How to Design Intrinsically Safe Circuits

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Example of Proof of the Intrinsic Safety of a Temperature Measurement Circuit

In process engineering applications, process variables such as temperature, pressure, flow rate, moisture, or pH value in the Ex area are collected by intrinsically safe measuring transducers (transmitters) and converted into a 4…20 mA standard electrical signal. Of these, temperature is certainly one of the most frequently measured physical variables.

Gallery

For that reason, we have provided an example showing (see Figure 2) how to verify the intrinsic safety of an intrinsically safe temperature measurement circuit up to Ex Zone 0 for a potentially explosive atmosphere resulting from a hydrogen/air mixture. The Ex i measurement circuit consists of an interconnection between a MACX MCR-EX-RPSSI-I repeater power supply as the active associated equipment item with a linear source characteristic curve and two passive intrinsically safe items of equipment: an FA MCR EX-HT-TS-I-OLP temperature transmitter and an FA MCR-EX-DS-I-I process indicator (Figure 2).

The following criteria must be met to demonstrate intrinsic safety:

• The Ex ia levels of protection for Zone 0 and the categories must match the zones. This criterion is met by the indication in the EU examination certificate for the field display that the device can be used within an Ex i circuit with level of protection ia without this affecting its level of protection.

• The material groups match. All Ex i devices are approved for a hydrogen/air mix Ex atmosphere in accordance with the IIC marking. The same applies for temperature class T4, so that this demand has also been met.

• The five criteria listed in Figure 1 for comparing Ex i criteria have been met.

This precondition is met according to the Ex i parameters shown in Figure 2. In addition, we are dealing with a mixed Ex i circuit for which there is no need to use the 50 % rule. Only the process indicator has a concentrated inductance of 35.1 μH. Therefore, based on calculations, a cable length of 750 m is possible with a specific cable capacitance C_c of 140 nF/km and a cable inductance L_c of 1 mH/km with a C_o value of 107 nF, and a cable length of 1964.9 m is possible with an L_o value of 2 mH and a C_i value of 35.1 μH.

This interconnection is thus intrinsically safe with a cable length of up to 750 m.

The proof of intrinsic safety described above must be implemented separately for the interconnection of the field-side connections of the head-mounted transducer with the thermocouple and the resistance temperature detector (RTD).

Simple Electrical Equipment

Thermocouples and RTDs are considered “simple electrical equipment” according to EN 60079-11. This means that, as an alternative, temperature sensors without approval can also be used. In this case, however, the user must ensure that the thermocouples and RTDs meet the characteristics specified in EN 60079-11 regarding energy storage and construction specifications. Moreover, the user must determine the surface heating of the device via the P_o value in order to assign them a temperature class.

Additional Consideration of Functional Data

When choosing devices, the dimensioning of the functional data must be taken into consideration along with the proof of intrinsic safety. For example, the MACX MCR-EX-SL-RPSSI-I repeater power supply provides the FA MCR EX-HAT-TS-I-OLP temperature transmitter with a supply voltage of 16 V at 20 mA. The temperature transmitter in turn requires a feed voltage of at least 11 V DC. The slight voltage drop for the FA MCR EX-DS-I-I process indicator of <1 V (<1.9 V with Hart) proves beneficial, because the feed voltage for Ex i sources is smaller than for non-Ex i devices. Consequently, 3.1 V are still available for a line-related voltage drop and as a reserve.

Multiple Approvals for Maximum Flexibility

Innovative Ex i isolator ranges such as the 12.4 mm narrow MACX MCR Ex SL… 8 stand out, thanks to their ideally coordinated I_o, U_o, and P_o values, making them compatible with a large number of Ex i field devices. While developing the product range, importance was also placed on the highest possible C_o values, given that this is an essential parameter for determining the maximum viable cable length (Figure 3).

The Ex i isolators of the MACX MCR Ex series are approved according to the current Atex directive 2014/34/EU and the corresponding harmonised standards for explosion protection with Ex II(1)G [Ex ia Ga]IIC and Ex II(1)D [Ex ia Da]IIIC marking for Ex i circuits up to Ex zone 0 (gas) and Ex zone 20 (dust). Other international approvals such as IECEx, EAC, and UL are prerequisites for their use in applications across the globe.

Functional Safety up to SIL 2 or SIL 3

If an MCR signal within the Ex system structure is part of the protection level, the system operator must, in addition to the proof of intrinsic safety, carry out a qualification of signal transmission with regard to availability and quality. This is why the MACX MCR-… Ex i series of isolators and the FA MCR… process indicators and field devices have been developed for use in safety-oriented circuits in accordance with EN 61508. Independent testing sites have certified them for use in applications up to SIL 2 or SIL 3 (Figure 4).

Summary: Whether a painting system or a refinery, plastics processing or the chemical industry — thanks to their features, the MACX isolators and Field Analog devices are proven cost-effective solutions when it comes to explosion protection and system safety.

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* The author is Product Manager Analog Ex at Phoenix Contact Electronics, Bad Pyrmont/Germany.

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