Health & Safety

Human Error – We All Make Mistakes

In my early twenties I lodged for 6 months with a married couple who I knew from work. They had just bought a terraced house and had a room going spare. The house needed a bit of work doing on it and Dave was a DIY enthusiast.

One wet Sunday afternoon he asked me if I would give him a hand with some electrical wiring work. Now I am not an electrician. Nor was he. But I wanted to show willing so jumped straight in to help.

As we got towards the end of the work we came to the critical task of making final connections into the fuse box (the consumer unit). This was a rather old-fashioned thing, typical of the 1980’s, with fuse carriers instead of the miniature circuit breakers found in a modern unit.

This is what it looked like:

Image of the inside of a fuse box

Sort Of…

Well, when I say that this is what it looked like what I mean is: it looked a lot like this, but not exactly like this. Not exactly. Because the unit in the photo, as you can clearly see, is switched ON. We didn’t want to work live (and run the risk of receiving a fatal electric shock) so we deliberately flicked the switch to the down position to turn the unit OFF.

We made our connections and then put the cover back on the fuse box in order to reinstate power and check that the light fittings worked.

When we put the cover back on this is what we saw:

Switch flicked on within the fuse box

Just to be clear. We did not put the cover on and then flick the switch down. We already had the switch down!


Look at the photos again. In the top photo it appears that if the switch is up the power is ON and if the switch is down the power is OFF. But when you put the cover on you can clearly see that it is, in fact, completely the other way round. Up is OFF and down is ON.

We had worked live.

Now here is exactly what happened next. I turned to Dave; he turned to me. The blood drained from our faces as we realised the mistake that we had made.

And we never said a thing about it.

And to this day we have still not spoken about it.

Lessons Learnt

Now there are several lessons to draw from this story.

Firstly, we were clearly incompetent and had no business doing what we were doing. We lacked the necessary training, knowledge and/or experience to properly understand the correct working methods and specifically we failed to understand exactly how the switch worked and how the condition of the switch (off or on) is reported by a plastic tab (the bit of plastic with ‘off’ and ‘on’ printed on it) that shows in a cutout window in the plastic case (the gap above or below the switch).

The life lesson has not been lost on me. At the time I felt embarrassed at my stupidity and put it out of my mind (i.e. I tried to forget about it). In retrospect, it was a valuable lesson that has taught me to be more aware of the edge of the envelope of personal competence.

Secondly, having a degree will not save your life. It is not a protective force shield. I had a degree. He was a doctor. Feel free to make comments about ‘common sense’ (whatever that is).

Thirdly, neither of us wanted to work live or in a dangerous manner. We thought we were being safe. We deliberately turned the switch ON thinking, at the time, that we were switching it OFF. We made a mistake.

Human Error

Now mistakes are just one of the several categories of human error that are modelled by the HSE in their guidance note HSG48: “Reducing error and influencing behaviour” (see here).

Human Error Model diagram (based on HSG48)
Human Error Model (Based on HSG48)

Other types of human error are included in this model. These are the skill-based errors (made up of slips of action and lapses of memory). When you accidentally press the wrong button on a calculator, or grind the gears in your car, you are committing a slip of action. When you leave your key in the front door (again) as you get home from work laden with bags you are committing a lapse of memory.

Mistakes are classified, in the model, as rule-based and knowledge-based. The difference between the two is largely to do with how hard you are thinking when you make the mistaken decision. If you are following a simple rule and that leads you to the wrong decision that is a rule based mistake. When you incorrectly spell the word veil as VIEL because you were taught the spelling rule “i before e except after c”, that is a rule-based mistake. If you are thinking quite hard about what to do and are basing your decision on background knowledge and underlying principles, then it is a knowledge based mistake.

Either way, you make the mistake deliberately. But, importantly, at the time you think that your action is the right thing to do. Not the wrong thing.


My young-adult idiocy may be of only passing interest to you but mistakes can clearly have very significant consequences. We got away with it by sheer luck. On another day, in a parallel universe, it might have been the last thing that I and/or Dave ever did. That’s why the HSE dedicate a part of their website to the topic along with other associated human factors (see here).

If you want an example of a mistake with horrendous consequences then look no further than the Kegworth air crash of 1989 (see here). The disaster happened because of a mechanical problem in one of the engines of the twin-engine aircraft shortly after take-off. By itself, the failure of a single engine should not have been a disaster because the plane was capable of flying on only one engine. But the aircraft crashed on the M1 motorway just a few yards short of East Midlands airport. It was only later, during the investigation, that it was realised that the pilot and co-pilot had deliberately shut down the perfectly functional engine and flown the faulty engine to final catastrophic destruction. Clearly the flight crew thought, at the time, that they were shutting down the faulty engine. In fact they were shutting down the good one.

There is a rather old, but nonetheless excellent, documentary about the disaster that catalogues the various failings that lead to the final human error (see here).

And though Kegworth happened 30 years ago you might draw some interesting parallels between it and the recent troubles that have beset the Boeing 737 Max (see here).

Dr Jim Phelpstead BSc, PhD, CMIOSH

RRC Consultant Tutor                                   

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