Power Line Telecommunications (PLT) – Current applications and the interferen...
Wednesday, February 2, 2011

Power lines have been used for telecommunications purposes for over forty years, principally for conveyance of supervisory control signals between High Voltage (HV) substations.   These systems were based on telephone “carrier” equipment which operated at frequencies generally below 300 kHz.    In more recent times intercoms in homes and some security and control apparatus have used the internal mains wiring as the communications bearer.   These systems also operated in a similar frequency range all of which are frequencies below normal broadcast frequencies.   Over the past 10 years or so, the potential use of the mains for internet, data and video communications has become feasible and systems are being produced which use frequencies up to and above 30 MHz. In-Home PLT –   These systems comprise two or more modems which are plugged into GPOs separated by no more than 100 to 200 metres.   The modems use Ethernet protocol for connection between PCs at either end or for audio/video communications between the two locations with the use of compatible terminal equipment. Data rates of around 200 Mbps are achievable. The frequency band used by these systems is typically 4 to 34 MHz. Access PLT -   These systems are used for provision of a public internet service.   Typically the last kilometre of distribution is over the Low Voltage (LV) mains power network into the home or office.   Communications to the fan-out point (node) is by optic fibre or microwave link. The data rates and frequencies used are similar to ‘in-home’ modems. Smartgrid -   The most recent application of PLT is to ‘smart’ metering of electricity consumption.   The system is similar to Access PLT but is dedicated to metering and supervisory control of appliances such as hot water systems, air conditioners, refrigeration etc to even out the demand on the power generators.   The frequencies used are lower than Access and unlikely to exceed 4 MHz. The interference caused by all of these systems can be considerable, that caused by ‘in-home’ and Access modems affects short wave radio reception including CB and is characterised by a tone in the region of 1 kHz.   Audio (hi-fi) equipment, may also be affected as well as AM   MF broadcast reception in fringe reception areas (a result of inter-modulation).   Interference of a less obvious nature can occur due to the incompatibility with switched mode power supplies, electronic ballasts of fluorescent lighting, video and TV displays etc.   Smartgrid   PLT is in its infancy and the potential for interference is still to be determined and characterised, however it is currently expected to be limited to AM MF broadcasting reception and audio equipment.   More details on PLT interference will be provided in a future bulletin.

Power line Electromagnetic fields (EMF) – Shielding of sensitive medical appa...
Wednesday, February 2, 2011

