Why must residential electrical systems be connected to Earth (soil)?

[gen x]

Why is grounded electrical system safer than isolated system?

 

[Averagesupernova]

Have you done a search here on PF? It's been discussed. You will get the same answers from the same people in this thread that have previously explained it.

 

[gen x]

Have you done a search here on PF? It's been discussed. You will get the same answers from the same people in this thread that have previously explained it.

No, maybe someone can put a link.

 

[DaveE]

google, dude. It works great...
https://staleyelectric.com/understanding-electrical-grounding-and-how-it-works/
https://en.wikipedia.org/wiki/Ground_(electricity)

 

[Baluncore]

Why is grounded electrical system safer than isolated system?

Because a common ground connection protects the insulation, by preventing overvoltage, that might break it down.

 

[sophiecentaur]

Why is grounded electrical system safer than isolated system?

If you have Googled this then you will have see a diagram of a grounded system. If every exposed piece of metal is connected to ground (and I mean every), then a person can touch any two thins and have only Zero Volts across them. =safe

 

[Lnewqban]

Why is grounded electrical system safer than isolated system?

Wherever persons can be in contact with electricity with voltage above 60 volts, the salty fluids inside their bodies can become a conducting path to electrons, which could interfere with the electrical control impulses reaching their muscles, especially the very important one named heart.

Creating an equal potential between the ground on which those persons stand, barefoot sometimes, and another surface, or between two separated surfaces which can be reached with the hands, eliminates the danger of having a dangerous flow of electrons through the path of their bodies.

Please, see:
https://www.nfpa.org/en/news-blogs-.../03/14/How-Grounding-and-Bonding-Are-Achieved

https://www.salinas.gov/files/sharedassets/city/v/1/fire/documents/grounding-bonding-fact-sheet.pdf

https://www.ecmag.com/magazine/arti...p-to-code-understanding-grounding-and-bonding

https://www.ecmag.com/magazine/articles/article-detail/what-on-earth-connecting-up-to-code-part-2

 

[DaveE]

@sophiecentaur , @DaveE... this is not simple question for google, there are lots of wrong answers at google.

It works better if you search for knowledge, not just answers. But kudos for recognizing that you must verify the information you find on the web. Answers are more appropriate for specific, detailed, questions. General subject matter questions, like what is grounding for, how does gravity work, etc., don't have simple answers. I think you may need to study electric circuits a bit more. The underlying concept is simple. In practice it can get rather complicated.

Finally, sorry, I didn't watch your videos. I'm not looking for more information about this myself.

 

[tech99]

Isolated systems are used for safety on construction sites etc. A transformer is used where the secondary is not connected to earth. A problem arises, however, if the system gets very big and an accidental connection to earth occurs on one wire. Then the system is dangerous, because if someone touches the other wire an earth path can exist via their body.

 

[Averagesupernova]

Isolated/floating systems are also used in places where a fault would otherwise trip a breaker and the loss of power would cause a large loss in revenue or other problems. The system is monitored and if a fault exists it is tracked down and dealt with while the power is on.
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Hospital operating rooms work with isolated systems. My memory isn't what it used to be but it seems I may have posted a link to a good YouTube video on the subject at some point in the last couple of years. I'll do a quick search.

 

[DaveE]

Also, note that isolation transformers aren't just your run of mill transformer. They will (should) have appropriate construction with either lots of spacing and insulation or grounded screens to prevent stray currents from causing excessive secondary voltage wrt earth.

Some of the reasons we favor grounded systems are that they are less expensive compared to alternatives, and are more fault tolerant (idiot proof) than isolated systems

 

[gen x]

@tech99 , @Averagesupernova

Do you understand and agree with Transistor answer? Why his isolated system can't use RCD to fix the fault?
Part "If we get an earth fault at D (not simultaneous with C above) then we have the same situation with LINE1 becoming N and LINE2 becoming live" ?

https://electronics.stackexchange.c...-adavantages-of-neutral-vs-isolated-grounding

 

[Averagesupernova]

@gen x the link you proved in post #12 makes sense. The user named transistor is basically correct although I will say that a fault the the actual soil is often not enough to trip a breaker or blow a fuse. It was implied in transistor's post that this would occur. However transistor did imply that resistance of the actual earth is high enough to not make a good conductor so I will just leave it at that.
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Why his isolated system can't use RCD to fix the fault?

An RCD can't ever fix a fault. It is designed to disconnect power to the circuit where the fault exists. Do you know how this occurs? Do you know how an RCD determines there is a problem? An RCD consists of a current transformer with both the hot and neutral passing through it. If the current on the hot and neutral is the same then there is no signal coming out of the current transformer. In a grounded system if someone is getting a shock then the current will not be the same on the hot and neutral. Some current is passing through the body of the person getting shocked and not going back to the source on the neutral. This imbalance of current through the current transformer causes an output from the current transformer and triggers a disconnect.
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Here's a Wikipedia article:
https://en.wikipedia.org/wiki/Residual-current_device
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Now consider the floating system with an RCD. If a person touches C or it becomes grounded in some way in the stack exchange post then no harm, no foul. It's isolated. Sounds safer right? Assume you are the one touching it. Now at the same time on the same circuit on the opposite side of the house I touch D. Now both you and I are in series across the line. The currents in each wire would be identical and there would be no reason for it to trip while you and I continue to get shocked.

