History
Putting isolation transformers into a UPS system is not new. In fact, some technologies relied on the transformer to create the power waveform (ferro-resonance). In most cases the need for the transformer was due to it’s ability to step up an AC voltage. This allowed an inverter to output a low AC voltage due to limitations on the voltage capacity on the power electronic components. More recently advances in power electronics have enabled the transformer to be removed and the inverter output connected directly to the load. This is called a high frequency design and is the basis for modern online double conversion UPS.
The 1/2V 10V Myth
Some UPS manufacturers include an isolation transformer to allow the Neutral to be bonded to the earth thereby creating a new N-E bond and eliminating any Neutral to Earth Voltage. Claims have been made that common mode voltage (and that means a voltage referenced to a common source – in this case the ground or earth conductor) of over V or normal mode (between live and neutral) of over 10V can cause equipment malfunction.
However, Neutral to Earth voltage does not cause malfunction with modern electrical equipment. This can be easily demonstrated by going to Germany and plugging something into the wall socket. You can then unplug it, turn the Schuko lead around by 180 degrees and plug it in again. Hey presto, what was once live is now neutral and vice versa and guess what – the electrical equipment works just the same without any problems at all, despite the neutral to earth voltage being 230V. Or you could simply rewire your mains lead to demonstrate.
