X1/Y1 and X1/Y2 describe the impulse withstand and insulation class a safety capacitor is
certified for, not its capacitance. Y1 parts are certified for double or reinforced
insulation across a line-to-ground barrier. Y2 parts are certified for basic or
supplementary insulation only. A Y1 part can stand in for a Y2 electrically. A Y2 part
cannot replace a Y1.
What is the difference between an X1/Y1 and an X1/Y2 safety capacitor?
The letter is the position. X capacitors sit line-to-line, where failure causes no shock
hazard. Y capacitors sit line-to-ground, where failure could energise a touchable surface.
The number is severity. Y1 is the highest Y subclass, certified for double or reinforced
insulation. Y2 is certified for basic or supplementary insulation.
The classes are defined in IEC 60384-14. The practical summary is this:
| Class | Position | Insulation duty | Typical peak impulse in service |
|---|---|---|---|
| X1 | Line-to-line | — | above 2.5 kV, up to 4 kV |
| X2 | Line-to-line | — | up to 2.5 kV |
| Y1 | Line-to-ground | Double or reinforced | 8 kV |
| Y2 | Line-to-ground | Basic or supplementary | 5 kV |
A part marked X1/Y1 carries both certifications. It can be used line-to-line as an X1 or
line-to-ground as a Y1. The same logic applies to X1/Y2 parts. That dual marking is why
Walsin Technology (Walsin) states the class as a combined UR field rather than a single
letter. Confirm the class limits against the current edition of IEC 60384-14 and the safety
agency certificate for the specific part — the table above is the framework, not the
certificate your approval body will ask to see.
Can I fit a Y2 capacitor where the schematic calls for Y1?
No. A Y1 designation on a schematic almost always means the capacitor bridges a reinforced
insulation barrier — typically primary to secondary across an isolation transformer. A Y2
part is not certified for that barrier. Going the other way, Y1 for Y2, is electrically
acceptable but still needs a dimensional and capacitance check.
Substitution here is not an engineering judgement. It is a certification question. The
capacitor’s own certificate has to cover the barrier it sits across. No amount of voltage
margin substitutes for the right class.
Here is the practical difference between the two Walsin small-size disc families at the
same capacitance value. The reference is the X1/Y1 part. The right-hand column states field
by field what matches and what does not.
| Walsin part number | Series | Dielectric | Capacitance | Tolerance | UR (class and voltage) | Match / mismatch vs SL0AH150J060 |
|---|---|---|---|---|---|---|
| SL0AH150J060 | AH, X1/Y1 small size | SL | 15 pF | ±5% | X1-400Vac / Y1-250Vac | reference part |
| SL0AC150J060 | AC, X1/Y2 small size | SL | 15 pF | ±5% | X1-440Vac / Y2-250Vac | match: capacitance, tolerance, SL dielectric, Y-side 250 Vac · mismatch: Y2 class not Y1, X1 rated 440 Vac not 400 Vac, AC series not AH |
| SL0AH180J060 | AH, X1/Y1 small size | SL | 18 pF | ±5% | X1-400Vac / Y1-250Vac | match: series, class, both voltages, dielectric, tolerance · mismatch: 18 pF not 15 pF |
| SL0AC100J060 | AC, X1/Y2 small size | SL | 10 pF | ±5% | X1-440Vac / Y2-250Vac | match: tolerance, SL dielectric, Y-side 250 Vac · mismatch: Y2 class, X1 440 Vac, AC series, 10 pF not 15 pF |
| SL0AH220J060 | AH, X1/Y1 small size | SL | 22 pF | ±5% | X1-400Vac / Y1-250Vac | match: series, class, both voltages, dielectric, tolerance · mismatch: 22 pF not 15 pF |
| S242N100J502 | S2, X1/Y2 surface mount | NP0 | 10 pF | ±5% | 5000 Vdc, 1808 case | match: tolerance, Class I dielectric family · mismatch: surface mount not leaded disc, Y2 class, NP0 not SL, 10 pF not 15 pF, DC-rated field not AC class field |
Note the X1 side of that first comparison. The AH part is X1-400Vac and the AC part is
X1-440Vac. The Y2 part has the higher X1 rating. If you selected on the X number alone
you would pick the wrong class for a line-to-ground position. Select on the Y letter first.
Which Walsin series are X1/Y1 and which are X1/Y2?
