At 07:15 on a cold morning at a Voltaic North site, an apprentice called Sem counted the padlocks on a locked switch and got five. There were six people on the work permit. He said so, quietly, to nobody in particular. I stopped the job.

Some background. Before anyone works on high-voltage equipment, it is switched off and locked. Each person puts their own padlock on the switch and keeps the key. The equipment cannot be turned back on until every person has removed their lock. It is simple, and it has saved more lives than any clever technology.

It took 50 minutes to find the answer

The sixth person was a contractor who had gone to fetch a tool and had not yet put his lock on. He was never in danger that morning. A few people thought I had wasted an hour of a tight schedule over nothing, and one of them told me so.

I understood the frustration. But a rule that is skipped when it is inconvenient is not a rule. If Sem had seen us shrug, he would have learned that counting locks is a formality. The next time, he would not have spoken. And the next time might be the one that mattered.

Safety culture is what happens when the newest person says something awkward.

At the end of that shift I thanked Sem in front of the whole team. No site I have led has had a lost-time injury. I do not think I am careful enough to deserve that by myself. I think it comes from many people making small stops like his.

What I ask of every site team

• Anyone can stop any job, and nobody has to justify it first.

• The person who stops a job is thanked, even when it turns out to be nothing.

• Locks are counted aloud against the permit, every time.

• If the schedule and the rule disagree, the schedule moves.

The schedule recovered by Thursday. Schedules usually do. People do not, which is why the rule comes first. Sem is now a commissioning technician, and he counts locks louder than anyone I know.