Unequal Voltage Readings to Earth on Isolation Transformer Secondary

Question

In Vigilohm IMD‑IT‑S* isolation transformer, when voltage measurements are made between secondary terminals L, N and Protective Earth (PE) they exhibit unequal values.

• L–PE ≈ higher voltage. Example 170V
• N–PE ≈ lower voltage. Example 60V
• L–N ≈ 230 VAC.

Why are the L–PE and N–PE voltages unequal?

Answer

1. Isolation Transformer Principle:

An isolation transformer provides galvanic isolation between its primary and secondary windings. This means there is no direct electrical connection between the secondary circuit and earth (PE). As a result, the secondary side is considered floating with respect to ground.

2. Why “Phantom” Voltages Appear:

Even though the windings are electrically isolated, they are physically close and separated only by insulation. This creates small stray/parasitic capacitances between:

• Primary and secondary windings.
• Windings and the transformer core.

Due to these capacitances:

• A small AC current is capacitively coupled from primary to secondary.
• When high-impedance digital Multimeter (≈10 MΩ) is used detect this very small leakage current it displays as a voltage.
• These readings are often referred to as floating or phantom voltages.

3. Reason for Unequal L-PE and N-PE Voltages:

The difference in measured voltages is caused by unequal stray/parasitic capacitances within the transformer:

• Capacitance values are influenced by winding geometry, position, and proximity to the core
• The two ends of the secondary winding are not perfectly symmetrical
• The side with higher capacitive coupling (L) shows a higher voltage relative to earth.
• The other side (N) shows a lower voltage.

4. Practical Significance of these voltages.

These voltages are not usable energy:

• They are caused by extremely small leakage currents.
• They collapse immediately under load.
• When Low‑impedance Multimeter is used to measure it would read very low or near Zero Voltage.

5. Summary:

• The observed voltage asymmetry is normal behaviour in floating isolation transformer systems.
• It is caused by capacitive coupling of the transformer windings, not by a fault.

פורסם עבור: שניידר אלקטריק ישראל