 

[Averagesupernova]

@gen x
Have a read. What I described in the above post was covered on the thread below. Not sure why I couldn't find it earlier.

Post in thread 'Following up on the recent thread about Earth Resistance measurements'

Aug 18, 2025

But earth is not necessary, the trigger could be conduction to another A or N circuit on another RCD.

Both you and @sophiecentaur seem to be missing my point. Of course I realize that an RCD works by sensing unequal currents in the live and neutral. Those currents will NEVER be unequal if the system is completely floating. The system has to be referenced to something else (the earth) upstream of the current transformer of the RCD in order for the system to allow an RCD to work correctly.
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Edit:
I do see your point about a pathway between two different RCDs on the same system...

• Averagesupernova

 

[gen x]

@sophiecentaur

@gen x the link you proved in post #12 makes sense. The user named transistor is basically correct although I will say that a fault the the actual soil is often not enough to trip a breaker or blow a fuse. It was implied in transistor's post that this would occur. However transistor did imply that resistance of the actual earth is high enough to not make a good conductor so I will just leave it at that

Can you expalin "If we get an earth fault at D (not simultaneous with C above) then we have the same situation with LINE1 becoming N and LINE2 becoming live" ?

An RCD can't ever fix a fault. It is designed to disconnect power to the circuit where the fault exists.

Of course I realize that an RCD works by sensing unequal currents in the live and neutral. Those currents will NEVER be unequal if the system is completely floating. The system has to be referenced to something else (the earth) upstream of the current transformer of the RCD in order for the system to allow an RCD to work correctly.

This is groudned system during fault, hot wire touch metal case, RCD trip:

This is isolated system during fault, same system without earth electrodes 1 and 2 in soil.
Most of current will travel via PE wire to neutral and back to source, so RCD will feel current imblance and trip as well.
Why you think it will not work ?

 

[Averagesupernova]

Both diagrams in post #15 will cause the RCD to trip as they are drawn. But the way they are drawn do not guarantee a floating system. The chassis of the appliance is wired to the neutral. There is no way to guarantee the chassis of every appliance is not in contact with the actual earth. These drawings are not the true floating systems represented by the drawings you link to in post #12. You are arguing about your position of an ungrounded system and then provide drawings that are obviously not ungrounded. You are contradicting yourself.

 

[gen x]

Both diagrams in post #15 will cause the RCD to trip as they are drawn. But the way they are drawn do not guarantee a floating system. The chassis of the appliance is wired to the neutral. There is no way to guarantee the chassis of every appliance is not in contact with the actual earth. These drawings are not the true floating systems represented by the drawings you link to in post #12. You are arguing about your position of an ungrounded system and then provide drawings that are obviously not ungrounded. You are contradicting yourself.

Isn't definition of isolated electric system, system that is not connected to Earth?

The chassis of the appliance is wired to the neutral. There is no way to guarantee the chassis of every appliance is not in contact with the actual earth.

How do you mean in contact with earth? Devices can't flaot in the air, becuase we are at planet Earth that has gravity.. ?

I show at diagram below, without earth electordes 1 and 2 works without any current travel through the earth, so I come to my first question what is the main purpose of connected system to earth(soil)? Lightning?

Why RCD need earth to trip or why RCD in isolated system would not trip?
I only need one parellel wire(green) to trip, not soil. Isn't it?

Why RCD will not trip in system below?

 

[Averagesupernova]

@gen x . Concerning your last diagram in post #17. Assume the chassis of your appliance is isolated from the actual earth. You've drawn with a fault from the red to the chassis. Remove that. Now have one person touch the red wire and a second person touch the blue wire. Both touch between the boxes drawn. They are standing on the earth without shoes. Do you understand how they will receive a shock and why the RCD will not trip?

 

[gen x]

@gen x . Concerning your last diagram in post #17. Assume the chassis of your appliance is isolated from the actual earth. You've drawn with a fault from the red to the chassis. Remove that. Now have one person touch the red wire and a second person touch the blue wire. Both touch between the boxes drawn. They are standing on the earth without shoes. Do you understand how they will receive a shock and why the RCD will not trip?

Yes now part of current will travel through one person through earth to other person. RCD will not trip. Breaker will not cut power, because current is too low/earth has too big resistance, but current is enough to kill a person.

If I now connect neutral to earth, then RCD will trip at 30mA, because part of the current bypasses RCD via earth.

Is correct?

 

[Averagesupernova]

Yes now part of current will travel through one person through earth to other person. RCD will not trip. Breaker will not cut power, because current is too low/earth has too big resistance, but current is enough to kill a person.
View attachment 368607




If I now connect neutral to earth, then RCD will trip at 30mA, because part of the current bypasses RCD via earth.

