Mitigating touch current in two-pronged AC without dedicated ground

I'm trying to work on a simple circuit where (simplified) I plug in a 240V 50Hz AC through a resistive coil to convert to heat. The coil is insulated and placed under a metal plate. I'm noticing that if I touch the plate, I would feel a slight buzzing/tingling sensation, AKA touch current. I have some questions about that:

1. Is this effect due to induction, or is it a capacitance phenomenon, (or something else??)

2. How should I design to ground the plate to eliminate/reduce the touch current? It's easy with 3-pronged plugs with a dedicated earth pin, but old EU plugs are round two-prong and indiscriminately reversible making grounding tricky. The solution would have to work when I plug it in either direction.

To be easily perceptible the current has to be fairly large (at least hundreds of uA). To get, say, 500uA by capacitive coupling with a 240VAC difference would require a capacitance of around 7nF, which is pretty huge. If the plate is like 25cm x 25cm and the insulation is rather thin (like 0.25mm) it's possible. UL considers 0.5mA to be the limit of perception and it's also their limit for allowable leakage in many situations.

The other possibility that that you are using an insulation type that leaks at high temperatures and you are allowing the insulation to get very hot. This is not uncommon with sheathed heating elements.

Your best bet might be to make the insulation a bit thicker, and find a dielectric material that transmits heat well enough that provides some separation. Even 1mm would be a big improvement. Lower dielectric constant is also better.

I'm not addressing issues of safety or standards here, that's more-or-less a separate issue. But you definitely do not want to connect the plate to your 2-pin plug.

• Thanks for the insight. I will check that out! – Arthur Mar 29 at 16:58
• EU had a 2.5mA leakage spec vs UL's 0.5mA. I'm not sure if it has changed but EU users are favouring UL rated GFCI's that have a lower trip threshold. – Sunnyskyguy EE75 Mar 29 at 18:25

No matter how it is coupled, the current you feel is a result is a potential voltage difference between you and the plate. You are coupled thru stray e-fields in the room thru the moist air and it goes thru your body's much lower imepdance without any sensation. But the impedance of the plate close to the coil increases the capacitance and puts a time-varying (high impedance) voltage on the plate.

This difference permits some xx to xxx uA of touch leakage.

Earth-bonding like metal stoves and ovens could be the prudent fix with a 3 wire cordset.

A photo may allow more detailed suggestions how to connect the ground to the plate.

An afterthought approach ... is a thermal BBQ grill , grounded via the cordset to shunt stray E-fields from reaching the plate, yet allow heat to go thru in terrahertz band ;)

• Thanks for explaining the impedance part! – Arthur Mar 29 at 16:59
• your welcome, dont forget to vote – Sunnyskyguy EE75 Mar 29 at 17:25

Given the polarityless nature of those reversible EU plugs, what is required here is double insulation. The various UL documents will talk about this, but the gist is two indepenent ways of assuring current can't get on the metal plate.