"earth" is connected to exposed touch-able parts on this coffee grinder. It does bite! It has the double insulated mark which per my standards is incorrect.
It has an unearthed input.
Schematic: https://i.stack.imgur.com/25FOs.jpg
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\$\begingroup\$ How many wires are inside the AC power lead? Also, are there two 4n7 capacitors, each from live or neutral to the earth point? How old is the appliance? \$\endgroup\$– Andy akaNov 27, 2016 at 14:03
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\$\begingroup\$ @Andyaka it might be a 1991 week 9 date code on that larger capacitor. \$\endgroup\$– KalleMPNov 27, 2016 at 14:28
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1\$\begingroup\$ Double insulated means pretty screwdriver, not post screwdriver. \$\endgroup\$– Scott SeidmanNov 27, 2016 at 14:31
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\$\begingroup\$ @ScottSeidman If I had to guess the chopping blades (and bowl) of the grinder are connected to the internal motor 'earth' point that is virtual for the purposes of interference suppression and not for user safety. If they stick their finger in the machine when it is plugged in it is against the common user instructions I have seen in grinders. Grinders I have investigated have had plastic parts between the motor shaft and cutter blades so never had this occur to me. \$\endgroup\$– KalleMPNov 27, 2016 at 14:37
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\$\begingroup\$ Can you post closeups of both sides of the 4.7nF caps? \$\endgroup\$– ThreePhaseEelNov 27, 2016 at 15:00
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1 Answer
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The tingle you feel is the current passing from the one interference supression capacitor connected to the live leg of the mains input. The metal parts are effectively held at half the line voltage through a very high impedance (small cap at 50/60 Hz).
It is unlikely to hurt you unless the caps were to fail. The double insulated symbol is probably optimistic. I believe there need to be two insulating barriers between live and any exposed metal parts before this symbol is appropriate, in this case you have one cap that needs to fail before you get full line voltage, a bit unsettling but not all that rare.
If you are up to the task you could replace the capacitors with two modern 500V units if you suspect they are faulty, the tingling may be reduced but unlikely to be eliminated.
EDIT:
I forgot to add that if you have an earthed plug (and as Andy mentioned 3 conductor power cable) and it is in an earthed outlet you should not be able to feel any tingle and any such tingling should trip the residual current circuit interrupter if it exceed about 10 to 30 mA.
Sadly in Europe you can plug an earthed Schuko plug into an old unearthed socket so the earth protection situation is a bit primitive.
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1\$\begingroup\$ I have no such RCDs. All our outlets are earthed and the plug is a molded europlug \$\endgroup\$– cccp3Nov 27, 2016 at 18:07
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2\$\begingroup\$ By EuroPlug you mean the two prong? If so then your device has no earth and will remain liable to have this tingling and also not conform to double insulation. If you unplug before sticking your finger inside you will be safe, if you want others to be safe as well consider replacing with an earthed plug and cable and connecting the earth to the point with the red wire. \$\endgroup\$– KalleMPNov 27, 2016 at 22:34
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1\$\begingroup\$ yes it is 2 prong. nobody else is using this grinder and besides i trust those capacitors enough \$\endgroup\$– cccp3Nov 28, 2016 at 5:53
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\$\begingroup\$ The double insulated requirement might be met if the lid of the appliance operates as a switch, keeping the capacitors unenergized until the lid is in place. One cannot touch the metal parts with the lid on. \$\endgroup\$– Whit3rdNov 28, 2016 at 9:40
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\$\begingroup\$ @Whit3rd The switch is often (but not always) mechanically interlocked with the lid (though not always, an east european unit I once had had the switch on the side). The double insulation would be true if both poles of the mains were switched but here it looks like just one is (as is often the case) and there is no way to know if it will be the live or neutral on any given occasion. \$\endgroup\$– KalleMPNov 28, 2016 at 12:06