E2S Knowledge Base - Intrinsically Safe Interface Solutions

Intrinsically Safe Interface Solutions for E2S Sounders and Beacons:

This is a guide to using the E2S Intrinsically safe sounders and beacons, it is based on the Atex certification but in general the guidelines apply to other intrinsic safety certifications including North American FM and IECEx.
It is not intended as a definitive guide to intrinsic safety and or a complete review of the subject.

Please refer to the Intrinsic Safety Certificate and installation instructions for individual equipment with reference to the relevant local and national standards.

Intrinsic Safety is a protection technique based upon the restriction of electrical energy to a level below that which can cause ignition by either sparking of heating effects. It is a system concept; unlike most other hazardous area protection methods, such as flameproof Ex d, all items in an intrinsically safe circuit must be taken into consideration.
An Intrinsically safe circuit is typically defined as:

"A circuit in which any spark or thermal effect produced in normal operation and specified fault conditions is not capable of causing ignition"

So the preservation of intrinsic safety depends on the strict control of energy in the circuit and therefore the whole circuit needs to be considered, not just the field apparatus in isolation.

Associated apparatus is the name given to the piece of equipment which is responsible for controlling the level of voltage and current which will be permitted to enter the intrinsically safe circuit within the hazardous area.
It preserves the integrity of the intrinsically safe field device (it does not make it safe) and takes the form of an interface between the safe area and the hazardous area and is usually a zener barrier or isolation interface colloquially both types are generally referred to in context as barriers or isolators.
The associated apparatus is located in the safe area near to where the intrinsically safe connections can be passed to the hazardous area. This of course may be inside another piece certified equipment such as an Ex d enclosure, which in effect creates a local safe area inside.

Additional certification (Ex N) for the associated apparatus may be utilised e.g. many zener barriers are approved to Ex N which allows the interface to be installed in a Zone 2 hazardous areas with appropriate enclosure. In this case in the context of the intrinsically safe circuit it is assumed the interface is in the safe area.

Cables have also to be taken in to consideration. Interconnecting cables include reactance which will have an effect on the storage of energy in the circuit. The calculation of cable requirements is out with the scope of this document. However, as intrinsic safety is low power, and is usually installed with small diameter cabling cable runs are rarely an issue. For more information on this please contact E2S
Unlike other protection techniques cable requirements for intrinsic safety are not onerous. Armoured cable is not necessary and the minimum requirements are for double insulated cable only. However local site standards may over-ride the minimum requirements.

IP Rating for an intrinsically safe installation is on IP20. However in practice a higher IP rating is usually required to meet environmental conditions or site standards.

Next....

►Introduction
►Interface selection
►Zener barriers
►Isolation interfaces

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