How to choose a safety barrier for Hazardous Areas

Products and components, used in Hazardous Areas (areas subjected to presence of potentially explosive atmosphere), must respect severe standards and have adequate security systems. The main European regulation in this field is the ATEX Directive 2014/34/EU, that has recently replaced the previous ATEX Directive 94/9/EC.

To avoid any explosion risk the market offers, among other products, intrinsic safety solutions: devices and circuits that are able to prevent explosions because they avoid hot spots, sparks or electrical arcs that could cause the ignition of a hazardous atmospheric mixture, both in normal use conditions and in case of failure.

When it comes to intrinsic safety, 3 are the basic elements to consider:

• intrinsically safe devices used in Hazardous Areas;
• intrinsically safe associated apparatus (safety interfaces);
• interconnection cables.

There are 2 types of intrinsically safe interfaces, or safety barriers:

1. Zener Barriers;
2. Galvanically isolated barriers (or Isolators).

What are the differences between them? And how can you choose the most suitable safety barrier for a given industrial application? Let’s see the main characteristics and parameters to take into account.

1. Zener barriers

Zener barriers are the most traditional interface type. They basically work diverting the potentially dangerous fault energy to earth. They consist in a very simple network of components including a fuse, a resistor and some zener diodes.

The main positive aspects of Zener barriers are the ease of installation, the small size and the relatively low cost, but there are also some negative aspects to consider:

• they need a good ground connection (with a resistance of less than 1 ohm) dedicated, which should be maintained over time;
• they are not reusable: if the fuse blows (for wrong or improper connections), the entire barrier must be replaced;
• isolation in the field must be guaranteed over time;
• they have a poor common mode rejection.

By using transformers, optoisolators, galvanically isolated barriers (or galvanic isolators) isolate excess energy, rather than divert it. The main advantages of this type of safety barrier are:

• the possibility of connecting devices to ground in a Hazardous Area (no dedicated ground connection is required);
• accuracy;
• simplicity of installation;
• elimination of ground loops;
• the possibility to use sensors connected to the structural ground of the system;
• full voltage available to field devices;
• high rejection to common mode disorders.
• Lower installation cost.

However, compared to Zener barriers, isolators have higher unit costs.