Knowledge / Ex protection
Every intrinsically safe circuit needs associated apparatus at the boundary of the ATEX zone. This article compares the three classic designs, shows alternatives such as intrinsically safe PLC output cards and valve control modules — and walks through the entity verification per EN 60079-14 with a concrete example.
An intrinsically safe field device alone does not make the circuit intrinsically safe. Only the associated apparatus on the safe side — recognisable by the marking in square brackets, e.g. “[Ex ia Ga] IIC” — limits voltage, current and power before the line enters the zone. The formal proof of intrinsic safety is carried out per EN 60079-14, Clause 16.
Fundamentals: intrinsic safety (Ex ia) in ATEX areasPassive network of Zener diodes, resistor and fuse: the Zener diodes limit the voltage to earth, the resistor limits the current, the fuse protects the diodes. There is no galvanic isolation — the protection concept only works with a low-resistance intrinsically safe earth.
Active device with galvanic isolation via transformer or optocoupler. Hazardous-area and safe-area sides are electrically separated; the 4–20 mA signal is transferred 1:1 to the safe side. An earth reference is not required for the protective function.
Combines supply and isolation in one device: it feeds the loop-powered 2-wire transmitter and provides galvanic isolation at the same time. A separate supply component in the cabinet is no longer needed.
Zener barrier, isolating amplifier and IS power supply cover the 4–20 mA measuring loop. In addition, further associated apparatus exists that can be the better choice depending on the application:
Many control-system and remote-I/O manufacturers offer input and output cards with integrated Ex i isolation (marking in square brackets, e.g. [Ex ia Ga] IIC). The isolation stage then sits directly in the I/O system — separate barriers in the cabinet are no longer needed. A space-saving option for 4–20 mA measuring loops such as the RW65_EX_II, provided the control system offers suitable cards. Entity verification remains identical: the card's output parameters against the field device's input parameters.
For switching intrinsically safe loads — solenoid valves, audible sounders, LED beacons — valve control modules are available: associated apparatus with an intrinsically safe, power-limited switching output. For driving the DKL01 signal beacon they are our primary recommendation: the module provides the limited output circuit, the beacon is connected directly, each of the three colors via its own channel.
View DKL01The proof of intrinsic safety compares the output parameters of the isolation stage with the input parameters of the field device (EN 60079-14, Clause 16; cable parameters per Annex J). Using the RW65_EX_II as an example:
RW65_EX_II: Ui ≤ 30 V DC, Ii ≤ 100 mA, Pi ≤ 620 mW. Ci and Li are negligible.
The following must hold: Uo ≤ Ui, Io ≤ Ii, Po ≤ Pi — i.e. Uo ≤ 30 V, Io ≤ 100 mA, Po ≤ 620 mW. The values are stated in the data sheet or the certificate of the associated apparatus.
Ci plus cable capacitance must stay below Co, Li plus cable inductance below Lo. Since Ci and Li of the RW65_EX_II are negligible, the entire Co/Lo budget of the isolation stage is available for the cable — a practical advantage on long runs.
The RW65_EX_II requires 11–30 V DC at its terminals. For Zener barriers, subtract the drop across the end-to-end resistance and the cable; for IS power supplies, observe the load specification in the data sheet.
The entity verification must be documented and forms part of the plant documentation. If several associated apparatus are combined in one circuit, EN 60079-14, Annex H applies — the result then reaches level of protection “ib” at best and is no longer suitable for Zone 0.
The RW65_EX_II can optionally be fitted with up to two intrinsically safe NAMUR contacts. They are evaluated via intrinsically safe switching amplifiers on the safe side (interface per EN 60947-5-6) and are independent of the 4–20 mA loop — typically, one contact triggers a local alarm while the analogue signal goes to the control system.
Existing plant with a monitored IS earth in place; cost in focus
New plant without an intrinsically safe earthing concept
2-wire transmitter, a supply is needed anyway
Driving an intrinsically safe signal beacon (DKL01)
Supply voltage, cable length, driving the signal beacon — for applications using our devices we support the selection of the right isolation stage. We do not offer general ATEX plant consultancy.
Note: The content of this page is provided for general guidance and replaces neither the operator's risk assessment nor case-by-case verification by qualified personnel. Responsibility for sizing, equipment selection, installation and operation lies solely with the operator. Despite careful preparation, we accept no liability for accuracy or completeness; the currently valid edition of the applicable regulations and standards prevails.