AH and AS are the X1/Y1 disc families. AC is the X1/Y2 disc family. S2 is the X1/Y2
surface-mount MLCC family. Each disc family is further split by size and by 500 V variant,
so the series name in Walsin’s data is longer than the family letter suggests.
From our own extract of the Walsin disc capacitor catalogue:
| Walsin series as listed | Class | Parts in our extract |
|---|---|---|
| Safety Certificated Caps X1/Y1_(AH series) small size | X1/Y1 | 74 |
| Safety Certificated Caps X1/Y2_(AC series) small size | X1/Y2 | 72 |
| Safety Certificated Caps X1/Y2_(AC series) | X1/Y2 | 42 |
| Safety Certificated Caps X1/Y1_(AH series) | X1/Y1 | 38 |
| Safety Certificated Caps X1/Y1_(AH series) 500V | X1/Y1 | 37 |
| Safety Certificated Caps X1/Y2_(AC series) 500V small size | X1/Y2 | 36 |
| Safety Certificated Caps X1/Y2_(AC series) Auto | X1/Y2 | 23 |
| Safety Certificated Caps X1/Y1_(AH series) 500V small size | X1/Y1 | 17 |
| Safety Certificated Caps X1/Y1_(AS series) | X1/Y1 | 15 |
| Safety Certificated Caps X1/Y2_(AC series) 500V | X1/Y2 | 13 |
That is 181 X1/Y1 parts across AH and AS, against 186 X1/Y2 parts across the AC variants —
close to evenly split. The Auto suffix on one AC line marks the automotive-grade variant,
and it is X1/Y2 only. There is no automotive AH line in our extract.
The class voltage combinations available across the whole disc catalogue are narrower than
the series list implies. Our extract shows 57 parts at X1-440Vac/Y2-250Vac, 57 at
X1-440Vac/Y2-300Vac, 56 at X1-400Vac/Y1-250Vac, 56 at X1-400Vac/Y1-400Vac, 54 at
X1-500Vac/Y1-500Vac 1500Vdc, 49 at X1-500Vac/Y2-500Vac 1500Vdc and 23 at
X1-440Vac/Y2-300Vac 1500Vdc. If your specification calls for a combination outside that
list, it does not exist in this catalogue and the requirement needs revisiting.
Is there a surface-mount safety capacitor, or is it all leaded discs?
There is. Walsin’s S2 series is a safety-certificated X1/Y2 MLCC in a 1808 case, rated
5000 Vdc. It removes the leaded disc from the board entirely, which matters for automated
assembly and for height-constrained designs. The catalogue covers both NP0 and X7R
dielectrics in that case size.
| Walsin part number | Dielectric | Capacitance | Tolerance | Rated voltage | L × W × T (mm) | Match / mismatch vs S242N100J502 |
|---|---|---|---|---|---|---|
| S242N100J502 | NP0 | 10 pF | ±5% | 5000 Vdc | 4.50 +0.80/−0.30 × 2.00 ±0.25 × 1.25 ±0.10 | reference part |
| S242N100K502 | NP0 | 10 pF | ±10% | 5000 Vdc | 4.50 +0.80/−0.30 × 2.00 ±0.25 × 1.25 ±0.10 | match: dielectric, capacitance, voltage, all three dimensions · mismatch: tolerance ±10% not ±5% |
| S242B101K502 | X7R | 100 pF | ±10% | 5000 Vdc | 4.50 +0.80/−0.30 × 2.00 ±0.25 × 1.60 ±0.20 | match: case outline L and W, voltage · mismatch: X7R Class II not NP0, 100 pF, ±10%, thicker at 1.60 mm |
| S242B471K502 | X7R | 470 pF | ±10% | 5000 Vdc | 4.50 +0.80/−0.30 × 2.00 ±0.25 × 2.00 ±0.20 | match: case outline L and W, voltage · mismatch: X7R, 470 pF, ±10%, 2.00 mm thick |
| S242B681K502 | X7R | 680 pF | ±10% | 5000 Vdc | 4.50 +0.80/−0.30 × 2.00 ±0.25 × 2.00 ±0.20 | match: case outline L and W, voltage · mismatch: X7R, 680 pF, ±10%, 2.00 mm thick |
| S242B102K502 | X7R | 1000 pF | ±10% | 5000 Vdc | 4.50 +0.80/−0.30 × 2.00 ±0.25 × 2.00 ±0.20 | match: case outline L and W, voltage · mismatch: X7R, 1000 pF, ±10%, 2.00 mm thick |
Thickness is the field to watch. The NP0 parts are 1.25 mm thick. The X7R parts at 470 pF
and above are 2.00 mm. Within one series, one case code and one voltage rating, the height
varies by 0.75 mm.
