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

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Discuss Will an RCD offer any protection in a building with NO earth circuit? in the UK Electrical Forum area at ElectriciansForums.net

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AmatuerinCambodia

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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! 😱
 
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Simon47 said:
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.
Click to expand...

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).
 
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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.
 
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GeeGee50 said:
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
Click to expand...

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.
 
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brianmoooore said:
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.
Click to expand...
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?
 
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GeeGee50 said:
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?
Click to expand...
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.
 
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RetiredNY said:
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.
Click to expand...

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:



[ElectriciansForums.net] Will an RCD offer any protection in a building with NO earth circuit?
 
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GeeGee50 said:
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
Click to expand...

GeeGee50 said:
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
Click to expand...
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.
 
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mainline said:
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.
Click to expand...
So what happened to the 10A active in the circuit before the circuit was broken?
 
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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.
 
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James said:
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.
Click to expand...
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?
 
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James said:
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.
Click to expand...

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.
 
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GeeGee50 said:
.....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...
Click to expand...

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.)
 
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GeeGee50 said:
So what happened to the 10A active in the circuit before the circuit was broken?
Click to expand...
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.
 
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RetiredNY said:
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.
Click to expand...

got it....tx


RetiredNY said:
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.....
Click to expand...

As in current starts to flow again because Joe touching the frame has created a circuit of sorts (with the earth)?
 
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ElectroChem said:
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.)
Click to expand...

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