Safety System Redundancy Options - GE Mark VIe System Manual

2.2.1 Modes of Operation
A demand mode is a mode of operation in which the safety function is called upon only on demand. IEC 61508-4 clause
3.5.12 defines two demand modes of operation:
• Low demand mode
• High demand or continuous mode
Low demand describes the mode in which safety function demand occurs no greater than once per year. In high demand
mode, the frequency of demand is greater than once per year. Continuous mode is regarded as very high demand and is
associated with the safety function operating to keep the EUC within its normal safe state.
The mode of operation is relevant when determining the target failure measure of a safety function. Low demand mode relates
to the PFD whereas high demand or continuous demand mode relates to measuring the probability of failure per hour
avg
(PFH) (there are approximately 10
referred to as the Safety Integrity Level (SIL).
SIL PFD
1
2
3
The SIL applies to all elements in the safety loop (sensors, logic solver, and final element) and their architecture. The loop
must be considered in its entirety.

2.3 Safety System Redundancy Options

Redundancy is a key feature in safety control system design that can enable on-line repair without interrupting the process.
The unique Ethernet back-bone of the Mark VIeS Safety system enables several options for redundancy to achieve required
SIL. This provides the safety control engineer unparalleled flexibility to meet the safety SIL requirements for each application
in the most cost effective manner.
70 GEH-6721_Vol_I_BP
4 hours in a year). IEC 61508 defines a scale of four distinct levels of risk reduction
Low Demand Mode
avg
≥ 10 -2 to < 10 -1
≥ 10 -3 to < 10 -2
≥ 10 -4 to < 10 -3
PFH High Demand Mode
≥ 10 -6
≥ 10 -7
≥ 10 -8
GEH-6721_Vol_I Mark VIe and Mark VIeS Control Systems Volume I
Public Information
to < 10 -5 > 10 to ≤ 100
to < 10 -6 > 100 to ≤ 1,000
to < 10 -7 > 1,000 to ≤ 10,000
RRF