JULIA CLARK /
01.01.2012 12:00 PM
RF safety
A British network puts safety first in high RF fields.

Britian's powerful terrestrial broadcast transmitters, such as London's iconic Crystal Palace tower, deliver television and radio signals to millions of homes across the country.

Arqiva owns, operates and maintains these landmarks of British broadcasting, with a history that can be traced back to the early years of the BBC. Although the company was conceived more than 50 years ago, the service-level agreements with the broadcasters — and the expectations of the general public — now require these towers to broadcast 99.99 percent of the time, and to 99 percent of the British population.

The company's experience and expertise in broadcast engineering enable it to maintain such stringent service levels, but with high power comes great responsibility to ensure the well-being of its field operatives. Although they do not pose a threat to the general public, at close range these broadcast transmitters emit significantly high levels of nonionizing radiation, meaning Arqiva engineers are exposed to power levels that are extremely close to safety guidelines.

The mobile phone masts dotted around the country require the same precautions, but in general, these are no major issue. They operate at a relatively low power of 60W to 120W — a small fraction of what is actually permitted. Compare this to the Sutton Coldfield transmitter, which has for many years been operating at 1000kW for analog television and 250kW for FM radio. The radio frequencies used for broadcast TV and radio share the thermal range of the electromagnetic spectrum with microwaves, and the human body absorbs this energy more readily than any other part of the spectrum. With that said, I need hardly spell out the dangers of climbing a broadcast mast with an antenna operating at full power.

National service

In order to meet its stringent service agreements, Arqiva needs to operate at full power as much as possible. It uses a simple hierarchy of control to manage health and safety, with each successive control growing weaker. The most stringent control, of course, is to design the antenna to be safe to climb through on full power, which the communications company has done with the new antennas it has switched over to digital. The majority of its older masts were not designed this way, however. The second control would be to turn off a transmitter while working near its antenna. Unfortunately, that simply is not something the company's service agreements usually allow, so the next stage is to implement engineering controls. The most important of these is to reduce the antenna's power, rather than turning it off. Major work in close proximity to the antenna requires a more significant decrease in power. To change a structure's mast lights, for example, the company would need to drop the power significantly, as the task requires engineers to climb right through and past the high-power antennas.

Even then, the engineers must be suitably prepared. Engineering controls may also include surveying and adding permanent or temporary screening to the mast. Arqiva's experience has enabled it to take the lead in devising a safe and fact-based set of guidelines for its workers; despite the dangers, the United Kingdom currently has no legislation for managing RF. Everything falls back on the Health and Safety at Work Act 1974 and general management regulations that require risk assessments to be completed. From the point of view of a health and safety executive (HSE), as long as companies follow the guidelines set by the International Commission for Non-Ionizing Radiation Protection (ICNIRP), they are doing the right thing.

Measuring the immeasurable

ICNIRP is a body that publishes world-standard safety guidelines for radio frequencies as part of its guidance for the whole nonionizing radiation spectrum. A few countries do something different, but most countries base their safety guidelines on ICNIRP. Every frequency range has a basic restriction, but this is usually in a form that is impossible to measure technically in a working situation. One example is SAR, which stands for Specific Absorption Rate and is quoted in terms of watts per kilogram of human tissue.

Mobile phone users may have seen “SAR” referred to in their phone's manual. Simply put, this is based on how much heat a device emits into a human head. However, there is no current way to measure this using the human body in real time — so devices such as mobile phones are SAR tested in a lab using a “phantom.” This is a head-shaped mould containing liquid that is used to simulate human tissue. A sensitive probe is then robotically moved around within the phantom with readings taken in different places. This might be a reasonable test for mobile phone manufacturers in carefully controlled conditions, but it is difficult to translate this testing method to human beings working on a broadcast tower.

Because of this, ICNIRP also publishes “reference levels,” known as “action values” in the EU directive. These are levels stated in terms of quantities that can be measured, for example, using professional survey meters to measure electric and magnetic fields or power density. If the company meets that level, it is guaranteed to be compliant with the basic restrictions. If it knows, for example, that it needs to be operating at less than 61V/m and it receives a reading of 50V/m, then it knows it can proceed safely. However, that doesn't mean that if levels of 70V/m are reached, the company has endangered its workforce. It simply means that the organization has to do more work to demonstrate that this higher level still results in compliance with the safety standard. This can be done by referencing published dosimetry information carried out by organizations such as the radiation protection division of the Health Protection Agency.

Tools of the trade

Test equipment has improved dramatically over the last few years. The company previously used devices without isotropic probes — meaning that readings were highly directional. Engineers also would need to contort their wrists in every angle possible in order to get a maximum reading for the area from a probe, which was far from ideal — as was reading that maximum from a needle on a gauge rather than from a digital readout.

The company now employs two pieces of high-tech measuring equipment: the Nardalert XT RF personal monitor and the Narda NBM-550 broadband field meter. The latter is equipped with a multidirectional isotropic probe, which provides a reasonably accurate numerical measure of RF levels measured in V/m (for the electric field) or A/m (for the magnetic field) in order to establish safe zones and measure field strengths to comply with ICNIRP guidelines. It is a reliable and important tool but, due to its size and visual requirement, engineers are protected only while they are staring at its screen. Because of this, all company engineers are required to wear a Nardalert XT RF personal monitor. About the size of a cigarette packet, this clips to a climbing harness, providing an audible alert and vibration when the engineer reaches 50 percent of the reference level and a different audible alert at 90 percent. The company's specialist RF surveyors take careful note of the readings, and their RF survey reports are made available to others who need to plan work on the structure.

Arqiva has invested money and time into the study of RF emissions, and its research demonstrates that, generally, its engineers can safely work above reference levels and still meet basic restrictions. It only does this occasionally, and only if it has explored all other possible avenues such as power reductions; working within reference levels makes for a simpler working regime.

As head of RF safety at Arqiva, I, or my deputy, have to approve every request to operate above reference levels. In this sort of situation, the company safety team will often employ further personal protective equipment such as RF protective suits. These are not widely used in the United Kingdom because they limit movement, but they are employed by the communications company when it is faced with a situation in which all other avenues of control have been explored and the safety team still feels a greater factor of safety is needed. For these projects — and the previous example of changing the mast lights comes into this category — I will always send one of my team to be solely responsible for overseeing RF safety throughout a job of this kind, leaving the other engineers to focus on their primary task, such as re-engineering the lights while the safety team checks and records the RF levels to which everyone is exposed.

Arqiva's ongoing research into RF has enabled it to lend significant insight into the European Commission's proposal to revamp rules protecting EU workers from harmful electromagnetic fields. This directive covered the entire nonionizing electromagnetic spectrum, including much lower frequency such as that employed in Magnetic Resonance Imaging (MRI) as well as the higher frequency used in broadcasting. First published in 2004, the directive should have been implemented in 2008 but was dismissed due to its unnecessarily stringent regulation at lower frequencies. The company assisted in the revision of the current version, working closely with the U.K. HSE and the EU to demonstrate that many of the proposed restrictions had no scientific foundation and to ensure that the resulting regulation was practical to implement.

Training for safety

The final, yet most important, control is simply training, assessment and information. Any work the company performs is meticulously planned, and it ensures that any contractors, as well as its own engineers, are trained to an extremely high level of RF awareness. It makes clear what the risks are and how to avoid them with sensible use of RF personal monitors. It is my belief that Arqiva's exemplary safety record is not attributable to the hardware it uses nor the levels and restrictions it abides by, but rather, it is in the company's dedication to training its staff to react knowledgeably and safely to any given situation.

Julia Clark is head of RF safety at Arqiva.



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