Author Topic: Power cuts and electric shocks  (Read 989 times)

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Offline G7PSK

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Power cuts and electric shocks
« on: March 29, 2020, 07:46:26 pm »
This morning we had a power outage while the wife was in the shower, the whole are went out. The strange thing is my wife got an electric shock as the power went out,the shower is a pumped type.
The power network engineers came round and tested the supply as I had reported the shock to UK power networks. the engineers told me the reason the power went out was due to a high voltage line coming loose in the wind and striking against the metal work of the sub station which is about twelve miles from the house. What I don't understand and neither did the power engineers is how or why the wife got the shock.
Our supply uses the company's earth which I assume is bonded right back to the sub station and beyond.
Did the high voltage line hitting the steel work send some power right along the earth/ground wires and so through the wife or is it down inductance between the live and earth/ground cables in the supply which is all overhead where I live.
 

Offline mzzj

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Re: Power cuts and electric shocks
« Reply #1 on: March 29, 2020, 07:53:17 pm »
If the "company earth" is not bonded to everything else in the house that is more or less expected.
In here it would be normal  to bond "company earth", house plumbing,  rebar(house foundation) and grounding electrodes to Potential equalization bar.
 

Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #2 on: March 29, 2020, 08:07:58 pm »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
 

Offline engrguy42

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Re: Power cuts and electric shocks
« Reply #3 on: March 29, 2020, 08:16:22 pm »
Hard to say for sure (especially since I'm only familiar with US, not UK), but when if the power line fell inside the substation, the substation has a big metallic ground grid mat in the earth underneath it. When the line falls on metal in the station it goes to the ground grid, but it has to go back to the source(s), where ever they might be. Probably there's a grounded transformer in the station where it fell, but there may also be other remote sources the current can flow back to. It can distribute itself in the earth and return to multiple remote sources far away.

Kinda surprising if it was 12 miles away, but I suppose it's possible there is another station that also has a grounding transformer near your house where some of the current flowed. Depending on the impedance of the ground around your house there may have been enough potential generated at your house (maybe the ground rod outside your house isn't connected very well to ground or something?).

Now if the power lines in your neighborhood have 4 wires (which include a dedicated ground wire), this is much less likely since the ground current will probably flow on those wires not in the earth.

If you want to do some detective work get on Google Earth and see if you can find a substation 12 miles away, and another one near your house  :D  And if you can zoom in to the one nearest your house and see the low voltage side of the transformer feeding the distribution system has 4 bushings, it's probably Y grounded and the current may have flowed back to that transformer.
 
Seems strange. Not good that you get a shock for something like that. Maybe have an electrician come and check your grounds in the house. A good ground in your house should limit any potentials like that. 
« Last Edit: March 29, 2020, 08:23:32 pm by engrguy42 »
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Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #4 on: March 29, 2020, 08:36:54 pm »
There is another transformer station around three miles away that feeds power from a solar farm that is only a quarter mile away from out house into the grid. Our LV supply comes from a pole transformer that is only five poles away from us.
 

Offline engrguy42

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Re: Power cuts and electric shocks
« Reply #5 on: March 29, 2020, 08:47:40 pm »
There is another transformer station around three miles away that feeds power from a solar farm that is only a quarter mile away from out house into the grid. Our LV supply comes from a pole transformer that is only five poles away from us.

It might be possible that the utility grounding isn't real good, and assuming the fault at the 12 mile away station was a ground fault (1 phase to ground) maybe the controls at the solar plant and lack of sufficient grounding (solar plants and their controls and grounding needs are often a big and complicated question mark for utilities) caused the unfaulted phases to go real high and cause an overvoltage in the area. There should be lighting arrestors that would protect against those for a fraction of a second then likely blow up (they're only designed for microsecond duration lightning). Did you hear a loud POP outside when this happened? Did others in the neighborhood have any problems?

Anyway, this is all just guessing. It depends on a lot of stuff, and only the utility guys can really figure it out.

