To find leakage between power domains, first map every conductive connection between them, then determine whether the current is flowing through insulation, a ground loop, an intentional capacitive path, or the measurement setup itself. Fix the identified path—not just the symptom. Never remove protective earth or a safety bond to silence noise; use isolation only when its ratings and signal function suit the installation.
What “leakage between power domains” can mean
An unexpected current between two domains does not automatically mean insulation has failed. Several different paths can produce a similar reading, and each calls for a different response.
| Possible mechanism | What carries the current | What to look for |
|---|---|---|
| Insulation leakage | Finite resistance through damaged, contaminated, aged, or otherwise imperfect insulation | Current between conductors or from a high-voltage circuit to chassis or earth; the path may change with operating conditions. |
| Intentional capacitive coupling | Capacitance, including components in equipment filters | Current associated with powered equipment or changing voltage. A current reading alone does not establish an insulation fault. |
| Ground loop | Two or more conductive connections between grounds, with voltage difference or induced noise driving current around the loop | Noise, data errors, or current through shields, chassis bonds, protective earth, or signal references. |
| Measurement leakage | Unwanted current through cable or instrument insulation in a high-impedance measurement | The reading changes with the measurement wiring or instrument configuration. |
Protective-earth leakage and a signal-ground loop are not interchangeable diagnoses. They can both involve current, but the current paths and safety implications differ. Analog Devices’ Understanding Common-Mode Signals (publication page identifies 2003) reports ground-point differences typically around 0.2 V RMS to 5 V RMS, and rarely as high as 65 V RMS between widely separated grounds. Those are reported observations, not expected limits or proof of a fault in a particular installation.
Trace the path before changing the circuit
- Name the domains. Identify the two circuits or systems in question, their power returns, chassis, and references. Note whether either domain connects to mains or a high-voltage source.
- Map every conductive connection between them. Include protective earth, chassis bonds, cable shields, signal-reference wires, power returns, and any measurement leads. Multiple ground paths can form a loop even when the signal wiring appears isolated.
- Record the operating and measurement conditions. Note whether the equipment is powered, what loads and signals are present, and where and how current or voltage is measured. Fluke’s Leakage Current Measurement Basics notes that equipment leakage may disappear when a device is turned off. That change is a clue about an operating-dependent path, not proof of a defect or a universal test procedure.
- Separate current from insulation resistance. A current measurement tells you what flowed under the particular voltage and conditions; it does not, by itself, identify the path or insulation quality. For a simple resistive path, current depends on both voltage and resistance. Capacitive paths and changing signals can make the relationship more complex.
- Check the measurement setup. In high-impedance measurements, cable insulation leakage can contribute to the measured current. National Instruments’ Using the Guard Terminals describes guarding as a way to reduce that contribution in appropriate measurements. Guarding is not a general fix for leakage in the equipment being tested.
This is a diagnostic framework, not a single safe maintenance procedure. Mains-connected and high-voltage measurements require equipment and procedures appropriate to the system and applicable requirements.
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How to interpret measurements
Current measurements
A leakage-current clamp meter can be useful when the circuit and measurement method are suitable. Fluke discusses clamp meters in its leakage-current guidance, which covers both insulation paths and equipment filters. The evidence here does not establish a particular model or guarantee that a clamp meter is suitable for every circuit; select an instrument for the conductor arrangement, expected signal, and required accuracy.
Insulation-resistance measurements
Insulation resistance is a different quantity from current in operation. Texas Instruments’ TIDA-00440 reference design page specifies a 0 to 100 MΩ insulation-resistance measurement range with 5% uncalibrated accuracy. These are specifications for that reference design, not a general performance promise for insulation testers or other instruments.
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For high-voltage battery-to-chassis applications, Texas Instruments’ TIDA035 application brief (October 2020) describes a two-point isolation leakage-measurement approach. The method illustrates why the test arrangement matters; it is not a universal procedure for unrelated power domains.
Accuracy and context
Reference-design figures should not be mistaken for the accuracy of a generic meter or a field measurement. Texas Instruments’ automotive high-voltage and isolation leakage reference design, revision B revised February 2023, reports room-temperature maximum observed deviations of 0.624% for voltage measurement accuracy, 0.621% for isolated leakage-current accuracy, 0.48% for ISO_POS, and 0.126% for ISO_NEG. Those results are design-specific measurements, not generic instrument accuracy claims.
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Choose a correction that preserves safety and function
If the path is an insulation problem
Investigate the physical insulation path and its condition rather than masking its current with a signal-ground change. For high-voltage battery systems, leakage to chassis can occur through parasitic insulation paths; the measured insulation resistance helps characterize that path. Use test equipment and an approved procedure suited to the system. Do not infer that a particular resistance or current is acceptable without the governing requirements for that application.
If the path is a ground loop
Identify which conductive connections form the extra return path and whether the loop is carrying interference or affecting data. Analog Devices states in Breaking Ground Loops with Functional Isolation to Reduce Data Transmission Errors (2011-12-01): “A ground loop can be a source of interference that can induce a noise voltage between the grounds on either end of the transmission.” Functional or galvanic isolation can break a conductive signal path while allowing information transfer, but it does not diagnose the underlying path or make a safety-ground connection unnecessary.
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Do not lift protective earth or defeat a required chassis bond to eliminate noise. Any change to a shield, signal reference, or bonding arrangement must preserve the installation’s safety and electromagnetic-compatibility requirements.
If the reading is caused by the measurement setup
For a suitable high-impedance measurement, check the cable and instrument connections and consider whether the instrument provides a guard terminal that is correctly configured for that measurement. If guarding changes the reading, that points to a measurement contribution; it does not establish that the equipment itself is free of leakage.
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When galvanic isolation is appropriate
Isolation is a design choice for interrupting a conductive signal connection, not a substitute for tracing the current path. The required isolation depends on the application’s working voltage, common-mode transients, installation category, pollution degree, and insulation level.
Analog Devices’ ADUM3220 product FAQ gives a 300 V RMS working-voltage example with a 1.35 kV RMS test-voltage requirement, expressly conditioned on installation category II, pollution degree 2, basic insulation, and a measurement/control application. It is an example for those conditions—not a rating-selection rule to transfer to another installation. Verify the requirements that apply to the actual equipment and jurisdiction before selecting an isolator.
Quick Recap
A practical diagnosis checklist
- Can you identify both domains and every conductive path connecting them?
- Does the current appear only under particular powered or signal conditions?
- Could intentional filter capacitance or capacitive coupling explain part of the reading?
- Are you measuring operating current, insulation resistance, or a voltage difference? Do not treat them as equivalent.
- Could cable insulation leakage or instrument configuration affect a high-impedance measurement?
- If considering isolation, have you matched its ratings and insulation function to working voltage, transients, installation category, pollution degree, and the required insulation level?
- Can the proposed correction be made without compromising protective earth, chassis bonding, or other safety requirements?
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