Just to make sure I'm being clear, I was hoping to achieve the following:

• Place 1st fuse - label F1

• Place 2nd fuse - label F2

• Place 1st breaker - label Q1

• place 3rd fuse - label F3

• place 2nd breaker - label Q2
  etc.

What actually happens is:

• Place 1st fuse - label F1

• Place 2nd fuse - label F2

• Place 1st breaker - label Q3

• place 3rd fuse - label F4

• place 2nd breaker - label Q5
  etc.

see example file here:
https://git.tuxfamily.org/qet/qet.git/t … labels.xml

~ % tree qet_git/elements/10_electric/11_singlepole/200_fuses_protective_gears
qet_git/elements/10_electric/11_singlepole/200_fuses_protective_gears
├── 10_fuses
│   ├── fusible.elmt
│   ├── qet_directory
│   └── sec_fus1.elmt
├── 11_circuit_breakers
│   ├── disjonct-m.elmt
│   ├── disjoncteur1.elmt
│   ├── jistic_1p.elmt
│   └── qet_directory
├── 12_magneto_thermal_circuit_breakers
│   ├── disjoncteur_magneto-thermique_1.elmt
│   └── qet_directory
├── 20_disconnecting_switches
│   ├── qet_directory
│   ├── sectionneur1-2.elmt
│   └── sectionneur1.elmt
├── 30_thermal_relays
│   ├── qet_directory
│   └── relais_therm1.elmt
├── 50_residual_current_circuit_breaker
│   ├── ddr1.elmt
│   ├── disjoncteur_differentiel_magnetothermique.elmt
│   ├── int_diff-1p.elmt
│   ├── int_diff1.elmt
│   ├── qet_directory
│   ├── rcbo.elmt
│   └── rccb.elmt
└── qet_directory
 
7 directories, 22 files

<category name="200_fuses_protective_gears">
      <category name="10_fuses">
        <prefix>F</prefix>
      </category>
      <category name="11_circuit_breakers">
        <prefix>F</prefix>
      </category>
      <category name="12_magneto_thermal_circuit_breakers">
        <prefix>Q</prefix>
      </category>
      <category name="20_disconnecting_switches">
        <prefix>Q</prefix>
      </category>
      <category name="30_thermal_relays">
        <prefix>F</prefix>
      </category>
      <category name="50_residual_current_circuit_breaker">
        <prefix>Q</prefix>
      </category>

My example that didn't work (where "circuit_breakers" and "fuses" are folders at root level of elements folder):

<labels>
  <category name="circuit_breakers">
    <prefix>Q</prefix>
  </category>
  <category name="fuses">
    <prefix>F</prefix>
  </category>
  <category name="miscellaneous"></category>
</labels>

and my example that did work (where "circuit_breakers" and "fuses" are sub-folders in a folder "circuit_protection"):

<labels>
  <category name="circuit_protection">
      <category name="circuit_breakers">
        <prefix>Q</prefix>
      </category>
      <category name="fuses">
        <prefix>F</prefix>
      </category>
  </category>
</labels>

What I am now struggling to understand is whether there is a way to keep the sequential numbering for different circuit element types separate (e.g. all fuses as F1, F2, F3, all breakers as Q1, Q2, Q3, rather than ending up with F1, Q2, F3, Q4 etc.). I realise you can create different numbering schemes for each type of circuit element, but this requires you to remember to change the numbering scheme each time you add an element to the folio - which I'm not very good at!

Thanks for any suggestions.

Scorpio, now, your example is working on my configuration - and my example also - BUT only if the user collection directory is on its default path.
If I try to change it to a custom location "Settings > Configure > Collections > User collection directory" then the content of the collection is correctly loaded but the qet_labels.xml file seems to be simply ignored.

put qet_labels.xml in the root of your elements folder.

Same result with your example : No prefix shown in QET.

Maybe qet_labels.xml should be placed elsewhere ?
I've tried to put it sequentially in :

• .\coils

• .\coils_prefix_FX

• .\coils_prefix_XYA

With no change in behavior.

── coils
│   ├── coils_prefix_FX
│   │   ├── bobine3.elmt
│   │   └── qet_directory
│   ├── coils_prefix_XYA
│   │   ├── qet_directory
│   │   └── relbistable.elmt
│   └── qet_directory

<labels>
<category name="coils">
      <category name="coils_prefix_FX">
      <prefix>FX</prefix>
      </category>
     <category name="coils_prefix_XYA">
     <prefix>YYA</prefix>
     </category> 
</category> 
</labels>

I'm desperately trying to define custom prefix for my own elements library but I failed until now.
I think I've understood - at least I hope to - what was explained in the previous posts and video.
I've made a lost of tests before following back to a really simple example. But it still doesn't work

• I'm using QElectroTech 0.7 on Windows

• I've kept the default path for my user library. It contains only one directory named "connecteurs"

• Inside of this directory, I've placed some .elmt files and a file named "custom_labels.xml" as follows :

<?xml version="1.0" encoding="UTF-8"?>
<labels>
  <category name="connecteurs">
    <prefix>XYZ</prefix>
  </category>
</labels>

But still, when starting QET and placing one of my custom elements on a new folio, when typing "%prefix" in the label formula, nothing is shown.
Am I still missing something ?