Neutral to Earth

[ForkEnigma]

how many ohms can you have neutral to earth without opening a 30mA RCBO.

My thinking is If=V/R so, If=230/0.03 = 7.6Kohms

So 7.7kohms will keep it closed

 

[James]

the above formula would be correct for a live to earth fault.

N to E fault depends on the loads that are being drawn at the time (creating an imbalance in the voltage between N and E and therefore a current flow between them)

 

[ForkEnigma]

the above formula would be correct for a live to earth fault.

N to E fault depends on the loads that are being drawn at the time (creating an imbalance in the voltage between N and E and therefore a current flow between them)

the above formula would be correct for a live to earth fault.

N to E fault depends on the loads that are being drawn at the time (creating an imbalance in the voltage between N and E and therefore a current flow between them)

A dead short of lets say 0.2 ohms N-E without current will not open RCBO, with a load it will open.

Now if the fault was >7666 ohms, with current will the RCBO open.

 

[James]

Think about how an rcd operates.

It is a simple comparison between live current and neutral current.

An imbalance between those measurements of >30mA will cause it to trip.

It is not possible to give a resistance between N and E that will cause it to trip or not trip.

A fairly low resistance fault could be tolerated if there is zero load on the entire installation
As loads increase, not only on the circuit protected by rcd but also others, a combination of parallel paths and differing volt drops in parts of the installation can cause it to trip.

 

[ForkEnigma]

Think about how an rcd operates.

It is a simple comparison between live current and neutral current.

An imbalance between those measurements of >30mA will cause it to trip.

It is not possible to give a resistance between N and E that will cause it to trip or not trip.

A fairly low resistance fault could be tolerated if there is zero load on the entire installation
As loads increase, not only on the circuit protected by rcd but also others, a combination of parallel paths and differing volt drops in parts of the installation can cause it to trip.

I'm not talking about imbalance, which we know is how an RCD operates. So forget that for now.

I'm talking about a Neutral to Earth resistance that is greater than 7666 ohms. On load will it open the RCD ?
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Anything less than 7666 ohms on load will open the RCD.

 

[James]

I'm not talking about imbalance, which we know is how an RCD operates. So forget that for now.

I'm talking about a Neutral to Earth resistance that is greater than 7666 ohms. On load will it open the RCD ?

depends on the load, that is what creates the imbalance

 

[ForkEnigma]

depends on the load, that is what creates the imbalance

A load of anything above 30mA will open a RCD when the fault is below 7666 ohms, but regardless of load above 30mA if the resistance is greater than 7666 ohms then RCD stays closed

 

[Mark Wright]

an impedance of 7666 Ohms would be the absolute maximum impedance like equivalent max Zs for a RCD if you ever had an impedance of 7.6 kiloOhms there is a serious problem.

 

[James]

Like I said in my first post, 7.6k is the maximum live to earth fault value that can be tolerated

 

[ForkEnigma]

an impedance of 7666 Ohms would be the absolute maximum impedance like equivalent max Zs for a RCD if you ever had an impedance of 7.6 kiloOhms there is a serious problem.

Indeed sir, it shows degradation of the insulation.
Max Zs is 1667 ohms on a 30mA which is touch voltage of 50V.

In doing EICRs its impossible to disconnect everything, especially with lighting loads.
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Like I said in my first post, 7.6k is the maximum live to earth fault value that can be tolerated

James the OP states N-E

 

[James]

so to trip the rcd Il needs to be different from Ina by greater than 30ma
Ina= In-If
If is dependant on both the load Rn, and the parralel resistors Rfault and Rn (nutrual conductor resistance.)
the 7.6k value you are mentioning would cope with a load of zero ohms or a dead short.

 

[Mark Wright]

I think if you get values above 1kohms you need to check your earthing conductor, supply or earth rod etc.

 

[ForkEnigma]

Like I said in my first post, 7.6k is the maximum live to earth fault value that can be tolerated

Surely you mean minimum

 

[Mark Wright]

any impedance under 7.6k will allow 0.03Amps you would need a maximum insanely high impedance of above 7.6k for there to be a problem so 7.6k is the maximum impedance that can be allowed etc.

 

[ForkEnigma]

I think if you get values above 1kohms you need to check your earthing conductor, supply or earth rod etc.

......WHAT, we are talking resistance between N & E! Should be >.5 meg ohm right? To comply with BS7671, Which is much higher than what we are talking about.

 

[Mark Wright]

I think you mean insulation resistance not resistance there different.

 

[ForkEnigma]

I think you mean insulation resistance not resistance there different.

Ok, i feel really stupid now.

 

[Mark Wright]

Why what did you do take care

 

[James]

now here is where it gets a little complicated.

depending on the other loads Rext and the resistance of the bond between E and N, the voltage of the N line can vary.
if it goes up, some of the current from Rext will pass back through the rcd and down the parallel path through Rn and Rfault SUBTRACTING from the current passing through the neutral line on the rcd.
this will INCREASE difference between Il and Ina and help the rcd to trip.

 

[ForkEnigma]

View attachment 59486
now here is where it gets a little complicated. give me 5 to edit

Oh i like it!
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Not sure of the R bond though
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loving the diagram, much respect