A friend of mine has a very strange condition with the electrical system in his room. Connected to mains, there is mainly an audio system, a computer and, most importantly electrical shutters. They are on all windows in the house.

Now, here is the problem: When the shutters stop moving, there is an audible click in the speakers. This is not a fault with the speakers, as they are relatively new and not the only thing affected. But that's not the best of it: Sometimes, when the click is very loud, the PC's drivers crash (LED illumination goes dark, audio driver crashes, ...). I could not reproduce this myself, but I have seen a video of it happening.

They had an electrician over to install some new outlets in the room and asked them about it, but the electrician did not know a solution to the problem either. I am a electronics hobbyist, and my friend asked me to look at the system. What I measured was as expected, that there is a voltage spike on the power line when the shutters stop.

This affects both L/N and the ground. I don't have pro equipment, and only an ebay oscilloscope and multimeter but even then there is a measurable voltage spike both on the ground conductor relative to a metal doorframe or air, and also between the mains wires when the shutters stop moving.

For testing, I isolated the ground prong for the speakers, and the click got much much weaker but still audible (I reversed this modification afterwards of course), so the problem is mainly with the ground line.

I assume that this is due to the coils in the motor of the electric shutter firing current back into the powerlines. Unfortunately, I don't have access to this shutter's wirings as it is mounted inside the walls. But I don't think that the unit is faulty, because the same occurs (although much weaker) when you move any other electric shutter in the house.

I have measured the mains voltage (between 230-235V at stable 50 Hz), the voltage between PE and N (~0.1-1V), and tried measuring if there was voltage on the ground line with the NCV on my multimeter, but everything seemed OK. Also, the house is pretty new.

Can this still be a fault of the power system, or is this just bad design of the shutters?

Also, what can be done to prevent the computer from being affected so strongly by this? I thought about an 1:1 isolating transformer, but that doesn't isolate the ground line IIRC.

Thank you in advance

  • It shouldn't be interacting with ground at all. If it is, that's a ground fault. Does it even have a ground? Commented Mar 14, 2020 at 3:39
  • @Harper-ReinstateMonica The thing is buried in the wall, so I don't know if it has a ground. But I think so because it is interacting with it. I also didn't understand why it is interacting with the ground wire, maybe it's some induced voltage from the coils? The whole floor has an FI (RCCB in english?) which does not trip, so it can't be a lot of current in the ground wire I think.
    – Twometer
    Commented Mar 14, 2020 at 12:18

2 Answers 2


At first I thought it was a cheap PC supply, but it's the stereo also. This is big enough to affect both.

It's voltage surges

When the curtain motor reaches position, it is disconnected abruptly. The motor is an inductor.

You may know a capacitor wants to keep voltage the same and will flow unlimited amounts of currrent in that effort. An inductor is the converse. It wants to keep current the same, and will flow unlimited voltage in that effort.

Interrupt a motor, the field current will want to keep going, and will spike voltage into thousands of volts.

This is the bane of hobbyists trying to control motors, as they usually fail to account for the voltages into the thousands of volts that can result.

It should, in theory, be inducting this onto the hot or neutral wire. In fact, if the switch was interrupting the hot wire, I would expect it to not happen; however Europeans are super slack about hot vs neutral (their plugs flip over, even)... So the people installing them may have inadvertently wired them to switch neutral. You could check that; it's an easy fix if it's wrong.

Fixing it

How to hack the motors to not do that is a question for electronics.se. (in DC you'd use a freewheeling diode, but that's a nonstarter in AC; but mayve some sort of VBO device in the right place to act like that.)

I have a feeling the curtains might've been built with such protection, but it has degraded, as MOVs will do. Fixing that is not hacking, it's repair.

To stay in our bailiwick, what you need is a commercial quality surge suppressor. The consumer jobs won't cut it; they use cheap MOVs which degrade via usage, i.e. become "used up". And this setup would "use them up" quickly. Meanwhile in America... NEC 2020 just made whole-house surge suppressors mandatory, so massive economies of scale are hitting the quality surge suppressor game. That's what I'd get; a whole-house surge suppressor, and stick it right on that branch circuit between the blinds and the PC/stereo.

Why is it affecting the ground wire? Ground faults. Consumer products typically have 500 or 1000 volt insulation between hot/neutral and ground. Well, this interrupt voltage is so high that it's leaping not only over the insulation on the blind motors themselves, but on the stereo's as well.

  • Thank you very much for this detailed explanation. That makes a lot of sense and I am going to try to find one of these surge suppressors to install. I'm also going to try to get to the wiring of the blinds to see if something can be done at the source as well.
    – Twometer
    Commented Mar 15, 2020 at 9:19

I'm sure the fault is in the speakers but it's probably easier to solve it at the source of the interference. The drive motors in the electric blinds are generating a spike of noise when they shut off. The speakers appear to be overly sensitive to it.

My recommendation is to get some EMI/RFI filtering devices and hook them to the power leads on the window drive motors that are causing the speakers to malfunction. Go to a local electrical supply shop and seek their recommendation for a specific product applicable to your location.

A surge/noise suppressor on the PC's power would likely help too but it's always better to eliminate the reduce the noise at its source.

  • Thanks for your reply. The spike also happens on the line with the speakers disconnected. I don't know if the computer is affected as well without the speakers because I have only seen that happen on a video. The problem is that this happens with all blinds in the house, and their electrical connectors are buried inside the wall. Would the noise filter also help with the spike on the ground line? Also, why is it even spiking the ground line? Is that just electromagnetic induction into the frame which is connected to ground? Thank you
    – Twometer
    Commented Mar 13, 2020 at 22:16
  • @Twometer - Keep in mind that in an electrical system the safety ground line is actually connected to the neutral wiring back at the power distribution panel. A spike on the neutral will easily be seen on the safety ground due to this connection.
    – Michael Karas
    Commented Mar 13, 2020 at 22:26
  • @MichaelKaras I thought so too, but the spike was higher on the ground line, and also the click was louder with the ground on, which I did not understand.
    – Twometer
    Commented Mar 13, 2020 at 22:34

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