Process transmitters in chemical plants, tank farms, refineries and other hazardous areas must fulfil more than the actual measuring task. In addition to measuring range, accuracy and media compatibility, the zone, equipment category, type of protection, gas or dust group, temperature class and ambient temperature must also match the measuring point.
With an intrinsically safe 4–20 mA transmitter, it is not sufficient simply to order an ATEX-approved field device. The transmitter, power supply isolator or Ex barrier, cable, PLC input, HART communication and connected test equipment together form an intrinsically safe circuit. Its safety must be demonstrated using the relevant certificates and electrical limit values.
Errors occur particularly frequently because nameplates have not been evaluated completely, unsuitable cable glands have been used, the ambient temperature has not been considered or a barrier transmits the 4–20 mA current but does not allow reliable HART communication.
Table of Contents
- ATEX, IECEx and operational explosion protection
- Determining the zone and required protection level
- Reading Ex markings correctly
- What does intrinsic safety mean?
- The complete intrinsically safe circuit
- Comparing Ui, Ii, Pi, Ci and Li correctly
- Ex barrier or galvanically isolated power supply?
- Designing the 4–20 mA current loop
- HART communication in hazardous areas
- Temperature class and ambient temperature
- Considering gas and dust groups
- Cables, glands and blanking plugs
- Earthing, shielding and equipotential bonding
- Process connection and wetted materials
- Which documentation is required?
- Commissioning and periodic inspection
- Loop check and HART diagnostics
- Typical planning and installation errors
- Practical example on a solvent tank
- Selecting the correct process transmitter
- Which products are suitable?
- Conclusion
- Frequently asked questions
ATEX, IECEx and operational explosion protection
ATEX and IECEx pursue similar protection objectives but fulfil different legal and international functions.
| Term | Meaning |
|---|---|
| ATEX 2014/34/EU | European product directive for equipment and protective systems intended for use in potentially explosive atmospheres |
| IECEx | International certification system based on the IEC 60079 series of standards |
| Zone classification | Assessment by the operator of how frequently and for how long an explosive atmosphere may occur |
| Explosion-protection document | Documentation of the risk assessment, zone classification and defined protective measures |
An ATEX approval for the transmitter replaces neither the zone classification nor the explosion-protection document. The operator must first determine which explosive atmosphere may occur and which equipment protection level is required.
IECEx and ATEX must not be treated as generally equivalent either. The legal requirements applicable at the installation location and the certificates actually available are decisive for the specific plant.
Determining the zone and required protection level
Zones 0, 1 and 2 are used for gases, vapours and mists. Zones 20, 21 and 22 apply to combustible dusts.
| Zone | Occurrence of the explosive atmosphere | Typical required protection level |
|---|---|---|
| Zone 0 | Continuously, for long periods or frequently | Category 1G or EPL Ga |
| Zone 1 | Occasionally during normal operation | At least Category 2G or EPL Gb |
| Zone 2 | Normally not present during normal operation or present only briefly | At least Category 3G or EPL Gc |
| Zone 20 | Combustible dust atmosphere continuously, for long periods or frequently | Category 1D or EPL Da |
| Zone 21 | Combustible dust atmosphere occasionally during normal operation | At least Category 2D or EPL Db |
| Zone 22 | Combustible dust atmosphere normally not present or present only briefly | At least Category 3D or EPL Dc |
Equipment with a higher protection level may generally also be suitable for a less demanding zone, provided that all other characteristics are appropriate. A transmitter approved for Zone 1, however, must not automatically be used in Zone 0.
The complete marking is decisive. The general description “ATEX version” is not sufficient.
Reading Ex markings correctly
A possible marking for an intrinsically safe gas device is, for example:
II 1G Ex ia IIC T4 Ga
| Component | Meaning |
|---|---|
| II | Equipment group for areas other than mines susceptible to firedamp |
| 1G | ATEX equipment category 1 for gas atmospheres |
| Ex | Explosion-protected design |
| ia | Intrinsic-safety type of protection with a very high protection level |
| IIC | Gas group with particularly stringent requirements |
| T4 | Maximum permissible surface temperature of 135 °C under the certified conditions |
| Ga | Equipment Protection Level suitable for Zone 0 |
The ambient-temperature range, certificate number and special information may also be stated.
