Safety Capacitor X1 X2 Y1 Y2 Class Y EMI Filter Power Supply Selection Guide 2026

There is a tempting shortcut every power engineer hears at least once: "EMI is a little high — just drop in another Y-cap and it'll pass." It often does pass EMI. And then the unit fails something else entirely. A USB-PD adapter we reviewed cleared CISPR 32 Class B with comfortable margin after the team added Y-capacitance to tame a stubborn common-mode peak — only to be rejected at CE touch-current testing with 0.32 mA against the 0.25 mA Class II limit, after an entire production batch had been built. The fix that solved one mandatory gate quietly broke another. That is the whole story of safety capacitors: an X-cap and a Y-cap are not just filter parts you size for impedance, they are safety-rated components governed by IEC 60384-14, and their value is bounded as much by electric-shock safety as by EMI performance.

This guide explains how to choose X1, X2, X3, Y1, Y2 and Y4 capacitors for a power-supply EMI filter — what each subclass means, where they sit in the filter, how Y-cap value trades against touch current, and the pitfalls that turn a "quick EMI fix" into a recall.

Why "Safety" Capacitor Is a Failure-Mode Word, Not a Marketing Word

An ordinary film or ceramic capacitor is rated by capacitance, voltage and temperature. A safety capacitor is rated by one extra thing that matters far more: how it fails. Because X-caps and Y-caps sit directly across the mains or bridge the isolation barrier to ground, a short-circuit failure would either start a fire or put mains voltage onto a touchable surface. The entire point of the X/Y classification under IEC 60384-14 ("Fixed capacitors for electromagnetic interference suppression and connection to the supply mains") is to guarantee a controlled, non-hazardous failure mode — an X-cap that fails open, not shorted, and a Y-cap whose insulation never breaks down enough to electrify the chassis. The capacitance is the easy part; the safety certification is what you are actually paying for.

Safety Capacitor Classes at a Glance: X vs Y

The split is simple once you know where each capacitor is connected:

• X capacitor — connected line-to-line (line-to-neutral), across the AC mains. It suppresses differential-mode (DM) noise — the noise that travels out on line and back on neutral. A shorted X-cap blows a fuse or starts a fire, so X-caps must fail open and are usually self-healing metallized film.
• Y capacitor — connected line-to-ground (or neutral-to-ground), bridging the primary-to-secondary or live-to-chassis isolation barrier. It suppresses common-mode (CM) noise — the noise that rides on both conductors together and returns through ground. Because a shorted Y-cap would energize a touchable surface, Y-caps must withstand large impulses and fail open, and they directly add to earth leakage / touch current.

Neither is interchangeable with a generic CBB (polypropylene) or general-purpose ceramic capacitor of the same value — those carry no safety certification and no guaranteed failure mode.

X-Capacitor Deep Dive: Differential-Mode, Self-Healing, Fail-Open

X-caps live across the mains and absorb differential-mode ripple and the switching fundamental's low harmonics. The dielectric of choice is metallized polypropylene film (MKP) because it is self-healing: a localized dielectric flaw vaporizes the thin metallization around the fault and isolates it, so a pinhole defect does not cascade into a dead short. Under abnormal over-voltage the device degrades to open circuit rather than shorting the line.

IEC 60384-14 splits X-caps by peak-pulse withstand:

• X1 — high pulse, peak impulse > 2.5 kV up to 4 kV, rated AC working voltage typically ≤ 760 V. Used where high surge energy reaches the input.
• X2 — general purpose, peak impulse ≤ 2.5 kV, rated ≤ 760 V (commonly 275/305/310 VAC parts). The workhorse of consumer and IT power supplies.
• X3 — peak impulse ≤ 1.2 kV, rated ≤ 660 V. Lower-stress positions only.

Typical X-cap values run 0.1 µF to 1 µF. Critically, IEC and the end-product safety standards require any X-cap with significant stored energy to have a discharge (bleed) resistor across it, so the plug pins are not left at a hazardous voltage after unplugging — commonly the residual voltage must fall below 34 V (or 60 V) within 1 second, as enforced through IEC 62368-1 / IEC 60335-1 capacitor-discharge clauses.

Y-Capacitor Deep Dive: Common-Mode, Reinforced Insulation, Impulse Withstand

Y-caps bridge the isolation barrier, so their insulation rating — not just their capacitance — defines the subclass. They suppress common-mode noise by giving CM current a controlled low-impedance path to ground at high frequency. Because they sit across the very barrier that protects the user, they must survive large impulses and fail open:

• Y1 — bridges reinforced/double insulation. Rated AC ≥ 250 VAC, impulse withstand 8 kV. The choice for primary-to-secondary across a single barrier in a Class II (double-insulated) design.
• Y2 — bridges basic insulation. Rated AC ≥ 150 VAC (commonly 250/300 VAC parts), impulse withstand 5 kV. The most common Y-cap in mains adapters.
• Y4 — rated AC < 150 VAC, impulse withstand 2.5 kV. Low-voltage positions only.

Y-cap dielectrics are typically ceramic (class-Y-certified disc capacitors) or metallized PP film. Values are deliberately small — usually 1 nF to 4.7 nF — because every nanofarad adds to earth leakage current, which is the hard ceiling discussed below.

