Ups systems

The use of the isolation transformers in UPS Systems is an area of debate among power quality professionals. Some UPS naturally contain a transformer for voltage conversion or to allow several output voltages (such as 208 / 120V in the US). In some cases an autoformer is used and in others an isolation transformer. Some argue that the isolation transformer is the only way to eliminate common mode noise and therefore provide the highest degrees of protection for electronic equipment. Taking that point, is electronic equipment inherently susceptible to common mode noise?
Some refer to common mode noise as "noise on the earth". Assuming they are referring to electrical noise, this is a misleading statement.

Electrical noise has to be measured as an electrical potential difference (i.e. a voltage) between 2 points. So this noise has to be between earth and neutral, or between earth and the live conductor. A modern electronic power supply is known as a "Switched Mode Power Supply" or SMPS. It works by rectifying the incoming AC power into DC and chopping this at a high frequency. This high frequency chopped DC (which is now AC!) is passed through a high frequency transformer and then converted to stable DC voltages of usually 5V and 12V positive and negative. The high frequency transformer provides isolation internally and the important point to note is that the incoming neutral and live are fully isolated from the output DC levels. This is a requirement for electrical safety.

As a result any "noise on the earth" will not propagate to electrical signals within the electronic apparatus. Electronic equipment is therefore not susceptible to common mode noise. Since an earth conductor is connected to the chassis of enclosures and in many cases this may be commoned with the zero volt line, it is wrongly assumed that a voltage measured on the input earth to the power conductors will be propagated internally. It cannot be.

What may occur is that attached equipment over a network may be at different earth potentials and as a result ground currents will form. This could cause issues with the operation of electronic equipment, however this is solved by good earthing practices, not by the use of isolation transformers, and certainly not by the use of gimmicky components added into the earth conductors.
In the UK, the IET Wiring Regulations (BS7671) allow for several types of power distribution. The most common of which is the TN-C-S system. In such installations the input neutral conductor is bonded to the earth conductor at the consumer unit. Therefore there is no neutral to earth voltage.

There may be some residual voltage measured due to high neutral currents in single phase applications (or harmonic currents in 3 phase applications but that is a different issue). In a typical installation voltage drops will be around 0.1mV per Amp per meter. Therefore a 100A circuit on a 100m ring main would result in a voltage drop of about 1V on the far end of the circuit, not enough to cause any issues with operation of electronic equipment.

It is therefore difficult to establish why there is a train of thought suggesting the need for the isolation transformer to protect against equipment that meets current legislative requirements and is connected to a properly installed electrical system. Of course, there may be times where the installation is not up to scratch and the peace of mind afforded by the isolation transformer may be a worthwhile investment. However, isolation transformers are large, heavy and undoubtedly expensive. Modern designs of UPS have removed the need for the isolation transformer and provide high quality power at all times in a smaller, more lightweight enclosure and of course at a much lower cost. This coupled with good earthing practices will ensure any electronic equipment is suitably protected.