Ερώτηση
Let's assume that we have 3 meters of a power cable that has the X capacitance per meter. Placing the cable to the floor does create different capacitance value as opposed to when suspended in the air?
Απάντηση
Yes — placing a cable on (or very close to) the floor **does change its effective capacitance** compared to suspending it in the air. The difference may be small or large depending on several factors.
Here’s the breakdown:
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**1. A cable already has a built-in capacitance per meter**
This is the **intrinsic capacitance** between the conductor(s) and the return path, determined by:
* insulation geometry
* conductor spacing
* dielectric material inside the cable
This part **does NOT change** with cable placement.
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**2. But cable position *does* affect its capacitance to ground**
Any conductor has parasitic capacitance to ground.
When the cable is:
### **➤ Suspended in air**
* The surrounding dielectric is mostly *air* (εᵣ ≈ 1.0)
* No close conductive surface
* Capacitance to ground is **lowest**
### **➤ Lying on a floor**
If the floor is:
* **Concrete, tile, or earth** → fairly conductive or at least high-εᵣ
* **Wood or insulation** → less conductive but still higher εᵣ than air
Then:
* dielectric constant around the cable increases (εᵣ > 1)
* the cable is closer to a quasi-ground plane
* capacitance to ground **increases**
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## ✔ **3. How big is the change?**
It depends on:
* how close the cable is to the ground (touching vs several cm away)
* the dielectric constant of the floor
* whether the cable shield/braid is grounded
For most power cables (thick insulation, relatively low external field), the effect is:
* **Small but measurable**
* Typically a few **pF to a few tens of pF** over 3 meters
For high-impedance or high-frequency circuits, this can matter.
For low-frequency AC mains (50/60 Hz), the difference is usually negligible *unless* you're analyzing noise, leakage currents, or EMC behavior.
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**Summary**
| Cable placement | Capacitance to ground | Explanation |
| ------------------------ | --------------------- | -------------------------------------------------------- |
| **Suspended in air** | Lowest | Surrounded by low-ε air, far from ground plane |
| **On or close to floor** | Higher | Floor acts as higher-ε dielectric + partial ground plane |
So **yes**, the capacitance *does* change with placement.
If you want, I can compute approximate values using typical cable dimensions and dielectric constants.
