My German residence is supplied with 3 phase. The "main" distribution panel, wherein the service feed is connected, supplies the "kitchen zone" hereafter zone A. From the main panel, a 5 conductor 10 mm^2 (3 phase) is connected to a secondary distribution panel. All 3 phases of this line are fused at 20 A in the main panel. In the secondary panel, this line is immediately connected to a Hager CDA440D. AFAIK, this is a Residual Current Device (RCD). Next are 16 A fuses for multiple zones in the secondary panel.

I recently purchased a basic 1500 W electric hot plate. Connecting it to any outlet supplied by the secondary panel trips the RCD*. It does not matter if the hot plate is set to Min or Max, it always immediately trips the RCD. Whereas zone A, which does not have a RCD, has no problem operating the hot plate under any setting.

Many high wattage devices have been operated from the secondary panel without this issue. 3000 W halogen work lights, 2000+ W drum sanders, etc. Big home renovation.

*Maybe the fuse too, I can't remember that part. Definitely the RCD tripping.

My questions:

1) Can this behavior be explained?

2) Are there any errors in this situation to be corrected?

Thank you for the advice :)

P.S. I want to learn how this works. Please save yourself the time and effort of posting dogmatic replies, i.e. "call an electrician". In Germany, it is legal for uncertified individuals to perform some electrical maintenance.

  • take a multimeter and check resistance between the power pins and the chassis/earth pin. If it less than a few megaohms then the hotplate is junk. Commented Mar 12, 2018 at 10:06
  • @ratchetfreak. That makes sense. It will be a week until I have a chance to do so. Commented Mar 12, 2018 at 10:08
  • @ratchetfreak if it fails that test, it's bad. However it's not enough to declare it good. it may be an insulation breakdown failure that only occurs at 230V. That test is what megaohmmeters do. Commented Mar 12, 2018 at 21:14

3 Answers 3


Can this behavior be explained?

The hotplate is faulty and leaking more than 30 mA of current to earth (ground). This is dangerous. This is what RCDs and GFCIs are designed to protect you against.

Your RCD is attempting to save your life.

For currents above 10 milliamps, muscular contractions are so strong that the victim cannot let go of the wire that is shocking him. At values as low as 20 milliamps, breathing becomes labored, finally ceasing completely even at values below 75 milliamps.

Ohio State University

Are there any errors in this situation to be corrected?

There's an error in the design, construction or operation of your hotplate. There's nothing to suggest your electrical installation is faulty.

You could report the faulty hotplate to your local equivalent of "trading standards". It is illegal to sell goods that don't comply with electrical regulations.

You could return the hotplate to the seller for a refund.


RCDs detect ground faults

Current travels in loops. An RCD looks at the flow on the intended current path (the two wires) and makes sure the flow is equal and all current is accounted for. If current is traveling a third path (e.g. through you, or via the safety earthing) that is called residual current. This type of leakage is called a ground fault. An RCD has one job: detecting this.

Occam's Razor: your device has a ground fault

It has the problem the RCD is designed to detect, which makes it dangerous. Stop using it and return it, fix it or smash it and throw it out. (So your trash bin is not looted by some unfortunate soul who is later killed by the device).


Faulty hot plate. If you're within 2 yrs from the purchase you can give it back to the shop you bought it and ask for a repair or replacement under the mandatory warranty. Check the (new) plate has the CE and the german equivalent of IMQ (Italian Certification of Quality) mark, or IMQ itself on it and refuse to buy if it hasn't them.

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