I have an electric coffee-grinder that plugs directly into the mains (not via an adapter). When I have finished using it, I unplug it, wrap the power cord around it and put it away in a cupboard.
Recently I’ve noticed that if I accidentally touch the prongs of the plug while putting it away, I get quite a zap. This happens even if it’s been unplugged for quite some time. What’s going on, and should I be concerned?
Edit: I should add that I’m in the UK, where we have three-pin plugs and earthed (grounded) outlets. Does this indicate a possible problem with the earthing on the power outlet I use to plug in the coffee grinder? I would have thought that any residual charge should be dissipated to ground once the grinder is switched off and still plugged in?
Just to be clear, you’re saying that you are receiving an electrical shock from the prongs of a coffee grinder’s electrical plug that is not connected to an electrical outlet.
Do you know which two, of the three, prongs is providing the zap?
Does the coffee grinder have a battery onboard? A battery and transformer could cause a strong enough discharge that you’d notice it.
A capacitor can also hold a charge.
However, both of these options indicate there is something very wrong with your coffee grinder. Test the leads with a volt/ohm meter to get some idea of what voltage you’re dealing with, and to find out which two leads are producing the zap.
What make and model coffee grinder are we talking about? There might be a schematic online?
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First, this is not a fault, but perfectly normal and safe (if slightly unpleasant!).
The large brushed motor in a food processor can generate small sparks and electrical noise (interference to radio signals) when it is running, so a small capacitor is connected across the AC mains to suppress this noise.
If you unplug the appliance when the AC voltage is at its peak, the capacitor is charged, and can hold its charge until you touch the pins.
It is a low value capacitor (typically 0.1 uF) so the charge stored should not be dangerous. (It is also a special construction - usually “class X2” so that if the capacitor breaks down it will fail safe instead of creating a short circuit)
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That is correct. I don’t know which prongs are responsible: I haven’t done any deliberate testing, all I know is if I pick up the plug while I’m putting it away I sometimes get a shock. I don’t have an electrical meter.
I wonder if it’s something more simple, like a combination of dry weather of static building up from sliding it across the shelf when you pull it out to use it.
If you want to experiment, try touching the prongs right after you use it. If the shock is originating from the wall, it should happen then as well.
Do everything as normal, but next time, don’t put it away. Do you get a shock this time too?
Have you been wearing different shoes? Try grounding yourself before you grab the coffee grinder for a few days. Maybe you’re the one with the static and the coffee grinder (with it’s motor windings) is big enough to let your static discharge.
Haven’t found a wiring schematic yet. I assume the Hampshire, England power company supplies 230vac at 50Hz. I’ve found 120vac grinder motors for the American market, and 230vac motors for the European markets but no schematics.
In the world of electric motors, there may be a run capacitor, a start capacitor, dual capacitors, or no capacitor. Capacitors are used to ease the start up load on the motor. The sudden application of a full 230v could cause the motor’s windings to burn out, or burn up the connecting wires, or blow fuses. (FYI - Trying to run a 120v device on 230v is an expensive way to test your smoke detectors.)
To the layman, the capacitor takes 99% of the load, and as it charges, it requires less and less electricity to charge. The capacitor’s load goes from 99% to 0%, when it’s fully charged. The effect is that the electric motor initially receives 1% and increases to 100%. The capacitor allows the electric motor to start spinning slowly. Less strain on the motor, gears, belts, bearings, etc…
Assuming there is a capacitor/motor, the capacitor would be fully charged when you turned the grinder off. It’s just waiting for you to complete the circuit that would discharge/drain the capacitor.
Would something as small as a coffee grinder need a start or run cap?
If it did and if it could backfeed into the prongs (or the mains if it was left plugged in and turned off), then my guess is that is has a bad bleeder resistor.
It sounds like there’s a ceramic cap inside the unit between the incoming neutral and hot lines that functions as an EMI filter. The designers should have included a 1 MΩ bleeder resistor across the cap, especially if the power switch is downstream from the cap.
Thanks for the replies. I’ll investigate at the weekend (I usually only have the time for fresh ground coffee at weekends!)
The grinder came via a family member some time ago so I can’t return it, and don’t really want to fork out for a new one, as it works fine.
I don’t think it’s static, as I never get any static shocks off anything else metal in my kitchen. Plus it’s a bit more of a jolt than a simple static zap.
