Why are we not always isolating the mains supply?

Question

Something struck me: to protect people, the neutral of the supply is connected on the supplier side to Earth, so that leakage through ground is detected when for example someone touches the phase (residual current devices do that). It doesn't protect people who would touch neutral+phase at the same time though.

But wait a minute, if the neutral wasn't connected to earth (which basically is the case when we isolate the mains supply), there would be no path through earth so no chance of leakage. So why is the supply made like this then, what do I miss?

Figure to illustrate - the dashed line separates the supplier side and the user side.

Answer 1

The electrical system we use was designed close to 100 years ago, and was modernized very little. Grounding and bonding was needed to blow fuses in case of short to an enclosure, or ground. The grounding was a reasonable option then, and it is unacceptable today.

The change to make it safer is not going to happen, because the misconception, that grounding is good an essential. It is too entrenched. Electricians are horrified of any change, and for years they were taught that grounding saves lives.

Grounding and bonding is actually the biggest cause of electrocutions.

But not all the circuits are grounded. Industrial 3 phase delta in North America is not grounded, and is potentially safer.

What we should have had by now is:

Circuit isolated by a transformer, grounded through a high resistance, and monitored for any leaks. This system already exists, but the main intension is not to protect people, but eliminate phase to ground arc.

For example currently the surface of an electric stove is connected to a bonding conductor, which is connected in the panel to a neutral, which goes to a transformer. The surface of the stove is connected to a leg of a transformer. The neutral is grounded at the transformer and at a building, but those grounds are not very good. Typically at least 5 to 10 Ohm. The solid connection is to the transformer.

This is why touching the stove and phase can be lethal.

Here is how it is done now in USA and Canada Here is a better solution. A ground fault circuit breaker would be used. A person touching a ground and a live wire would have a limited current running through them, and the circuit breaker would disconnect the power. This system doesn't protect from line to line shock. The resistor sensing works on the principle of a voltage drop, the resistor also limits the fault current to a safe level. The ground fault works on a principle of magnetism and it compares current in the incoming wires. If one current is higher, it shuts off.

Answer 2

The vast majority of people only get in contact with electricity via an appliance, tool or other device. If you connect the neutral to the earth and earth all the devices a circuit breaker will likely trip if the earth resistance is low enough. If you'd isolate the neutral you would not know unless you touch the neutral or the neutral makes contact with the earth accidentally, in which case you get the shock, not the circuit breaker. The 1st option is still the safer one.

This is also the reason why in many countries the earth resistance needs to be below a certain value before you get certification.

The only exception are double insulated appliances and tools which are build in such a way that the enclosure can never become live. These device are not to be earthed in any way because that would again increase the risk of shock if the earth wire would become live through another equipment that would accidentally leak.

Answer 3

By not connecting earth mains to neutral, you're allowing the wall's line voltage to "float". I.e., although the hot and neutral line will always be 120VAC with respect to each other, there's nothing stopping them from rising above ground potential. The stick figure on the right will get a nasty shock if the line is floating far above earth ground--which is certainly possible (it's happened to me! although the mechanism is different, see my question here: Connecting center-tapped transformer to earth ground; or, why am I being electrocuted? ).

edit: the figure is touching the live wire! I didn't see that. He's going to get zapped even if you connect neutral to Earth. But if you don't connect neutral to Earth, touching it can zap you. Furthermore, most of the things you plug into the wall are "grounded" via the neutral pin. If that's floating, you can get electrocuted just touching your toaster!

Answer 4

You are right that isolation would prevent a shock from directly touching a live wire, but brings with it other problems. This is also something that is used in applications where one needs additional failure safety in case of shorts to ground (no immediate need to switch off in the case of a single fault), but with care that the problems below are handled correctly. Cases this is done are e.g.:

• in ICUs in hospitals, where most devices are completely insulated anyways.
• in the chemical industry where a power outage might even be dangerous (e.g. exothermic reactions)
• areas that need special protection against explosions (e.g. coal mining)
• on ships
• sometimes by fire brigades when using portable generators

It is also not done as simple as you drew is, e.g. metal cases are still grounded, and the secondary side of the transformer has a connection to earth, but only in the range of a few kΩ.

Such supply nets are normally limited in area, since finding a fault is relatively hard. Most of the time those are also permanently monitored for isolation (with a insulation monitoring device) as to detect single faults before they cause problems.

