How to ensure intrinsic safety for wireless systems

Intrinsic safety (IS) is not always widely understood, at least not in North America, where other protection methods are more popular. However, the core principle of IS technology is to limit the energy entering the potentially explosive area below the level that could cause ignition, thus making the system inherently (intrinsically) safe.

Intrinsic safety is achieved either with passive (Zener) barriers, or “active” systems often called galvanic or optical isolators that operate similar to air gap systems, isolating the low-energy circuits in the potentially explosive area from the circuits in the non-explosive area. Ironically, many IS diagrams label the circuits in the explosive area as safe (because they are now safe due to the energy limitations), so be aware of this when looking at any IS system documentation. IS installations are always treated as a “system” which, fortunately, the IS equipment providers make applicable to a broad enough range of end devices that the system itself covers a wide variety of installations.

Because they are simple devices, Zener barriers, which introduce an impedance in the network to limit energy, are the most commonly used form of IS protection. Unfortunately, for digital signals, because of signal attenuation and distortion, the Zener barrier's capabilities mean that this principle can only be used on lower-frequency (less than 500 kHz) signals.

Though glass is often considered a viable option for allowing RF signals into an enclosure, it highly attenuates the higher frequencies, such as 2.4 and 5 GHz WLAN signals. With a typical output of only approximately 100 mW from the device in the case of an 802.11b/g/n WLAN, this is far from an ideal solution.

With wireless systems that have self-contained power, the link to the outside world is the radio signal itself and the associated antenna. As a result, it’s this connection that must adhere to the requirements of the approval standards, including the CLC/TR 50427, EN 60079 series, IEC 60079 series and others, such as FM3600. These standards’ agreed area of radio frequency (RF) under the explosion-protection standards only considers the range from 9 kHz to 60 GHz.

Table 1: Power and energy thresholds per IEC 60079-0:2011

Equipment for	Threshold power (W)	Threshold initiation time μs	Threshold energy Z_th, μJ
Group I	6	200	1,500
Group IIA	6	100	950
Group IIB	3.5	80	250
Group IIC	2	20	50
Group III	6	200	1,500

CLC/TR 50427 explains the methods that can be used to assess whether an RF installation is safe to operate in a hazardous area, while IEC 60079-0:2011 provides RF power or energy threshold tables for the various IEC Zone classifications (Table 1).

Fortunately, wireless technology that is of great interest to users to deploy in their plants, such as Wi-Fi, IEEE 802.15.4 radios, or RFID, has RF power levels far lower than the 2 W limit for the most stringent Group IIC, Acetylene & Hydrogen environment. Providing the antenna gain doesn’t cause these levels to be exceeded, and they are installed with adequate protection (e.g. Ex d enclosure), they can be used in a hazardous area without restrictions.

However, because devices have antennas, the maximum effective isotropic radiated power (EIRP) must be calculated to ensure the power radiating from the antenna with its associated gain remains below the limits for the relevant hazardous area.

Other considerations when performing the system calculations include:

In case of device with multiple outputs and multiple antennas, each threshold power is calculated separately for each output/antenna.
Gain of multiband antennas should be evaluated separately at each individual frequency.
High-gain, directional antennas on the same device should not be directed in the same direction.

Just like we have isolators to separate powered devices from an IS circuit to the field, several manufacturers have developed isolators that can be installed between an antenna and the powered device, such as an access point, to ensure the IS requirements of the above standards can be achieved without requiring onerous calculations on the part of the end user.