I'm currently automating a machine for my bachelor thesis and I'm running into some questions about properly fusing my electrical installation. I'll attach a photo of my PLC configuration so you can see what I'm working with.
I'm using a Siemens S7-1500 (CPU 1512C-1 PN) with several additional I/O expansion modules. My plan is to split the 24VDC supply into two separate circuits:
- **PSU 1** → feeds the PLC CPU + all I/O modules
- **PSU 2** → feeds all field devices (sensors and outputs)
Both power supplies are Siemens SITOP units fed from 230VAC.
**Field devices:**
- 83 digital inputs (all PNP inductive sensors and photoelectric sensors, 24VDC)
- ~30 digital outputs (all either solenoid valves or relays, 24VDC coils)
**My questions:**
**1. Fusing the 230VAC side (before the SITOP)**
**1. 230VAC side (before the SITOP)**
My issue is figuring out the right amperage. The SITOP for the sensors is rated 2.5A output at 24VDC, so around 60W — that's less than 0.3A on the 230VAC side, so a 1A breaker should be fine there. But for the PLC supply I'm less sure — the CPU itself draws 1A max but the I/O modules can draw up to 8A per group according to the specs. So should I size the breaker based on the total possible load of all modules combined, or just the SITOP output rating?
**2. Fusing the 24VDC output side**
I know I should group my 83 inputs into separate fused groups rather than putting everything on one fuse. My plan is to group by station (I have 5 processing stations), so roughly 15-20 sensors per group with a 1A single-pole breaker each.
But I'm not sure:
- Should these be 1-pole or 2-pole on the DC side?
- Is 1A the right value or should I go higher?
- For the outputs (solenoid valves, ~100mA coil each), should I fuse per group of 8 outputs or per module?
**3. Fusing the PLC itself**
The CPU spec sheet says max current consumption is 1A without load, 19A with full I/O load. The inrush current is listed as 1.9A. So I'm thinking a 4A slow-blow fuse on the 24VDC line to the CPU — does that make sense? And should I fuse each I/O module separately or group them?
**4. General approach**
Is it correct that on a 24VDC system you only fuse the + line (single pole) and leave the 0V common unfused? Or are there cases where you'd fuse both poles on DC?
Any advice is appreciated — I want to do this properly and not just slap a 10A fuse on everything. Thanks in advance!