Introduction
This guide helps you diagnose and resolve inaccurate measurements when using Shelly EM or 3EM devices. It covers CT clamp orientation, wiring issues, voltage reference tips, and calibration best practices.
Legend:
CT = Current Transformer
L/N = Live / Neutral
A→B→C = Standard 3-phase wiring order
EM/3EM = Energy Meter, 1-phase or 3-phase
1️⃣ Symptom: A channel shows negative power when the load is consuming power
Cause: CT clamp is installed backwards (arrow is opposite the current flow).
Action:
Flip the CT (correct K–L direction), or
Use the “Reverse CT measurement direction” setting in the Web UI for that channel.
✅ After correcting, readings should become positive.
2️⃣ Symptom: Channel shows zero or near-zero reading, despite active load
Cause: CT is installed on neutral, or on both live + neutral wires — no differential current detected.
Action:
Move the CT to the live (phase) conductor only.
Avoid placing the CT on neutral (unless the device has a dedicated N CT port).
Confirm each CT is on the intended circuit.
✅ Proper placement will restore accurate readings.
⚠️ Important: This guide involves working with live electrical panels. If you're unsure or unfamiliar with wiring practices, please consult a qualified electrician.
3️⃣ Symptom: Incorrect or inconsistent phase readings, or phase sequence error
Cause: Phases (A, B, C) are mismatched with CTs, or CTs are swapped.
Action:
Verify each phase wire matches its CT input.
For 3-phase systems, correct the order to A → B → C.
On Pro 3EM, enable “Monitor phase sequence” in diagnostics.
✅ After correction, voltage and current values should align properly.
4️⃣ Symptom: Device shows no voltage or power on one or more phases
Cause: Missing or incorrectly wired neutral or phase - device cannot power on or measure correctly.
Action:
Confirm neutral is connected to the N terminal.
Ensure the correct phase lines (L1, L2, L3) are connected to their respective voltage inputs.
Check for loose or swapped L/N wires.
✅ Once neutral and the respective voltage inputs are wired in the device, the power readings will become available.
5️⃣ Symptom: Power/energy readings are too high or too low
Cause: Incorrect CT clamp rating is set (ct_type mismatch) or using a non-Shelly CT without calibration.
Action:
In the Web UI/API, select the correct CT type (e.g., 50A, 80A).
For 3EM, use
/em/phaseToPhaseCalibfor manual calibration.If calibration fails, CT may be wrong or miswired.
✅ Match CT type and retry.
6️⃣ Symptom: Inaccurate or unstable readings, even with correct CT placement
Cause: Voltage reference (L terminal) is not from the same phase being measured — common in split-phase setups or different breaker sources.
Action:
Power the Shelly EM from the same circuit or phase you are monitoring.
In 3-phase systems, verify L1 powers the meter and matches CT input A.
✅ This ensures voltage and current are in phase, improving accuracy.
Tip: For accurate readings, always power the EM/Pro EM-50 from the same phase you want to measure.
✅ General Best Practices
Always follow official wiring diagrams
Use correct CT types
Keep firmware up to date
After physical corrections, always reboot the device
Collect Debug logs and Diagnostics data when reporting complex cases
⚙️ Diagnostic Tools
Use
/calib.datto verify calibration on Shelly 3EM (Gen1 only)Use the Shelly Web UI to inspect CT assignment and phase alignment
On Pro 3EM, enable “Phase sequence” monitoring in Diagnostics