General > General Technical Chat
Stupid voltage leak question
Zero999:
--- Quote from: ebastler on January 13, 2024, 01:56:39 pm ---
--- Quote from: Zero999 on January 13, 2024, 01:49:40 pm ---Except, if you re-read the thread carefully, you'll find the correct answer. I also dispute part about parasitic capacitances.
--- End quote ---
Yes, you pointed out the relevant question before. But I felt that you also jumped to a particular conclusion, which may be the wrong one; hence thought that I should elaborate a bit.
Considering the boundary conditions under which the tingle was felt (barefoot on a cement floor), I would certainly not rule out harmless capacitive coupling to a metal part that is floating by design. What constitutes a "tingle" is a very subjective assessment; it might just be the 60 Hz "vibrations" which are typically felt. Let's wait whether the OP comes back with more details about his espresso machine and the specific metal parts that caused the sensation.
--- End quote ---
The reason why I doubt it's parasitic capacitance is because I can touch one side of a toroidal 1kVA transformer with one hand and earth with another and don't feel a thing and such a transformer will have far greater parasitic capacitance than a coffee maker. Mechanical vibration is a possibility, but doubt it would have raised the same level of alarm, because it would be audible i.e. the pump running.
I prefer to err on the side of caution, especially given the wording of the original post indicates it was written by a beginner..
ebastler:
--- Quote from: nctnico on January 13, 2024, 02:20:26 pm ---On a grounded device, all metal parts should be connected to the ground pin.
--- End quote ---
I very much doubt that. As I said a few posts above, e.g. some decorative metal panels glued onto the plastic enclosure might well be unconnected to anything (and rightfully so, since they are double-isolated from mains). At the same time, the boiler and group head need to be grounded, and hence the machine comes with a grounded plug. My little portafilter espresso machine has a full metal enclosure which obviously is grounded, but the metal drip tray cover is not, since it resides outside of anything electrical. Etc...
Siwastaja:
--- Quote from: nctnico on January 13, 2024, 02:20:26 pm ---On a grounded device, all metal parts should be connected to the ground pin.
--- End quote ---
Not true at all. The world is full of appliances which are grounded but have floating metal parts; in fact I would dare to say 99% have some, it's very difficult to ground every metal piece. In no legislation they are illegal. Decorative stainless steel surfaces on knobs etc. are a great example.
Of course for any such floating metal part, it has to conform to double insulation requirements (or something similar; exact details depend on country). Every metal part which is user-accessible AND which can touch a live (neutral or line) wire due to a single fault has to be grounded to protective earth. But this does not comprise every metal part.
Nominal Animal:
On some non-electrical "devices", like the aluminium pencil tray on a whiteboard, you can sometimes find static dissipation resistors (1 - 2 MΩ) to a protective earth (true ground potential, or any much larger metal structure).
Fully isolated conductive surfaces (anodized and brushed aluminium in particular, having that isolating oxide layer on top) can get statically charged to very high relative potentials in dry air conditions (e.g., indoors during cold winters here in Finland), so if they are intended to be touched by humans, static dissipation resistors can be used to stop the painful zaps one would otherwise get. (My "best zap yet" was an arc ~ 10 cm long from a door handle to my middle finger, which numbed my hand to my elbow for about a quarter of an hour. Painful. Very dry air, plus static charge accumulating clothing, socks, and carpeting, all contributed.)
This is a particular problem with class II and class III (or SELV, separated extra-low voltage, powered devices), since they do not have a ground conductor to the actual device/appliance at all.
There are two mechanisms – capacitive coupling and static charge accumulation – such isolated but highly conductive surfaces can cause a tingle or a zap when touched by a hunam. Capacitive coupling carries a low AC current, sub-milliamp range, via the surface through the human to a ground potential, and produces a tingle. Large enough potential difference causes a single zap, which equalizes the potentials. The zap is typically in the kilovolt to a few dozen kilovolt range, but with very little current, in a very short pulse. (The instant wattage can be surprisingly high for a very small fraction of a second.)
Non-electrical devices are easy, as they can be safely grounded via a large enough resistor, but very conductive decorative panels on electrical devices gives me the heebie-jeebies: I don't know how to deal with those properly, unless I've built the device/enclosure myself, and understand the grounding and protection scheme. (I only do class III/SELV devices, hobby-grade, though: routers, single-board computers, and such.)
I suspect, but have not verified, that a paint, coating, or replacement panel that is only slightly conductive (i.e., "static dissipative"; sufficient resistance) is the only generally safe option in all cases. This dissipates some of the electrical power, and spreads out the zap pulse in time, leading to lower voltage and current on contact, helping with both the tingle and the zap, compared to a completely isolated well-conducting surface.
Plastics and metals are easy to prep – sand, then use proper primer for the plastic/metal at hand, then thin layers of surface paint or laquer with fine sanding (600 grit and better) in between to remove bumps and hairs, with optionally a final linear pass with a scouring pad to get a brushed steel/aluminium effect; and not even the spouse will complain –, so it is only a question of finding a surface paint or lacquer in a rattle can form, providing a slightly conductive, static dissipative surface.
Does anybody know of any specific paints or lacquers for this, for home/hobby use? I'm particularly interested in durable conductive carbon acrylic/polyurethane aerosol paints in rattle cans myself.
soldar:
--- Quote from: Zero999 on January 13, 2024, 02:30:03 pm ---The reason why I doubt it's parasitic capacitance is because I can touch one side of a toroidal 1kVA transformer with one hand and earth with another and don't feel a thing and such a transformer will have far greater parasitic capacitance than a coffee maker. Mechanical vibration is a possibility, but doubt it would have raised the same level of alarm, because it would be audible i.e. the pump running.
I prefer to err on the side of caution, especially given the wording of the original post indicates it was written by a beginner..
--- End quote ---
Many years ago the steel kitchen sink was giving me the tingling. The sink was electrically isolated because it had PVC drain. After some confusion I determined the sink was touching some aluminum foil which, in turn, was touching a toaster oven. The all-metal toaster oven had two heating elements which were metal tubes with, I assume, resistive wires inside, isolated from the outside metal tube which was held in place and electrically connected to the metal structure of the oven. Measuring resistance between the internal heating element and the external metal tube the reading is infinite resistance. But there was enough capacitance to give that tingling. The oven was supposed to be connected to earth and that would solve the problem but my apartment at that time did not have protective earth wire in the supply. This was long time ago. This was a 6th floor apartment and the building did not have earth wiring.
I bought some heat resistant flexible tubing and put it between the metal tube and the metal clamps which hold it in place and that resolved the problem.
I think it might be asbestos but at that time asbestos was not yet dangerous. I still have and use this toaster oven but now it is grounded.
Just an example if capacitive leak. With the oven grounded there is no problem but with the metal housing floating it does give the tingles.
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