Isolation Transformer with 115kV primary [duplicate]

Question

Supposed you are using an Isolation Transformer with 115kV primary, should you worry about the capacitive current going through it? What would happen if you touch one of the output leads? Remember an isolation transformer secondary is independent from ground.

I always used isolation transformer when diagnosting circuits so I'd not get a shock or the equipment broken by having the path to ground.

To get better understanding. I'd like to know what would happen if the ground rods in North American utility pole were say stolen and at the same time your service panel ground rods didn't work because say the contacts got molds or disconnected and contact to ground completely lost . This would make the utility tansformer secondary floating, so your appliances enclosure could become live. What would happen if you touch the appliances metal enclosure like metallic washing machine body, would you get a shock? or be electrocuted theoretically? But how can you get shock when you were isolated from ground already.

Answer 1

Let's get something straight: are you REALLY talking about a 115 kV i.e. 115 000 volts transformer? You should NEVER touch any parts of such transformers when live nor even go near any for that matter. Power transformers of such voltages almost never have a floating ground because they're almost always made as autotransformers i.e. with a single winding where the secondary taps into the winding somewhere, thus they share a common ground. The reason they do this is because they need to have voltage regulation on one or both sides.

Answer 2

Leakage to Earth ground may consist of many sources and the bottom line is how much leakage current exists in your residence if a ground fault should occur. i.e. the short circuit ground current, Isc. Isc could be quite high so redundant grounds are used. Units are often rated for 250uA and Systems with multiple power supplies limited to 1.5mA of ground noise current. Converted to impedance depends on the current frequency, line, f or 2f or SMPS rate which is much higher.

• Moisture leakage impedance from the line and neutral to earth ground of all appliances ( e.g. washing machine in basement, extension cord on wet grass )
  • with resulting open circuit voltage on floating ground
• insulation impedance of your feet to earth (salty sweat & leather soles might be 100k~10M vs water-proof rubber shoes 10M~10G) but capacitance depends on noise current frequency
• resulting current I(f) = Voc(f) /Z{load leakage + feet)
  • each SMPS might be up to 250uA leakage on its line filter to earth grounded units which can add up significantly if powered on.
  • worst case might be touching earth ground water-tap or SS sink and floating stove with high short-circuit current. ( but these are usually plastic now for cost reasons)

Redundant earth grounds mitigate this hypothetical question with earth bonding at distribution transformer, possibly at the service entrance and to underground conductive water/natural gas pipes.

Anecdotal.

Having felt the leakage current of an ASUS laptop on the tip of my kneecap sitting in a lawn chair with bare feet on damp grass, the current at high frequency ( 40kHz?) feels more like a burn than at low line f. Now if you have dozens of computers with earth bonded line filters for its SMPS, this burn sensation would rise sharply and you touch both an ungrounded stove to an earth bonded water sink. The high f current might not necessarily excite the muscles to freeze in contraction as they respond to low audio spectrum rates down to DC. I suspect this experiment would hurt you.

BTW the feeder 115kV primary transformers are usually floating DELTA windings and it is the DT secondary service low voltages that are Y or "WYE" connected with Neutral centre earth-bonded at the Distribution transformer (DT)