# Example Using SCT to Analyze Standby with Delay

The purpose of this example is to illustrate how to model standby with delay using State Change Triggers (SCT).

Problem Statement

Three devices A, B and C are in parallel. 2 out of 3 are required to perform the task. Initially A and B are working and C is in standby status. If A or B fails, C will replace the failed one with some delay. After the failed device is restored, it will become a standby device. For example, if A fails, with 10 hours delay C will become active and replace A. After A is restored, it will become a standby device for B and C.

BlockSim Solution

The BlockSim modeling of this system is shown in the figure below.

In order to model the delay, three extra blocks AA, BB and CC are added to the diagram. Each of them fails every 10 hours (normal distribution with mean 10 hours and standard deviation 0.0000001 hours). They operate even if the system is down and repair durations for them are 0 (immediate repair). Block AA(BB/CC) belongs to maintenance group AA(BB/CC). They all have state change triggers. The initial state for each is OFF and the state upon repair is "Always OFF." If Block A (B/C) goes down, then activate Block AA (BB/CC).

Block A fails every 100 hours. It belongs to maintenance group A. It has state change triggers. The initial state is ON; the state upon repair is "Default OFF unless SCT overridden." If Block BB or Block CC goes down, Block A is activated.

Block B fails every 200 hours. It belongs to maintenance group B. It has state change triggers. The initial state is ON; the state upon repair is "Default OFF unless SCT overridden." If Block AA or Block CC goes down, Block B is activated.

Block C fails every 300 hours. It belongs to maintenance group C. It has state change triggers. The initial state is OFF; the state upon repair is "Default OFF unless SCT overridden." If Block BB or Block AA goes down, Block C is activated.

The repair duration for Blocks A, B and C is 30 hours.

Block Up/Down Plot

The system event log is shown in the figure below and is as follows:

1. At 100 hours, Block A fails and activates Block AA.
2. At 110 hours, Block AA fails and activates Block C.
3. At 230 hours, Block B fails and activates Block BB.
4. At 240 hours, Block BB fails and activates Block A.
5. At 340 hours, Block A fails and activates Block AA.
6. At 350 hours, Block AA fails and activates Block B.
7. ...