An MOV placed across line and neutral does not solve the same problem as an MOV placed from line to protective earth. The terminals, surge mode, earthing system and safety requirements determine which current path the protection creates.
This guide uses the common abbreviations L (Line), N (Neutral) and PE (Protective Earth). Final design must follow the equipment standard, installation category and regional wiring system.
Differential-Mode and Common-Mode Surges
A differential-mode surge appears between current-carrying conductors, such as L and N. A common-mode surge appears between one or more current-carrying conductors and earth.
A surge generator and coupling network may apply different test modes. The circuit should be reviewed against the actual test setup rather than the generic phrase “surge protected.”
MOV From Line to Neutral (L–N)
An L–N MOV provides a shunt path for differential-mode transients. It is common near an AC input, after the appropriate upstream overcurrent protection, with a short low-inductance connection.
Key checks include:
- maximum continuous AC voltage including mains tolerance;
- temporary-overvoltage exposure;
- clamping level at the specified surge current;
- fuse and thermal-disconnect coordination;
- distance from heat-sensitive or flammable material.
MOV From Line to Protective Earth (L–PE)
An L–PE branch can divert common-mode surge current toward protective earth. That function also introduces safety considerations because the component bridges a hazardous conductor and accessible-earth network.
The device construction, approvals, creepage, clearance, fault behavior, leakage and equipment class must be reviewed. Do not add an L–PE MOV merely because an L–N test passed poorly.
MOV From Neutral to Protective Earth (N–PE)
An N–PE branch may be used as part of a coordinated common-mode protection network, but neutral is not universally at earth potential. Wiring faults, installation topology and regional systems can produce significant N–PE voltage.
Selection must consider the worst continuous and temporary voltage across this branch, not only the ideal normal condition.
Why Three MOVs Are Not Automatically a Complete Solution
A network using L–N, L–PE and N–PE MOVs changes surge-current distribution. The branch that conducts first can affect the current and residual voltage seen by the others. Lead and trace inductance can add to the measured peak voltage.
Coordination requires system-level testing. It also requires a defined response if one branch is disconnected by thermal protection.
Equipment With No Protective Earth
Class II or unearthed equipment does not provide a PE node for an ordinary L–PE MOV branch. Protection topology must respect the insulation system and applicable standard. Do not create a pseudo-earth connection that compromises reinforced or double insulation.
Leakage and Insulation Testing
Although a correctly selected MOV is high resistance during normal operation, it still has leakage current and capacitance. Earth-connected protection can influence leakage-current measurements, dielectric testing and insulation coordination.
Use device limits across temperature and end of life, not only a typical room-temperature value.
Where Should the Network Be Placed?
The surge branch should usually be close to the entry point and the intended current return path. Keep the loop short and avoid routing surge current through sensitive signal or control ground. The exact position relative to filters, switches and fuses depends on the required safety and EMC behavior.
Design Review Checklist
- Identify every required test mode and generator connection.
- Document the earthing and equipment insulation class.
- Calculate maximum continuous and temporary voltage for each branch.
- Check component approval and abnormal-operation requirements.
- Coordinate overcurrent and thermal disconnection.
- Model or measure residual voltage at the protected load.
- Review leakage, capacitance, creepage and clearance.
- Test the assembled product under the applicable standard.
Related Guides
- MOV Protection Circuit Basics
- MOV Failure Modes and Thermal Protection
- MOV Surge Waveforms
- How to Select an MOV
Reference
For a private topology review, use Ask UBAEC and include the input diagram, earthing arrangement, equipment class, mains range and surge-test modes.
