Articles and whitepapers

Mains Filtering, Distribution and Protection (2/8/2004)

By Gainsford Holness

Wherever sound or signal transmission by cable is required, such as home cinema, recording studios, monitoring equipment, television, hi-fi, computers, and electronic equipment, cable connections can be problematic.

The demand for high-technology communication has highlighted the need for superior filtration qualities in line connections, and while for decades we have accepted that using better cables and connections will improve the signal transmission or sound at the point of delivery, we did not necessarily understand why these improvements occurred. Experience has shown that all we were doing was creating filtration at different frequencies, depending on the quality of the wires used in the cable, and the material used in the connectors.

In other words, to ensure that the original qualities of signal or sound will be transmitted with minimum interference, we need an optimum combination of materials.

Interference

Electrostatic fields are an unavoidable consequence of voltages in wiring. The significance of RF (radio frequency) interference cannot be overstated. It can overload a stage, demodulating the noise and translating it into the equipment. Where it is a significant problem, it can be addressed by means of an electrostatic screen. This prevents RF interference on the mains from being capacitively coupled to the circuitry that follows.

Interference can also occur through plating the wire, which changes its natural frequency. This is sometimes accepted in the hi-fi industry as sound improvement, when different kinds of plating such as gold, nickel, tin, and silver are used. It requires a running-in period for the combination to achieve stability of its permanent frequency.

Mains cable

Ideally, we need wire that is low in resistance, high in current capacity, unplated and also made from oxygen free copper (OFC) that is tightly twisted. The best mains cables are screened-braided, use OFC, and employ high-performance ferrites (Nickel-Zinc) that are effective against high-frequency noise generated by office automation equipment and other digital equipment.

Having selected the wire, we need to ensure that the insulation is protected from all unwanted external interference or frequencies by using conductive fabric shielding tape that has been specially designed for the purpose over its full length. Materials such as thermoplastic elastomer (TPE) teflon or reinforced halogen-free silicone are suitable for such insulation.

32A mains cable with silicone insulation

A high-performance cable also requires high-performance connectors. These should be tarnish-free, and their plating must reject unwanted frequencies to the same high standard as the cable.

Mains filters

The mains, or power line filter, is the key element in eliminating mains-borne interference. This filter not only has to meet the requirements of Electro-magnetic compatibility (EMC), but safety aspects as well. For some applications, the filter also has to prevent the radiation of classified information from the line, while other applications require the filter to protect equipment from destructive voltages on the power line.

The two simple forms of filtering are aluminium enclosure and EMC cable gland. Mains filters use different techniques for reducing the interference levels of each type of emission. These include:

Capacitors between the line (live and neutral) - the X capacitors.

Capacitors between each line and ground - the Y capacitors.

Common mode chokes - each coil is wound in same direction so that the flux created by the 50Hz power current (which is differential) will cancel and not saturate the core.

Differential mode chokes, with each line having its own choke.

The Experience Filtration V250f mains filter

Mains isolation transformers

Mains isolation transformers, also known in hi-fi industry as mains conditioning transformers, are designed to provide an additional barrier between the mains supply and the electrical system. These transformers must be housed in enclosures complying with the relevant standard, and input and output components must be chosen to keep leakage currents within limits.

Mains distribution

A good mains distribution unit should offer protection against power surges and RF interference. The Experience Filtration mains distribution unit for example, is housed in an aluminium enclosure and has an EMC cable gland for shielded cable entries. It includes a surge suppressor, offers protection against high voltages, lightening strikes and transients, and can withstand a surge current of 4000A. Its attenuation of RF interference is down to approximately 30dB at 1MHz, and it offers noise damping of 20db.

Mains distribution unit

Conclusion

While general-purpose filters, cables and mains distribution units for domestic use can be purchased from any major high street retailer, high-quality mains filters and cables should be sourced from a specialist manufacturer. This way you will get the right product for the job, and enjoy the purest and most perfected form of filtration.

Gainsford Holness is the Managing Director of Experience Filtration Ltd, manufacturer of mains filtration and cables.

www.experiencefiltration.com