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IEC 60384-14-2: Fixed Capacitors for Electronic Equipment — Safety Requirements for EMC Suppression Capacitors
X Capacitors and Y Capacitors — Why These Two Power Supply Input Capacitors Must Never Be Interchanged
IEC 60384-14-2:2016 specifies fixed capacitors for electromagnetic interference suppression (X and Y capacitors). Every SMPS input has two safety-rated capacitors — X capacitors across L-N (differential mode), Y capacitors from L/N to PE (common mode). Their positions dictate completely different safety requirements.
| Type | Connection | Failure Consequence | Safety Class |
|---|---|---|---|
| X1/X2/X3 | L-N (across mains) | Short → fuse blows → safe failure | May fail short-circuit |
| Y1/Y2/Y3/Y4 | L/N-PE (line to earth) | Short → chassis live → shock hazard | Must fail open-circuit |
A Y capacitor connects between the primary side and protective earth (PE). If it fails short-circuit, mains voltage appears on the equipment metal enclosure — the operator touching it receives a potentially fatal electric shock. Y capacitors must be designed with extremely low short-circuit failure probability (thicker dielectric, precise self-healing metallization control), and if they do fail, the failure mode must be open-circuit.
TNLab — An X capacitor failing short-circuit is acceptable (the fuse will blow). A Y capacitor must never fail short-circuit — because it directly affects personal safety.
