Varistor Disc Size vs Peak Current for Italy: 79 of 2,340 Same-Voltage Pairs Rate the Larger Disc Lower

In Walsin’s 379-part MOV range, varistor disc size peak current is not monotonic. We compared all 2,340 same-voltage pairs of different diameter: 79 rate the larger disc lower, all between 130 and 680 Vac. A 20 mm Standard disc rated 6,500 A sits below an 18 mm High Energy disc at 9,000 A.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, holds the 379-row varistor export published by Walsin Technology (Walsin), Walsin_MOV_ALL.xlsx. This post is written from that file for an engineer or buyer in Italy who picks a surge device by disc diameter and assumes a bigger disc is a stronger one. We parsed all 379 rows, grouped them by Varistor Voltage, and compared every pair of different diameters inside each group. Every part number below is real and appears in the export.

Does a bigger varistor disc always carry a higher peak current rating?

No. At one varistor voltage, 79 of 2,340 cross-diameter pairs in Walsin’s export put the lower peak current rating on the larger disc, 3.4 percent. Another 19 pairs tie and 2,242 rise with diameter as expected. The 79 involve 119 of the 379 part numbers, 31.4 percent of the range, and every one sits between 130 Vac and 680 Vac.

Where the pairs come from	Cross-diameter pairs	Larger disc rated lower	Tie	Match / mismatch against “bigger disc, higher rating”
Standard (D) series	365	0	0	Match on all 365
High Energy (E) series	42	0	0	Match on all 42
Super High Energy (V) series	60	0	0	Match on all 60
Industrial series	173	39	19	Mismatch on 39 pairs, tie on 19
Between two different series	1,700	40	0	Mismatch on 40 pairs
All pairs	2,340	79	19	Mismatch on 3.4 percent

We have not seen this counted anywhere. Inside each of the Standard, High Energy and Super High Energy series the order never breaks, across 467 pairs. The rule fails only when a selector compares across series or inside the Industrial series. Our earlier work on series selection lists each series on its own; this post counts the comparisons between them.

Which four pairings invert, and at which voltage steps?

Four pairings account for all 79. We listed every inverted pair and grouped it by the two series and diameters involved. A 20 mm Standard disc rated 6,500 A is beaten by an 18 mm High Energy disc at 9,000 A on 20 voltage steps and by a 14 mm Super High Energy disc at 8,000 A on another 20; the 34 mm square Industrial disc at 80,000 A beats the 40 mm disc at 40,000 A on 19 steps and the 53 mm disc at 70,000 A on 20.

Pairing	Larger disc, as published	Smaller disc, as published	Steps	Larger-disc rating relative to smaller	Match / mismatch	Your incumbent part
Standard 20 mm against High Energy 18 mm	SR391K20D, 6,500 A	SR391K18E, 9,000 A	20	0.72 times	Mismatch — bigger disc, lower rating	______
Standard 20 mm against Super High Energy 14 mm	SR391K20D, 6,500 A	SR391K14E, 8,000 A	20	0.81 times	Mismatch — bigger disc, lower rating	______
Industrial 40 mm against Industrial 34 mm square (34RD)	SR391K40D, 40,000 A	SR391K34RD, 80,000 A	19	0.50 times	Mismatch — half the rating	______
Industrial 53 mm against Industrial 34 mm square (34RD)	SR391K53D, 70,000 A	SR391K34RD, 80,000 A	20	0.88 times	Mismatch — bigger disc, lower rating	______

Each row is shown at the 390 Vdc varistor voltage step, which the export pairs with a 250 Vac working voltage. Within each of these pairings the ratio repeats on every step, because both parts carry the same single rating at every voltage. The steps run from 205 Vdc and 130 Vac to 1,068.5 Vdc and 680 Vac. Below 205 Vdc only the Standard series exists, so nothing can invert there.

Where does the rating order hold, and where does it break?

It holds inside three of the four series and breaks mainly in the Industrial series, whose members differ in suffix as well as diameter. Of 34 voltage steps offered in more than one diameter, the largest disc is also the highest rated on 14 and is not on 20, 59 percent. We counted the largest disc at each step against the highest peak current at that step.

