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I was trying to get rid of the humming noise coming from my speakers in my hi-fi system. It is an old piece of equipment from back when the UK appliances did not come with the mains plug and you had to add it yourself.

I noticed that the amplifier was not grounded so I added a cable from the chasis to the ground pin in the plug and violá: no more humming. I did test the continuity between the 3 pins in the plug using a multimeter, and also tested between live and the chasis of the amp and everything was nice and dandy.

I then connected (via rca) my CD player and I got hum again... so effectively, after checking the plug I found no ground in this one either, so I repeated the same procedure.

When testing, I found that between live and neutral in the plug, there was a little bit of continuity! So I checked my work, as I assumed, I messed up and accidentally made a contact between live and neutral somewhere... however, this contact was nowhere to be found.

Maybe there is something wrong with the cable I used for ground? I thought. So I unplugged it and put the device back to how it was earlier. But this small continuity was still present.

When measured in terms of resistance, it gave me a value of around 22 KOhm.

Is this normal at all? I am afraid of plugging the device to mains now!

I’m new to electronics and electrical theory, I just know very little from high school and I just finished my first semester of electrical engineering at university.

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Measuring continuity on a circuit can be a false measurement.

A multi-meter tests continuity by measuring if a certain amount of DC current flows from the Positive to the Common terminal.
On a wire, with low resistance, this current will flow without a problem.

However components like a capacitor or transformers can also act as a wire for the DC test current of the meter.
A capacitor will allow the DC test current to charge it.
A transformer looks like a resistor for a DC test current.

If the capacitor is small, it will beep shortly and the resistance will quickly rise. The capacitor is charged. But if you have a large capacitor, the meter might not be able to charge it, and read as a short circuit for a DC continuity test.

Measuring continuity on the Live and Neutral or Positive and Negative of a device might not provide the result you expect.

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  • \$\begingroup\$ Thank you! After reading your answer I plugged my CD player and it worked with no problems. Also the humming noise is gone. \$\endgroup\$ – Javier González Jan 2 at 11:26

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