Marking with “X”
An “X” at the end of the certificate number indicates special conditions for safe use. These may relate, for example, to:
- a restricted ambient-temperature range,
- protection against electrostatic charging,
- special cable glands,
- mechanical protection,
- process or housing earthing,
- permissible material combinations.
The device may be installed only after these conditions have been evaluated and implemented.
Marking with “U”
A “U” normally identifies an Ex component that has not been assessed as a complete device suitable for independent use. Such a certificate must not be treated as a complete equipment approval.
What does intrinsic safety mean?
With the intrinsic-safety type of protection, the electrical energy in the circuit is limited so that no ignitable sparks or impermissible heating can occur under the specified fault conditions.
Typical protection levels are:
| Type of protection | Typical use |
|---|---|
| Ex ia | Depending on the complete approval, up to Zone 0 |
| Ex ib | Depending on the complete approval, up to Zone 1 |
| Ex ic | Depending on the complete approval, up to Zone 2 |
Intrinsic safety does not mean that the transmitter may be connected arbitrarily to a normal 24 V power supply. Suitable associated apparatus, such as an intrinsically safe power supply isolator or Ex barrier, is required between the safe area and hazardous area.
The complete intrinsically safe circuit
The intrinsic-safety assessment covers the complete interconnection:
- process transmitter,
- power supply isolator or Zener barrier,
- connecting cable,
- terminals and plug connections,
- display or isolation modules where applicable,
- connected test and communication equipment.
An individual intrinsically safe field device does not guarantee an intrinsically safe circuit. Likewise, an approved barrier is not sufficient if its output values do not match the transmitter’s input values.
HART modems, handheld terminals or current-loop calibrators connected subsequently also become part of the interconnection being assessed. To connect them in the hazardous area, they must have the appropriate approval and be included in the documentation.
Comparing Ui, Ii, Pi, Ci and Li correctly
For the interconnection, the safety-related limit values of the associated apparatus and field device are compared.
| Barrier or power supply isolator | Field device | Basic condition |
|---|---|---|
| Uo: maximum output voltage | Ui: maximum permissible input voltage | Uo ≤ Ui |
| Io: maximum output current | Ii: maximum permissible input current | Io ≤ Ii |
| Po: maximum output power | Pi: maximum permissible input power | Po ≤ Pi |
| Co: permissible external capacitance | Ci plus cable capacitance | Co ≥ Ci + Ccable |
| Lo: permissible external inductance | Li plus cable inductance | Lo ≥ Li + Lcable |
This comparison forms the basic assessment but does not replace the information in the respective certificate. Additional restrictions may apply if relevant capacitance and inductance occur simultaneously or if special output characteristics are involved.
The cable capacitance and cable inductance are determined from the manufacturer’s data and the actual cable length. A subsequent extension of the cable can therefore invalidate the original assessment.
Ex barrier or galvanically isolated power supply?
Zener barriers or galvanically isolated power supply isolators are frequently used to limit the available energy.
| Design | Typical characteristic | Important planning consideration |
|---|---|---|
| Zener barrier | Simple energy limitation using Zener diodes, resistors and fuses | Usually requires a particularly reliable intrinsically safe earth connection |
| Galvanically isolated power supply | Galvanic isolation between the field side and control side | Check supply voltage, output power, HART transparency and channel isolation |
Galvanically isolated power supplies often simplify earthing and potential problems. However, they must be suitable for the intended transmitter, the Ex assessment and the communication type.
An existing power supply isolator must not be reused merely because it already supplies another 4–20 mA transmitter. Its safety-related output parameters and available voltage range must be checked again.
