Electrocuted via stray capacitance of mains isolation transformer?

Hello,

Please can you explain the current flow path in my recent AC mains electrocution?.........

I was working at a Flat Screen television company.

We were testing the Flyback SMPS that supplies the TV electronics....We had supplied the flyback through an isolation transformer , but when i touched the flyback circuit with a single finger, i got an electric shock....how?

One thing i do know is that the senior engineers, for a joke, often used to snip off the 1 Megohm resistors which connected the primary and secondary of the flyback. (from primary gnd to secondary gnd)
I believe that thes 1Meg resistors were there to discharge the seconday, (ie. to prevent the secondary from floating up to a high voltage via the Y caps which connect primary to secondary)

So with the 1Meg resistors snipped away, my secondary circuit floated up to a high voltage , then when i touched it, a capacitive current flowed through the Mains, through the winding capacitance of the isolation transformer, through the Y Cap, through me, then to earth ground?

Is this right?

Sounds right. You were a Cap in the capacitance divider series circuit. Also all electrolytic caps have an equivalent circuit that shows a high value with a high ESR in parallel with the main cap. Which can be proven by shorting the cap momentarily and measuring the memory effect as the voltage rises again. WHich is why they suggest only work with one hand on a TV set so you do not ground yourself. But there is still parasitic path to ground, so this only reduces the current to a "safer" level. This is why they use bleeder resistors as well to shunt the memory effect. But it is a relatively long time constant.

So does the secondary ground float up to the value of the peak of the mains voltage?

The secondary ground or TV chassis can float to any particular value if the leakage is asymmetrical (nonlinear like a diode clamp) But generally it is symmetrical leakage and isolation caps are rated for high voltage. Generally transformer leakage must be minimized by design and attention to leakage from moist dust must be observed as a risk. The old sets did not have a ground connection but had severe warnings on the back to not open set unless you were qualified to do so.

THis experience of the one hand ungrounded for TV repair techs is worth remembering. reminds me of my Shop supervisor in Grade 11 said to another student, yes when the student asked. Can I touch this? He did and said ouch. and said why did you say I could. His reply was you asked the wrong question. YOu ought to have said "Should I touch this?
I think many of us have had similar experiences.. I once as a newbie was removing staples on an AC mains cable with 15A breaker. My huge screw driver acting as a crowbar, punctured the insulation and melted the tip in milliseconds.. ( YOu can get 15KAmps short circuit spikes from AC mains here.) Good thing my safety reading glasses were on as they coated with a fine spray of embedded copper vapour which had melted into the plastic like spray paint.

Besides iolation transformer stray capacitance, you can also expect an Y-capacitors in the SMPS to add some leakage current. Total leakage current should be far below potentially dangerous levels, but you can possibly sense it when touching the circuit with a fingertip. High frequency leakage currents can add if you touch a hot SMPS node, e.g. primary drain.

I think, that it's generally not a good idea to touch the floating circuit parts in operation. Besides the risk of "jumpy" movements you may also damage sensitive circuit parts by just touching it.

I bet FvM has had first hand experience on his recommendations of touching RF leakage on primary drain. Our brains seem to have more excellent storage of such events. (meaning low loss , dynamic refresh analog memory)

Thanks,

I was wondering, was i right that snipping the 1 Meg resistors causes this kind of electrocution.?

They were always up to tricks, and i wonder about this........no matter now, we all got sacked some time back.

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Thanks,

I was wondering, was i right that snipping the 1 Meg resistors causes this kind of electrocution.?

They were always up to tricks, and i wonder about this........no matter now, we all got sacked some time back.