New Assessment Method for IS Active Power Supplies

The aim of this project was to develop an alternative assessment and testing method to be used when certifying active intrinsically safe (I.S.) power supplies for use in potentially explosive locations.  The work was to provide testing Authorities and institutions with a procedure for the certification of active power supplies.

The project proposed the development of a range of equivalent circuits that would be tested to provide the same response to stimuli as an active power supply.  The equivalent circuits could then be evaluated against intrinsic safety standards.  It would then be possible to safely certify/approve an active power supply for use in a hazardous location. 

The project was divided into two focus areas.  The first area was the development of the equivalent models and the second, the investigation of actual active power supplies.  The components used in the equivalent circuits were resistors, capacitors and inductors as individual components or different combinations of the components.  The equivalent models were theoretically analysed, simulated with a simulation package and physically constructed.  The same waveforms for each equivalent circuit, using the three different techniques, were obtained.  A collection of 33 active power supplies were investigated.  The transient characteristics of the active power supplies were measured and analysed when a short circuit was placed across the output.  This was compared to the results when a short circuit was applied to the equivalent circuits.

It was not possible to determine a direct relationship between the response of the active power supplies and the equivalent circuits to a short circuit.  Based on the procedures and results of this project, it is not feasible to use an equivalent circuit to represent an active power supply in the assessment of the intrinsic safety of the system.

During the testing the inexact nature of the Spark Test Apparatus was highlighted.  When testing a particular power supply it would pass a specific test for intrinsic safety on one day, but fail the same test on a different day.  These results suggest that the success or failure of equipment to comply with intrinsic safety testing is not dependant on the equipment alone.