For an electrical product to be ‘Intrinsically Safe’ it must be incapable of igniting an explosive atmosphere by either spark or heat.
This is achieved by ensuring only low currents and voltages enter the hazardous area, and therefore the whole circuit must be considered, not just the device in isolation.
An ‘Intrinsically Safe Circuit’ therefore must restrict the electrical and thermal energy such that ignition of a hazardous atmosphere (explosive gas or dust) cannot occur.
Understanding Hazardous Areas
The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR), defines a hazardous area as:
Any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers.
EN60079-10-1 explains that Hazardous areas can be classified into zones based on the amount of time explosive gases, vapours and mists can be found in the atmosphere.
An area in which an explosive gas atmosphere is present continuously or for long periods.
An area in which an explosive gas atmosphere is likely to occur in normal operation.
An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.
There is also zone classification for combustible dust:
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is 
likely to occur in normal operation occasionally.
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Intrinsically Safe Device Categories
When selecting an intrinsically safe device, installers must ensure that the product has the appropriate certification for the zone required.
Equipment categories are different to zones; please refer to how the zones and categories correspond below:
Please note:
Category 1 equipment is also suitable for zones 1, 2, 20 and 22.
Category 2 equipment is also suitable for zones 2 and 22.
For example, Hochiki’s SOC-E-IS and DCD-1E-IS are
certified for use in Category 1. This means they are
suitable for zones 0, 1, 2, 20, 21 and 22.
The differences between a standard device and an Intrinsically Safe device
Aesthetically, the shape and size of an intrinsically safe smoke detector, heat detector or multi sensor can be very similar to standard devices, however, you will notice additional warning information and zone information printed on an intrinsically safe device.
The low power levels also mean that there is no chance of receiving an electric shock due to excess thermal energy and arcing.
And remember, when installing an intrinsically safe device, it is crucial that an intrinsically safe mounting base is used.
Barriers and Modules
We have established that standard devices cannot be used in hazardous areas as there is a risk of ignition; therefore, low current intrinsically safe devices must be used instead.
Of course, buildings do not tend to be completely hazardous; they will have safe areas that require standard devices and hazardous areas that require intrinsically safe devices.
Therefore, an intrinsically safe compatible module is designed to interface between the two. Some modules also provide line monitoring for open or short circuits on the wiring connected to both the safe and hazardous areas.
It is important to note that a module does not reduce the current in any way, a module is simply used to interface between standard devices and intrinsically safe devices.