Are MLCCs really suitable for safety isolation (Y-cap) use?

Question

I am looking at a ~1nF capacitor for Y1 safety use. I am aiming for SMT so as to avoid a THT step.

I see various MLCCs that fit the bill, for instance the following : SCC4012X102K8C2TS , DK1E3EA102M86HAH01

Digikey Links: https://www.digikey.com/en/products/detail/holy-stone-enterprise-co-ltd/SCC4012X102K8C2TS/17952318 https://www.digikey.com/en/products/detail/murata-electronics/DK1E3EA102M86HAH01/22528377

My understanding is that Y caps are designed to fail open and thus avoid risk of short. However, MLCC are notorious for cracking and failing short if, e.g., they are placed near an edge, are twisted, etc.

Is it safe to assume that these MLCCs are somehow different and will fail OPEN when cracked? I'm thinking -- no.

How then are these able to be used as Y1/Y2/etc. capacitors when there is a well-known single fault (PCB flexing resulting in cracking resulting in a short) that would result in shorting across the isolation gap?

Answer 1

The first thing you should look for is the safety approval(s) for the capacitors for EMI purposes. It's UL/BS/EN/IEC 60384-14 for the EU and the UK (or Europe in general, but not for the US or China). Both capacitors have these approvals.

Is it safe to assume that these MLCCs are somehow different and will fail OPEN when cracked?

From a safety point of view, X- and Y-rated capacitors are designed not to fail at all under any of the test conditions defined in the standards. The standard mentioned above covers electrical, environmental, mechanical and endurance tests, and these are common for all as far as I'm aware. Check if your application involves any higher stresses.

Because the only difference between X- and Y- ratings is the peak impulse voltages (i.e. 8 kV for Y1, and 4 kV for X1 – you can think of the Y as double-insulated X) if a capacitor survives all the tests and gets a Y1 rating then it can also have an X1 rating easily, hence the X1/Y1 capacitors you see in the market. Similarly, there are capacitors with X1/Y2 rating, quite possibly they can withstand up to 5 kV pulses (Y2) so why not have an X1 as well? (*)

When it comes to product safety (e.g. IEC 62368-1), things can be different. For example, you may be limited to using only Y1 caps (or two Y2 in series instead of one Y1 cap) across the barrier or across L/N and earth.

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(*) I find this a marketing trick but it is what it is. If I put a capacitor across the L & N for EMI, I wouldn't want it to fail open despite this doesn't present a safety concern. I would like it to trip the breaker rather than run with less (or no) filtering.