Designing the 4–20 mA current loop
In addition to an intrinsically safe energy supply, the process transmitter requires sufficient operating voltage. The following must be considered:
- minimum operating voltage of the transmitter,
- voltage drop across the power supply isolator,
- resistance of the PLC analogue input,
- HART communication resistance,
- cable resistance,
- additional displays or isolation modules.
In simplified form, the maximum available loop resistance can be calculated as follows:
Rmax = (Usupply − Utransmitter − additional voltage drops) / 0.02 A
For intrinsically safe circuits, the actual characteristic available at the output of the power supply isolator must be used for this calculation. The nominal 24 V control-cabinet supply is not automatically the voltage available at the field device.
A transmitter may appear to operate correctly at a low current but enter an undervoltage condition at 20 mA or at an alarm current above 20 mA. The complete loop should therefore also be tested at the upper end of the signal range.
HART communication in hazardous areas
With HART, a digital frequency signal is superimposed on the analogue 4–20 mA signal. The average value of the analogue measuring signal remains unchanged.
For reliable communication, the following components must be HART-compatible:
- process transmitter,
- power supply isolator or Ex barrier,
- PLC input or HART multiplexer,
- communication resistor,
- cable and shielding,
- HART modem or handheld terminal.
A loop resistance of approximately 250 Ω is used in many applications. This may already be integrated into the PLC input, power supply isolator or test instrument. An additional resistor must not be added without checking because it reduces the available voltage range.
A power supply isolator may transmit the DC current correctly while still attenuating HART communication. Explicit HART compatibility must therefore be stated in the data sheet.
The connection point is also relevant. A USB HART modem without Ex approval must not simply be connected to the field circuit inside the hazardous area. Communication is frequently performed on the safe side of the power supply isolator or at designated test connections.
Temperature class and ambient temperature
For gas atmospheres, the temperature class specifies the maximum permissible surface temperature of the device under the certified conditions.
| Temperature class | Maximum surface temperature |
|---|---|
| T1 | 450 °C |
| T2 | 300 °C |
| T3 | 200 °C |
| T4 | 135 °C |
| T5 | 100 °C |
| T6 | 85 °C |
A lower numerical temperature-class designation means that a higher surface temperature is permitted. T6 therefore represents more stringent requirements than T4.
The achievable temperature class may depend on the ambient-temperature range, process pressure, process medium and process temperature. A transmitter may, for example, meet T4 at a moderate ambient temperature but be approved only for a different classification at a higher ambient temperature.
The tables and special conditions in the certificate or Ex operating instructions are decisive.
Considering gas and dust groups
Gas atmospheres are divided into groups IIA, IIB and IIC. Within this series, IIC represents the most stringent requirements and includes particularly easily ignitable gases.
Equipment approved for IIC may generally also be suitable for IIB or IIA, provided that the remaining marking is appropriate. Equipment approved for IIA is not automatically approved for IIB or IIC.
Dust atmospheres use groups IIIA, IIIB and IIIC. In addition to the dust group, the permissible surface temperature, possible dust layers and the enclosure’s degree of protection must be considered.
A gas approval must not automatically be used for a dust area. The marking must explicitly cover the relevant atmosphere.
Cables, glands and blanking plugs
The cable gland forms part of the explosion-protection concept. Among other requirements, it must match:
- the enclosure’s type of protection,
- the gas or dust application,
- thread type and thread size,
- cable diameter,
- temperature range,
- mechanical tensile loading,
- the enclosure’s degree of protection.
A metric thread must not be confused with an NPT thread. Adapters may be used only if they are permitted for the specific Ex application.
Unused enclosure openings must be sealed with suitable certified blanking plugs. A standard industrial plastic plug does not automatically maintain the Ex approval of the enclosure.
After installation, the seal, clamping and strain relief must be checked. A cable gland provides reliable protection only within its specified cable-diameter range.After installation, the seal, clamping and strain relief must be checkedp>
Earthing, shielding and equipotential bonding
Metal transmitter housings, process connections and pipelines must be included in the equipotential-bonding system in accordance with the plant concept.
