The Trap of "Intuitive" Tagging
Many engineers are tempted to design their own tagging systems. I’ll make it intuitive, they think. I won’t make the same mistakes that I’ve seen on all the other tagging systems. I’ll make it flexible, yet consistent across everything across this system.
They will most likely assign M for motors, PU for pump, FN for fans, R for relay, CB for circuit breaker, CP for compressor etc. Numbers are then assigned: 100 for cooling, 200 for heating, etc. Devices associated with these get letters, such as PT for a thermistor or SN for solenoid.
Solenoid 1 on the first pump in the cooling circuit becomes PU100SN1. The circuit breaker for the pump is CB100. Then they add the compressor CP100. They can’t assign CB100 to the compressor, so the neat alignment where the number would correspond to the CB is broken.
In desperation they might assign the compressor to be CP101, so that they can make CB100 go to PU100, and then CB101 go to CP101. But this means that the pumps start at 100, while the compressors start at 101. Then they want to assign a letter to a control panel. ”How about CP?” No, that’s already assigned as a compressor. Add in a few last minute design changes, and the system is no longer intuitive.
The tag system is broken, and it will only get messier over time. You eventually need an extra document that tells you what the exceptions are.
The Suburban Analogy
Consider the engineer who tries to label a room in two identical buildings as 100Kitchen01 and 200Kitchen01. This seems logical until you realize the kitchen itself hasn't changed—only its location has. If you decide to move that kitchen design to a third building, you have to rename every single drawing and tag associated with it.
Now imagine two identical suburbs: Northwood and Southwood. In both suburbs, there is a street called "Maple Avenue," and on both streets, there is a "House Number 12." Under a manual system, you might try to make a unique name to distinguish it. Under IEC/ISO 81346, the kitchen is simply -K1. Its full identity is derived from its position in the tree structure.
The Three Aspects
IEC/ISO 81346 uses three prefixes to keep the logic consistent without ever needing to "break" the numbering alignment:
- = Function: What the object does.
- + Location: Where the object is physically located.
- - Product: What the object is (the specific component).
Because the structure is a tree, you can have a "Circuit Breaker 1" for the Pump and a "Circuit Breaker 1" for the Compressor. They are differentiated by their parent branch. You can copy-paste a design for a standardized pump station 100 times, and every internal tag remains identical. You only change the "House Number" at the top of the branch.
The advantages are enormous. Copying systems means only changing one upstream value. Once the relevant branches have been assigned, people can independently assign tags within those branches. Duplicate tags never occur, and you never run out of tags. You can always create a branch (system) for a device if you need to break a system down further. For example, if your motor has a thermistor, brake, cooling motor, etc, you can just break it down further -M1 would be the main motor, -M1-M1, might be the cooling motor fan, -M1-R1 might be the brake etc.