There are two ways to lose plant documentation. One is not maintaining it. The other is converting it to a new labelling system in the middle of operation.
What the standard governs – and what it does not
IEC 81346 defines the structure: = for functional assignment, + for mounting
location, − for the product, : for the terminal. Part 2 adds the letter codes:
Q for switching energy, F for protective functions, B for sensors, K for
signal processing.
That is the standardised part, and it is quickly explained. What remains open is everything that follows: how is numbering handled? Does each cabinet start at 1, or does the count run across the whole plant? Does a contactor belong functionally to the conveyor line or to the cabinet it sits in?
Exactly these questions decide whether maintenance finds anything during a fault. And exactly these the standard does not answer.
The most expensive mistake: converting halfway
In plants grown over decades you regularly meet two systems side by side – the old one from DIN 40719 and the new one to IEC 81346, introduced during a rebuild and then never finished. The effect is worse than either system on its own: you have to know both, and additionally know where the boundary runs.
So our recommendation for existing plants is almost always to continue the system in place, even where it does not match the current standard. A consistently applied in-house order is worth more than a compliant one that covers half the plant.
Conversion belongs to a new plant or to a migration that touches the entire documentation anyway – not to a side project.
What makes a system hold
It is written down, not only in people’s heads. It is simple enough for a fitter to apply without training. And it survives the plant being extended – anyone numbering straight through the whole plant has a problem at the first addition.
On a site of several square kilometres we established a labelling structure that locates any component to the metre. The effort for that was not in the standard, but in the decisions taken beforehand.