5 steps for verification of intrinsically safe circuits
Proving intrinsic safety is not an easy task – after all, in contrast to most other types of protection, intrinsic safety must always consider the entire electrical circuit, as well as the combination of current and voltage sources, consumers and connecting cables. To make it easier for you to get started, we've listed five helpful tips that will give you an overview of the work involved.
1. Be in the know when it comes to zone classification
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.
2. Learn more about equipment grouping, temperature classes and ambient temperatures
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.
3. Find the information you need about selected devices and cables
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 Ui, Ii, Pi, Ci and Li. 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 Uo, Io, Po, Co and Lo.
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.
4. Do you use simple apparatus?
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.
5. Establish verification of intrinsic safety
Intrinsically safe circuits require verification and documentation of their intrinsic safety in accordance with the installation regulations set out in IEC 60079-14 . This process involves verifying that ignitable sparks and hot surfaces have been safely avoided.
The process of obtaining proof involves calculations, for which the values for the voltage, current and power, as well as the capacitances and inductances of the energy storage device (incl. the cable) are evaluated. Proving intrinsic safety can be a difficult undertaking. R. STAHL provides cost-effective verification of intrinsic safety, including the corresponding documentation – even for complex interconnected systems consisting of a number of sources. To do so, the company requires a specification, the circuit diagram, data sheets for the field devices and isolators including their approval certificates (ATEX, IECEx, etc.), operating instructions, and images of the devices' type plates.