X/Y Safety Capacitor Parameter Comparison Table

Rule of thumb: X2 + Y2 covers the vast majority of external power supplies; step up to X1/Y1 only where surge energy or the insulation scheme demands it.

Y-Cap Leakage Current vs IEC 60990 Touch Current

This is where EMI and safety collide. A Y-cap connected line-to-ground passes a small leakage current at mains frequency, and that current is part of the touch (contact) current measured per IEC 60990 and limited by the end-product safety standard:

• Class II (double-insulated, no earth): touch current ≤ 0.25 mA
• Class I (earthed): touch current ≤ 3.5 mA

The leakage through a Y-cap is I = V × 2πf × C. The table below shows the line-to-ground leakage of common Y-cap values at 230 V / 50 Hz (single capacitor; parallel Y-caps add directly):

Two lessons jump out. First, a single 4.7 nF Y2 already busts the Class II budget at 230 V. Second, two 2.2 nF Y-caps in parallel (a frequent "let's just add one more" move) sum to ~0.32 mA — exactly the recall scenario from the introduction. Class I designs have more headroom because the earth conductor carries the leakage, but Class II is unforgiving.

How X and Y Caps Work Inside the EMI Filter Topology

A textbook input EMI filter is a π-type network built from three blocks, each owning a different noise mode:

  L ──┬──[ X-cap ]──┬───CM choke───┬──[ X-cap ]──┬── to rectifier
      │             │   (2 windings)│             │
   [Y-cap]          │               │          [Y-cap]
      │             │               │             │
  N ──┴─────────────┴───────────────┴─────────────┴── 
      │                                             │
     PE (chassis / ground) ◄── Y-caps return CM here

• The common-mode choke presents high impedance to CM current (the two windings' fluxes add for CM, cancel for DM/load current).
• The Y-caps give the CM current that the choke blocks a low-impedance path back to ground/PE — they are the main weapon against common-mode (radiated and high-band conducted) noise.
• The X-caps bypass differential-mode ripple line-to-neutral — the main weapon against differential-mode noise — and form the capacitive arms of the π filter.

Get the division of labor wrong (e.g., trying to kill a CM problem with a bigger X-cap, or a DM problem with more Y) and the filter underperforms while you burn your leakage budget. For the regulatory targets these filters chase, see our CISPR 32 vs FCC Part 15 Class B EMI compliance guide.

Class I vs Class II: Why the Y-Cap Ceiling Differs

The single biggest driver of how much Y-capacitance you can use is the protection class:

• Class II (double insulation, no protective earth): there is no earth wire to carry leakage, so the user's body is the return path under fault assumptions. The touch-current limit is the tight ≤ 0.25 mA, which in practice caps total line-to-ground Y-capacitance around ≤ 4.7 nF (and often lower with margin). Most USB-PD and desktop adapters are Class II.
• Class I (protective earth): the earth conductor carries the leakage current, allowing ≤ 3.5 mA, so total Y-capacitance can be relaxed to roughly 22–47 nF, giving much more CM filtering headroom — provided the earth connection is intact. That proviso matters: a Class I product's safety leans on earth-conductor integrity, which is itself a tested parameter.

So the same EMI problem has a very different solution space depending on class — a Class II design must solve common-mode noise mostly with the choke and layout, reserving the small Y-cap allowance for the last few dB. To see how the leakage ceiling is actually measured and verified, read our Hi-Pot, insulation resistance and touch-current verification guide.

Dielectric Material: Why MKP and Ceramic — and Never Electrolytic

Material choice is not interchangeable:

• Metallized polypropylene (MKP) — the standard X-cap dielectric. Self-healing, low loss (low ESR/low dissipation factor), excellent high-frequency behavior, stable with temperature. Also used for film-type Y-caps.
• Class-Y ceramic — the common Y-cap dielectric in disc form, certified for the impulse and insulation requirements in a tiny package. Watch the temperature coefficient: X7R dielectric shifts capacitance with temperature and bias, while C0G/NP0 is far more stable but offers less capacitance per volume.
• Electrolytic / aluminum-polymer — never. They are polarized, leaky, fail short, dry out, and carry no X/Y certification. Using one across the mains or barrier is a safety violation, not just poor practice.

The reason is the same failure-mode logic from the start: only self-healing film and certified class-Y ceramic give the guaranteed fail-open behavior the standard demands.

The Hi-Pot Test Y-Cap Trap

A classic false failure: during a Hi-Pot (dielectric withstand) test, a primary-to-secondary AC test voltage of, say, 3 kV AC drives a substantial current through the Y-cap (its reactance at 50/60 Hz is low at that voltage), and the tester reads several milliamps of leakage and flags a breakdown — when the insulation is perfectly fine. The capacitor is doing exactly what it is built to do.