But most of all I'd suggest you have a good electrician come thru again and make sure you're good. If you're lucky he'll say "oh yeah, I had a lot of calls recently because of that" :)  and his utility technician buddy told him the scoop :)
« Last Edit: March 29, 2020, 08:55:19 pm by engrguy42 »
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Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #6 on: March 29, 2020, 09:07:14 pm »
There is another transformer station around three miles away that feeds power from a solar farm that is only a quarter mile away from out house into the grid. Our LV supply comes from a pole transformer that is only five poles away from us.

It might be possible that the utility grounding isn't real good, and assuming the fault at the 12 mile away station was a ground fault (1 phase to ground) maybe the controls at the solar plant and lack of sufficient grounding (solar plants and their controls and grounding needs are often a big and complicated question mark for utilities) caused the unfaulted phases to go real high and cause an overvoltage in the area. There should be lighting arrestors that would protect against those for a fraction of a second then likely blow up (they're only designed for microsecond duration lightning). Did you hear a loud POP outside when this happened? Did others in the neighborhood have any problems?

Anyway, this is all just guessing. It depends on a lot of stuff, and only the utility guys can really figure it out.

But most of all I'd suggest you have a good electrician come thru again and make sure you're good. If you're lucky he'll say "oh yeah, I had a lot of calls recently because of that" :)  and his utility technician buddy told him the scoop :)
It is quite likely that the utility ground is not that good as the soil around here consists largely of sand. Did not here any pops or other sounds aside from the alarm systems warble that indicates a power outage on the system. We have several solar farms in the area along with wind farms as the old WW2 airfields make good sites for such. I will get an electrician out not sure how soon as we are self isolating at the moment due to the wife being in a wheel chair and having asthma so wish to avoid people coming into the house the line engineers did not come in they just did their tests from the meter box outside and handed me an electronic socket tester to check inside the house,all light were green on it which indicates there was nothing wrong.
 

Offline engrguy42

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Re: Power cuts and electric shocks
« Reply #7 on: March 29, 2020, 09:17:32 pm »
Hey, sorry to hear that. Hope all goes well with you and your family.

BTW, regarding the utility grounding, with stuff like solar (and other) power plants they have to design their system to make sure there's no overvoltage problems during faults and stuff. Often they install special equipment (eg, grounding transformers) to make sure there's sufficient grounding so you don't get overvoltages. Especially with solar and other inverter-based generators (wind, etc.) this can be a challenge, especially when you have to figure out how the solar inverter controls will respond. It gets real complicated.

My (rather biased) hunch is that there may have been an unexpected overvoltage due to some unexpected behaviour of the solar plant and/or lack of sufficient grounding on the system. Either that or the thing I mentioned first where currents might have returned to a station near you and your earthing rod or ground impedance need some attention.

Good luck.
« Last Edit: March 29, 2020, 09:36:12 pm by engrguy42 »
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Online tautech

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Re: Power cuts and electric shocks
« Reply #8 on: March 29, 2020, 09:33:39 pm »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
Big mistake !
Reinstall a dedicated earth extending down into the watertable.
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Offline engrguy42

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Re: Power cuts and electric shocks
« Reply #9 on: March 29, 2020, 10:10:32 pm »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
Big mistake !
Reinstall a dedicated earth extending down into the watertable.

I'm curious...anyone know what code is in the UK for residential earthing?
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Online tautech

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Re: Power cuts and electric shocks
« Reply #10 on: March 29, 2020, 10:23:34 pm »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
Big mistake !
Reinstall a dedicated earth extending down into the watertable.

I'm curious...anyone know what code is in the UK for residential earthing?
Does that matter when mongrels are going around stealing the copper earth cabling from Powerco's installations ?
Your PE installation is your first line of defence against the risk of shock. Period.
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Offline engrguy42

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Re: Power cuts and electric shocks
« Reply #11 on: March 29, 2020, 10:35:56 pm »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
Big mistake !
Reinstall a dedicated earth extending down into the watertable.

