Author Topic: No shock from grounded 500VA isolation transformer (Solved)  (Read 190 times)

Offline DeLorean

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No shock from grounded 500VA isolation transformer (Solved)
« on: April 19, 2022, 09:42:11 AM »
Hi,


for a while now, i'm breaking my head on something that maybe has a simple answer,
but i have searched the net, and ask many people that know much more from Electricity then me,
and nobody can give me the answer.
So i hope that somebody here can give the answer.

Before i get reactions about grounding a isolation transformer, i'm aware that this will bypass
the safety feature of a isolation transformer, but this grounding is for testing purpose only.

Here in Belgium we have the TT net, the Neutral that comes to my house is bounded in the substation,
and the earthwire here in house is connected to a copper rod in the ground.
Further there is no connection in the breaker panel between this Neutral and earthwire, and if somewhere
in the house both touch eachother, the 30mA RCD trips, so the grounding is working.

Now my question :
When i use a 500VA isolation transformer, connect the primary to the 230V mains grid,
then the output on the secundairy has no neutral anymore, but 2 floating phases (lets call them L1 and L2 from now).
When i connect L1 to the earthwire of a wall outlet, the isolation transformer is grounded.
But when i touch L2, that carries 230V and is now earth referenced, i don't feel nothing at all.
When i place a DMM in µA range in series with one lead in my hand, and the other lead touching L2, i measure a current of 11µA,
in voltage range, i measure 90V in high impedance mode, and arround 4V in Low impedance mode (300K on my DMM).
I can light up a incandescent light bulb or a 1200Watt hairdryer between L1 and L2, and between L2 and earthwire, so the transformere
is capable of delivering enough current.

When i do a body capacitance measuring, with one lead in my hand, and the other lead touch the earthpin of a wall outlet,
i measure 150pF. Standing on a floor with tiles and rubber sole shoes.
When i use my 30V linear bench powersupply, holding the negative in one hand, and the positive in my other hand,
i measure arround 100µA current. So my body resistance is at 30V DC arround 300K.

What is the difference between touching the phase of the mains grid, that carries 230V and is earth referenced,
and getting a zap, and touching L2 of the isolation transformer that also carries 230V is referenced to the same earth,
where i don't feel anything?
So what am i'm missing here ?

Thanks in advance

Greetings from Belgium
DeLorean
« Last Edit: April 27, 2022, 09:31:26 AM by DeLorean »

Offline davekni

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Re: No shock from grounded 500VA isolation transformer
« Reply #1 on: April 20, 2022, 05:54:33 AM »
Quote
i measure a current of 11µA
Quote
i measure 150pF
Those two measurements match almost perfectly.  At 50Hz, 150pF is 21.2Mohm.  11uA across 21.2M is 233V, surprisingly close to 230V.  11uA is too small for almost any person to feel, by a factor of 10-100.

Quote
What is the difference between touching the phase of the mains grid, that carries 230V and is earth referenced,
and getting a zap, and touching L2 of the isolation transformer that also carries 230V is referenced to the same earth,
where i don't feel anything?
So, the puzzle is why you feel any shock touching a single mains wire.  I suggest retesting touching mains (only hot wire) while insulated in exactly the same way as you were for your isolation transformer experiments.  If you still feel something, then repeat the same measurements (current and voltage).
David Knierim

Offline DeLorean

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Re: No shock from grounded 500VA isolation transformer
« Reply #2 on: April 27, 2022, 09:27:39 AM »
Quote
i measure a current of 11µA
Quote
i measure 150pF
Those two measurements match almost perfectly.  At 50Hz, 150pF is 21.2Mohm.  11uA across 21.2M is 233V, surprisingly close to 230V.  11uA is too small for almost any person to feel, by a factor of 10-100.

Quote
What is the difference between touching the phase of the mains grid, that carries 230V and is earth referenced,
and getting a zap, and touching L2 of the isolation transformer that also carries 230V is referenced to the same earth,
where i don't feel anything?
So, the puzzle is why you feel any shock touching a single mains wire.  I suggest retesting touching mains (only hot wire) while insulated in exactly the same way as you were for your isolation transformer experiments.  If you still feel something, then repeat the same measurements (current and voltage).

Hi Davekni

Sorry for the late reply, i had notifications not enabled  :o

You are right, i have repeated the test (i don't recommend this to others, unless you know what you are doing).
I have used in decending order a resistor of 1M, 860K, 480K, 300K, 220K, 100K, 47K and finnally no resistor, to touch only the hot wire.
And i came to the same results, arround 90V between hot wire and myself, and arround 11µA.
So it seems, that in the past years when i got a zap, i was touching the hot and chassis of a device at the same time, otherwise i wouldn't get a zap.
I always where the same type of shoes, so that could not be the difference.
But the puzzle is now solved, it seems that i have a decent isolation compared to earth/ground.
Now i can focus on the "touch protection" that i want to add to my isolation transformer project.
I have already done some testing with a BC547 and BD139 in Darlington configuration as a wireless detector :
/>The goal is to use a relay instead of the LED , to interrupt the power to the contactor coil, so that the output of the transformer is cut off.
This breadboard design needs further testing, if build on PCB and placed inside the alu cabinet, of the detection still works,
and if the alu cabinet does not act like a Faraday cage (what i suspect). If so, i can bring the input of the detector with a short piece
of RG174 coax to the outside of the cabinet, and add a isolation wire (no shielding) to the inner conductor of this coax,
like a antenna of a Wifi router.

Grtz
DeLorean

High Voltage Forum

Re: No shock from grounded 500VA isolation transformer
« Reply #2 on: April 27, 2022, 09:27:39 AM »

 


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