The US electrical system

[richard.cs]

Now, I could be wrong, but as far as I'm aware most electric cars do not connect the charging CPC to the chassis. That coupled with plastic handles and extremely high quality paint finishes leaves me entirely unconcerned about taking a belt off a car. Oh, that and like most people I walk around on a big sheet of rubber, not my bare feet.

Unfortunately most electric cars do have that CPC to chassis connection. I suspect only because it makes EMC design easier, or reduces costs over double/reinforced insulation, or even just because of an "earth everything" mentality. I agree there is no sound technical reason why it is required.

Personally I think plastic handles, paint and shoes make the risk small compared to other things (outside taps are the classic worry, often being associated with wet skin and bare feet). I don't worry at all when mine is plugged into a known TNC-S supply.

I do, however, think we should never have used TN-C-S as an excuse not to properly control potential at the installation. It's a perfectly good system for ensuring a nice low impedance fault path for currents in normal operation without using lots of extra copper (or lead, or alu), but a poor excuse for not having good earthing at the property (and the American method of whacking a rod in quickly doesn't cut it).

Fundamentally it is a poor system for that reason, and if we were starting from scratch today this is not where we'd end up. Personally I'd be inclined to go for something with an impedance-earthed neutral and then we could distribute a low-current CPC at negligible cost. This would be somewhat similar to UK HV distribution. Such a system allows RCD-like fault protection with much greater reliability than common 30 mA RCDs due to having a few tens of amps of fault current to play with, and gives negligible rise of earth potential during a fault.

From where we are today though I think that will never happen, we'd have to return to TNS or more likely give up on supplier earthing and TT everything - and that has problems with unreliable RCDs.

[tom66]

I think the reason for the CPC being earthed to battery negative is the sheer convenience of the communication interface of the pilot signal and cable detection. And for vehicles with DC-CCS, then they also need a communication bus with the charging station.  So you can keep all of this logic on the non-insulated side which reduces costs and complexity.

Of course the high voltage battery is always isolated from the rest of the chassis and this system is continuously monitored,  isolation drifting out of the specification will flag a service warning or disable drive.

[Monkeh]

From where we are today though I think that will never happen, we'd have to return to TNS or more likely give up on supplier earthing and TT everything - and that has problems with unreliable RCDs.

The reliability (or lack thereof) of RCDs is why I'm not a fan of TT systems. Although I suspect a lot of that has to do with the price point of them usually being used for secondary protection. Quality of protective devices is a problem which sorely needs addressing (more so than 'ooh, AFDDs sound good', and 'oh no, plastic boxes are bad in fires').

reduces costs and complexity.

Increases cost and complexity to the end user for the sake of the profit margin of the vehicle manufacturer, you mean. They know full well what earthing systems are in place, just like the manufacturers of bathroom fans know full well they'll be installed on 6A circuits.

[tom66]

Well the costs only come to users for countries where PME is not standard.  I don't know what the situation is in the rest of Europe but I imagine this would have been a consideration.

It's worth noting that in the Type2 standard the CP/PP signals are referenced to PE, and a secondary pain happens should there be a short that leads to the ~10mA DC current from these saturating a downstream RCD's current transformer, preventing it from operating correctly.

What this really is showing is how outdated our electrical infrastructure is. Frankly given all of this I'm shocked (no pun intended) that "granny chargers" (i.e. portable EVSE) are still permitted and sold with cars, despite the well known fire hazards.

[Monkeh]

It's worth noting that in the Type2 standard the CP/PP signals are referenced to PE, and a secondary pain happens should there be a short that leads to the ~10mA DC current from these saturating a downstream RCD's current transformer, preventing it from operating correctly.

Ah, yes, because we still use type AC RCDs in blissful ignorance of the loads around us changing. Well, those who even have RCDs, because spending money on your electrical installation is daft, "It's worked fine for all these years".

[richard.cs]

Frankly given all of this I'm shocked (no pun intended) that "granny chargers" (i.e. portable EVSE) are still permitted and sold with cars, despite the well known fire hazards.

The risk approaches for cars and electrical installations are so fundamentally different. All EVs could come supplied with a pair of yellow rubber washing up gloves and be connected to the mains with permanently-live M10 wingnuts and the majority of deaths would still come from allowing minimally-trained persons to manoeuvre tonnes of metal at high speed in public places rather than electrical shock.

