It also allows maintenance work and conversions to be carried out during operation. In addition, the use of simple electrical equipment without special approval is possible.

The protection principle of the Ex i type of protection is based on the limitation of the energy conducted into the potentially explosive atmosphere and stored there. The energy of a possible spark is therefore always less than the minimum ignition energy of the surrounding explosive atmosphere. In addition, no impermissibly hot and therefore ignitable surfaces, for example of electronic components, can occur. In contrast to all other types of ignition protection, Ex i in accordance with EN/IEC 60079-11 does not refer to a single item of equipment, but to the entire intrinsically safe circuit.

'Ex i' separator required

The intrinsically safe circuit usually consists of the following components:

- the intrinsically safe equipment, i.e. a consumer installed in the 'Ex i' area, for example an 'Ex i' temperature transmitter;

- the associated equipment, which is a source installed in the non-'Ex i' area ('Ex i' isolator),

- and the connecting line (cable).

In accordance with EN/IEC 60079-0 and -11, 'Ex i' isolators galvanically isolate the intrinsically safe circuit from all other non-intrinsically safe circuits and are therefore mandatory in every 'Ex i' MSR circuit. They also limit the energy fed into the Ex area - the maximum open-circuit voltage Uo, the maximum short-circuit current Io and the maximum power Po - to a non-incendive level. At the same time, they use the Co and Lo specifications to determine the maximum additional energy storage - concentrated capacitance Ci and concentrated inductance Li in the field device, line capacitances Cc and line inductances Lc - that may be connected without jeopardizing the intrinsic safety of the circuit.

Another important aspect of the Ex i type of protection is the reliability of the energy limitation even under the assumption of certain faults. Intrinsically safe electrical equipment and the 'Ex i'-relevant circuit parts of the associated equipment are therefore designed according to the required reliability and divided into different protection levels, which in turn are adapted to different zones of the Ex area. The Ex ia protection level (two-fault safety) is suitable for use in Zone 0 and therefore also in Zones 1 and 2, the Ex ib protection level (single-fault safety) is suitable for use in Zone 1 and therefore also in Zone 2, and the Ex ic protection level (zero-fault safety) is only suitable for use in Zone 2.

Proof of intrinsic safety

Figure 2: An example of an intrinsically safe temperature measuring circuit.

© Phoenix Contact

To ensure that the respective interconnection cannot generate any ignitable sparks or hot surfaces, the user or system operator must carry out and document the 'proof of intrinsic safety'. This is specified in the ATEX Directive RL1999/92/EC and the BetrSichV or the new GefStoffV and described in the normative requirements for electrical explosion protection, for example EN/IEC 60079-14. The procedure offers the user the advantage that 'Ex i' field devices and isolators can be selected and combined according to the specific requirements, regardless of the manufacturer.

Figure 2 shows an intrinsically safe circuit in blue, which consists of an associated equipment (source) with a linear or resistive source characteristic, an intrinsically safe equipment and the connecting cables. The safety parameters required to demonstrate intrinsic safety and the criteria that must be met for the circuit to actually be intrinsically safe are also listed. The parameters can be found in the EU type examination certificates and operating instructions or data sheets.

In accordance with the current editions of DIN EN 60079-11 and the installer standard DIN EN 60079-14 (VDE 0165-1), it must also be assessed whether the 50% rule must be applied. This is because the certified Co and Lo values of the associated equipment may only be fully utilized in the following cases:

- in simple intrinsically safe circuits without concentrated capacitances (= Ci) and without concentrated inductances (= Li),

- in a mixed intrinsically safe circuit with concentrated capacitances and/or concentrated inductances under the condition that Li < 1 % of Lo or Ci < 1 % of Co.

If Li is ≥ 1 % of Lo and Ci ≥ 1 % of Co in the mixed intrinsically safe circuit, the certified Co and Lo values must be reduced by 50 %. The following then applies:

Ci + CC < 0.5 Co

Li + LC < 0.5 Lo

In some cases - as with the MACX MCR-EX product line - Co and Lo value pairs specially determined by the test laboratories are available for this case, which are up to a factor of 1.5 greater than the halved values.

Example of an intrinsically safe temperature measuring circuit

Figure 3: The 12.5 mm narrow 'Ex i' disconnector series MACX MCR-EX-SL... is characterized by coordinated Io, Uo and Po values, making it compatible with a large number of 'Ex i' field devices. During its development, emphasis was also placed on the highest possible Co values, as this is the key parameter that determines the maximum cable length that can be realized.

© Phoenix Contact

In process engineering applications, process variables such as temperature, pressure, flow rate, humidity or pH value in hazardous areas are recorded by intrinsically safe transmitters and converted into an electrical standard signal of 4 to 20 mA. Temperature is one of the most frequently measured physical variables. In the following, the proof of intrinsic safety is therefore carried out using the example of an intrinsically safe temperature measuring circuit up to Ex zone 0 of an Ex atmosphere consisting of a hydrogen/air mixture. The 'Ex i' measuring circuit is a combination of the MACX MCR-EX-SL-RPSSI-I power supply isolating amplifier as the active associated equipment with a linear source characteristic and two passive intrinsically safe items of equipment: the FA MCR-EX-HT-TS-I-OLP-PT temperature transmitter and the FA MCR EX-FDS-I-OLP process indicator (see figure on page 2).

The following points must be fulfilled for the verification of intrinsic safety:

- The protection levels Ex ia for zone 0 and the categories correspond to the zones. This criterion is met with the note in the EU type examination certificate of the field display that the device can be used within an 'Ex i' circuit of protection level ia without affecting its protection level.

- The substance groups correspond. All 'Ex i' devices are approved for an Ex atmosphere consisting of a hydrogen/air mixture in accordance with the IIC marking. The same applies to temperature class T4, which is why this requirement is also met.

Figure 1: Overview of the parameters and criteria of an intrinsically safe circuit.

© Phoenix Contact

- The five criteria listed (see figure on the right) for comparing the 'Ex i' parameters are fulfilled. This requirement is met according to the 'Ex i' parameters shown in the figure on page 2. In addition, this is a mixed 'Ex i' circuit in which the 50% rule does not have to be applied. Only the process indicator has a concentrated inductance of 35.1 μH. Therefore, with a specific cable capacitance Cc of 140 nF/km and a cable inductance Lc of 1 mH/km with a Co value of 107 nF, a cable length of 750 m is possible, with a Lo value of 2 mH and a Ci value of 35.1 μH, a cable length of 1,964.9 m is possible.

This interconnection is therefore intrinsically safe with a maximum cable length of 750 m. For the interconnection of the field-side connections of the head transmitter with the thermocouple and the resistance thermometer, the proof of intrinsic safety described above must be implemented separately.
Thermocouples and resistance thermometers are considered 'simple electrical equipment' in accordance with EN 60079-11. Alternatively, temperature sensors without approval can also be used. In this case, however, the user must ensure that the thermocouples and resistance thermometers comply with the characteristic values of the energy storage and construction regulations specified in EN 60079-11. In addition, the user must determine the surface heating of the devices via the Po value in order to assign them to a temperature class.