I'm curious...anyone know what code is in the UK for residential earthing?
Does that matter when mongrels are going around stealing the copper earth cabling from Powerco's installations ?
Your PE installation is your first line of defence against the risk of shock. Period.

My question was surrounding whether the original electrician was following UK code or not. And I'm curious what exactly is required in the UK. So yes, it  matters to me. And it's also why I suggested he contact a good electrician to make sure he's okay.

As far as people stealing copper, while that may be true in very limited cases, I sincerely doubt it has any relevance whatsoever to someone getting shocked in their home. Generally they steal rolls of uninstalled copper during construction, or short lengths of copper they cut by hand. While that can affect safety in the station, it's otherwise irrelevant here.

Though if you have any actual cases in which this has occurred I'd be interested to hear it. 
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Offline Monkeh

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Re: Power cuts and electric shocks
« Reply #12 on: March 29, 2020, 10:46:35 pm »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
Big mistake !
Reinstall a dedicated earth extending down into the watertable.

I'm curious...anyone know what code is in the UK for residential earthing?
Does that matter when mongrels are going around stealing the copper earth cabling from Powerco's installations ?
Your PE installation is your first line of defence against the risk of shock. Period.

No, not period, it depends on the method of earthing and protection you use. Rods are really, really crap, frankly.

That said, his electrician shouldn't have touched anything without verifying the type of earthing facility available (which may be 'none').
 

Online tautech

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Re: Power cuts and electric shocks
« Reply #13 on: March 29, 2020, 10:52:07 pm »
@ engrguy42
Sure if the code was followed but the house occupant was still shocked without a clear source then the the powerco's PE or the UK code is flawed.
New installations here are megger tested by the powerco inspector before they are livened and a local electrician should also be able to do a megger check as a sanity check that an earth is sound and if not add further pegs or longer ones to get to the watertable.
In a shower like the OP's wife the source of the shock could come from via a faulty electric hot water heater (improper or broken earth) and best advice would be to return the PE installation to local and check all PE bonding.

Here in NZ, 25mm2 copper droppers down poles to the transformer earth field installations have been stolen so frequently the powerco's now use a copper coated soft steel dropper for the earth and tag label it as 'Not Copper' !
« Last Edit: March 30, 2020, 12:57:07 am by tautech »
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Offline blacksheeplogic

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Re: Power cuts and electric shocks
« Reply #14 on: March 30, 2020, 12:52:49 am »
local electrician should also be able to do a megger check as a sanity check that an earth is sound and if not add further pegs or longer ones to get to the watertable.

When I had the house rewired they replaced the rod as part of completion. Don't know if that was because of some compliance issue with the existing or just part of their their process when doing this work.
 

Online tautech

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Re: Power cuts and electric shocks
« Reply #15 on: March 30, 2020, 01:05:00 am »
local electrician should also be able to do a megger check as a sanity check that an earth is sound and if not add further pegs or longer ones to get to the watertable.

When I had the house rewired they replaced the rod as part of completion. Don't know if that was because of some compliance issue with the existing or just part of their their process when doing this work.
TBH it's good practice IMHO.
After decades of dealing with stray voltages and non-perfect powerco earthing the PE rods erode and I have seen some pulled that were barely as thick as a pencil for most of their length.
Today we use 12mm hot dipped galv rods where once 20mm/3/4" galv pipe was used and it lasted much much longer.

Come to think of it I must ask my sparky mate if galv pipe for PE still meets regs.  :-//
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Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #16 on: March 30, 2020, 08:19:09 am »
@ engrguy42
Sure if the code was followed but the house occupant was still shocked without a clear source then the the powerco's PE or the UK code is flawed.
New installations here are megger tested by the powerco inspector before they are livened and a local electrician should also be able to do a megger check as a sanity check that an earth is sound and if not add further pegs or longer ones to get to the watertable.
In a shower like the OP's wife the source of the shock could come from via a faulty electric hot water heater (improper or broken earth) and best advice would be to return the PE installation to local and check all PE bonding.

