree rise
  • Class E = 90 degree rise
  • Class F = 115 degree rise
  • Class H = 140 degree rise

These figures are based on an ambient not normally exceeding 25 degrees but occasionally reaching 35 degrees.

What’s the difference between an isolating transformer and an autotransformer?

An isolating transformer has no electrical connection between the input and output windings, an autotransformer has a common winding. The two windings on an isolating transformer are often separated by an earth screen for additional protection.
schematic

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