Have you ever had a panic re-design? I did once and this is how it happened.
We’d been given a new turnkey TV station to design and install. The client briefed us and gave us a set of plans of the building. Everything looked nice and organised so I settled down to design the system. Pretty soon I was at the stage where I could design the rack layout of the machine room. So I got the floor plan and set out the racks in a nice arrangement on my CAD model. Checked with the client, got the OK and started populating the racks. After a couple of weeks we were ready and the installation team moved in.
At my desk finishing off the details, the phone rang… my lead installer: “I’ve just measured the machine room and it’s 2 metres short !!!”
And there where I thought everything was under control I found myself having to re-think my entire design. So
NEVER EVER TRUST ANYONE’S ARCHITECTURAL PLANS – ALWAYS MEASURE
We engineers tend to take it for granted that all wires should be labelled in an installation – but is it actually worth the effort?
Thinking about this one day I decided to try and do the math… Clearly it depends on how many cables we are talking about. But say you have a bunch of 10 cables going from A to B and they haven’t been labelled, what effort does it take to sort them out?
So for our thought-experiment let’s get two technicians one at one end, one at the other. Give them some test gear and a walkie-talkie each. Now they can start testing wires. To find the first wire among 10 could take up to 10 tests, to find the second wire may take up to 9 tests, to find the third wire 8 tests and so on. So the worst case would be:
Worst case = 10 + 9 + 8 … + 3 + 2 + 1 = 55 tests
This is very pessimistic. No-one is that unlucky, right? Let’s, say they are lucky and pick the right wire every time.
Best case = 1 + 1 + 1 … + 1 + 1 + 1 = 10 tests
Nice but quite unlikely. I’m not a statistician, but the most probable outcome is in middle.
Average case = 5.5 + 5 + 4.5 … + 2 + 1.5 + 1 = 32.5 tests
So how long is it going take our techies to sort out this mess? If each test takes them about 30 seconds. What with handling the cables, connecting the tester, communicating and putting labels on, we are being optimistic but let’s go with 30s.
Best case 5 minutes Average case 16.25 minutes Worst case 27.5 minutes
If it takes 30s per cable to put labels on before you pull them, then labeling before we pull the cables saves 11 minutes. So what’s the fuss about?
Well once the numbers of cables get a little bigger scary things happen. Look at this.
20 cables need on average 115 tests – nearly an hour’s work
40 cables need an average of 430 tests – that’s 3 1/2 hours !!!
Forty cables is a small system. Yet without labeling you can lose half a day for nothing. And that’s without fault-finding…
So there it is. It’s not a case of OCD, it really does pay to label your wires.