The way you drew the system you could have multiple problems (not an exhaustive list):

• You can not use RCDs to prevent from touching e.g. cases of devices where the live wire is shorted to the case. Which would not be a problem by itself, but becomes one if you touch anything that is grounded (water pipes, heating, being in your garden, maybe even your living room - depending on your isolation to earth in that case) or e.g. another device where neutral is connected to the case (either by design or also by accident).
• You have to monitor that your net is never connected to ground at any point (either by design (made for a different country) or by accident).
• You would have problems with the shields of e.g. network cables. You have to connect them somewhere, but you can't connect them to ground anymore. Maybe connect them to neutral? Then you get a compensation current over the shield, because of the different neutral level of each device (because of the return current and the ohmic resistance of the neutral wire)

Your view also ignores capacitive coupling between the power lines.

Answer 5

Every object that can carry a charge is at some potential referenced to earth. Thundercloud tops, for example, can reach a surface potential of over a billion volts. Choosing zero volts as the mean potential really is the safest option; any other voltage would be higher and thus the potential for an electric shock would be worse.

For some related trivia, nowadays three phase electric power is carried to houses preferably in a cable which has five conductors: Line voltages L1, L2, L3; the Neutral and Earth. But back in the old days at the countryside when copper was scarce, there would have been only four: L1, L2, L3 and a much smaller in diameter Neutral, to save material. And Neutral would have been literally connected to the earth, using a copper nail of at least one meter in length, driven into the soil at the side of your house. To my knowledge, this is not done today. But it gives some sense to the term "earth" in electric wirings and sheds some light to the practicality of using earth.

When the neutral is tightly connected to earth, there's another safety plus: when electric cables do break, by the action of a digger or a falling tree, the earth gives a conducting path for the current, which can be detected and fault relays at the power station can switch off the power in the cable. Similarly, if an electric appliance of yours breaks down, there's a good chance that it will blow a fuse in your fuse box if the shield in your appliance is earthed.

Answer 6

It is very desirable to construct devices such that when they are switched off or have a fuse blow, no part of them will be able to feed current into ground, even if there exist faults within other devices on the same circuit. This effectively requires that all power leads which could source or sink significant current when their potential substantially differs from ground must always be switched or disconnected simultaneously. It is much easier and cheaper to have a hot wire fused and a designated neutral wire unfused than it would be to guard hot and neutral wires with a "double-pole fuse" constructed such that an over-current condition on either wire would disconnect both. Although it's possible to construct fuses in such fashion, such designs are much more expensive than independent fuses.

In the absence of a designated/forced neutral, an appliance which developed a short between one of its power leads and ground could cause the shorted power lead to behave as "neutral" and the other to behave as "hot". If another appliance on the same circuit only switched or fused the same power lead as the one on which the first appliance developed a short, the entire second appliance would be live (relative to ground) even when it was switched off--a potentially dangerous condition.

If all appliances switched or fused both power-input leads, it wouldn't matter which was hot and which was neutral. Having a designated neutral, however, makes it possible to have appliances safely switch or fuse only a single lead, rather than having to ensure a simultaneous disconnect of both leads. That's a pretty big win.

Answer 7

The electrical system is, as you say, old. It is not unacceptable though. (I am in Australia so it is a little different than the US, but not that much in principle.)

The neutral being connected to the earth potential is, as has been explained, so that if there is a short circuit to the ground/chassis/earth in a piece of equipment, the fuse will blow.

Fuses were indeed the only protection we had for many decades. Now, in addition, we have residual current and earth leakage current devices that literally measure the difference in instantaneous current in the active and neutral conductors.

If there is a difference, it means that some current is going through the equipment to ground, like in the case of a refrigerator where moisture is causing tracking of current through dust and dirt to the frame of the fridge. The other possibility is that current is going through a person to the ground through their shoes... and maybe their heart!

That is why this relatively recent innovation is so important and operates in addition to normal circuit protection in the form of fuses and circuit breakers.

In Australia we call RCDs (Residual Current Devices) "Safety Switches". They are exactly that!

That ends the lesson.

Answer 8

Unfortunately none of the answers on this page seem to mention one rather important thing about high-voltage transformers, which makes them fairly cheap/affordable: graded insulation. From what turns out to be one of my favorite books (in answering questions here), Grounds for Grounding (by Joffe and Lock) here's the relevant figure:

Without earthing the neutral, you can't use graded insulation. I've only included here the figure for the initial high-voltage transformer at the generator site, but the same graded insulation applies to all the substation transformers on the distribution network toward the consumer.