It's very hard to argue that car manufacturer's should spend anything on electrical safety when they would save more lives spending that same money on improved brakes/seatbelts/airbags/etc., let alone that they should worry about TNC-S touch voltages.

[themadhippy]

IF were ripping out the  network and starting from a clean sheet we should spent a few quid on  distributing the neutral from the source to the point of use,would make things lot safer.

The risk is that if PEN fails due to cable damage, corrosion, poor maintenance etc

In the uk the biggest cause of failure of the bond from neutral to earth at substations was the same cause of train signalling failures, being tied to the towbar of a 4x4 or transit and driven off down the road at  high speed

as far as I am aware there are only two ways to solve the problem:
- Earth rods until.

Its a "problem" thats well document and covered in sevral parts of the uk wiring regs,put simply if your taking power outside the house dont take the earth,source it locally.

[IanB]

I'm a bit puzzled about the discussion of potential hazards from unexpected earth potentials.

Isn't it an important requirement that earth is bonded to all exposed metalwork (water pipes, gas pipes) in an installation?

If I could get a shock from touching my charging EV, couldn't I also get a shock from touching a garden tap for a hosepipe?

What is the difference between these cases?

[helius]

If I could get a shock from touching my charging EV, couldn't I also get a shock from touching a garden tap for a hosepipe?

What is the difference between these cases?

Conceptually they are the same: the metal object is bonded to the equipotential zone of the building, but you are standing on a section of earth that is outside and can be at a different potential in case of significant earth leakage, or if the building's link to earth is broken because of poor maintenance. People can and do get shocked by touching outside faucets, and worse problems are found around boats.

[tom66]

If I could get a shock from touching my charging EV, couldn't I also get a shock from touching a garden tap for a hosepipe?

Yes you could, but you don't tend to find two-tonne water garden taps parked on driveways accessible to the public, postman and neighborhood cat.

More seriously I think this is the reason plumbing tends to be grounded.

[TimFox]

Electricity and plumbing is not a trivial problem.  During the American invasion of Iraq, faulty grounding caused over 18 accidental electrocutions of Army and contractor personnel in new showers.  See
https://www.huffpost.com/entry/toll-rises--at-least-18-u_b_124863
When installing equipment in foreign countries with temporary portable Diesel generators for AC mains, I encountered ungrounded three-wire outlets.  Since all our low-power equipment used a conventional 3-prong IEC line filter at the single-phase mains input, the chassis of computers or other equipment ended up near half the mains voltage with respect to the heavy hardware that was grounded.  Even though the current through the victim is limited by the small capacitance of the filter, it can still cause one to fall off a ladder when surprised.

[AlbertL]

Figure 1-1 ("Multi-Grounded Common Neutral" Distribution System) on this page https://www.electrical-contractor.net/forums/ubbthreads.php/ubb/printthread/Board/15/main/16309/type/thread.html illustrates the system used by my utility, and which I think is the most common in the US.

So, here's a question: what would be the resulting voltage at the customer's premises (bottom of the drawing) in the case of an accidental short between the ungrounded side of the distribution transformer primary (7200V) and one end of the 240V secondary?

[duckduck]

Non-heat-pump condenser dryers use more energy than vented, not less: https://www.cda.eu/laundry/condenser-vs-vented/

In cold climates, vented dryers are the least efficient after accounting for heat lost from the building.

In cold climates all appliances are 100% efficient

[CatalinaWOW]

Non-heat-pump condenser dryers use more energy than vented, not less: https://www.cda.eu/laundry/condenser-vs-vented/

In cold climates, vented dryers are the least efficient after accounting for heat lost from the building.

In cold climates all appliances are 100% efficient

Not an outside vented cloths dryer.

[helius]

So, here's a question: what would be the resulting voltage at the customer's premises (bottom of the drawing) in the case of an accidental short between the ungrounded side of the distribution transformer primary (7200V) and one end of the 240V secondary?

Which end, the side linked in-phase with the primary, or the other side?

[richard.cs]

So, here's a question: what would be the resulting voltage at the customer's premises (bottom of the drawing) in the case of an accidental short between the ungrounded side of the distribution transformer primary (7200V) and one end of the 240V secondary?

