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

[AmatuerinCambodia]

I live in Phnom Penh, Cambodia and people die here regularly by electrocution. There is no earth wiring in the buildings, just live and neutral.

I am a member of an expats forum and the question arises: Will using an RCD device offer any protection at all?
I understand RCD's will go off with any leakage and that may have to include the human being the earth. At the moment my apartment has 8/16/32 amp circuit breakers and nothing else.

I can buy plug-in type RCD units to go between an appliance (like the washing machine) and the wall socket. Will this offer any protection even it is me being the earth!

 

[mainline]

In the scenario I was responding to (see post above mine) poster says live touches frame, frame is not earthed, current flows through neutral. Poster was describing a dead short.

A live touching a metal frame that is connected to neutral or earth doesn’t mean it’s going to be a short circuit and cause the ocpd to trip, it’s going to depend on the impedance of the circuit.

It could be for instance that the disconnected neutral side of a lighting circuit contacts the metal frame.

 

[RetiredNY]

A live touching a metal frame that is connected to neutral or earth doesn’t mean it’s going to be a short circuit and cause the ocpd to trip, it’s going to depend on the impedance of the circuit.

It could be for instance that the disconnected neutral side of a lighting circuit contacts the metal frame.

I assumed neutral was solidly connected to metal frame, in this hypothetical example.

 

[mainline]

I assumed neutral was solidly connected to metal frame, in this hypothetical example.

And as already said that still doesn’t mean it’s going to be a short circuit and trip the breaker.

 

[Simon47]

However, this contradicts the video which states that when a Live wire touches a metal frame, the current can still flow back to the RCD, causing it to trip. I get the video is making an example, but is this a plausible real-world scenario?

The scenario would be:

• Frame is not earthed
• Live touches metal frame
• Current flows through metal frame
• Current flows from metal frame to neutral
• RCD doesn't trip as there is a complete circuit and no loss of current

Can this situation exist in the real world (@westward10 / @Simon47)?

OK, "live touches frame" is a fault - it can happen and that's why we design in features to deal with it.

"Current flows from metal frame to neutral" is a different issue.

Here in the UK, that would need a separate fault as the frame(case) would be earthed. So we've be looking at two faults for that to occur - and the device would have stopped working with either one of them. Otherwise it would need a fairly improbably fault - e.g. power cord pulled such that all 3 cores get disconnected, L&N both tough the frame together, but E does not touch the frame.

However, I believe in some countries that's not the case - I suspect your's may be one of them. If you only have 2 wires, then the only way to "earth" the frame will be to use the neutral and rely on neutral being tied to earth "somewhere". So in that situation, yes a current could flow from L to N via the frame - and because that would be balanced it would not trip an RCD.

Such systems are inherently dangerous though. If the neutral connection fails outside of the equipment (and it could be outside of your house), then the frame will be connected to live via the internal load. That means a single (and not uncommon) fault can make the case live - there will be no current as the neutral is broken, so the RCD will not trip. If you touch the case and provide a path to earth then an RCD should trip and protect you - so adding it will provide protection from some faults, but not others.

 

[GeeGee50]

If Neutral is connected to metal frame, RCD will not trip but this is a short circuit condition and then the breaker will trip.

Cool, tx.

A live touching a metal frame that is connected to neutral or earth doesn’t mean it’s going to be a short circuit and cause the ocpd to trip, it’s going to depend on the impedance of the circuit.

It could be for instance that the disconnected neutral side of a lighting circuit contacts the metal frame.

Ok, noted.

I assumed neutral was solidly connected to metal frame, in this hypothetical example.

You assumed right.

If frame is not earthed, and live touches metal frame, current will be present on metal frame and RCD will not trip.

I'm assuming you mean where the metal frame is not connected to Neutral?

If so, I still don't understand why the RCD would not trip.

Here is my logic:

• Metal frame is not earthed AND frame is not connected to Neutral
• Appliance is on
• Live touches metal frame
• Live-neutral circuit is broken and therefore a complete circuit does not exist
• Current is unable to make its way back to RCD
• RCD notices imbalance and trips <------------ is this correct?

GeeGee

 

[GeeGee50]

OK, "live touches frame" is a fault - it can happen and that's why we design in features to deal with it.

"Current flows from metal frame to neutral" is a different issue.

Here in the UK, that would need a separate fault as the frame(case) would be earthed. So we've be looking at two faults for that to occur - and the device would have stopped working with either one of them. Otherwise it would need a fairly improbably fault - e.g. power cord pulled such that all 3 cores get disconnected, L&N both tough the frame together, but E does not touch the frame.

Yes, agreed in terms of earthing. I do understand and appreciate why earthing is critical for safety.

My scenario is purposefully leaving out any earthing of the frame as I want to understand how an RCD will behave (even if such a setup is highly unlikely and not advised).

 

[brianmoooore]

If a complete circuit does not exist then the RCD will not trip.
If you use your body to create a complete circuit, by simultaneously being in contact with the conductive ground and the frame of the faulty appliance, then the RCD will trip if the resulting current is high enough. This will be extremely painful, but in most cases won't be fatal.
If the current flowing is insufficient to trip the RCD, the effects will range from a barely perceptible tingle to a continuous pain that will encourage you to let go as soon as possible.

 

[RetiredNY]

If so, I still don't understand why the RCD would not trip.

