Using Wireless Devices in Hazardous Areas
The trend in industrial technology is moving towards wireless devices, resulting in an increase in the number of wireless devices installed in hazardous areas.
The wireless devices are radio devices that emit electromagnetic radiation – clearly a potential source of ignition in an explosive atmosphere. The main risk lies in the induction of currents in metallic objects or inadequately EMI-protected electronic circuits. These currents can result in excessively high temperatures and the formation of sparks.
These devices are in applications that include reading RFID tags, instrument asset management and condition monitoring of mechanical equipment, such as rotating equipment. This article will cover the design of these devices, and the risks that they pose when installed in hazardous areas. The paper will also attempt to give details of the requirements for safe operation of wireless technology in hazardous areas.
With the ever increasing demand for process measurement points, the need for industrial measurement devices is continually rising. Wireless technology is said to be replacing the current industrial cable-based networks as it offers many advantages over its cables counterpart:
- Substantial reduction in costs, as cabling costs increase drastically with increasing lengths and flexibility.
- Wireless offers versatility in that the devices can be placed anywhere without the need of intensive equipment to carry out an installation i.e. scaffolding, excavation etc.
- Reduction in timeline of the projects, spent on installation and reinstallation
- Difficult to reach places or moving measuring points like rotating kilns
Thus the trend to meet the high demand for process measurement is moving towards the installation of wireless instrumentation. With this development comes the need to consider the safety implications for wireless applications in hazardous locations.
The concern arises from the fact that wireless instrumentats are essentially radio devices, and therefore are sources of electromagnetic radiation – a potential source of ignition in hazardous areas. This article will outline the relevant standards, risks and procedures incurred when using wireless instrumentation in hazardous locations.
Wireless applications in hazardous locations
Due to the mobility, cost and ease of installation of wireless instrumentation, it is found in many applications in industry, including hazardous locations. Applications include process measurement indications, RFID tagging and marking; and portable handheld terminals and many more.
Industry standards
There is currently no industry standard that explicitly deals with the risks around wireless technology used in hazardous locations. International standards mainly deal with the exposure levels of RF radiation on the human body.
- The USA’s Federal Communications Commission gives guidelines as to the regulations of transmitters but again the emission thresholds they supply are based on settings for human exposure.
- Wireless standards such as ISA100 and wireless HART give details of wireless functionality of the devices but don’t elaborate on their installation in hazardous areas.
- The standard for hazardous areas namely, IEC 60079-0 gives specific tables that have specific parameters for power and energy levels thatare allowed in hazardous locations.
- British Standard BS6656 – assessment on inadvertent ignition of flammable atmospheres by radio frequency radiation gives recommendations to the subject matter but mainly discusses the RF sources outside a hazardous location and transmitting into the relevant area.
Risks of wireless technology in hazardous locations
Wireless instrumentation typically refers to radio devices that operate in the frequency band of 30 kHz – 300GHz. These devices are able to emit electromagnetic radiation, which is a source of ignition in an explosive atmosphere. A risk also exists in the design of the device, i.e, inadequate circuit protection against electromagnetic induction and interference.
Plant construction metal surfaces can act as conductors and absorb energy from radio frequency radiation fields from wireless instrumentation. This stored energy can be released as a spark or arc in hazardous locations.
Other risks involved with wireless instrumentation are the batteries that are used to power the portable devices. Some devices cease to be intrinsically safe when the device is opened to replace the battery, thus the device has to be removed from service in order for the battery to be replaced. Some vendors have opted to use intrinsically safe battery packs that allow the battery to be replaced while the device is in the hazardous location. Battery packs are sealed enclosures that are incapable of causing sparks in the hazardous locations due to power limiting circuitry and should be taken into consideration when evaluating wireless instrumentation.
Wireless adaptors are the new solution for obtaining HART diagnostics from already hardwired instrumentation. These devices connect onto the existing hardwired instrument and extract data/diagnostics from it.
There are concerns regarding the incorporation of wireless adaptors into the instrument loops.
