Explosive Atmospheres – Part 28: Protection of Equipment and Transmission Systems Using Optical Radiation

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DIN EN 60079-28: Why Fibre Optics Require Special Attention in Hazardous Areas

A standard whose most recent edition dates back more than ten years is once again moving into the spotlight: DIN EN 60079-28 – Explosive atmospheres – Part 28: Protection of equipment and transmission systems using optical radiation.

The reason is obvious: digitalisation does not stop at hazardous areas. Modern communication and automation systems require ever-higher bandwidths and increasingly need to cover longer distances. While fibre-optic technology has long been established in conventional office and IT environments, it is now also being used more frequently in industrial applications—and therefore in potentially explosive atmospheres.

But isn’t a fibre-optic cable inherently safe?

After all, no electrical current flows through a copper conductor. And where no electrical spark can occur, there should be no ignition hazard—right?

Unfortunately, it is not quite that simple.

Fibre Optics Are Not Automatically “Ex-Safe”

Fibre-optic cables offer numerous advantages over copper cabling. Distances of several kilometres can be covered without difficulty, high transmission rates well into the multi-gigabit range are possible, and electromagnetic interference is virtually irrelevant.

These are decisive advantages, particularly in industrial plants.

However, different standards apply in hazardous areas. Here, it must always be ensured that an explosive atmosphere cannot be ignited by technical equipment.

With electrical conductors, the principle is easy to understand. A short circuit, a damaged conductor, a connector being unplugged carelessly, or another fault condition can create an ignition source. This is why suitable types of protection are used—for example, intrinsic safety.

The situation with optical radiation is less obvious.

A beam of light does not create a conventional electrical spark. However, the energy emitted by modern laser diodes or other optical sources can be absorbed by surfaces or particles. This can cause local heating. If an explosive gas, vapour, mist, or dust atmosphere is present at the same time, this energy may, under certain conditions, be sufficient to cause ignition.

This makes one thing clear:

With optical radiation, the issue is not the spark—but the transmitted optical energy.

And this is precisely where DIN EN 60079-28 comes into play.

Which Protection Concepts Are Defined by DIN EN 60079-28?

The standard describes different ignition-protection concepts for equipment and transmission systems using visible or infrared optical radiation.

Three concepts are particularly relevant:

op is – inherently safe optical radiation
op pr – protected optical radiation
op sh – optical systems with interlock

Which concept is applicable depends, among other things, on the type of optical source, the available power, the environment, and the required Equipment Protection Level (EPL).

1. “op is” – Inherently Safe Optical Radiation

With “op is”, the optical radiation is limited in such a way that, under normal operating conditions or under specified fault conditions, it does not have sufficient energy to ignite the relevant explosive atmosphere.

The basic principle is therefore similar to the concept of intrinsic safety for electrical circuits:

The available energy is limited so that no hazardous ignition energy is available, even under defined fault conditions.

Self-Limiting Optical Sources

One option is to use optical sources whose behaviour is self-limiting under fault conditions.

This may include laser diodes, LEDs, or other optical sources that overheat and fail if their output power rises beyond a permissible level. This behaviour must be demonstrated through appropriate testing.

Depending on the design, the relevant EPL requirements can therefore be met.

Optical Sources with Power Limitation

Another option is to limit the radiant power by means of a suitable driver circuit.

In this case, the fault analysis must be extended to include the power-limiting circuit. What matters is not only the optical source itself, but the interaction between the source and the limiting device.

A typical example is an LED whose operating current is restricted by the driver circuit to a value within the permitted specification.

This illustrates why the assessment of optical systems must not consider only the maximum output of the light source. The decisive factor is the optical power that can actually become available in a fault condition.

Limiting Optical Power

For relevant op is applications, the permissible optical power and irradiance are particularly important.

With suitable design, systems can be implemented for different equipment groups and EPLs. However, the specific classification must always be assessed based on the normative requirements and the technical design of the individual system.

2. “op pr” – Protected Optical Radiation

The protection concept “op pr” takes a different approach.

Here, the optical power is not primarily limited. Instead, it is assumed that the optical radiation remains enclosed within an optical fibre or another transmission medium.

The level of safety therefore depends largely on the reliability of this containment.

Under normal operating conditions, the optical fibre or fibre-optic cable prevents optical radiation from escaping into the explosive atmosphere.

Mechanical Protection Becomes Critical

This is where one of the key practical challenges lies.

For certain EPL requirements, an ordinary, unprotected fibre-optic cable is not sufficient. Additional mechanical protection may be required, for example:

Continuous armouring
Protective conduits
Cable trays
Cable ducts or equivalent protective measures

For applications with higher requirements, the word “continuous” is particularly important.

And this seemingly small word can have significant practical consequences.