The cable shield is connected in accordance with the plant’s EMC and earthing concept. Arbitrary earthing at both ends may cause equalising currents, while unsuitable single-ended shielding may impair HART communication or interference immunity.
With Zener barriers, the intrinsically safe earth connection has a particularly important safety function. It must not be treated as equivalent to an arbitrary functional earth.
Blue cables are not intrinsically safe merely because of their colour. The colour marking supports identification but replaces neither separation, documentation nor the calculated assessment.
Process connection and wetted materials
The Ex approval does not confirm whether the transmitter is suitable for the process medium. The following must additionally be checked:
- pressure reference and measuring range,
- maximum process pressure and overload pressure,
- diaphragm and connection material,
- sealing material,
- corrosion and hydrogen permeation,
- process temperature,
- cleaning and flushing media,
- flush process connection or diaphragm seal,
- dead spaces and hygienic requirements.
For hot media, a cooling element, impulse line or diaphragm-seal concept may be required. The influence of the process temperature on the transmitter housing and the permissible Ex temperature class must be considered.
The diaphragm-seal design, filling fluid and diaphragm area additionally influence the measuring range, temperature error and response time.
Which documentation is required?
A traceably planned measuring point should include at least the following documents:
- zone plan and explosion-protection document,
- measuring-point data sheet with measuring range and process data,
- complete transmitter ordering code,
- EU declaration of conformity,
- ATEX or IECEx certificate,
- Ex operating instructions and special conditions marked with “X”,
- nameplate and marking data,
- data sheet for the power supply isolator or barrier,
- calculated intrinsic-safety assessment,
- cable type, length, capacitance and inductance,
- circuit and wiring diagram,
- information on cable glands and blanking plugs,
- earthing and equipotential-bonding concept,
- commissioning and inspection report,
- calibration and maintenance records.
After changes to the barrier, cable, transmitter or connected device, it must be checked whether the existing Ex assessment remains valid.
SIL and Ex documentation fulfil different purposes. SIL suitability does not replace Ex approval, and ATEX approval does not automatically confirm a particular functional-safety integrity level.
Commissioning and periodic inspection
Before initial commissioning, at least the following points should be checked:
- The nameplate corresponds to the planning documentation and zone.
- The certificate’s special conditions have been implemented.
- The process connection, seal and materials are suitable for the medium.
- The barrier and transmitter satisfy the intrinsic-safety assessment.
- The cable type and cable length correspond to the calculation.
- The cable glands and blanking plugs are installed correctly.
- Earthing, shielding and equipotential bonding correspond to the planning.
- The 4–20 mA scaling and HART parameters are correct.
- Alarm current, alarm limits and PLC diagnostics are tested.
- The actual installation is documented.
In addition to the measuring function, periodic inspections should also assess the condition of Ex-relevant components. These include corrosion, loose glands, damaged cables, impermissible enclosure openings and undocumented changes.
Loop check and HART diagnostics
A suitable current-loop calibrator can be used to test the electrical measuring chain independently of the process pressure.
Typical test points include:
- 4 mA as the lower measuring-range value,
- 8 mA for 25 % of the span,
- 12 mA for 50 % of the span,
- 16 mA for 75 % of the span,
- 20 mA as the upper measuring-range value,
- configured alarm currents where applicable.
The Druck UPS4E current-loop calibrator can measure and generate 0 to 24 mA, provide loop power and support HART testing with an integrated 250 Ω resistor.
For work inside a hazardous area, the appropriately approved instrument version must be used. The permissible connection type, Ex marking and safety parameters of the complete circuit must also be considered.
A loop check tests the wiring, analogue input, scaling and alarm functions. It does not replace pressure calibration of the transmitter. An appropriate pressure reference and pressure source are additionally required for this purpose.
Typical planning and installation errors
Only “ATEX” is specified in the order
The zone, gas group, temperature class and type of protection have not been clearly defined. The supplied version may therefore be unsuitable for the measuring point.
