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EMI suppressor and protective earth

Chris Pearson
I would be most grateful if the boffins in here could explain something please.


This suppressor was fitted to my lathe on the load side of the VSD. Something went wrong when I was parting off the other day - the tool dug in and the motor stalled - and now it is running roughly. Back to square 1!


In an attempt to determine whether the VSD has been damaged, I tried connecting another motor, but I was unable to demonstrate protective earth continuity. The cable from the green terminal on the suppressor goes to the motor's earth terminal. I expected that the case of the suppressor would be the supply side of the PE, but apparently not. So with the suppressor fitted there is no PE to the motor, which I find a little disconcerting.


FWIW, the capacitance between red and black is about 0.5 µF, the resistance is 470 kΩ, and the capacitance between each of red and black to green is 9 nF.


Would somebody kindly explain whether this is safe please?

83550124b8831caa7e200218ed497ebc-huge-20200629suppressor.jpg
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    Simon Barker:

    The green terminal of the suppressor is there to aid in its suppressing function.  It's placing capacitors between each of the supply lines and ground.  These will present a low impedance to high frequency noise, effectively shorting it to ground.


    Usually, in Class I appliance, yes the Class Y capacitors go to PE. In Class II appliances, they can be connected to help ideal with eddy-type noise currents in the frame or rotor of the motor, and may not be connected to PE at all.


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  • 0
    Simon Barker:

    The green terminal of the suppressor is there to aid in its suppressing function.  It's placing capacitors between each of the supply lines and ground.  These will present a low impedance to high frequency noise, effectively shorting it to ground.


    Usually, in Class I appliance, yes the Class Y capacitors go to PE. In Class II appliances, they can be connected to help ideal with eddy-type noise currents in the frame or rotor of the motor, and may not be connected to PE at all.


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