Two facts explain most of the breaks. First, the 34 mm square Industrial disc exists twice at every step, as 34R at 40,000 A and 34RD at 80,000 A, so a 40 mm disc at 40,000 A ties the first and loses to the second. We counted 19 such ties and 19 matching losses. Second, the 20 mm Standard disc is rated 6,500 A, the highest tier of its series, while the 18 mm High Energy and 14 mm Super High Energy discs carry 9,000 A and 8,000 A as their single figures. Our post on 40,000 A against 80,000 A Industrial varistors covers the 34R and 34RD split in detail.

We repeated the grouping on Max Working Volt. in place of Varistor Voltage. The two map one-to-one across 35 groups and the result is identical: 79 inversions, 19 ties, 2,340 pairs. The voltage disagreement in 65 of 379 varistor rows does not change it, because we grouped on the columns and not on the part number.

What does one voltage step look like when you line it up by diameter?

Seventeen parts share the 390 Vdc, 250 Vac step, and three of them rate below a part with a smaller disc. We sorted the step by diameter and checked each rating against the best rating of every smaller diameter. SR391K20D, SR391K40D and SR391K53D fail that test; the other fourteen pass.

Part number	Series	Diameter in Size field	Peak current	Best rating among smaller diameters	Match / mismatch	Your incumbent part
SR391K05D	Standard (D)	5 mm	400 A	none smaller	Match	______
SR391K05E	High Energy (E)	5 mm	800 A	none smaller	Match	______
SR391K07D	Standard (D)	7 mm	1,200 A	800 A	Match	______
SR391K07E	High Energy (E)	7 mm	1,750 A	800 A	Match	______
SR391K10D	Standard (D)	10 mm	2,500 A	1,750 A	Match	______
SR391K10E	Super High Energy (V)	10 mm	4,000 A	1,750 A	Match	______
SR391K14D	Standard (D)	14 mm	4,500 A	4,000 A	Match	______
SR391K14E	Super High Energy (V)	14 mm	8,000 A	4,000 A	Match	______
SR391K18E	High Energy (E)	18 mm	9,000 A	8,000 A	Match	______
SR391K20D	Standard (D)	20 mm	6,500 A	9,000 A	Mismatch — lower than an 18 mm disc	______
SR391K20E	Super High Energy (V)	20 mm	13,000 A	9,000 A	Match	______
SR391K25D	Standard (D)	25 mm	18,000 A	13,000 A	Match	______
SR391K32D	Industrial	32 mm	30,000 A	18,000 A	Match	______
SR391K34R	Industrial	34 mm square	40,000 A	30,000 A	Match	______
SR391K34RD	Industrial	34 mm square	80,000 A	30,000 A	Match	______
SR391K40D	Industrial	40 mm	40,000 A	80,000 A	Mismatch — half of 34RD	______
SR391K53D	Industrial	53 mm	70,000 A	80,000 A	Mismatch — lower than 34RD	______

Note the suffix. SR391K20E is Super High Energy and SR391K18E is High Energy; both end in E, and only the size code separates them. The 20 mm row that fails is the D part, not the E part. Selecting by diameter alone at this step returns two different 20 mm parts, rated 6,500 A and 13,000 A.

Nominal area does not rescue the rule. A 20 mm round disc is about 314 square millimetres against about 254 for 18 mm and 154 for 14 mm; a 40 mm round disc is about 1,257 against about 1,156 for a 34 mm square.

What does the peak current figure not tell an engineer in Italy?

Most of what a surge design needs. The export carries one peak current per part and no energy rating in joules, no pulse waveform, no pulse count and no clamping voltage. We opened all 10 columns of Walsin_MOV_ALL.xlsx: Packing, SPQ and MOQ are the only others besides part number, series, size, working voltage, varistor voltage, tolerance and peak current. Tolerance reads +/-10% on all 379 rows.

So peak current is the only surge number published, and an engineer in Italy sizing a mains-input device cannot rank two discs on energy from this file. We hold no varistor datasheet, and a datasheet would carry the waveform conditions. For what a datasheet adds beyond the export, see what a varistor datasheet tells you that the export does not.

How should a buyer use this when crossing a varistor to Walsin?

Compare the voltage first, then the series and size code, and treat diameter as a footprint field and never as a rating proxy. Of the 379 parts, 119 sit in an inverted pair, so a diameter-first filter misranks nearly one part in three. Our free cross-reference tool states field-by-field differences from your original; for varistors it cannot rank by joules because the file has none.

