The PSU Switching Unit 9012-8 ADV has eight inputs and eight outputs, handles a total current of 90 amps, and up to 25 amps per channel. These three figures determine the entire planning — and they are regularly considered in the wrong order.
The three limits and their hierarchy
With a digital switching module, there are three upper limits that must be observed simultaneously:
- Channel current. Up to 25 A per output. The largest individual consumer on a channel must remain below this — including its inrush current.
- Total current. 90 A continuously across all channels. Eight channels at 25 A theoretically yield 200 A; this scenario is not intended and does not need to be.
- Number of channels. Eight outputs. In practice, this is the limit you encounter first.
The correct order for planning is precisely the opposite of intuition: first the number of channels, then the total current, and finally the channel current. This is because the number of circuits is fixed at the beginning, while the currents can still be distributed.
Concurrency instead of addition
The 90 A total current is no cause for concern, as long as you calculate realistically. The decisive factor is not the sum of all connected consumers, but the sum of those that run simultaneously.
An example from an expansion: Four dimmed lighting circuits at 10 A each, a reading lamp at 10 A, a cool box at 15 A, a switched inverter release circuit, and a continuous positive feed at 5 A. On paper, this adds up to more than 90 A — in reality, the lighting circuits rarely run simultaneously at full brightness, and the cool box cycles.
Therefore, the planning parameter is the realistic concurrency case, not the theoretical maximum. If you still want to be safe, plan for the worst realistic case: the evening when all the lights are on, the cooling is running, and the pump starts.
Assigning consumers to channels
Two principles have proven effective.
Group by function, not by location. Everything that is switched on and off together belongs to one channel. Everything that needs to be switched independently requires its own channel. The most common planning mistake is to combine two circuits that later need to be operated separately — and the realization comes after installation.
Set the special cases first. Outputs with special requirements — dimmable, motor control via H-bridge, continuous positive without shutdown — are planned first. The simple on-off circuits can then be fitted anywhere.
NoteThe inputs are permanently assigned to the outputs: input 1 belongs to output 1 and so on. When planning the buttons, it is worth considering this assignment from the beginning, instead of trying to find it later.
An assignment to write down
The assignment belongs in a table before the first cable is laid. Four columns are sufficient: channel, consumer, current limit, function.
Its usefulness only becomes apparent later. A digital switching module has no labeled fuses that would indicate what goes where — the assignment exists in the configuration and in the mind of the person who made it. A printed table in the technical cabinet is therefore not bureaucracy, but a substitute for the labeling that used to be on the fuse holder.
Plan for reserve
Eight channels sound like a lot and are quickly occupied during expansion. A channel that remains free is therefore not a waste, but the only reserve that can be used without modification.
If the system expands beyond eight circuits, multiple units can be operated in series or parallel. This is the cleaner way — cleaner, at least, than putting two consumers on one channel that really belong separately.
Check before installation
- Each channel has exactly one purpose and this purpose is noted.
- The largest consumer per channel, including inrush current, is below 25 A.
- The realistic concurrency case remains below 90 A.
- Dimmable circuits are planned as such — not every consumer tolerates dimming.
- At least one channel is free or expansion is planned.
Typical planning errors
- All currents added up. Leads to the conclusion that the module is too small — although the real concurrency case is far below.
- Inrush current forgotten. Pumps and compressors draw multiple times their continuous current at startup.
- Two circuits on one channel. Saves an output but costs separate operability.
- No assignment table. Without it, the assignment can be reconstructed after two years, but only with difficulty.
- All eight channels occupied. The first additional consumer then forces a replanning.