Here in NZ, 25mm2 copper droppers down poles to the transformer earth field installations have been stolen so frequently the powerco's now use a copper coated soft steel dropper for the earth and tag label it as 'Not Copper' !
The engineers who came yesterday Sunday 29th tested the earth bonding with a megger and all tested OK and yes it is up to code to use the company earth, wherever there is a tap from the overhead there is an earthing/grounding on that pole by a metal shoe that fits on the bottom of said pole and there is one on the pole that our power comes from as I saw the pole go in when they fitted a new one nine years ago when we had the pole moved to allow for a garage to be built, the pole is at one end of our property and the feed in is at the other end of the house via about 30 odd meters of underground cable.
 

Offline mzzj

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Re: Power cuts and electric shocks
« Reply #17 on: March 30, 2020, 09:15:17 am »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
I'd double-check everything is bonded to equipotential as intented.  Cast iron sewers? water supply?
Really old houses don't have rebar at all  and only new ones have the rebar tied to potential equalization. (last 20 years or so here)
 
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Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #18 on: March 30, 2020, 10:35:35 am »
Every thing is bonded, up until a couple of years ago we had our own earthing via the usual rod in the ground but one day when playing with the scope I realised we no longer had a very good earth so I called an electrician who moved the outgoing earth wire from the rod onto the company incoming earth. We have a pole just 30 meters from the house where our power comes from and at the base of that is a large earthing plate as well an earth line going back along the overheads.
I'd double-check everything is bonded to equipotential as intented.  Cast iron sewers? water supply?
Really old houses don't have rebar at all  and only new ones have the rebar tied to potential equalization. (last 20 years or so here)

All drains are plastic, the house was rebuilt 20 years ago but the original fabric goes back to the 1800's. The GFD/RCD did not trip The pulse that gave my wife had to have come down the ground line.
 

Online tautech

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Re: Power cuts and electric shocks
« Reply #19 on: March 30, 2020, 10:38:57 am »
The pulse that gave my wife had to have come down the ground line.
Or via the hot water cylinder if it's grounding is not up to scratch. Mentioned earlier.
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Offline Gyro

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Re: Power cuts and electric shocks
« Reply #20 on: March 30, 2020, 12:00:50 pm »
Hard to think how a fault 12 miles away could cause an electric shock. The local substation, stepping the voltage down to 240V would be within a few hundred meters of your house. The 240V mains neutral would definitely be grounded at that point.

If your house uses the most common TN-C-S (PME) earthing system, then Earth and Neutral will be bonded together at the house main fuse, and all pipework will be bonded to it, and each other.

The sheer number of appliances loading the mains in the immediate neighbourhood (of your local substation) should have clamped any sort of inductive kick.

Was it an over-bath shower (and a metal bath!) or a plastic shower tray or bath? It's difficult to see where any earth return path for a shock. Is it possible that the light suddenly going out and the shower pressure jumping could have been perceived as a shock?

P.S. Even if her toe had been in contact with the metal drain / plug-hole, the waste pipe is almost invariably plastic. Maybe a case of what the Victorians would have diagnosed as 'female hysteria'  :D
« Last Edit: March 30, 2020, 12:09:04 pm by Gyro »
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Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #21 on: March 30, 2020, 12:19:56 pm »
The pulse that gave my wife had to have come down the ground line.
Or via the hot water cylinder if it's grounding is not up to scratch. Mentioned earlier.
Hot water cylinder is heated by an oil fired boiler, the immersion heater is turned off. Other possibilities that come to my mind are filter capacitors, the GFD's have a fairly high resistance to ground compared to a directly bonded system and a wet body might be a lower resistance, it only takes a very small current and voltage to give a wet person a shock they can feel, I have had a shock from a single cell wet acid battery 2.5 volts only.
 

Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #22 on: March 30, 2020, 12:21:08 pm »
Hard to think how a fault 12 miles away could cause an electric shock. The local substation, stepping the voltage down to 240V would be within a few hundred meters of your house. The 240V mains neutral would definitely be grounded at that point.

