Intrinsically Safe (IS) Active Power Supplies

Intrinsic safety is an explosion protection method applied to electrical apparatus. The basic concept behind intrinsic safety relies on energy limitation to ensure that an ignition of an explosion cannot occur by either spark ignition or thermal ignition.

Generally intrinsically safe (IS) power supplies are categorised for compliance as associated electrical apparatus. Because the active power supplies behave in a non-linear manner they cannot be assessed using the ignition curves published in the standards and the intrinsic safety must be determined using the spark test apparatus (STA).

To form a basis from which the project started, the terms of active, passive, linear and non-linear were defined in the context of IS power supplies. In addition the existing assessment and testing methods used to establish conformance to intrinsic safety standards were examined.

The project then set out to develop an assessment method for active power supplies. The assessment method utilises both the steady state and transient output characteristics to define the topology and component values of a simple circuit model. The simple circuit model contains only a few components, which are all linear. The steady state and transient output characteristics are determined from simple tests. The transient output characteristic is obtained using a relay contact and an oscilloscope. As the simple circuit model contains only a few linear components it can be assessed using existing methods and the ignition curves published in the standard.

The assessment method was applied to an active power supply and the result compared to results obtained from STA testing of the same active power supply. The comparison indicated a correlation between the developed assessment method and the results from using the STA.