How earthing actually works in a fault

[ulfr]

Hi gents,

It's straight forward that in an event of a fault earthing will deliver current from exposed metal parts(electrical accessories) to the Earth, and it grounds to Earth potential.

So the question is, if for example, in lighting circuit a fault occurs, how it will go from a ceiling rose(which is plastic) to the earth if the earth wire is not connected directly to the lamp? Does it mean that the ceiling rose will become an exposed part and then earth will be in the game?

 

[Bobster]

Hi gents,

It's straight forward that in an event of a fault earthing will deliver current from exposed metal parts(electrical accessories) to the Earth, and it grounds to Earth potential.

So the question is, if for example, in lighting circuit a fault occurs, how it will go from a ceiling rose(which is plastic) to the earth if the earth wire is not connected directly to the lamp? Does it mean that the ceiling rose will become an exposed part and then earth will be in the game?

View attachment 29048

The most important part of your question I've highlighted.

Have a little think.

 

[telectrix]

you can't have an earth fault on a 2 wire lamp. only a L-N fault will trip the OCPD.

 

[ulfr]

The most important part of your question I've highlighted.

Have a little think.

Plastic is a good insulator. I can't really see how the ceiling rose can become exposed.. only if it still conducts some electrical charge.

you can't have an earth fault on a 2 wire lamp. only a L-N fault will trip the OCPD.

Why then earthing is necessary for the circuit ? Because of potential difference?

 

[telectrix]

earthing is required up to the end of the circuit cable to protect against earth faults in the cable itsef. it's only connected to fittings if they have conductive parts. otherwise the cpc is just terminated in a blank connector or connected to a terminal provided for it. e.g. as you find in a plastic patress.

 

[ulfr]

earthing is required up to the end of the circuit cable to protect against earth faults in the cable itsef.

Could you please elaborate this? Can't really grasp the meaning..

otherwise the cpc is just terminated in a blank connector or connected to a terminal provided for it. e.g. as you find in a plastic patress.

So the cpc is not necessary there?

 

[telectrix]

to put it over simply, if someone bangs a nail into the cable, shorting out the L and cpc. the OCPD will interrupt the supply

 

[happysteve]

Could you please elaborate this? Can't really grasp the meaning..

If the cable deteriorates and causes a short or (more likely) accidental/careless misuse by others (screwing/nailing things in the wall).

So the cpc is not necessary there?

Regulation 411.3.1.1 tells you that "A CPC shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point."

So the "such a point" is the ceiling rose, and the "lampholder having no exposed-conductive-parts" is the bit of plastic you attach your lamp to, suspended by the white bit of two core flex.

The reason for this regulation is that, even if all your accessories have no exposed-conductive-parts, they could at any time in the future be replaced for metal ones. Wiring systems are installed to last a long time.

Hope that's helpful

 

[ulfr]

to put it over simply, if someone bangs a nail into the cable, shorting out the L and cpc. the OCPD will interrupt the supply

But the OCPD will interrupt the supply even without the cpc, isn't it? And if someone will bang the nail just in the cpc, nothing will happen to the circuit and the OCPD will not be fired, right?

If the cable deteriorates and causes a short or (more likely) accidental/careless misuse by others (screwing/nailing things in the wall).



Regulation 411.3.1.1 tells you that "A CPC shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point."

So the "such a point" is the ceiling rose, and the "lampholder having no exposed-conductive-parts" is the bit of plastic you attach your lamp to, suspended by the white bit of two core flex.

The reason for this regulation is that, even if all your accessories have no exposed-conductive-parts, they could at any time in the future be replaced for metal ones. Wiring systems are installed to last a long time.

Hope that's helpful

Thanks that was really helpful! Much appreciated.


But I can't really see why the cpc is protecting "against earth faults in the cable itsef". Even if the cpc will be faulty in the circuit, it shouldn't matter because it's not used at all?

 

[telectrix]

But I can't really see why the cpc is protecting "against earth faults in the cable itsef". Even if the cpc will be faulty in the circuit, it shouldn't matter because it's not used at all?



​you will get the fault if the L is shorted to the cpc. this is because the cpc is connected to the MET @ 0V and so a short will give rise to a fault current of sufficient magnitude to trip the OCPD. ( we hope).

 

[ulfr]

But I can't really see why the cpc is protecting "against earth faults in the cable itsef". Even if the cpc will be faulty in the circuit, it shouldn't matter because it's not used at all?



​you will get the fault if the L is shorted to the cpc. this is because the cpc is connected to the MET @ 0V and so a short will give rise to a fault current of sufficient magnitude to trip the OCPD. ( we hope).

I understand that if L is shorted to the cpc then more current will be dragged because of the additional cpc length, therefore =more current used =there's a fault = OCPD in use. Am I right ?

The question is, if only the cpc will be nailed in that circuit, it won't compromise the circuit. Therefore the cpc doesn't have any practical usage over there, isn't it? It more has "decorative" function, just because it's stated in Regulations that : "A CPC shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point." only because of that we need the cpc there.

Thanks!

 

[telectrix]

i advise some serious reading, as you don't seem to have a grasp of the fundementals.

 

[ulfr]

i advise some serious reading, as you don't seem to have a grasp of the fundementals.

Could you suggest specific topics?
--

Sorry I said a bull**** about the length, it's all about resistivity..

 

[telectrix]

several you tube videos by chris kitcher are worth watching. also google earthing and bonding. see what you find

 

[Simonslimline]

https://www.google.co.uk/webhp?sour...&ie=UTF-8#q=principles of electrical earthing

 

[ulfr]

Hi everyone,

Finally I got it.

1) In this circuit it can operate without the cpc, because there are no metal exposed parts nor in the ceiling rose, nor in the switch and a plastic is good insulator. The only fault that can occur is L-N, which can lead to an over current.

2) The cpc is needed because of regulations, and for a future use. If for example someone will install a metal spotlights or something like that, then it should be earthed.

3) Earth fault can occur if the cpc and the line are shortened. For example if one of the cables are melted, or a nail was banged into the wall shortening them both, or something like that.

 

[telectrix]

Hi everyone,

Finally I got it.

1) In this circuit it can operate without the cpc, because there are no metal exposed parts nor in the ceiling rose, nor in the switch and a plastic is good insulator. The only fault that can occur is L-N, which can lead to an over current.

2) The cpc is needed because of regulations, and for a future use. If for example someone will install a metal spotlights or something like that, then it should be earthed.

3) Earth fault can occur if the cpc and the line are shortened. For example if one of the cables are melted, or a nail was banged into the wall shortening them both, or something like that.

you're getting there. :yes:. don't forget that an earth fault can also occur if the N and cpc are shorted or have a finite low en ough resistance between them. a 30mA RCD willl trip at a leakage resistance of 8000 ohms ( R=V/I, or R=240/0.03 = 8000. Where 0.03 is thr 30mA tripping threshold)).

 

[ulfr]

you're getting there. :yes:. don't forget that an earth fault can also occur if the N and cpc are shorted or have a finite low en ough resistance between them. a 30mA RCD willl trip at a leakage resistance of 8000 ohms ( R=V/I, or R=240/0.03 = 8000. Where 0.03 is thr 30mA tripping threshold)).

Thanks.

Sure.. because current passes through N as well, it makes a continuity with L.

Thanks telectrix, much appreciated.

 

[telectrix]

The RCD monitors the L and N currents. if no leakage from either to E, then it holds. a difference between the L and N currents in excess of the tripping threshold, and it will let go. so a RCD will still work if there's no cpc. as long as there is another path to earth. e.g. through your body.

 

[ulfr]

very helpful. will do some serious reading on that..