If your house uses the most common TN-C-S (PME) earthing system, then Earth and Neutral will be bonded together at the house main fuse, and all pipework will be bonded to it, and each other.

The sheer number of appliances loading the mains in the immediate neighbourhood (of your local substation) should have clamped any sort of inductive kick.

Was it an over-bath shower (and a metal bath!) or a plastic shower tray or bath? It's difficult to see where any earth return path for a shock. Is it possible that the light suddenly going out and the shower pressure jumping could have been perceived as a shock?


P.S. Even if her toe had been in contact with the metal drain / plug-hole, the waste pipe is almost invariably plastic. Maybe a case of what the Victorians would have diagnosed as 'female hysteria'  :D

Tiled floor type wet room.
 

Offline richard.cs

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Re: Power cuts and electric shocks
« Reply #23 on: March 30, 2020, 02:14:38 pm »
Tiled floor type wet room.

That's a big part of your problem then.

You shouldn't really have a tiled wetroom on the ground floor like that unless you have a metallic grid in the concrete floor that's bonded to the main earth terminal of your supply, especially not with a PME earth as you now have. The problem is that the concrete floor you are standing on with wet feet is inevitably going to stay at true earth potential whilst the rest of the "earthed" stuff in your house (including your plumbing) bounces relative to it. Essentially your feet are outside the equipotential zone. Technically the DNO should have refused to provide you a PME earth terminal because of that wetroom (hopefully your electrician didn't just connect to their neutral or cable sheath!). The whole safety case for PME hinges around maintaining an effective equipotential zone so the "earth" terminal they provide occasionally wondering up to a few hundred Volts doesn't kill people, hence all the worries about using a PME earth terminal for outdoor things like car chargers.

The best improvement you can make here without digging up the floor would be to put insulating sections in all the plumbing that enters the wetroom, so you can't get a dangerous shock off the water pipes whatever voltage they are at. You should also seriously consider reverting to TT earthing.

These symptoms from a fault 12 miles away are a bit odd though. It is possible that the HV and LV earths are combined at your local substation (which will be a few hundred metres away at most) but this is only done when they can get a good enough connection at that point to hold the voltage down (to some hundreds of Volts) during reasonably foreseeable faults (it could of course have been an unreasonable fault). Did you say it was a pole mount transformer? It would be unusual to have combined HV and LV earths then because it's difficult to get a low enough earth resistance in the limited space. It is possible that something very weird happened, with like a 132 kV line at a major substation falling across an 11 kV output and raising its potential enough to flash over 11 kV : LV transformers all over the place. It would still be odd to get so much rise of earth potential so far away because it would flash over in huge numbers of places giving multiple parallel paths to ground.

For the rest of you who are not UK based, earthing in the UK generally falls into the following categories:

TT. You have an earth rod that is your property and gives you an earth reference. It's generally got a resistance of a few tens or hundreds of ohms so RCDs (GFCIs) are mandatory.

PME / TNC-S External to your house there are four wires. Three phases and a combined neutral and earth conductor (CNE). At your property this splits out into earth and neutral. The network is supposed to be designed with high reliability connections on the CNE conductor (multiple redundant crimps, etc.) and with regular earth rods throughout its length. This gives you a nice low impedance for tripping MCBs on earth faults, and people tend to notice when the neutral fails so it gets repaired quickly, but when it does fail your "earth" can go up to full line voltage. It can also sit 10 V or so from true earth in normal operation which can then divert lots of amps down any earth connections you have such as plumbing. Sometimes electricians create this connection on networks that don't have the extra earth rods and double crimps, needless to say this is a Bad Thing(TM)

TNS - There is a 5th connection back to the substation providing an earth connection separate from the neutral, often the lead sheath of the cable. They repair these types of networks with 4 core cable, so in the modern world you should just treat this like TNC-S as all the same risks can then apply.
 

Offline G7PSK

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Re: Power cuts and electric shocks
« Reply #24 on: March 30, 2020, 03:08:36 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.
 


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