Which switch do you buy?
Switch A breaks about every 250 hours and takes 2 hours to fix. Switch B breaks about every 100 hours and takes 1 hour to fix. Which is better?
Two numbers help you choose. One says how long a part runs between breakdowns. The other says how long a repair takes.
Key words
| Word | What it means | Example |
|---|---|---|
| Mean time between failures | The average running time between one failure and the next. | MTBF |
| Mean time to repair | The average time it takes to repair a failure and get working again. | MTTR |
| Average | The total divided by how many items there are. | |
| Failure | When something stops working as it should. | |
| Reliability | How rarely a part fails. | |
| Downtime | The time a system is not working. | |
| Network | Computers and devices joined together so they can share data. |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can explain MTBF and MTTR, work them out from simple numbers and use them to compare parts.
Starter: what you already know
Have a go
Starter: what you already know
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Cycle 1: MTBF
MTBF is the average running time between failures. A switch ran for 1000 hours and failed 4 times. Divide 1000 by 4 to get an MTBF of 250 hours.
A high MTBF is good. The part fails less often. It is a measure of reliability.
Cycle 2: MTTR
MTTR is the average time to repair. The four repairs took 2, 3, 1 and 2 hours. Add them: 8 hours. Divide by 4 repairs to get an MTTR of 2 hours.
A low MTTR is good. Repairs are quick and downtime is short. Spare parts, clear instructions and trained staff all lower it.
Worked example: two switches
| Switch A | Switch B | |
|---|---|---|
| Running hours | 1000 | 1000 |
| Failures | 4 | 10 |
| MTBF (1000 divided by failures) | 250 hours | 100 hours |
| Total repair hours | 8 | 10 |
| MTTR (repair hours divided by failures) | 2 hours | 1 hour |
| Total downtime | 8 hours | 10 hours |
The cells students fill in are left blank in this preview.
Check: read the numbers
Have a go
Check: read the numbers
4 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.