Just looking at the diagram I would say it depends on the saturation behaviour of the distribution transformer when it sees 7200 V on its secondary winding, and the impedance of the HV line and neutral at that point. Overall the secondary winding probably looks mostly like a short to the HV, and the shared neutral is probably pulled up to around half the HV line voltage. Clearly it should draw enough fault current to trip the HV, but until that happens I don't think I'd want to be touching any line, neutral or earth at any of the six consumers shown, including the ones on other transformers.

An impedance-earthed star point on the HV source would be much nicer.

[james_s]

I've never heard of that happening although I suppose anything is possible.

Back in the 90s a high tension line fell on a 7200V line and resulted in a substantial overvoltage in my friend's neighborhood. It blew out a bunch of light bulbs and fried some of their electronics.

[IanB]

I've never heard of that happening although I suppose anything is possible.

Apparently it can happen that the 7200 V line can fall onto the telephone or CATV line strung lower down the pole, and cause problems (occasionally fatal) in connected homes.

[richard.cs]

Apparently it can happen that the 7200 V line can fall onto the telephone or CATV line strung lower down the pole, and cause problems (occasionally fatal) in connected homes.

We don't share telephone etc. with HV poles here (UK), poles can have HV and LV or LV and signal stuff (but is possibly discouraged now?) but not HV and signals. Generally we have less overhead distribution in urban areas compared to what I have seen of the USA, and near-zero overhead HV in towns.

We have a lot of overhead LV and overhead telephone in older suburban areas, but all but the most rural newbuilds have all the cabling underground. Likely more to do with maintenance costs than safety, all the urban overhead LV was installed when labour was a lot cheaper.

Typical UK suburban area with overhead LV (note that line and several others is fed from a ground mounted transformer ~50m away via that PILC cable): https://goo.gl/maps/ruSgiQZ6PCM4PnNu9
A few hundred yards away it's been updated to aerial bundled cables: https://goo.gl/maps/Rxujc9uwFYUqKMDs5
Typical UK rural setup with a small pole mount transformer and HV and LV sharing that pole: https://goo.gl/maps/Ph3DQrHQmp7W6wMh7

[TimFox]

In residential areas of Chicago, with alleys between rows of houses, there are usually poles along the alley that carry the overhead power line, pole transformers, telephone, and cable lines.  Typically, though, they are fed from underground cables.  Total underground distribution is typical in industrial parks in the suburbs.  If you want to see a sky extremely polluted with overhead wires, go to urban Japan.

[themadhippy]

If you want to see a sky extremely polluted with overhead wires, go to urban Japan.

Their mere amateurs,try Thailand Cambodia or Vietnam to see how the professionals do it.

[james_s]

Apparently it can happen that the 7200 V line can fall onto the telephone or CATV line strung lower down the pole, and cause problems (occasionally fatal) in connected homes.

We don't share telephone etc. with HV poles here (UK), poles can have HV and LV or LV and signal stuff (but is possibly discouraged now?) but not HV and signals. Generally we have less overhead distribution in urban areas compared to what I have seen of the USA, and near-zero overhead HV in towns.

We have a lot of overhead LV and overhead telephone in older suburban areas, but all but the most rural newbuilds have all the cabling underground. Likely more to do with maintenance costs than safety, all the urban overhead LV was installed when labour was a lot cheaper.

Underground wiring is far more expensive to install and maintain than overhead. While it's true that it isn't as prone to damage in wind storms that knock trees down it's not immune, a root ball from a fallen tree can really do a number on underground wires. It's also far more prone to damage from flooding and tremendously more difficult to troubleshoot and repair. It works well in areas that are dry and have relatively few trees though.

[TimFox]

A big advantage of underground wire in my part of the country is less problems with ice storms.

[AlbertL]

So, here's a question: what would be the resulting voltage at the customer's premises (bottom of the drawing) in the case of an accidental short between the ungrounded side of the distribution transformer primary (7200V) and one end of the 240V secondary?

Which end, the side linked in-phase with the primary, or the other side?

Good point - I hadn't considered the phasing aspect.   I think in the electrical power world they call that "polarity" even though it's referring to AC.

[james_s]

Underground certainly has advantages, but it's not a panacea. Every time we have a big storm, especially the once in a decade kind a bunch of people start moaning that we should redo everything to underground. They have no clue how much it would cost and think it will solve all the problems, but it won't.