The question is whether this adaptor is part of the existing loop or is it to be considered as a second loop with its own certification?
If it is partof the existing loop, it changes the dynamics of the existing loop and thus the loop needs to be re-certified. The other risk is that some wireless adaptors are loop- powered, thus care must be taken in the loop design to compensate for the power consumption of the device and the rest of the loop, so as to function properly.
Antenna sources – EIRP
Antennas are sources of the power radiation and thus the EIRP of the antenna must be considered.
EIRP – Effective isotropic radiated power is a measure of the power radiated from an antenna. The IEC 60079 standard does not give any indication of the energy limits for EIRP of devices in hazardous areas.
CISCO stated in a technical White Paper “Low power radios operating at less than 100 mW/-10 dBm EIRP for frequency of 2,4/5 GHz should not pose any risk if operated under normal circumstances”.
Methods of risk mitigation
Many of the wireless devices used in industry today are low power devices that are designed according to international standards and specifications.
One should also conduct a general safety assessment prior to installing wireless instrumentation in a plant or hazardous area.
As part of this safety assessment the following should be taken into account;
- Identify the classification of the hazardous area in which the equipment will be installed;
- Identify the threshold of ignition energy for the area;
- Review the instrument certification and specifications to verify that the power transmitted is less than the limits of the hazardous area, even under fault conditions, to minimise the risk of an explosion occurring.
IEC 60079-0 gives tables which detail power and energy thresholds for radio frequency devices in hazardous areas.
| Equipment for | Threshold power (W) | Thermal initiation time (Averaging period) (μs) |
|---|---|---|
| Group I | 6 | 200 |
| Group IIA | 6 | 100 |
| Group IIB | 3.5 | 80 |
| Group IIC | 2 | 20 |
| Group III | 6 | 200 |
IEC 60079:2002 Table 5 – Radio Frequency Power Threshold
| Equipment for | Threshold energy Z1 (μJ) |
|---|---|
| Group I | 1500 |
| Group IIA | 950 |
| Group IIB | 250 |
| Group IIC | 50 |
| Group III | 1500 |
IEC 60079:2002 Table 5 – Radio Frequency Energy Threshold
Wireless adaptors pose the same risks as that of other wireless instrumentation and must be treated as such. Since some adaptors are loop-powered, the power requirements for a loop must be considered before the adaptor is installed. The adaptor will be an additional load on the loop and must be catered for with regard to power requirements.
Risk mitigation methods for batteries of wireless instrumentation should be included with the maintenance schedule of the instrumentation; batteries are rated to work for longer than four years depending on the update rates of the wireless network.
Battery packs are preferred as they are sealed enclosures that contain a power limiting circuit that give the battery pack its intrinsically safe rating.
Conclusions
You cannot control what you cannot measure.
The need for process measurement points is in high demand to achieve optimum process control. Wireless instrumentation is currently used mostly in indication application however; it will soon be more widely used in control applications.
Thus with the use of wireless technology and instrumentation being used to meet this high demand we must consider the safety implications incurred by the use of wireless networks in hazardous applications.
In most circumstances, low power devices are used in order to operate i.e. WLAN. These devices use low voltages, less than 100 mW effective radiated power and operate in the 2,4 GHz range and do not pose any risk if operated under normal conditions of use.
However, there are no standards for the end users to comply with for the use of wireless networks in industrial hazardous locations.
Until such a stage that standards are released and accepted for use of wireless technology in hazardous locations, it is recommended to follow the procedures to check and ensure that the radiated power from the instrumentation and ultimately the wireless network falls within the ignition energy levels of the hazardous area.
This will ensure that the wireless networks will not pose any risk to the hazardous location and will ensure the safe operation of the industrial processes.
References
[1] IEC 60079:2002 Electrical Installation in Hazardous Areas[2] www.fcc.gov 25 July 2010[3] www.hazardexonthenet.net/ 27 July 2010[4] hazardexonthenet.net/article.aspx?AreaID=4&ArticleID=23206 27 July 2010[5] Electrical Installations in Hazardous Areas by Alan McMilan
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