Power lines produce and electricity wiring produce both electric and magnetic fields which can be harmful to human health at high levels but more often are a concern to the operation of electronic equipment, particularly medical diagnostic equipment even at very low levels.   Most notable of these are MEGs (magnetoencephalographs), used for studies of brain wave activity.   Other sensitive equipments include EEGs (electroencephalographs), ENGs (electronystagmography), EMGs (electromyographs) and ECGs (electrocardiographs). MEGs (magnetoencephalographs) are ultra sensitive to magnetic fields having a similar sensitivity to that of an electron microscope.   The EMG employs ‘squids’ or superconducting quantum interference devices to detect very low magnetic fields.   MEG arrays of around 300 sensors are set in helmet covering most of the head. The brain's weak magnetic fields of 10 femto tesla (fT) for cortical activity and 10 3 fT   or 1.0 pT for the human alpha rhythm, are considerably smaller than the typical magnetic environment of 10 6 pT or 1.0 µT.   Consequently, the area where a MEG or electron microscope is to be located requires specialised shielding and design of the electrical services required to supply the MEG and surrounding equipment. An EEG (electroencephalograph) measures small voltages across the cranium and   can only operate with impunity in a low magnetic field environment of levels below that found in a normal room environment. Although the signals measured (as low as 0.01 volt) are filtered, masking (or desensitisation) is not readily detected. This necessitates special precautions be taken to either shield the area from magnetic fields or design the room to avoid the creation or intrusion of magnetic fields.   The latter is the more economic approach but usually requires early intervention by a specialist consultant in the design of the EEG test area. Electronystagmography ( ENG ) is a test which measures eye movements to diagnose symptoms of dizziness, balance, or vertigo.   The ENG test performs measurements on the muscles that control eye movements . Electrodes placed around the eye record movements due to the voltage between the retina and cornea, these voltages change as the eyes move.   ENG diagnostic equipment requires a similar environment to that required for EEGs but with more emphasis on the absence of low frequency and slowly varying fields. Similarly EMG (electromyography), a process which measures muscle stimulation, requires precautions much the same as those required for an EEG or ENG test area however there is less likelihood that the area will need shielding.   Because of its lesser sensitivity to external fields and a lower expectation of problems, fewer precautions are taken by users, consequently EMG measurements may be affected without realisation that external fields of a sporadic nature have intervened. As some but not all effects of magnetic fields will be observable by a medical practitioner, a specialist EMF consultant should be engaged to advise on the suitability of the test location. The least sensitive of this group of diagnostic equipments is the, ECG (electrocardiograph).   ECGs are the most widely used of all these equipments. Fortunately, an impugned plot is more obvious causing the operator to repeat the exercise or question the circumstances of the test when an irregularity is observed, nevertheless the prospect of misleading or unrecognisable interference should not overlooked. Normally it is adequate to exert caution in the use of the ECG, however where it is to be used repeatedly in the same location, it is wise to take expert advice on the area’s suitability. It is unwise to accept short term test results as a basis for acceptance of a site for medical equipment.   Expert theoretical as well as practical analysis and opinion is required to explore the variables which may exist at the time of the assessment and going into the future. Every situation is different, generic shielding solutions should be avoided as they offer no guarantee of success.   EMC Services engineers have wide experience in providing tailored (best fit and lowest overall cost) solutions for the location of medical diagnostic equipment. Other medical apparatus generate high levels of electromagnetic fields such as MRIs,   CT scanners Ultrasounds and Quarterizing devices .   Some of these need extensive shielding and others require well designed areas to prevent interference or malfunctioning of other equipments used in the same area or an adjoining area.   We will address these problems in our next Bulletin.

Power Line Telecommunicarions (PLT) – Current EMC requirements
Wednesday, August 25, 2010

Since the year 2000, extensive work has been carried out by the IEC/CISPR to both assess the impact of and to produce an EMC emissions standard for telecommunications equipment which communicates over the powerline including both the low voltage (240/415V) mains distribution system and the power distribution wiring of the home or office. Potentially, this form of communications offers an attractive low cost alternative to other copper and fibre-optic line based networks. After the deliberations of two successive working groups of the CISPR Subcommittee I each tasked with producing a standard within 5 years, the second of these groups has reached the end of its term with no clear agreed recommendation. As a consequence the dilemma of what standard should be applied remains however there are some established principles and regulations which apply in the interim. In Australia and New Zealand, the existing requirements of CISPR 22 apply and will continue to apply at least until a specific requirement is promulgated by the CISPR.  Such a requirement is unlikely to occur in less than 2 years. When applying CISPR 22, the emission limits of the power port apply to the injected RF communications signals when the modem is communicating.  The limits and measurements methodology normally applying to the communications port are not applicable to the mains port but do apply to the dedicated Ethernet port connecting to a PC or other equipment. Currently there are no systems operating in Australia which communicate over the external mains power distribution system but PL communications internal to a building are becoming increasingly common for data and video communications from room to room.  These “in-home” modems must comply with CISPR 22. The situation in Europe and the Americas is less demanding, “in-home” modems produced for the European market are not required to meet the CISPR 22 requirements in the transmit  mode and will in most instances not comply with the Australian requirements.  Most, if not all products marked CE will fail the requirements of the Australian regulations.  In the USA, the FCC requirements apply only to radiation from the mains distribution infrastructure and consequently modems and other PLT terminal equipment sourced from or intended for the American market will likely fail the Australian requirements.