Measurement Technology for Potentially Explosive Areas How to Design Intrinsically Safe Circuits
The intrinsic safety (Ex i) type of protection has established itself worldwide in the field of measurement and control technology in systems with potentially explosive areas. For one thing, it is much more cost-effective when it comes to construction when compared to other types of protection. For another, it allows for maintenance work and modifications during operation. Moreover, it is also possible to use simple electrical equipment without special approval.
The protection principle behind the Ex i type of protection is based on limiting the energy that is being conducted in the potentially explosive area and that can be stored there. This means that the energy from any potential spark is always less than the minimum ignition energy of the surrounding potentially explosive atmosphere. In addition, it prevents the creation of any impermissible hot surfaces that could cause an ignition, such as electronic components. As opposed to all the other types of protection, Ex i refers not only to a single item of equipment but to the entire intrinsically safe circuit, in accordance with EN/IEC 60079-11.
Special Significance of the Ex i Isolators
Intrinsically safe circuits are generally composed of the following elements:
- the intrinsically safe equipment, that is, a consumer installed in the Ex i area (e.g., an Ex i temperature transmitter);
- the associated equipment, which involves a source in the non-Ex i area (Ex i isolator);
- the connecting line (cable).
In accordance with EN/IEC 60079-0 and -11, the Ex i isolators provide the intrinsically safe circuit with galvanically safe isolation from all other non-intrinsically safe circuits and are therefore a necessity in every Ex i MCR circuit. Moreover, they limit the energy conducted to the Ex area, i.e., the maximum off-load voltage Uo, the maximum short-circuit current Io, and the maximum power Po, to a non-incendive level. At the same time, via Co and Lo, they determine which maximum additional energy storage elements — concentrated capacitance Ci and concentrated inductance Li in the field device, line capacitances Cc and line inductances Lc — can be connected without jeopardising the intrinsic safety of the circuit.
Another important aspect of the Ex i type of protection is the reliability of the energy limitation itself under the assumption that certain faults will occur. For that reason, intrinsically safe electrical equipment and the Ex i-related switching parts of the associated equipment are designed in accordance with the required reliability and divided into different protection levels, which in turn are aligned with different zones in the Ex area. The Ex ia level of protection (double-fault tolerance) is suitable for use in Zone 0 (and therefore in Zones 1 and 2 as well), the Ex ib level of protection (single-fault tolerance) for use in Zone 1 (and therefore in Zone 2 as well), and the Ex ic level of protection (zero-fault tolerance) only for use in Zone 2.
Combining Ex i Field Devices and Isolators from Different Manufacturers
To ensure that the respective connection cannot produce incendive sparks and hot surfaces, the user or system operator has to demonstrate and document “proof of intrinsic safety.” This is laid out in the Atex Directive RL 1999/92/EC and the BetrSichV or the new GefStoffV and described in the standardisation requirements for electrical explosion protection (e.g., EN/IEC 60079-14). This process offers the user the advantage of being able to select and combine Ex i field devices and isolators from different manufacturers in accordance with the specific requirements.
Figure 1 (see picture gallery) shows an intrinsically safe circuit marked in blue consisting of an associated item of equipment (source) with a linear or ohmic source characteristic curve, an intrinsically safe item of equipment, and the connecting cables. The figure also shows the safety technology parameters necessary for demonstrating intrinsic safety, along with the criteria that must be met to ensure the circuit is actually intrinsically safe. The parameters can be found in the EU examination certificates and user guides or data sheets.
In accordance with the current versions of DIN EN 60079-11 and the DIN EN 60079-14 construction standard (VDE 0165-1), an evaluation must also be carried out to determine whether the 50 % rule must be used. This is because the certified Co and Lo values for the associated equipment may only be fully utilised in the following cases:
- in simple intrinsically safe circuits with no concentrated capacitances (= Ci) and no concentrated inductances (= Li);
- in a mixed intrinsically safe circuit with concentrated capacitances and/or concentrated inductances on condition that Li <1 % of Lo or Ci <1 % of Co.
If Li ≥1 % of Lo and Ci ≥1 % of Co in a mixed intrinsically safe circuit, the certified Co and Lo values must be reduced by 50 %. In this case, the following applies:
Ci + CC ≤ 0.5 Co
Li + LC ≤ 0.5 Lo.
For this situation, there are in some cases — such as with the MACX MCR-EX product line — Co and Lo value pairs available that have been specially derived from the testing sites that are greater than the half-value by up to a factor of 1.5.