RFI testing of fluorescent lighting

Energy-efficient fluorescent lamps produce high levels of radiofrequency interference (RFI), as well as significant levels of harmonics. The RF conducted component has the potential to interfere with the operation of sensitive control and IT apparatus as well as potentially interfering with communication systems.

Lighting used in modern office, shop and hospital operating theatre environments has the potential to severely impact sensitive IT and medical apparatus. The mechanisms of interference include direct radiation but also the effects of radiation from conducted emission that find their way onto the power reticulation network.

For a fluorescent tube to operate at high frequency, an electronic switching ballast is required. When compared to mains frequency-operated fluorescent lamps, those with electronic ballasts provide an increased efficacy of 20% or more. Typically, electronic ballasts operate at frequencies in the region of 20–40 kHz. The high-frequency ballast is essentially a switching power supply and has the potential for RFI in the same way as any switched mode power supply. Dimming is achieved by reducing the power applied to the fluorescent tube. To maintain the discharge, the frequency of operation must increase as the light output level is decreased. Thus, a typical electronic ballast which operates at around 30 kHz for maximum light output will supply the lamp at increasing frequencies up to approximately 100 kHz for minimum light output.

Harmonic currents are the result of employing electronic ballasts in addition to the radiofrequency spectrum conducted interference. The table below indicates typical harmonic contributions in terms of total harmonic distortion (THD) in comparison to typical desktop computers.

RFI needs to be tested to a frequency of 30–300 MHz, according to the latest requirements of the EMC standard for lighting CISPR 15. As regards harmonics, various limits are imposed depending on the power authority involved, but generally at points of common coupling, total voltage THD is not to exceed 5%.

Electronic ballasts can cause an increase in interference in long- and medium-wave bands to portable radio receivers and similarly conducted interference can affect line-connected equipment. Modern lighting technologies and techniques do give rise to significant emissions in the VHF band. In practice, these emissions are seen to be limited to an upper frequency of less than 300 MHz at present, but since CISPR 15 gives no tests above 30 MHz, these emissions are effectively uncontrolled by any legislation.

The measurement of conducted interference from the fluorescent light/electronic ballast requires the use of a coupling-decoupling network, which shields the measuring receiver from mains power and at the same time prevents conducted RFI from travelling up the power line.

In measuring harmonic content, the analyser to be employed must be able to perform accurately under conditions of very high crest factor input currents typical of electronic ballasts. Furthermore, the sampling frequency needs to be such as to provide a bandwidth far exceeding the basic switching frequency of the power supply being analysed.

Westek Electronics
www.westek.com.au