Will an RCD offer any protection in a building with NO earth circuit?

[Rockingit]

International distribution systens are fascinating, and completely dispel the myth that Brit is best, too. For example I'm currently in Australia, in basically a Brisbane city suburb. Walking down the road and I realised when glancing at the poles that the same pole is carrying both HV and LV - Conventional delta HV on top and then intermittently it drops down via a TX onto the LV lines and appears as 3PN. Earthing (that I've seen, anyway) is TT. A simple idea that must save on costs massively whilst delivering constancy over long distances.

 

[Mike Johnson]

The power providers in France leave the earthing arrangements to the home owner so TT systems are the norm, most electricians carry large SDS drills for that purpose.

 

[ElectroChem]

International distribution systens are fascinating, and completely dispel the myth that Brit is best, too. For example I'm currently in Australia, in basically a Brisbane city suburb. Walking down the road and I realised when glancing at the poles that the same pole is carrying both HV and LV - Conventional delta HV on top and then intermittently it drops down via a TX onto the LV lines and appears as 3PN. Earthing (that I've seen, anyway) is TT. A simple idea that must save on costs massively whilst delivering constancy over long distances.

Australia's LV distribution is almost exclusively PME/TNCS, here called MEN (Multiple Earthed Neutral). You'll find very few TT systems here.

 

[pc1966]

Australia's LV distribution is almost exclusively PME/TNCS, here called MEN (Multiple Earthed Neutral). You'll find very few TT systems here.

But you do have single-wire eath return HV systems where the Earth is your return path?

That is about TT as you can get LOL!

 

[GeeGee50]

If the appliance is not connected to earth then the RCD will not have any earth leakage current to detect, until your body provides a path when you touch it. So the RCD with no earth won’t trip.

But isn't there still a leakage / loss of current if the appliance develops a ground fault (i.e. the active/live wire touches the appliance chassis/casing)? Therefore, this will create an imbalance of current between Live and Neutral and therefore the RCD will still trip?

 

[brianmoooore]

But isn't there still a leakage / loss of current if the appliance develops a ground fault (i.e. the active/live wire touches the appliance chassis/casing)? Therefore, this will create an imbalance of current between Live and Neutral and therefore the RCD will still trip?

Only if the unearthed appliance is in contact with the general mass of earth, which is by no means certain, since many appliances sit on rubber or plastic feet.

 

[westward10]

But isn't there still a leakage / loss of current if the appliance develops a ground fault (i.e. the active/live wire touches the appliance chassis/casing)? Therefore, this will create an imbalance of current between Live and Neutral and therefore the RCD will still trip?

Not if it is not connected/isolated from earth. Live will touch the casing, have no where to go and not create an imbalance.

 

[Thrubstack]

Surely when you touch a live circuit, your body provides a path to earth, that will result in an imbalance and the RCD trips.

 

[brianmoooore]

Yes, there's a good chance that your body will provide a path to earth and imbalance the currents, with the result that the RCD trips.
This will probably save your life, but it will still hurt.

 

[HappyHippyDad]

Yes, there's a good chance that your body will provide a path to earth and imbalance the currents, with the result that the RCD trips.
This will probably save your life, but it will still hurt.

Can you really say a 'good' chance? If you see post number 9 your body will only provide a pathway to earth if the neutral point of the transformer is earthed. I have absolutely no idea if this would be common practise in the OP's country.

I suppose it couldn't actually do any harm by using a plug in RCD. It would either not work, so you're no worse off, or it would work and probably safe your life. So I guess it's a good idea. However, by using an RCD you may feel a sense of security and take greater risks!

 

[GeeGee50]

Only if the unearthed appliance is in contact with the general mass of earth, which is by no means certain, since many appliances sit on rubber or plastic feet.

Not if it is not connected/isolated from earth. Live will touch the casing, have no where to go and not create an imbalance.

Ok.

But this is what I still don't get (apologies for my ignorance)---> your responses imply that the current is still flowing to Neutral? But how is that possible if the current has left the intended live-neutral loop?

Note: My logic / assumption is that if a Live wire touches a metal chassis, then that means it is no more connected to neutral. And if it’s not connected to neutral, then there’s no way for the current to flow back to neutral (hence my reasoning why the RCD will trip). What am I missing here?

 

[ElectroChem]

Ok.

But this is what I still don't get (apologies for my ignorance)---> your responses imply that the current is still flowing to Neutral? But how is that possible if the current has left the intended live-neutral loop?

Note: My logic / assumption is that if a Live wire touches a metal chassis, then that means it is no more connected to neutral. And if it’s not connected to neutral, then there’s no way for the current to flow back to neutral (hence my reasoning why the RCD will trip). What am I missing here?

If the connection has broken inside the appliance and contacted the chassis instead, then with no neutral return path there will be no current and the appliance will not work. The casing will be live and if someone touching it completes a path to earth/neutral they will get a shock, but the RCD will not automatically trip.

This is one reason why if e.g. a light or heater doesn't start when you plug it in and turn on the power (assuming its own power switch is on) then it should be unplugged before being examined as a broken internal wire may be touching the casing.