1. Match working and varistor voltage. The export has 35 voltage steps from 18 Vdc to 1,181 Vdc.
2. Read the series and the size code together. The final letter alone does not identify the series.
3. Compare peak current directly. Do not infer it from the diameter. 79 of 2,340 pairs show why.
4. Put your incumbent part in the blank column of the tables above and check each field, including the ones the export leaves out.

Stock is a dated snapshot. The ready stock page lists 1,125 part numbers as at 23 September 2026, and varistors are not a product line in it, so ask us about availability rather than assume. The full range sits on the MOV page. Documentation we hold is 28 Walsin datasheets, 451 pages, none for varistors.

The short version

In Walsin’s 379-part varistor export, 79 of 2,340 same-voltage pairs of different diameter rate the larger disc lower, 3.4 percent, touching 119 parts. The Standard, High Energy and Super High Energy series never invert across 467 pairs; the breaks are 39 inside the Industrial series and 40 between a 20 mm Standard disc and smaller High Energy and Super High Energy discs. At 20 of 34 multi-diameter voltage steps the largest disc is not the highest rated. The export has peak current and no joules, so it cannot settle the question further. We will not tell you a varistor matches because its diameter does. We will show you the rating the file holds. Confirm against both datasheets and qualify in your own circuit.

Send us the varistor line off your bill of materials with diameter, voltage and surge rating, and we will tell you which pairs in this file invert for your part. And a question back: when you last chose a varistor, did you pick the rating or the disc?

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Frequently asked questions

<!– wp:rank-math/faq-block {"questions":[{"id":"faq-question-01","title":"Does a larger varistor disc always have a higher peak current rating in Walsin's range?","content":"No. In Walsin's 379-part export we compared 2,340 same-voltage pairs of different diameter. 79 of them rate the larger disc lower, 3.4 percent, and 19 more tie. The inversions sit between 130 Vac and 680 Vac, and they involve 119 of the 379 part numbers, so diameter is a poor proxy for rating.","visible":true},{"id":"faq-question-02","title":"Which Walsin varistor series never invert on disc size?","content":"Standard (D), High Energy (E) and Super High Energy (V). We counted 365, 42 and 60 cross-diameter pairs inside those three series respectively, 467 in all, and the larger disc is rated higher in every one. Inversions appear only inside the Industrial series, 39 of 173 pairs, and between series, 40 of 1,700 pairs.","visible":true},{"id":"faq-question-03","title":"Why is a 20 mm Standard varistor rated lower than an 18 mm High Energy one?","content":"The export publishes 6,500 A for the 20 mm Standard disc and 9,000 A for the 18 mm High Energy disc at the same varistor voltage, on 20 voltage steps from 130 Vac to 680 Vac. The file gives no reason. We hold no varistor datasheet, so the explanation is not in the documents we hold.","visible":true},{"id":"faq-question-04","title":"Is the 34 mm Industrial varistor stronger than the 53 mm one?","content":"On the published figure, yes, for the 34RD suffix: 80,000 A against 70,000 A at 20 shared voltage steps. The 34R suffix is rated 40,000 A. A 40 mm disc at 40,000 A ties the 34R on 19 steps and is half the 34RD. Check the suffix, not just the diameter.”,”visible”:true},{“id”:”faq-question-05″,”title”:”Does Walsin publish a joule energy rating for these varistors?”,”content”:”Not in the export we hold. Each of the 379 rows carries one peak current figure and no energy in joules, no pulse waveform and no clamping voltage. We hold no varistor datasheet, so confirm the conditions behind the peak current against Walsin’s own documentation for the exact part number.”,”visible”:true},{“id”:”faq-question-06″,”title”:”What should an engineer in Italy check before crossing a varistor to Walsin?”,”content”:”Match the working and varistor voltage first, then read series and size code together, then compare peak current directly rather than inferring it from diameter. 119 of 379 parts sit in an inverted pair. Which standard or requirement governs your design is not in the documents we hold, so confirm it yourself.”,”visible”:true},{“id”:”faq-question-07″,”title”:”Are Walsin varistors on the ready-stock list?”,”content”:”The snapshot dated 23 September 2026 lists 1,125 part numbers, and varistors do not appear as a product line in it. Stock is a dated snapshot and not a live feed, so contact us with the part number you need and we will check what is held in Hong Kong.”,”visible”:true}],”listStyle”:””,”titleWrapper”:”h3″,”titleCssClasses”:””,”contentCssClasses”:””,”sizeSlug”:””} –>

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