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Today we've had two WS-12-250-AC which, when first connected to power, had something blow up next to where mains power is connected at the internal power supply.

The picture shows that part: there's a black F1 fuse(?) block and just above it, next to the mains posts, there's a blue coil(?) or whatever. After the flash the blue thing had some black stuff on it, which was also spread to the mains cables (not shown in the picture).

The first switch also blew our fuse in the lab when it did that, the second did not. Both switches seem to work normally, though, so whatever blew up is not vital for operation (of the power supply brick).

(Sorry if this was documented before, but I wouldn't know what keywords to use, and a quick scan through the subject lines didn't reveal anything similar.)

The blue thing is a Metal Oxide Varistor. They're normally used for transient suppression on AC lines: if you open up a cheap surge protector, all it is is a switch and two of those wired across line/neutral.

Kinda confused here, normally it takes a lot to blow one of those. I'm not ruling out faulty components, but two in a row is hard to wrap my head around as a normal failure case.

When you put a meter between line and ground on your mains supply, what do you get?

I don't think the blue MOV has really blown up - the black material seems to be on its outside only. OTOH, the fuse F1 hasn't blown either or at least we couldn't see how - it could be something else. When it happens, a flash comes out through the case. I was thinking there has been a short on the board which has been melted away - mains power could do that.

We will put a meter between line and ground, but I wouldn't expect to find anything we could judge as being unusual - remember: the switches are now working just fine.

I linked this post to our engineer at the power supply company that build them to get a response but probably will not hear back from them until Monday.

My engineer came up with a theory. In fact, the first switch was first connected in the lab - and it didn't "flash" there. Next itw as brought to my desk for me to configure and it was there that it flashed. The second switch was brought directly to my desk and again it flashed. Both are working normally after the flash but why would the first one not flash in the lab?

European power plugs can be put in either way, so you're never sure what is "neutral" and what is "line". However, if there's a short between one of them and ground, you could connect it to power and nothing happens because the short is between neutral and ground. But if it is next connected to a power cord which has neutral and line reversed, as it may be the case for the power cable one on my desk, the short is between line and ground - and boom.

Yeah, we can wait. The third WS-12-250-A we tried didn't blow, btw, like many before.

tma wrote:My engineer came up with a theory. In fact, the first switch was first connected in the lab - and it didn't "flash" there. Next itw as brought to my desk for me to configure and it was there that it flashed. The second switch was brought directly to my desk and again it flashed. Both are working normally after the flash but why would the first one not flash in the lab?

European power plugs can be put in either way, so you're never sure what is "neutral" and what is "line". However, if there's a short between one of them and ground, you could connect it to power and nothing happens because the short is between neutral and ground. But if it is next connected to a power cord which has neutral and line reversed, as it may be the case for the power cable one on my desk, the short is between line and ground - and boom.

That is a good hypothesis. Decades ago I had a power supply that had a short between neutral and ground. It passed my bench test before I delivered it to the customer but the moment I connected it at the customer's location, it blew. The customer's outlet was wired backwards.

I got curious, since that hypothesis seemed to have some merit to me, and swapped line and neutral on the input of 5 of our 250w supplies; I was unable to confirm, unfortunately. I used supplies from several different production runs, none of them had any observable issue, no sparks.

This is why I love electronics.. everything makes sense, until it doesn't.

I guess we wait until Monday to see what our supplier has to say.

I "hope" to have an answer from supplier by Monday

Dear All, thanks your accompany with our power supply. We did study this issue and try to duplicate the "Spark" including using reverse polarity cable. But we have not managed it. Our QC has issued a analysis report but the cause is not clear. They think- the switch inlcuding PSU has no problem since PSU still has output and work normally after "Spark", it should result from the outer paraments. Potential possibilities: First, there are conductive substance drop to the input side of PSU, It spark when power up, and leave the black stuff. The substance could be: residual tin of PSU, residual metal fillings of housing of switch etc. Second, the relative ambient temperature varies between office and lab, surface moisture condense, decrease the insulation capability of AC side, it spark...

Anyway we could say all are hypothesis, we might need the real parts to do further study if we need to find the real cause.