Why Heat Pumps “Leak” Electrical Current
Part 1 — Simple Explanation
The short answer
Your heat pump doesn’t “leak” electricity because it’s broken. It leaks a tiny amount because of how modern electronics must work.
💡 Think of it like a flexible membrane
Imagine:
- Two tanks of water
- A rubber sheet between them
If you push water into one side:
- The rubber flexes
- Water moves on the other side
👉 No water crosses the rubber
👉 But movement still happens
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⚡ How this relates to your heat pump
Inside your heat pump are components called capacitors. They exist between:
- Live wire → Earth
- Neutral → Earth
When voltage is applied:
- An invisible electric “pressure” (electric field) builds
- As the voltage changes, that pressure changes
- This causes tiny amounts of current to move in the wires
👉 Even though electricity never “jumps across” the insulation
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🔁 Why it happens more when running
- When the heat pump is idle → slow electrical changes → small leakage (~4 mA)
- When it’s running → very fast switching electronics → more movement → higher leakage (~10 mA)
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🏠 Why this can trip your RCD
Every modern device in your home does the same thing:
- TVs
- phone chargers
- LED lights
- computers
Each adds a tiny amount of leakage.
Add them together:
- House background leakage
- + Heat pump leakage
👉 Total gets close to the safety limit (30 mA)
👉 Your RCD (safety switch) trips
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✅ Key takeaway
> Your heat pump isn’t faulty — it’s behaving exactly like modern electronic equipment should.
> The issue is usually lots of small leakages adding up, not one big problem.
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🔧 Part 2 — Technical Explanation (For Engineers & Installers)
1. The root cause: displacement current
Leakage in inverter-driven heat pumps is dominated by capacitive coupling to earth, not resistive insulation failure.
From Maxwell–Ampère law, current has two components:
Conduction current
Displacement current
The capacitive current is:
I = C (dV/dt)
This is the key mechanism.
2. Where the capacitances exist
In a split heat pump system, capacitance to earth arises from:
Intentional components
EMI suppression Y-capacitors (Line–Earth, Neutral–Earth)
Parasitic capacitances
Inverter switching node → heat sink (earthed)
Compressor windings → stator/frame
Interconnecting cable (core-to-earth capacitance)
PCB traces → chassis
3. Operating modes and leakage levels
Standby (~4 mA typical)
Mains still present at EMI filter
50 Hz sinusoidal voltage across Y-capacitors
I = ω C V
This produces steady, low-level leakage.
Running (~8–10 mA typical)
Dominated by high dV/dt switching:
PWM inverter (2–20 kHz switching frequency)
Fast voltage edges (hundreds of V/µs)
Result:
Significant displacement current through all parasitic capacitances
Increased common-mode current to earth
4. Common-mode current path
The inverter generates common-mode voltage relative to earth:
Output phases swing relative to chassis
Capacitively couple to earth
Current path:
Inverter → parasitic capacitances → earth → supply earth → back to source
This current does not return via neutral.
5. RCD operation mechanism
An RCD measures:
IΔ = IL − IN
Under ideal conditions:
IL = IN → no trip
With capacitive leakage:
Some current diverts to earth
IL ≠ IN
Detection principle
Residual current creates net flux in toroidal core
Changing flux induces voltage in sensing coil (Faraday’s Law)
Trip mechanism activates when threshold is exceeded
6. Why nuisance tripping occurs
Design guidance
Recommended: ≤ 30% of RCD rating as standing leakage
For a 30 mA RCD → ≈ 9 mA design limit
Real-world scenario
Background electronics: 10–20 mA
Heat pump running: 8–10 mA
Total: 18–30 mA
This sits within the realistic trip range of a 30 mA RCD.
7. Waveform considerations
Inverter systems produce:
Non-sinusoidal residual currents
High-frequency components
Pulsating DC components
Implications:
Type AC RCDs are unsuitable
Type A is the minimum in many cases
Type B may be required where DC components are present
8. Engineering interpretation
This behaviour is a direct consequence of:
Maxwell–Ampère law (displacement current)
High dV/dt switching in power electronics
Mandatory EMC filter design
There is no contradiction:
The system is electrically sound, yet still produces measurable earth current.
🔚 Final takeaway (technical)
Leakage current in inverter heat pumps is an unavoidable result of capacitive coupling and high-frequency switching, not an indicator of insulation failure.
RCD tripping in modern homes is typically due to cumulative system leakage rather than a single defective appliance.