SIL2 Certification

The product for which the module / unit testing was done with TESSY, is an active explosion barrier system for gassy underground mines. This is a safety-system for use within a working mine, mainly consisting of a large reservoir of water. In case of a gas explosion underground, this system will detonate and produce a shield of water from the local reservoir. The water shall shield the men and material in the mine against the effects of the fire and the explosion. Obviously, such a system can only be ignited once, like an airbag. Therefore, it is very important that it is not used prematurely by a “false  alarm”. On the other hand, the system must not fail, in case it is needed.

Because of the safety-related functionality, the system had to be certified according to IEC 61508, Safety Integrity Level 2.

Because of the safety integrity level, module testing of the software is mandatory. Becker Mining Systems selected TESSY as the tool for module testing. The system under test is a multiprocessor system, consisting of three microcontrollers: two MSP430 devices from Texas Instruments and one M16C from Renesas. The software for the three microcontrollers was written in C and comprised of about 270 functions. From these, about 80 were selected to be tested by Becker Mining Systems and also about 80 to be tested by Hitex as a test service for Becker Mining Systems. (The rest was tested as part of the hardware tests).

To determine the necessary test cases for each function, the Classification Tree Method was used. This allows an easy review of the testing ideas. Often, test data could be specified in the classification tree. For a function, typically 15 test cases were necessary, but some functions needed up to about 100 test cases. For each function the branch coverage (also known as decision  coverage) was determined. When 100% coverage was not achieved, the reason for this was carefully investigated, and documented. For each function, comprehensive test documentation was  created.

TESSY turned out to be essential in automation of the module tests. Without the help of TESSY, testing surely couldn’t be finalized in time and probably some error would be gone undetected. A short time after the tests with TESSY were finalized, the active explosion barrier system for gassy underground mines got its certification according to IEC 61508 from the Dekra Exam GmbH. Becker Mining Systems would like to thank Hitex for meeting the tight schedule of this testing project, for which TESSY was prerequisite.