S2 series datasheet —
https://www.passivecomponent.com/wp-content/uploads/datasheet/WTC_MLCC_Safety_Certified_S2.pdf
Why can I not find Walsin’s X2 series in their search tool?
Because it is not indexed. Walsin’s S3 series is the X2 safety MLCC family, and it returns
zero parts on Walsin’s own public parametric search. It exists as a datasheet only. An
engineer searching for an X2 surface-mount part will conclude Walsin does not make one.
Walsin does.
This is not the only gap in that tool. Walsin’s disc capacitor export is missing body
diameter, body thickness and lead pitch on every row. Those are the three dimensions you
need to lay out a through-hole footprint, and they exist only in the drawing tables inside
each datasheet PDF. A parametric search that gives you capacitance, class and voltage but
no body size cannot complete a mechanical selection.
Venatech Limited, an authorised Walsin distributor in Hong Kong, holds the underlying
catalogue data for the indexed series and reads the missing dimensions off the datasheet
drawings on request. If you need the AH or AC body dimensions for a footprint, ask and we
will pull them from the drawing rather than leave you to guess.
Frequently asked questions
Does a Y1 capacitor cover a Y2 requirement?
Electrically and in certification terms, Y1 is the more onerous class, so it covers a Y2 position. It is not automatically a drop-in. Body size, lead pitch, capacitance value and the agency marks your approval body accepts all still have to match. Check the footprint before assuming the swap.
What happens if I use an ordinary ceramic capacitor line-to-ground?
It will fail safety approval, and it can fail dangerously. A safety-certificated capacitor is designed and tested to fail open rather than short. An ordinary MLCC has no such requirement, and a shorted capacitor across a line-to-ground barrier energises the chassis. Never substitute a general purpose part into a Y position.
Why do X1/Y2 parts show a higher X1 voltage than X1/Y1 parts in this catalogue?
The X and Y ratings are independent. In Walsin’s disc catalogue the AH X1/Y1 small-size parts are X1-400Vac while the AC X1/Y2 small-size parts are X1-440Vac. The Y subclass is what governs the line-to-ground barrier, so select on the Y letter first and treat the X figure separately.
Is the Walsin S2 series a direct replacement for a leaded Y2 disc?
No, and it should not be treated as one. S2 is a 1808 surface-mount MLCC rated 5000 Vdc. A leaded disc is a through-hole part with an AC class rating. The footprint, the mounting method and the way the rating is expressed all differ. It is an alternative approach, not a substitution.
What capacitance values do the safety disc series cover?
The small-size AH and AC parts in our extract sit in the low picofarad range — 10 pF to 24 pF in the rows we hold. Higher values exist in the full-size AH and AC series and in the 500 V variants. Send the value and class you need and we will check the catalogue against it.
Does an automotive safety capacitor exist in this catalogue?
Yes, in X1/Y2 only. Our extract lists 23 parts under the AC series automotive line. There is no automotive AH (X1/Y1) line in the data we hold. If your programme requires an automotive-grade Y1 disc, flag it early because the catalogue may not support it.
Where do I find the body diameter and lead pitch?
Not in the parametric data. Walsin’s disc capacitor export omits body diameter, body thickness and lead pitch entirely. Those dimensions appear only in the drawing tables of the series datasheet PDF. Venatech can extract them for a specific part on request.
Next step
If you are specifying across an isolation barrier, send us the schematic reference, the
barrier type, the class your approval body requires and the capacitance value. We will
return the Walsin AH, AC or S2 candidates with a field-by-field match and mismatch table,
plus the body dimensions that Walsin’s own parametric data does not publish. We will not
tell you a part is equivalent. We will show you where it differs.
Send us your BOM → · Browse Walsin disc and safety capacitors →
Related reading: NP0 vs X7R vs X5R: which MLCC dielectric ·
High-Q, low-ESR MLCC for RF
Cross it yourself. The cross-reference tool takes a single part number or a whole BOM column and returns the Walsin candidates with every field that differs stated plainly — qualification stays with you. Ready stock shows what is held in Hong Kong today, with an RFQ against any line.