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

[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?