Author Topic: Power cuts and electric shocks  (Read 960 times)

0 Members and 1 Guest are viewing this topic.

Online richard.cs

  • Frequent Contributor
  • **
  • Posts: 809
  • Country: gb
  • Electronics engineer from Southampton, UK.
    • Random stuff I've built (mostly non-electronic and fairly dated).
Re: Power cuts and electric shocks
« Reply #25 on: March 30, 2020, 04:00:30 pm »
Our feed is from a pole transformer but the HV fault occurred at the main sub station which is of course ground mounted and would have a 275 or 400 KV input, it was one of these phase wires that contacted the supporting steel work as I understand it from what the UK power network engineers told me.

So an EHV to earth fault at the main substation (where earths will be shared) would push up the star point of the 11 kV network by quite a lot (unsure by how much, there will be a resistor/inductor at the source(es) that limits the earth fault current but it's still probably a lot of kiloamps into perhaps an Ohm). That star point isn't distributed, but all the phases will jump together. At your pole mount transformer (and many other places) that might cause one or more of the 11 kV phases to flash to the tank, but that will be on the HV earth. You shouldn't see your earth jump unless that HV earth flashes to the neutral or your substation combines the HV and LV earths. I suspect that the latter is true (which is somewhat uncommon for a pole mount, more often the LV earth is on the next pole along).

Generally the system is designed with limited earth fault currents to keep the voltages down when this sort of thing happens, but they can still be surprisingly high for the duration of the fault, and some types of HV faults can take seconds to clear. The assumption is that faults are rare and that the vast majority of the end customers are protected by being in an equipotential zone. And in reality whilst standing on a wet earthy floor in bare feet your wife may have been able to get a painful shock from just a few tens of volts.
 

Offline jmelson

  • Super Contributor
  • ***
  • Posts: 1528
  • Country: us
Re: Power cuts and electric shocks
« Reply #26 on: March 30, 2020, 07:19:55 pm »
At my parent's old house that I grew up in, we had a basement window well that tended to flood in storms.  The procedure was to rig a garden hose outside, turn on slow, and put in well.  When water came out of hose, disconnect from tap and drop on ground, to siphon out the water.
One time, I had my hand on the tap to disconnect, and lightning hit a couple blocks away.  I got a pretty good jolt through my body.
That was the effect of the lightning current spreading through the ground.

I'm guessing a similar effect was what your wife experienced.

Jon
 

Offline dmills

  • Super Contributor
  • ***
  • Posts: 1766
Re: Power cuts and electric shocks
« Reply #27 on: March 31, 2020, 06:43:19 pm »
It don't take much voltage with wet skin to bloody hurt! Possibly only 50V or so, not exactly dangerous, but wet skin, wet concrete floor, possibly some soap.... Yea, that will tickle.

My betting is that the suppliers earth moved, and your concrete floor didn't.

Incidentally, given your user name, you might wish to pay attention to the earthing on any aerial structures outside the equipotential zone (Can of worms), but IIRC the suppliers recommendation for the earthing of radio stations is to go TT with an RCD instead of TN-C-S.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf