Degraded mode of operation for ac800mHI
Normally the degraded mode for a QUAD CONFIGURATION IS 4-2-0.In AC800M HI we normally mention the degraded mode of operation to be 4-3-2-0.How is this theoretically explained when during fault of 2 PM or 2SM modules the system does a fail safe operation.
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First of all, HI safety system has operation modes and not degradation mode. This is due to the fact that HI safety system is certified for SIL-3 in single configuration and redundancy ie using 4 processors is meant for high availability and does not affect SIL.
4-3-2-0 mode is applicable for cases wherein one PM and one SM fails rather than 2 PM or 2 SM. This is better than what other Quad safety system offers.