5 steps for verification of intrinsically safe circuits

The zone classification reflects the probability of the occurrence of an explosive atmosphere. You can find information and specifications relating to zone classification e.g. in IEC 60079-10-1 for gas hazardous areas and in IEC 60079-10-2 for areas with combustible dust. The maximum risk potential for each area must be taken into account when dividing hazardous areas into zones and determining the necessary protective measures. The devices used in the defined zone must meet the requirements of the corresponding assigned equipment category and/or the equipment protection level.

As well as the zone classification, the equipment grouping, temperature class and ambient temperature are also important factors when selecting suitable devices. The equipment grouping and temperature class are determined using the expected explosive atmosphere. In other words, these parameters are closely linked to the materials that may form an explosive mixture together with the air in the system.  In the gas explosion protection sector, available choices are groups IIA, IIB and IIC, combined with temperature classes T1 to T6. In the dust explosion protection sector, these choices are groups IIIA, IIIB and IIIC alongside the maximum surface temperature.

The ambient temperature is also key when selecting the device. The reason behind this is twofold. Firstly, the device used must cover the corresponding range; secondly, the value is important when proving intrinsic safety.

Once it is clear how the hazardous area is defined, it is important to find suitable devices. With respect to verification of intrinsic safety, this involves finding the safety characteristic values (entity parameters). These values can be found in the operating instructions, data sheet and EU Type Examination Certificate. The field device will generally be a current sink (consumer) and you should keep an eye out for the values for U_i, I_i, P_i, C_i and L_i. This is where the ambient temperature of the system comes into play – often, it determines the Pi value.

The device for intrinsically safe separation (barrier, isolator or intrinsically safe I/O module) sets out the starting parameters, such as U_o, I_o, P_o, C_o and L_o.

It's easy to forget about the cable – however, this item plays an important role in obtaining verification of intrinsic safety thanks to its capacitance and inductance. You can request the values from the cable manufacturer; alternatively, values of 1 mH/km and 200 nF/km can be used. Now, all you need to do is determine the length of the cable, and you're ready to get started.

Normally, an EU type examination is sufficient to ensure that users are on the safe side, as they can rest easy knowing that the device has been tested and can be used safely. Exceptions to this rule are simple apparatus such as Pt 100, LEDs, etc., which do not require an EC type examination. But be careful! Factors such as creepage distances and clearances or electrostatics with regard to the enclosure must be assessed and documented, even for simple apparatus in intrinsically safe circuits. This means that, if it ever becomes necessary, you can prove that you have considered the intrinsic safety of all devices – even devices that you wouldn't expect to pose an issue.