Here is my logic:

• Metal frame is not earthed AND frame is not connected to Neutral
• Appliance is on
• Live touches metal frame
• Live-neutral circuit is broken and therefore a complete circuit does not exist
• Current is unable to make its way back to RCD
• RCD notices imbalance and trips <------------ is this correct?

GeeGee

OK, there is no current in your example. In order to have current flow, a circuit must exist. Voltage would be present on the metal, un-earhted, un-neutral referenced frame, but there is no current flow. RCD sees no imbalance because current needs to flow through both the neutral and the live conductors it is monitoring. Should, however a person who is earthed reach out and touch the live metal frame, then current will flow through the person to earth, and not return through the neutral, so the RCD sees current flow, but no equal current return, that creates an imbalance or a magnetic field in the RCD coil, which generates a micro-voltage to trip the RCD off.

 

[GeeGee50]

If a complete circuit does not exist then the RCD will not trip.
If you use your body to create a complete circuit, by simultaneously being in contact with the conductive ground and the frame of the faulty appliance, then the RCD will trip if the resulting current is high enough. This will be extremely painful, but in most cases won't be fatal.
If the current flowing is insufficient to trip the RCD, the effects will range from a barely perceptible tingle to a continuous pain that will encourage you to let go as soon as possible.

tx mate...

I think I'm missing the finer technicalities of how an RCD operates.

In my mind (and in my scenario), there IS a complete circuit initially BEFORE the live wire touches the frame.....so at the instance the live touches the frame, then the current that went out via the RCD will fail to come back into the RCD....doesn't that meet the definition of an imbalance?

 

[RetiredNY]

tx mate...

I think I'm missing the finer technicalities of how an RCD operates.

In my mind (and in my scenario), there IS a complete circuit initially BEFORE the live wire touches the frame.....so at the instance the live touches the frame, then the current that went out via the RCD will fail to come back into the RCD....doesn't that meet the definition of an imbalance?

Your scenario leaves out any earthing of the frame. So it's assumed the frame has no connection to earth or neutral. The live touching the frame doesn't complete any circuit, therefore there is no current flow for the RCD to measure. Therefore, all's right with the world from the perspective of the RCD.

Now along comes poor Joe, who is barefoot on a moist kitchen floor, and touches the live frame. Now some current flows through the RCD's coil, but it doesn't return through the circuit's intended path, the neutral, to cancel the current the live is experiencing. This current will be detected by the RCD if it exceeds it's trip point.

 

[GeeGee50]

Your scenario leaves out any earthing of the frame. So it's assumed the frame has no connection to earth or neutral. The live touching the frame doesn't complete any circuit, therefore there is no current flow for the RCD to measure. Therefore, all's right with the world from the perspective of the RCD.

Now along comes poor Joe, who is barefoot on a moist kitchen floor, and touches the live frame. Now some current flows through the RCD's coil, but it doesn't return through the circuit's intended path, the neutral, to cancel the current the live is experiencing. This current will be detected by the RCD if it exceeds it's trip point.

Yes, correct assumption. That is, my scenario has always been:

• appliance frame not connected to earth
• frame not connected to negative

OK, so everyone keeps saying an RCD needs a complete circuit to work (and I'm not disputing that).

However, It still makes no sense to me why an RCD can't detect when a perfectly working circuit is suddenly lost (with regards to my scenario), especially if it's lost whilst current is running through the circuit.

I've drawn a diagram to better illustrate my point:

 

[mainline]

Yes, correct assumption. That is, my scenario has always been:

• appliance frame not connected to earth
• frame not connected to negative

OK, so everyone keeps saying an RCD needs a complete circuit to work (and I'm not disputing that).

However, It still makes no sense to me why an RCD can't detect when a perfectly working circuit is suddenly lost (with regards to my scenario), especially if it's lost whilst current is running through the circuit.

I've drawn a diagram to better illustrate my point:



View attachment 112956

Yes, correct assumption. That is, my scenario has always been:

• appliance frame not connected to earth
• frame not connected to negative

OK, so everyone keeps saying an RCD needs a complete circuit to work (and I'm not disputing that).

However, It still makes no sense to me why an RCD can't detect when a perfectly working circuit is suddenly lost (with regards to my scenario), especially if it's lost whilst current is running through the circuit.

I've drawn a diagram to better illustrate my point:



View attachment 112956

In the 1 st scenario the return current in both the L and N are the same (balanced) so the rcd won’t trip.

In the 2nd scenario there is no current flow at all because the frame doesn’t connect to E or N so the rcd won’t trip.

 

[GeeGee50]

In the 1 st scenario the return current in both the L and N are the same (balanced) so the rcd won’t trip.

In the 2nd scenario there is no current flow at all because the frame doesn’t connect to E or N so the rcd won’t trip.

So what happened to the 10A active in the circuit before the circuit was broken?

 

[James]

It stopped flowing the moment you broke the circuit.
in the same way that the current (Amps) flowing through your lounge light stops flowing the moment you turn off the light switch.

 

[GeeGee50]

It stopped flowing the moment you broke the circuit.
in the same way that the current (Amps) flowing through your lounge light stops flowing the moment you turn off the light switch.

hmmm

so it stopped flowing, ok.....but what about the current that was already present in the circuit after the circuit was broken??......it just disappears into thin air?