View attachment 368608




Is correct?

Yes. You have the correct idea. In the USA we refer to the RCD as a a GFCI. A receptacle that contains GFCI protection is protected at 5 mA. Any device whether it's a circuit breaker or receptacle that's designed to prevent a shock through GFCI action is supposed to have a threshold of 5 mA. Receptacles have both line and load terminals. This way whatever is connected to the load side is protected as well.

 

[gen x]

Yes. You have the correct idea. In the USA we refer to the RCD as a a GFCI. A receptacle that contains GFCI protection is protected at 5 mA. Any device whether it's a circuit breaker or receptacle that's designed to prevent a shock through GFCI action is supposed to have a threshold of 5 mA. Receptacles have both line and load terminals. This way whatever is connected to the load side is protected as well.

Why my diagram from post #15 without earth electrodes 1 and 2 is not isolated system?

 

[Averagesupernova]

Why my diagram from post #15 without earth electrodes 1 and 2 is not isolated system?

You cannot guarantee that the chassis of an appliance will forever remain isolated from the earth. A washing machine is connected to piping and eventually that piping is connected to the earth. Even with plastic pipe in your home, eventually the water that those pipes carry will contact the earth.

 

[gen x]

You cannot guarantee that the chassis of an appliance will forever remain isolated from the earth. A washing machine is connected to piping and eventually that piping is connected to the earth. Even with plastic pipe in your home, eventually the water that those pipes carry will contact the earth.

But you can't guarantee at any isolated system that will remain isolated to the earth, here connection to the earth we treat as fault? So this is fault case?

 

[gen x]

@Averagesupernova

Do you agree with his example why we need earth? I don't, because if his case has RCD, it will trip.

https://electronics.stackexchange.c...on-in-home-electrical-system-really-necessary

 

[Averagesupernova]

I don't intend to chase every link you provide since there are endless misconceptions with someone either spouting off how they think it should be or someone else correcting them.
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I have run across a situation in a basement where a wire got stuck between the screw that holds the light switch in and the side of the box that enclosed the switch. The house was wired in the 1950s and I found this in the mid 1980s. The box was not grounded and follows the exact scenario described above about not self revealing. It was revealed when someone touched it.

 

[gen x]

I don't intend to chase every link you provide since there are endless misconceptions with someone either spouting off how they think it should be or someone else correcting them.
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I have run across a situation in a basement where a wire got stuck between the screw that holds the light switch in and the side of the box that enclosed the switch. The house was wired in the 1950s and I found this in the mid 1980s. The box was not grounded and follows the exact scenario described above about not self revealing. It was revealed when someone touched it.

Of course it wasnt self revealed, because it didnt have RCD, if it had it will trip. For this case earth is not needed to RCD trip.

 

[Averagesupernova]

Of course it wasnt self revealed, because it didnt have RCD, if it had it will trip. For this case earth is not needed to RCD trip.

The point was that this sort of thing happens "in the wild". The real world example I gave did not have the conduit or switch box connected to the neutral back at the main panel. It was not required at the time. The metal box and pipe were floating. The USA has used a ground (earthing) rod since the beginning. Had the conduit and box been connected to the neutral in the main panel the breaker would have tripped at the time. Prior to the switch to breakers, the fuse would have blown.
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Any installation that is relying on a GFCI to trip to protect people when they touch it rather than stopping an unsafe situation prior is asinine.

 

[DaveE]

Any installation that is relying on a GFCI to trip to protect people when they touch it rather than stopping an unsafe situation prior is asinine.

This!!! RCD/GFCI should not be used as primary protection. They are an additional safety feature which simply isn't as reliable as good construction. Other code requirements are still necessary.

 

[gen x]

This!!! RCD/GFCI should not be used as primary protection. They are an additional safety feature which simply isn't as reliable as good construction. Other code requirements are still necessary.

What is good primary protection and good construction?

 

[Averagesupernova]

In the case that I gave the conduit was a protective sleeve that terminated into the switch box. I won't comment about if it was safe at the time of original installation. The conduit did not run all the way to the breaker panel.
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As far as what is good primary protection and good construction:
Ideally in my opinion the safest would be to have what we have now in the USA. The neutral is tied to the chassis of the meter can as well as the main breaker panel. After this the protective ground is kept separate from the neutral. Every piece of metal associated is bonded to the protective earth. Water piping is also bonded to the protective ground. Ground rods are driven and connected to the main panel. Concrete encased electrodes are also used. A length of rod is encased in the concrete footing of the building. GFCIs have become quite reliable concerning nuisance trips. One thing that could be changed is to keep the neutral and protective ground separate all the way to the transformer. I've thought this could be an added layer of safety but I am not so sure. The protective ground wire en route to the transformer could be broken without being known until it's really needed. This could result in the chassis of every appliance developing a voltage in the event that somehow the protective earth system in the house becomes energized.