Calculating the Max RF Power for a Hazardous Area Wireless Device

Wireless networks for hazardous areas are becoming increasingly commonplace as the digitalisation of process industries gains pace. There are many considerations to be made for hazardous area wireless devices, including the right way to calculate its maximum radio frequency (RF) power. Additionally, it is important to assess if this max RF power is below acceptable limits for specific hazardous areas.

Your choice of wireless device must have suitable Ex approvals to prevent the risk of ignition from electrical faults, sparks, or hot surfaces. The device will either be Ex-approved or it will be installed in a wireless enclosure system that provides the Ex protection.

Effective Isotropic Radiated Power (EIRP) Levels in Hazardous Areas

There are maximum EIRP (Effective Isotropic Radiated Power) levels allowed in hazardous areas. This affects the deployment of hazardous area wireless devices.

To see the full safe RF limits tables, please refer to IEC 60079-0.

The first thing you need to determine is the max RF power from the radio transmitter. Standards accept the manufacturer’s technical specifications for max power as a given. You are not required to consider the likelihood that RF amplifiers could – under fault conditions – output more power than stated.

There are two key reasons for this approach. Firstly, ignition from RF alone is rare compared with ignition from electrical faults. This is because the RF of hazardous area wireless devices is not the main risk. The risk is that RF induced in plant structures can act in the same way as an antenna, with the subsequent energy having the capacity to spark at ignition-capable levels.

Secondly, the RF signals of hazardous area wireless devices can be made intrinsically safe by using an RF galvanic isolator, such as the iSOLATE501 and an explosion proof connector transit, such as the iSOLATE-CT, meaning non-Ex antennas can be used in hazardous areas.

It is important to note that you are not allowed to use any user accessible parameters in the radio device to limit the max RF power. If a radio is rated at 2W RF power at the antenna port, then that is the value that must be used for the assessment. This is because there is a risk that user-accessible parameters could be inadvertently changed or default back to maximum setting as part of a device software update.

Read more about deploying wireless device into hazardous areas here

How to Calculate EIRP

The calculation used to determine the EIRP is.

Radio max output power + antenna gain – losses in the RF path such as cable and connectors

If we look at an example WiFi Access Point deployed for connectivity, we can see how this calculation works in practice. Typically, a device of this nature will be 100mW at 2.4 GHz, but in the 5GHz band higher powers are allowed so the radios often have 200mW or even up to 500mW radio power.

In most instances these calculations are in dB.

Table 1: dB to W conversion chart

It is important to determine the losses in the RF cable from the manufacturer’s data sheet. The common low loss cable types for Sub 10GHz RF applications are RG58, LMR400 or LMR600. Table 2 gives an example for a high quality LMR400 RF cable specification.

Table 2: An example for a high quality LMR400 RF cable specification

Below are three examples based on real world applications to help demonstrate how RF limits can impact hazardous area wireless deployments.

Example 1 – Safe to Deploy In Hazardous Areas

In this example, we are deploying the 2.4GHz WiFi device using omni directional antennas in a Gas Group IIC hazardous area.

Radio power 100mW/ 20dBm

Antenna gain: 6dBi

Cable loss per metre: 0.222dB (2 metres used for a remote mounted antenna)

Type N RF connector: 0.2dB (x 2)

Max EIRP = Max Radio Power + Antenna Gain – Cable Losses – Connector Losses

Max EIRP = 20dB + 6dB – 0.444 – 0.4 = 25.156dB/ 327.79mW

The maximum RF power allowed in a IIC hazardous area is 2W (2000mW), so in this example the RF power level is well below the safe level specified in the standards and, therefore, safe to use in the IIC hazardous area.

In most applications where wireless devices have RF power less than 500mW and are used with Omni Directional antennas, they do not often exceed the safe RF power limits for hazardous area use. Where it becomes more of an issue is in applications that involve the use of high gain directional antennas for creating long distance wireless links.

Example 2 – Unsafe to Deploy In Hazardous Areas

Radio power 500mW/ 27dBm

Antenna gain: 18dBi

Cable loss per metre: 0.222dB (2 metres used for a remote mounted antenna)

Type N RF connector: 0.2dB (x 2)

Max EIRP = Max Radio Power + Antenna Gain – Cable Losses – Connector Losses

Max EIRP = 27dB + 18dB – 0.444dB – 0.4dB = 44.156 dB/  26.03W

This level exceeds both the IIC and IIA gas group limits of 2W and 6W respectively. Therefore, this would not be safe as a hazardous area’s wireless device under the 60079-0 standard.

The solution in this example is to reduce the max RF power by selecting a radio device with a less powerful RF amplifier, add insertion loss, and reduce the antenna gain. However, this could mean that the link may not work because there is not enough power to overcome the free space losses that occur. It could also mean there is not enough signal fade head room, which should be at least 6 dBi or better for a reliable link.

In this case a user would need to take other measures to install this link, such as moving the antenna out of the hazardous area or using other protective measures for uses in hazardous areas. Further details can be found in CLC/TR 50427 assessment of inadvertent ignition of flammable atmospheres by radio-frequency radiation guide.

Learn more about making signals intrinsically safe here

Example 3 – Unsafe to Deploy In Hazardous Areas Until Revised

Radio power 1400mW = 31.5dBm

Antenna gain: 7dBi

Cable loss per metre: 0.222dB (10 metres used for a remote mounted antenna)

Type N RF connector: 0.2dB (x 2)

Max EIRP = Max Radio Power + Antenna Gain – Cable Losses – Connector Losses

Max EIRP = 31.5dB + 7dB – 2.22dB – 0.4dB = 35.88dB/ 3.87W

This level exceeds the IIC, IIB gas group limits of 2W and 3.5W respectively. Therefore, this would not be safe as a hazardous area’s wireless device under the 60079-0 standard for use in a IIC or IIB hazardous area but is safe to deploy in a IIA hazardous area i.e. <6W. If the application is in a IIC or IIB hazardous area the solution in this example is to reduce the max RF power by selecting a radio device with a less powerful RF amplifier, add insertion loss, and reduce the antenna gain.

In this example to permit the safe installation into a IIB hazardous area with a maximum EIRP the most effective way to achieve this would be to add an in-line attenuator as it is often not possible to find an alternative wireless device with a reduced power output, especially as the radio technology itself determines what the maximum power output is of the device.

Inserting a 2dB attenuator into the line would result in the following calculation which clearly demonstrates it would be safe to deploy in a IIB hazardous area;

Max EIRP = 31.5dB + 7dB – 0.4dB – 2.22dB – 2dB (attenuator) = 33.88dB =3.08W, which is below the max 3.5W EIRP allowed in a IIB hazardous area.

Try out our RF link calculator for your application

For more information or to see how Extronics can help you with your hazardous area wireless connectivity, please contact a member of our team on +44 (0)1606 738 446 or email us at info@extronics.com.