The correct responses: (1) use a DC Hi-Pot voltage (e.g., ~4242 VDC as the DC equivalent of 3 kV AC) so the Y-cap charges and stops conducting, or (2) short out / lift the Y-cap connection during the AC Hi-Pot and test the barrier alone, then verify the Y-cap separately. A related wiring mistake: the X-cap discharge resistor must sit across the X-cap, never across the Y-cap — a bleed resistor across a Y-cap adds a permanent DC leakage path to ground and corrupts both the touch-current and insulation-resistance results. The relationship between Y-cap impulse rating and real surge events is covered in our IEC 61000-4 surge/ESD/EFT immunity test guide.

Five Common Safety-Capacitor Pitfalls

1. Faking an X2 with a generic CBB cap. A same-value polypropylene capacitor looks equivalent but has no X-class certification and no guaranteed fail-open behavior — it can short across the mains and start a fire. Only use parts carrying the X/Y safety marking.
2. Confusing Y1 and Y2 and under-insulating the barrier. A Y2 (5 kV, basic insulation) substituted where the isolation scheme requires Y1 (8 kV, reinforced) leaves the user one insulation layer short. Match the Y subclass to the insulation it bridges, not just to capacitance.
3. Passing EMI but busting 0.25 mA touch current. Adding Y-capacitance to fix common-mode noise pushes earth leakage over the Class II limit. Fix the noise source and choke first; spend the Y-cap allowance last.
4. Y2 impulse breakdown at high altitude. Air's dielectric strength falls with altitude; a Y-cap and its surrounding clearances rated at sea level can flash over under the same surge at 3,000–5,000 m. De-rate impulse and clearance for the altitude in the product's use profile.
5. Discharge-resistor open circuit or parallel-Y oversight. A broken X-cap bleed resistor leaves hazardous residual voltage on the plug (and fails the 1-second discharge check), while paralleling multiple Y-caps adds their leakage directly — two 2.2 nF parts are ~0.32 mA, not 0.16 mA. Always sum every line-to-ground capacitor.

Sanyi Power Supply Ecosystem — X2 + Y2 EMI Filter, ≤0.25 mA Touch Current

Sanyi designs its USB-PD, GaN and desktop/industrial adapter lines around a disciplined X2 (460/760 VAC) + Y2 (reinforced, ~2.2 nF) limited-leakage EMI filter, so common-mode noise is suppressed primarily with the common-mode choke and layout while total line-to-ground Y-capacitance stays inside the ≤ 0.25 mA Class II touch-current budget. For high-power applications, the HP high-power adapter series (up to 240W) carries the input filtering and barrier design needed to hold EMI margin without overshooting leakage. The APN desktop adapter series applies the same X2/Y2 discipline across mid-power desktop and IT loads. For multi-port and workstation charging, the SY-C260W multi-mode charger and the higher-output SY-C500W high-power charger balance dense GaN switching against the touch-current ceiling.

Because EMI suppression and electric-shock safety are a single trade-off — not two separate files — our adapters are engineered to satisfy both at once. For the broader safety-standard context, see our IEC 62368-1 power supply safety standard migration guide. Contact our power engineering team with your power, protection class and destination-market requirements and we will recommend a compliant EMI-filter platform with the matching X/Y safety-capacitor scheme.

FAQ

Can an X-cap and a Y-cap be swapped? No. They occupy different positions and carry different safety duties. An X-cap is line-to-line and rated to fail open across the mains; a Y-cap is line-to-ground and rated to bridge the isolation barrier with reinforced (Y1) or basic (Y2) insulation and survive 8 kV/5 kV impulses. Putting an X-cap where a Y-cap belongs leaves the user-protecting barrier uncertified, and a Y-cap's small value is the wrong tool for differential-mode bypassing.

A bigger Y-cap improves EMI — why can't I just keep adding capacitance? Because every nanofarad of line-to-ground Y-capacitance adds earth leakage / touch current. A Class II adapter is capped at ≤ 0.25 mA per IEC 60990, which limits total Y-capacitance to roughly ≤ 4.7 nF at 230 V — a single 4.7 nF part or two parallel 2.2 nF parts already exceeds it. Beyond a point, more Y-cap means a failed safety test, not better EMI.

Is the X-cap discharge resistor really mandatory? Yes, wherever the stored energy can present a hazard at the plug. Safety standards require the residual voltage to fall below a safe level (commonly < 34 V or < 60 V) within about 1 second of unplugging, which an X-cap of meaningful value cannot do on its own. The bleed resistor (or an active discharge IC) provides that path — and it must sit across the X-cap, never the Y-cap.

How do I choose between ceramic and film Y-caps? Class-Y ceramic discs are compact and cheap and dominate small adapters, but watch the temperature/voltage coefficient — X7R drifts, C0G/NP0 is stable but lower density. Film (MKP) Y-caps are larger but offer lower loss, tighter tolerance and better stability for higher-reliability or higher-power designs. Both must carry the proper Y1/Y2 certification; the choice is about size, stability and cost, not safety class.

Can I still use a Y2 cap at high altitude? Only with de-rating. The impulse withstand and the surrounding clearance/creepage both depend on air's dielectric strength, which drops with altitude. A Y2 rated for sea level may flash over under the same surge at 3,000–5,000 m, so the impulse rating and spacing must be de-rated (or stepped up to a higher subclass) for the product's actual use altitude.