EDIT: I guess I shouldn't have left this dangling. Since you can't (cheaply) get rid of earthing on the high-voltage distribution, when you couple an ungrounded low-voltage supply system to it, you have capacitive coupling as mentioned at the end of Andreas Wallner's answer. The effects of a ground fault in a large ungrounded system are more severe than one (leaky) home transformer experiment may suggest. According to IEEE Std 141-1993 you can see transient over-voltages five times the nominal voltage in such a case when a phenomenon called arcing ground occurs. (The same info is repeated in http://www.hv-eng.com/2010IASGrounding.pdf) There's a somewhat intuitive explanation of arcing ground phenomenon at http://www.electrotechnik.net/2011/05/arcing-grounds.html I'm not really sure about the reliability of that latter source, but it says basically that the arc forms and breaks up many times that's why you get the overvoltages (said to be 3-4 times the nominal in that source). There are ways to prevent those, but an ungrounded low-voltage distribution isn't as simple as an isolation transformer in a Class II wall-wart.

EDIT2: The IAS presentation actually explains the overvoltage due to arcing more scientifically in terms of a resonant circuit that forms. It's actually a very good presentation, but rather long and my EE time today was rather limited... Towards the middle (of the 100+ slides) it gets to talk about high-resistance grounding (HRG) which is a way to get some benefits of no grounding (reduced risk of shock) without much of the downsides, but alas it's only applicable to three-phase loads; HRG can't be used with single-phase loads. So we probably won't see it in residential distribution.

Answer 9

First and foremost, grounding is needed, period. People who say grounding is not needed have no idea what they are talking about. Once you truly comprehend how dangerous high voltage live AC is, how electricity behaves and works and why grounding is valuable, you'll understand why grounding is mandatory.

It is about safety. Yes, it is true that touching the live 125 volts of an AC conductor will travel through you to ground since you are completing the circuit, that's the whole point of it. This is only true of live AC electricity coming directly from the electricity company. They need to ground the neutral wire, its for the safety of people who works with high AC voltages, the safety of people living near by high voltage AC lines and also to provide a better quality of electricity delivery service, grounding the neutral wire also equalizes the electricity's potential from pole to pole during delivery if a short occurs. We need an electrical short to easily travel to ground. When a short does not travel to ground, this brings a lot of problems. Electricity by nature, a large sum of electrons of short will always want to dive down to earth (earth as in deep into the soil). This is how thunders work. 7ft deep into Earth's soil has the least potential amount of energy and this is where high potential of energy (high voltages of current, a short) will always wants to go.

In USA we have 3 pins, the 3rd prong is to provide an extra layer of security and safety from stupid ideas of not having a grounded line. I have to admit, USA has the state of the art most advance residential electrical standards and is future proof for advancement. Other countries which uses the 2 pins outlet system is straight up a bad standard and needs to follow the USA 3 prong system. Because countries who uses the 2 pin prong system will always follow the old bad standard of grounding the neutral wire at their homes, rather than having a dedicated grounded wire, because as again, there must always always needs to be a path for a short to travel to least resistance. Which also means 2 prong electrical live wire systems can never have a safe isolated system since a safe dedicated grounding implementation will never exist. Therefore a 3rd prong is needed just for this safety measure. USA can have all their homes fitted with isolated transformer since we have a future proof electrical standards.

In the future residential homes should have isolated wiring, where the natural wire is not grounded. Which means taking the supply line from the electricity company which is grounded and have to invest into a good isolated transformer system which removes the grounding and supplies isolated electricity. But there must be a grounded wire, literally grounded 7 feet under into earth's soil. The electrical outlets for the entire home must utilize the 3rd earth pin and must be connected to this grounding wire. This also means products made from companies that utilizes mains voltage must be more strict and serious how they design their products. This also means electricians and home builders will also need to strictly design how they build homes to be safe and utilize effectively of the 3rd prong grounding due to isolated live wiring. This will take years upon years for everyone to follow. It will be a much safer design in the future but having everyone to follow this standard will be tough.

The way we have both the neutral and 3rd ground pin connected to earth is really safe and fools proof in the long run, but it will be much more safer if the world can isolate the ground from the natural wire and direct it to a dedicated earth prong for their residential powering system. To simply avoid retarded shocks from people sticking their fingers on live AC conductors or wires with bare feet with completing the circuit to ground. It will also reduce accidental electrocutions with careless electricians.