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

[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.

ok, let me put it another way ---> why is an RCD unable to detect when a circuit is broken (with regards to my scenario)?

People say because the circuit is incomplete and or current has stopped flowing.......but so what......I still don't get why RCD's cannot recognise such an event?

I guess my point/question is not such much around 'why' it doesn't trip (I accept what you guys are saying in general) but moreso 'how' come it doesn't trip..........how can an electrical device such as an RCD, with our incredible modern technological advances and science and knowledge, not be able to detect when current stops flowing....seems hard to fathom that they can't be designed/programmed to handle such situations.

PS, I know I'm probably getting into some deep discussion here because of my ignorance, so apologies in advance.

 

[ElectroChem]

.....how can an electrical device such as an RCD, with our incredible modern technological advances and science and knowledge, not be able to detect when current stops flowing...

The simplest reason is - that's not an RCD's job. The only job an RCD has is to compare the current in and current out through it's measurement coil. If IN=OUT then all is well. If not then trip.

There are absolutely devices who's job is to detect if current stops flowing. I've used them on heating elements to detect burnouts. But that application is specific. A device measuring current couldn't tell the difference between a broken wire and a switch being turned off, because all it sees is current or no current.

Regarding your earlier question about what 'happens to' the current already in the circuit when you break a wire / open a switch. Nothing 'happens' to it, it was never there. Current isn't something that fills up a wire like water in a pipe then has to either go somewhere or sit in place. The electrons are already in the wire, a flowing current just shuffles them around a bit and as soon as external voltage is removed they just stop. (Macroscopically, don't @ me physicists.)

 

[mainline]

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

That’s returning down the N via the appliance so the circuit is balanced.

Remember that shorting the L to N should cause the fuse /mcb /rcbo to trip due to the high current and not the rcd as the circuit current would still be balanced.

 

[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.

got it....tx

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.....

As in current starts to flow again because Joe touching the frame has created a circuit of sorts (with the earth)?

 

[GeeGee50]

The simplest reason is - that's not an RCD's job. The only job an RCD has is to compare the current in and current out through it's measurement coil. If IN=OUT then all is well. If not then trip.

There are absolutely devices who's job is to detect if current stops flowing. I've used them on heating elements to detect burnouts. But that application is specific. A device measuring current couldn't tell the difference between a broken wire and a switch being turned off, because all it sees is current or no current.

Regarding your earlier question about what 'happens to' the current already in the circuit when you break a wire / open a switch. Nothing 'happens' to it, it was never there. Current isn't something that fills up a wire like water in a pipe then has to either go somewhere or sit in place. The electrons are already in the wire, a flowing current just shuffles them around a bit and as soon as external voltage is removed they just stop. (Macroscopically, don't @ me physicists.)

tx mate.....this was really helpful. Helps me better understand what's going on (and better comprehend previous replies from other posters).