The field device has Ex approval, but the barrier is unsuitable
Uo, Io or Po exceed the permissible transmitter input values.
The cable data are missing
The cable capacitance and inductance are not included in the assessment.
HART does not work despite a correct 4–20 mA signal
The power supply isolator, PLC input or loop resistance is not HART-compatible, or the available impedance is unsuitable.
A non-approved HART modem is connected in the hazardous area
The additional device changes the documented intrinsically safe circuit.
The ambient-temperature range is overlooked
The Ex marking is valid only up to a temperature below that which actually occurs at the installation location.
An incorrect cable gland is used
The thread, cable diameter or type of protection does not match the enclosure approval.
Unused openings are sealed incorrectly
A standard blanking plug does not satisfy the requirements of the Ex enclosure.
SIL and ATEX are treated as equivalent
Functional safety and explosion protection are not assessed separately.
The transmitter is replaced, but the Ex assessment is not updated
Even if the measuring range and output signal are identical, Ui, Ii, Pi, Ci or Li may differ.
Practical example: Pressure measurement on a solvent tank
The gas-space pressure of a storage tank containing a flammable solvent is to be measured. The measuring point is located in Zone 1. The signal is transmitted to the control system as 4–20 mA with HART.
The planning data are:
- Zone 1, gas atmosphere,
- required gas group IIB,
- temperature class at least T4,
- ambient temperature from −20 to +55 °C,
- measuring range from 0 to 400 mbar gauge,
- medium containing solvent vapour,
- 4–20 mA with HART,
- cable length 180 m.
A process transmitter is selected with suitable Ex ib or Ex ia approval for at least EPL Gb, an appropriate gas group and an ambient-temperature range approved up to +55 °C.
The output parameters of the galvanically isolated power supply are compared with the transmitter’s input parameters. Cable capacitance and cable inductance are considered for the complete 180 m cable length.
The following are also checked:
- available voltage at 20 mA,
- HART transparency of the power supply isolator,
- existing loop resistance,
- material compatibility of the diaphragm,
- correct Ex cable gland,
- equipotential bonding of the metal housing.
During commissioning, the current signal works, but HART communication is initially unstable. Testing shows that the PLC input has a very low input resistance and that no additional HART resistor is present.
After a communication resistor designed for the measuring chain is added on the safe side, communication operates reliably. The additional load is then included again in the current-loop voltage calculation and documented.
Selecting the correct process transmitter
At least the following information is required for a reliable selection:
- measured variable and pressure reference,
- minimum, normal and maximum process pressure,
- process medium and material requirements,
- process and ambient temperature,
- gas, vapour, mist or dust atmosphere,
- zone and required EPL or equipment category,
- gas or dust group,
- temperature class or maximum surface temperature,
- required type of protection,
- 4–20 mA, HART or fieldbus,
- existing power supply isolator or barrier,
- supply voltage and PLC input,
- cable type and cable length,
- process connection and sealing material,
- housing material and degree of protection,
- diaphragm seal or impulse line,
- SIL and diagnostic requirements,
- calibration and documentation requirements.
A meaningful enquiry could read as follows:
Gauge-pressure transmitter, 0 to 6 bar, ethanol medium, process temperature −10 to +80 °C, ambient temperature −20 to +50 °C, Zone 1, IIB T4, 4–20 mA with HART, intrinsically safe two-wire circuit, G ½ process connection and stainless-steel wetted parts.
Which products are suitable?
Process transmitters and differential-pressure transmitters
The process transmitters / differential-pressure transmitters category includes instruments for gauge pressure, absolute pressure, differential pressure, level and flow measurement.
Depending on the model, 4–20 mA, HART, fieldbus communication, different measuring cells, diaphragm seals, SIL versions and ATEX or IECEx approvals are available.
Siemens SITRANS P300
The SITRANS P300 process transmitter is a digital pressure transmitter for gauge and absolute pressure.
