In intrinsically safe systems, associated equipment plays a key role in the safety of the system. Their positioning can be foreseen outside the hazardous area [1] or, alternatively, in explosion-proof enclosures that can provide them with adequate protection for installation in Zone 1.

In figure 1, we see a representation of how a system integrating an intrinsically safe system could be implemented where the associated equipment has been integrated into a flameproof enclosure according to the 'Ex db' protection method.

Figure 1: Example of an intrinsically safe electrical system where the associated equipment has been integrated into a flameproof enclosure

Certified associated equipment acts as an interface between control and command devices, located in a safe area, and simple electrical constructions or certified intrinsically safe constructions, located in a hazardous area.

Their function is to prevent dangerous energy levels from being transferred from the control panel to devices installed in the dangerous area.

Certified associated equipment has a special marking that gives us an indication of the protection level (EPL) of the construction (simple or certified) that can be connected to it.

• [ia Ga] means that the certified intrinsically safe construction or the connected simple construction can be installed in Zone 0.
• [ib Gb] means that the certified intrinsically safe construction or the connected simple construction can be installed in Zone 1.

Normally, the certified associated equipment does not have its own level of protection and must therefore be installed in a safe area.

However, there are also associated equipment that have their own level of protection achieved by flameproof enclosures 'Ex db' or by potting as foreseen by the 'Ex mb' protection mode. For example, certified associated equipment bearing a marking 'Ex db [ia Ga] IIC T4 Gb' is integrated into an 'Ex db' enclosure, which can be installed in Zone 1 with a gas of any group having an ignition temperature higher than 135°C.

The certified associated equipment, also called safety barriers, are divided into two large groups:

• Zener diode safety barriers
• Galvanic separation safety barriers.

Zener diode safety barriers are very common, simple in construction and cheap. They use a Zener diode to limit the output voltage and a fuse to limit the current. The same elements can be repeated to ensure, through the redundancy of the circuit elements, operation even in the presence of one or two faults.

The limit of the Zener barrier is the management of any fault currents that discharge into the ground, this aspect implies additional considerations that do not apply in the case of the galvanic separation safety barrier. In fact, galvanic separation barriers integrate a separation between the electrical circuit upstream of the barrier and the one downstream, on the field device side [3].

Intrinsically safe circuits consist of multiple components, among which certified associated equipment is a key aspect of protection.

It can be difficult for the layperson to understand the purpose and markings of these devices as it requires a thorough understanding of how the entire intrinsically safe system works.

Although the topic is complex, the spread of these devices makes it necessary for the personnel involved in their selection, installation and maintenance to study the concepts underlying their operation.

Notes and bibliographical references

[1] See Figure 1 in the article “The Ex-i protection method: general aspects of intrinsically safe electrical systems”

[2] 3.4 CEI EN 60079-0:2013-09

[3] The term field is used, in the context of intrinsically safe circuits, to indicate the installation area in a dangerous zone, think of the term fieldbus