Receiving 120VAC from frame of double insulated TV, is this normal?

[admiral-ackbar]

Got a jolt from the metal frame of my 2020 Sony KD-65X9000H TV.

Measuring 120VAC at all times unit is plugged in. Now I know there's often some leakage with double insulated items but I thought that 120v seemed fairly high.
Is that somewhere around the expected amount to leak?

Contacted Sony to confirm if its a fault and they said, nope perfectly normal.

Any help?

 

[littlespark]

Not normal.

What did you test with? Something that gives an actual voltage reading or just an indicator light.

Perth UK or Perth Oz?

 

[admiral-ackbar]

Not normal.

What did you test with? Something that gives an actual voltage reading or just an indicator light.

Perth UK or Perth Oz?

My hobby test meter, reliable meter.

Went from the frame of the TV to the earth on the same power board the TV is fed from.
What sort of leakage might you expect from a normal class 2 appliance?

Aus, so half the single phase voltage of round here.

 

[Avo Mk8]

My hobby test meter, reliable meter.

Went from the frame of the TV to the earth on the same power board the TV is fed from.
What sort of leakage might you expect from a normal class 2 appliance?

Aus, so half the single phase voltage of round here.

This question has arisen before.
If there is an EMI filter circuit in the tv, as below, and the TV chassis is not connected to earth, the Cy capacitors act as a potential divider, and you get half the mains voltage on the 'chassis'.
The capacitors won't supply much current, so you get a tingle.
Your multimeter is high impedance so displays the 120V. If you continue measuring and hold on firmly to the TV I would expect the voltage to diminish!

 

[Lucien Nunes]

Exactly. Leakage is not measured as voltage but as current. A tiny current flows through the Y-cap from active to chassis, and from chassis to neutral. Since the caps are of equal value, in the absence of any other connections or leakages you would expect the chassis to be halfway between active and neutral, i.e. 110-120V to earth. Once you start making other connections to the chassis that increase the stray capacitance or reduce the resistance to earth, the voltage to earth decreases due to the lower impedance of the bottom limb of the voltage divider. What matters is the maximum current that flows out of the chassis when that other external connection is your body.

For example, I have a utility 12V DC socket on the bench fed from a class-II PSU. The 12V DC output normally sits around 110V AC to earth, measured with a conventional multimeter. Touching the two meter probes with my fingers drags it down to 50V. The voltage in itself is pretty meaningless. The important figure is the leakage current which is 130μA, correct for this model of PSU and around the lower limit of what you can feel.

What leakage current can you measure from the TV to earth? (This will depend on how low your AC current ranges go, meters vary widely in AC current spec)

 

[admiral-ackbar]

Wow thanks @Avo Mk8 and @Lucien Nunes, super detailed explanations that's definitely put me at ease.

Yeah not enough grunt to give you more than the little jolt/tingle but I'll see if I can get the current value tomorrow morning.
I find it surprising that the only area that this leakage is occurring is on the thin bit of metal on the surround of the TV and not anywhere else on the external chassis.

 

[Avo Mk8]

I find it surprising that the only area that this leakage is occurring is on the thin bit of metal on the surround of the TV and not anywhere else on the external chassis.

I used the word 'chassis' rather loosely, and it may be that internally certain metalwork or screening is connected to that EMI filter 'ground'. And possibly there is capacitive coupling between that and the bezel you touch, rather than there being any direct connection.
The low voltage/signal processing side of the TV will likely be isolated from all this, so I wouldn't expect the antenna input, HDMI or USB's to share this "ground".
It would be interesting to see the circuit diagram, but a (very) quick search didn't reveal one!

 

[brianmoooore]

Much more fun back in the 60s, with TVs from that decade or earlier. Chassis connected directly to one side of the mains, and two pin reversible power plugs. 50 -50 chance of all the metalwork being at full mains voltage with plenty of current available.

 

[Lucien Nunes]

With valve sets the rectification was half-wave, so that on AC mains you could always make the chassis neutral if you wanted to. On DC you could only make it neutral if the line was positive. Negative line required the chassis to be live as it has to be negative. Arguably it got more interesting later with solid-state sets where the chassis was at full-wave rectified mains negative. It was never possible to have it not live.