Its key features include:
- measuring ranges from low pressures up to 400 bar,
- 4–20 mA with HART,
- alternatively PROFIBUS PA or FOUNDATION Fieldbus,
- stainless-steel housing,
- threaded and flush process connections,
- local configuration using three buttons,
- diagnostic and simulation functions,
- intrinsically safe versions for Zone 0 or Zone 1, depending on the ordered approval.
The P300 is particularly suitable for hygienic and demanding process applications. The Ex approval, output signal, process connection and materials must be defined in the complete ordering code.
Additional process transmitters for Ex applications
The ICS category additionally includes, for example, the WIKA IPT-20/IPT-21, CPT-20/CPT-21 and Siemens SITRANS P320/P420.
Depending on their configuration, these instruments offer different measuring cells, housings, HART functions, SIL suitability, diaphragm-seal options and Ex approvals. Selection depends on the measuring range, medium, accuracy, diagnostic scope and explosion-protection concept.
Druck UPS4E current-loop calibrator
The Druck UPS4E is suitable for servicing, commissioning and troubleshooting 4–20 mA current loops.
Among other features, it offers:
- measurement and generation of 0 to 24 mA,
- internal 24 V loop power supply,
- step, ramp and valve-test functions,
- integrated 250 Ω resistor to support HART applications,
- data logger,
- ATEX/IECEx version for work in hazardous areas.
Before connection to an intrinsically safe measuring point, the specific Ex marking, approved operating mode and effects on the complete circuit must be checked.
Conclusion: The complete measuring point, not only the transmitter, must comply with the Ex requirements
A process transmitter may be used only if its complete Ex marking matches the zone, substance group, temperature class and ambient temperature of the measuring point.
In intrinsically safe 4–20 mA circuits, the transmitter, power supply isolator, cable and connected devices form a common safety system. Uo, Io and Po of the associated apparatus must remain within the permissible Ui, Ii and Pi values of the field device. Cable capacitance and cable inductance must also be included.
HART additionally requires suitable loop impedance and HART-transparent barriers or power supply isolators. A functioning analogue signal does not confirm reliable digital communication.
Cable glands, blanking plugs, earthing, process materials and temperature conditions are just as relevant as the measuring instrument itself. Special conditions in a certificate marked with “X” must be implemented without exception.
A safe and auditable measuring point is therefore achieved only through coordinated equipment selection, a documented Ex assessment, correct installation, controlled commissioning and maintenance.
Frequently asked questions about process transmitters in hazardous areas
Is a general ATEX approval sufficient for Zone 1?
No. The equipment category or EPL, type of protection, gas or dust group, temperature class and ambient temperature must also match the measuring point.
Can an Ex ia transmitter be connected directly to a normal PLC input?
A suitable Ex barrier or galvanically isolated power supply is normally required. The complete interconnection must be assessed using the safety-related parameters.
What do Ui, Ii and Pi mean?
They specify the maximum permissible input voltage, input current and input power of the intrinsically safe field device. The barrier’s output values must not exceed these limits.
Why must cable capacitance and cable inductance be considered?
Cables can store electrical energy. Their values are therefore included in the intrinsic-safety assessment together with the field device’s internal capacitance and inductance.
Does HART work through every Ex barrier?
No. The barrier or power supply isolator must be suitable for HART and transmit the digital signal adequately. Suitable loop impedance is also required.
May a standard HART modem be connected in a hazardous area?
Only if the device and the specific connection method are approved for this purpose. Communication devices without Ex approval are normally connected on the safe side of the circuit.
What does the “X” after an Ex certificate number mean?
The “X” indicates special conditions for safe use. These must be identified in the certificate and operating instructions and implemented before installation.
Is a SIL transmitter automatically suitable for hazardous areas?
No. SIL relates to functional safety. A separate Ex approval suitable for the measuring point is required for explosion protection.
Which information does ICS Schneider require for selection?
The required information includes the measuring range, pressure reference, medium, process and ambient temperature, zone, gas or dust group, temperature class, required type of protection, output signal, existing barrier, cable length, process connection, materials and requirements for SIL, calibration and documentation.