What are Y-Capacitors? Understanding the Different Classes and Ratings

[Guineafowl]

Hi all,

An interesting old device for you, circa 1984. It comes in a nicely varnished plywood cabinet:

To restore it, I had to do some cabinetry to fix some broken seams, and a relay had gone bad inside. All working now.

On the inner lid is a sheet with full operating instructions. On the right is the tester itself, with a neat compartment for the test leads:

The unit switches on with a satisfying ‘clunk’ of a relay, and the upper orange neon glows to show the power supply is OK. You plug the DUT into the socket, connect the flying Earth lead to its metalwork (if applicable) and select:

0.1ohm Earth circuit test/1250V flash:
First applies 8 Vac at 26 amps through the appliance’s Earth circuit. Displays Earth path resistance on meter. Then applies 1250V between L+N to Earth to check insulation resistance. If all is well, the two green pass neons glow.

0.5 ohm Earth circuit test:
Same as above, but only 10 A through Earth path and allows up to 0.5 ohm resistance there. For appliances with long leads.

0.5 ohm Earth circuit test/500V IR test:
Again, a gentler Earth test and only 500V IR test for delicate appliances.

3000 and 4000V Flash test:
Applies these voltages between L+N and the high voltage probe (see below). You probe any metalwork on the (double insulated) appliance to check for flashover. If so, the machine will cut the test short and the lower right fault lamp will glow. Sparks and crackling happen while you’re doing this.

The load test switch activates a contactor and actually runs the appliance, so you can check the current is within spec. A 16A trip is present. The button above divides the scale by 10 for smaller appliances.

Showing the probe, Earth clip and two fault simulator plugs.

Hope this is interesting to someone - it was hard to find info online about the machine, so that pasted instruction sheet was very useful. It’s so much more fun testing an appliance with this - the clunks and clicks, the glowing lamps - you really feel like you’re doing something. Better than a modern tester which is just plug in, press test, BEEP.

 

[dlgoff]

Hope this is interesting to someone

Thanks for sharing. This is right down my hobby line.

 

[Guineafowl]

Thanks for sharing. This is right down my hobby line.

Yes, I posted a hand-cranked 500V insulation tester from the 40’s there a while back. A very valuable thread.

I’m not sure why the insulation tests are at 1250V - today, 500V is used as it is just higher than the expected peak voltage of 330V or so from a 240V rms supply.

The Earth test seems a good idea - a shaky Earth connection would surely fuse at 26A, but may appear good using a simple multimeter continuity test. Things get warm when you test them.

Would you dare test a USB wall wart with the 3000/4000V flash test? This would put that voltage between L+N and the probe, with the latter touched to the USB casing. I ask because I think it’s popped my Apple phone charger. As you can imagine, I’ve been playing with it. The probe hurts when I apply it to my leg.

 

[dlgoff]

I ask because I think it’s popped my Apple phone charger.

Live and learn I guess. That is a lot of voltage.

As you can imagine, I’ve been playing with it. The probe hurts when I apply it to my leg.

Live and learn I guess. That is a lot of voltage.

 

[Guineafowl]

Live and learn I guess. That is a lot of voltage.

Live and learn I guess. That is a lot of voltage.

On the autopsy (of the Apple charger!) there was a shorted diode on the primary side; presumably for rectifying the bootstrap power from the auxiliary winding.

From further reading, the 3000V setting is for normal testing of equipment, the 4000V being for repaired items. It says 4000V is (was) the standard flashover rating for manufacturers of double insulated appliances. ‘Twas the 4000V setting that killed the charger.

Now, I guess switch mode power supplies were around in 1984, but maybe not the miniaturised type we see today. Is it possible that standards have been lowered to accommodate these newer wall warts? It has a bearing on some threads on PF about the safety of phone chargers - after all, the phone’s metal casing is in contact with the USB’s, and that is coupled to mains via a class Y capacitor.

What is the flashover voltage rating for a good USB SMPS? I expect it depends on that class Y, the optopcoupler, transformer and the general clearance/creepage of the PCB. Also, what was the point of the older 4000V standard? Lightning strike?

 

[dlgoff]

I expect it depends on that class Y, ...

I did a little Googling and found this in Back to Basics: What are Y-Capacitors?

Y-capacitors, which are used to bridge operational insulation, are classified as Y1, Y2, Y3 or Y4 according to type of bridged insulation as well as AC and peak voltage ratings. Y1 class capacitors are rated up to 500 Vac, with a peak test voltage of 8 kV. Y2 capacitors have 150 to 300 Vac ratings and a peak test voltage of 5 kV. Y3 capacitors are rated to 250 Vac with no peak test voltage specified. Y4 capacitors are rated to 150 Vac with a peak test voltage of 2.5 kV.