 

[GeeGee50]

I entered this thread because video below states that when a Live wire touches a metal frame there will be current leakage causing the RCD to trip. No earthing of the metal frame, no person touching the frame. RCD still trips.

This video seems to contradict the initial responses I received above, which states the RCD will trip when a ground fault happens but only if the metal frame is connected to earth.

Confused as ever....

starts at 3:00

 

[westward10]

There is a few ifs and buts here. If the frame is not earthed, say it is floating in the air and completely isolated then there is nowhere for any current to go and as such the RCD shouldn't sense an imbalance.
Another scenario, again the frame is not earthed by way of the electrical system but has a tenuous contact to earth. For instance a washing machine sitting on a damp floor which is likely to allow leakage to flow hence you get an imbalance.

 

[GeeGee50]

There is a few ifs and buts here. If the frame is not earthed, say it is floating in the air and completely isolated then there is nowhere for any current to go and as such the RCD shouldn't sense an imbalance.

You're pretty adamant there won't be any current leakage as the current travels through the body of the frame....not saying you're wrong, just interesting to hear different viewpoints.

Follow on question ---> so how does the current make its way back to neutral from the frame?

 

[westward10]

If it has no possible return to earth no current will flow. If I touch a live wire to a metal spoon on a rubber mat there is nowhere for any current to go, this is the same scenario.

 

[GeeGee50]

If it has no possible return to earth no current will flow.

Sorry, I wasn't being clear enough ---> I'm not talking about the current going back to neutral via earth. I'm saying how does the current that is now buzzing around the metal frame/body make its way back to the RCD?......because the point is if we say the metal frame is not earthed AND we say there is no imbalance in current, then that must mean the current is making its way back via the circuit's neutral wire....correct?

 

[plugsandsparks]

You're pretty adamant there won't be any current leakage as the current travels through the body of the frame....not saying you're wrong, just interesting to hear different viewpoints.

Follow on question ---> so how does the current make its way back to neutral from the frame?

As mentioned before, the DNO supply transformer is expected to have a centre point Neutral which is then referenced to Earth at the Substation and in some cases along the route to the home/businesses. Any conducting surface like a toaster casing that has become live, if the toaster is earthed at the home, the current will either flow along the home earth to the DNO earth or Earth Rod and make its was back to the Substation to complete the cct. As this is a long path/highish resistance path it may not trip a breaker or blow a fuse but just sit there. Along comes a human , touches the case and creates a parallel cct through the human body to the floor or if touching an earthed component like a radiator or kitchen tap a highish current will flow through the human. If an RCD was in cct, it would see current down the live and not coming back to the neutral through the RCD as its been diverted through the human to a parallel path back to the substation, hence it can trip if the impedance is low enough.

 

[loz2754]

Sorry, I wasn't being clear enough ---> I'm not talking about the current going back to neutral via earth. I'm saying how does the current that is now buzzing around the metal frame/body make its way back to the RCD?......because the point is if we say the metal frame is not earthed AND we say there is no imbalance in current, then that must mean the current is making its way back via the circuit's neutral wire....correct?

Perhaps you're confusing current and voltage. There will be a voltage at the metal frame, but in order for a current to flow anywhere there needs to be a complete circuit, either back through the neutral to the transformer, or back through the general mass of earth to the neutral point of the transformer.
Current does not 'buzz around', it is the flow of electricity in a conductive material. If there is no path for current to flow, there is no current.

 

[Simon47]

I'm saying how does the current that is now buzzing around the metal frame/body make its way back to the RCD?

What "current buzzing around the frame" ?
Are you familiar with Kirchoff's 1st (or Current) Law ? Put simply, all the current going into/out of the device (via whatever routes) must sum to zero. If there is no path to ether from the frame (lets say it's sat on a perfectly insulating surface and has no earth connection) then there can be no flow of current between frame and earth. If the live supply wire comes free from it's terminal and touches the frame then two things happen :
1) The supply current will stop. There's no longer a path from live to neutral via the device load, and no path to earth from the frame, therefore when you apply Kirchoff's law, there can be no current in the live. No current in the neutral, no current in the live, therefore no current imbalance and the RCD will not trip.
2) The frame will become live. Simply, there's no a connection from the live supply to the frame, so the frame will be sat there at mains live voltage.

......because the point is if we say the metal frame is not earthed AND we say there is no imbalance in current (meaning the RCD won't trip), then that must mean the current is making its way back via the circuit's neutral wire....correct?

No, as above, there will be no supply current - it has nowhere to go to.
Remember, the supply does not "push current" through the loads, it forces a voltage onto the loads, the current is drawn according to the supply voltage and load characteristics. That's a common misunderstanding - people thing that you cannot use (say) a 2 amp power supply to for a device that only draws (say) 1/2 amp, they believe that somehow the 2A supplyt will force 4 times the current through the device and fry it. Assuming the voltage is the same, the device will simply draw it's normal 1/4A from the supply.

Now, there can be a discussion as to whether a large device sat on an insulating mat really will pass zero current to earth. In practice, the frame, mat, and floor/ground will form a small capacitance which in the presence of an AC voltage will pass a very small current. But for this discussion that current will be small enough to ignore.