Redundant components that share an environment, design, or supplier fail together, voiding the redundancy.
Two generators from the same batch, in the same flooded basement, are not redundant. Fukushima Daiichi in 2011 lost backup power this way — the redundancy was real and the independence was not.
In practice
- A data centre with two power feeds from the same substation has one power feed with extra cabling.
- An airline rosters two pilots who both ate from the same galley, which is why cockpit meals are deliberately different.
- Subject
- Risk & Reliability
- Essay
- How accidents actually happen