Mains hum

[geoff]

But only if you're (in the eyes of the peelers) a shifty looking forriner

[geoff]

In message , Piers Finlayson writes

Never been a morris dancer, then ?

[John Rumm]

Indeed. Some info on testing and tracing:

[John Rumm]

One typically chooses filter capacitor values such that the reactance at mains frequency is such that they can only pass relatively small currents, certainly not enough to kill or injure.

(also you need to be able to have a bunch of them on a circuit and not trip a 30mA threshold RCD!)

[Dave Plowman (News)]

I've known quite a few small value ceramic capacitors fail short circuit - but on low voltage DC circuits. Do they use some special type that positively can't do this?

[fred]

In article , Dave Plowman (News) writes

Yes, you can get safety rated capacitors for this job:

Class X capacitors are safe to use between line and neutral where failure would not expose the user to a shock but where a fault would need to be cleared in a safe manner.

and

Class Y capacitors are safe to use between line and earth or other positions[1] where failure could expose the user to a shock.

[1] eg across a double insulated (or reinforced insulation) barrier such as between input and output of an unearthed (class 2) switched mode power supply, as discussed on this thread.

Commonly available as metal film or ceramic, ceramic better for HF suppression due to their construction.

Class X:

X/Y (dual purpose):

[John Rumm]

They do. Many are designed to be internally fusible as well. There are also various rules requiring them to be self extinguishing as well IIRC.

[Jules]

And holy cow do they ever generate a lot of smoke and make a mess when they do fail :-) (I've replaced many a one in vintage equipment - and they have a habit of seeming fine in something which hasn't powered up in years, but then abruptly let go after an hour or two of running)

All the old ones seem to be rated to 250V - a lot of the newer ones are rated to 275V. I'm not sure if the failures of the original parts are purely down to age, or if the lower operating margin contributes in some way.

cheers

Jules

[The Wanderer]

Unfortunately, many major manufacturers and suppliers source quite a lot of their stuff from China, and I wouldn't like to assert that everything that comes out of China meets all the various rules and regulations that hold in the west.

[John Rumm]

No, probably not. Although if you get ones certified to the relevant IEC standard I suppose they ought to be ok.

[john]

Indeed. At this location though, I've now so much SMPS kit hooked up (I'm stuck with the s**te), that just an extra 5-10ma of leakage trips the lab 30ma RCD!.

As a good reference source for low power SMPS designs I use ...

a good discussion of the SMPS power supply leakage problem is here ... Rod Elliot site shows the common 2 filtering capacitor setup, hence the reason for 120Vac sitting on the DC output. The TopSwitch people seem to use EMC cap' values up to 4n7, which implies about 0.4ma mains leakage ,(rising to amps under transient connection conditions) but I've a suspicion I've seen research into 'onset of fibrillation ma levels' not too far up from this value. Also, I had a lab wall wart PSU, that I found had a 275Vac Varistor fitted instead of the X rated filter cap. Fine for months, then the Varistor started leaking badly and blew =A3200 of connected test kit.

Gimme Victorian technology, 50Hz, heavy metal transformers, any day!

[geoff]

Smelly tar, don't forget the smelly tar

[D.M. Procida]

So why does it not happen using the same equipment on another socket? That's the slightly troubling puzzle.

Daniele

[D.M. Procida]

Using a plug-in tester - it reports everything correct.

Daniele

[Dave Plowman (News)]

Socket in the same room? I'd check them out. If in another room more likely the floor material providing a better earth path.

[D.M. Procida]

No, different rooms. In both cases though the floors are wood (one carpeted) and I was wearing rubber-soled footwear.

Daniele