The Conventional Fibre Reserve Becomes a Problem

In industrial plants, it is common practice to provide a reserve length or fibre loop in front of operator terminals, control cabinets, or other connection points.

This reserve is technically sensible: it facilitates later modifications, repairs, or the re-termination and splicing of connections.

In the context of “op pr”, however, it must be assessed whether this fibre loop continues to be adequately protected.

An unprotected fibre reserve may, under certain circumstances, interrupt the required protection concept.

This means that established installation practices may need to be reconsidered.

It is not only the cable itself that must be evaluated, but the entire cable route.

Further Installation Challenges

In addition to fibre loops, several other practical issues must be considered.

Armoured Cables

When armoured cables are used, the installation must be designed in such a way that no impermissible potential transfer can occur. Particular attention should be paid to transitions between different plant areas, buildings, and fire compartments.

Protective Conduits and Conduit Systems

With conduit systems, it must be ensured that they are reliably and continuously closed. Otherwise, the issue is not only the protection of the optical radiation. The propagation of an existing explosive atmosphere through the conduit system may also become a relevant concern.

Cable Trays and Cable Ducts

Cable trays and cable ducts must be checked for completeness and continuity.

A typical real-world problem is subsequent installation work. A cable is added later, the cover of the duct is opened—and then not correctly refitted. In a normal industrial environment, this may merely be considered an installation defect. In a hazardous area, however, it can compromise the intended protective effect.

The protective measure does not end with selecting the correct cable. It must be maintained throughout the entire installation.

3. “op sh” – Optical Systems with Interlock

Here too, it is assumed that the optical radiation is enclosed within an optical fibre or another transmission medium. In addition, however, an interlocking system with shutdown is used.

The system must detect when the protection provided by the enclosure fails and optical radiation could be released.

In this case, the unprotected radiant power must be reduced to a safe level within a defined period of time.

This means:

The system must not only detect that optical containment has failed—it must also react quickly enough.

Shutdown Time as a Decisive Factor

The permissible response time of the shutdown system depends on the specific application.

For certain applications, the shutdown times must, for example, be shorter than the ignition delay time of the explosive atmosphere present.

For other applications, delay times relevant to eye safety may play a role.

This makes “op sh” a protection concept in which not only the mechanical design of the fibre-optic system is safety-relevant, but also the function of the monitoring and shutdown system.

Depending on the EPL, different requirements apply to the interaction between optical containment, monitoring, and safe shutdown.

Why “op sh” Has Limited Appeal in Production Plants

From a technical perspective, “op sh” is an interesting protection concept. In productive facilities, however, it can have one decisive disadvantage:

A protective shutdown may also cause a production shutdown.

In chemical, pharmaceutical, or petrochemical plants in particular, unplanned shutdowns can have substantial economic consequences.

For this reason, the use of such a concept in a production environment must be carefully assessed.

Typical areas of application may include training and educational facilities or laboratory environments, where a shutdown is easier to accept and different boundary conditions apply.

What Does DIN EN 60079-28 Mean in Practice?

The growing use of fibre-optic technology means that optical transmission systems are becoming increasingly relevant in hazardous areas as well.

One important insight should remain at the centre of the discussion:

Fibre-optic cables are not automatically explosion-protected simply because they do not transmit electrical energy.

What matters is the optical energy that may be released in a fault condition.

DIN EN 60079-28 provides different protection concepts for this purpose:

Type of protection	Basic principle	Main protection approach
op is	Inherently safe optical radiation	Limitation of power and energy
op pr	Protected optical radiation	Reliable containment of the radiation
op sh	Optical radiation with interlock	Containment plus safe shutdown

For planners, installers, and operators, this means that selecting a suitable fibre-optic cable alone is not enough.

Instead, the entire transmission system must be considered—from the optical source and the cable to its mechanical protection, connectors, cable reserves, cable routes, and, where applicable, monitoring and shutdown equipment.

Digitalisation Meets Explosion Protection

The digitalisation of industrial processes will continue to advance. Higher bandwidths, greater transmission distances, and modern communication architectures make fibre optics attractive even where copper was previously used exclusively.

However, particularly in hazardous areas, technological progress must not lead to established explosion-protection principles being overlooked.

Optical transmission technology offers enormous benefits—but it is not a free pass to use arbitrary components.

The key question is therefore not:

“Is this a fibre-optic cable?”

But rather:

“Which protection concept reliably prevents ignition under the intended operating and fault conditions?”

This is precisely where DIN EN 60079-28 becomes relevant.

Anyone planning, installing, or operating optical transmission systems in hazardous areas should therefore look beyond the data sheet of the fibre-optic cable. What matters is the assessment of the entire system and its specific installation environment.

Because even when data is transmitted using light, the same fundamental rules still apply in hazardous areas: