Electronics > Beginners

How to Kill Yourself with an Isolation Transformer

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Isolation transformers can provide a degree of safety when working with high voltage equipment (device under test or DUT).   Here, I’m defining high voltage as a voltage as one that can give a harmful shock via dry skin contact.  However, the safety provided by isolation transformers can be easily defeated.  Worse, isolation transformers can nullify the benefits of  better protection … a GFCI/RCD-protected power source.

It’s easy to re-reference an isolation transformer output to ground via test equipment … with potentially lethal results.  For example, connect one side of the transformer output to the DUT and connect the ground lead of a mains-powered oscilloscope to a part of the DUT that has low resistance to where the transformer output is attached.  Attach the other output of the transformer to a part of the DUT that has a high resistance to where the first output is attached.   Pay your life insurance, and touch this second part of the DUT with one hand and something grounded with the other.

The diagram and pictures below illustrate the scenario.  In the pictures, there is a light bulb instead of a body.  I do not recommend doing the setup in the pictures unless you are very familiar with working with mains power.  In the pictures, you are the light bulb.  In the first picture, the DUT, a plug cord in this case, is connected to the 120 VAC output of an isolation transformer.  Your left hand (the white lead of the bulb socket) is touching something conductive connected to one side of the isolation transformer.   Your right hand (black lead) is touching a part that is grounded … the red clips, green and then black wire go to the ground pin of the orange plug in the receptacle.  What could go wrong   … you’re isolated   … right?  Wrong in this case!  The other side of the transformer secondary is also grounded via the oscilloscope.

The isolation transformer in the picture was plugged into a GFCI-protected socket.  The GFCI didn’t trip because there was no ground fault that it could detect … it was isolated from the fault by the transformer.  The GFCI receptacle on the isolation transformer secondary provides little extra protection.  Something would have to go seriously wrong inside the transformer for it to trip.  Its ground socket is not connected to anything.

The scope was plugged into a the same GFCI-protected receptacle as the transformer.  So why didn’t that trip the GFCI?  It didn’t because current was passing through the scope ground chassis to the ground line of the cord and pin.  From there, it passed into the ground contact of the receptacle and into mains wiring ground.  A GFCI monitors hot and neutral currents and trips if they aren’t the same; it does not monitor current in ground wires directly.

In the second picture, the DUT is plugged directly into an outlet protected by a GFCI breaker.  The bulb is not lit.  Why?  Because the GFCI tripped.  No shock to the body here!

If the DUT was plugged directly into a functioning GFCI/RCD, a real body might feel a brief tingle before power was removed.  I experienced this when a garage door installer put a lag screw into a mains cable.  I’ve seen GFCI receptacles fail live.  Rare, but it happens.  That’s why they have test buttons.  When’s the last time you tested yours?  I thought so … you’re like me.   :)  But yeah, a DUT plugged directly into a functioning mains-powered GFCI is shock-safer with regard to grounding than using an isolation transformer.  All my shop receptacles are GFCI-protected.
So, will GFCI protect test equipment against a ground fault?  With regard to starting a fire or shocking the operator … yes.  However, GFCI’s operate very quickly, but not instantly.  It might be possible for a ground fault to cook some semiconductors before power is removed.  Battery powered test gear is best for high voltage work.  However, some battery powered instruments are not designed for high voltage use, especially some oscilloscopes.  For mains-powered oscilloscopes, differential probes or probe isolators are recommended.  And plug any mains-powered test gear into GFCI/RCD-protected receptacles.

Now, all you have to do to get shocked is touch two hands to high voltage parts of a DUT.


Maybe the best way to avoid all these problems is to simply isolate the test equipment.

Most test equipment anyhow is isolated, apart from the ground reference they have for the signals.
The only thing that one would lose with isolating the test equipment, is maybe the power supply
fault detection.

The load is almost always higher power than the test equipment. To have the RCD where the energy and
a mistake could kill you, is most probably the better thing to do. In the end we have isolated test equipment
and they are called multimeters.  ::)

Or use isolated probes. Not the Differential ones, with the 10MΩs. Isolated.

I think there's a Darwin award lurking here.
Why on earth (pun intended) would you want to touch the high voltage secondary end of an isolation transformer when the other end is grounded?
I mean, if you have a 500 V battery, you don't touch both ends at once (unless you're a moron).


--- Quote from: TrickyNekro on April 14, 2024, 08:46:55 pm ---Maybe the best way to avoid all these problems is to simply isolate the test equipment.

--- End quote ---
That will get you killed.

I think most experienced technicians were aware of this type of fatal flaw just as we / they are aware that even with an isolation transformer if you happen to be working on a Carver PM-2.0T that incorporates both a + and - 130VDC power rail with a kilowatt of power available you can still be killed and D.C. doesn't like to let go. I guess the real problem comes through the rumor mill that connecting your unit under test to an isolation transformer protects you in every imaginable situation. It seems many noobs get their information 3rd or 4th hand from folks who weren't very electrically savvy in the first place. It is amazing on Iceland folks working with 240vac may tend to take chances working a hot circuit but when faced with 240vdc which exists in some areas they take insane amounts of precautions!!! A 240vdc arc flash will sustain forever and the plasma ball can get huge when copper vapor is involved. An incredibly bigger fireball than a 240vac arc flash.


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