RCBOs vs RCDs Pt2

[Deleted member 26818]

Take one bit of 2 core flex, connect both cores at one end to a neutral, then connect the other ends of the two cores together.
What current will flow?
Where will it flow to.
That is essentially what you have with the N & E linked on the distribution side, and your N-E fault in the installation.

 

[ImpededLoop]

The neutral current will divert through the fault to earth in accordance with the laws of resistances in parallel.

I would agree with this.

However, how many circuits are there in existence that are not RCD protected but have had neutral to earth faults for years with no apparent effects?

A single pole RCBO would hopefully provide protection to somebody receiving an electric shock, so I think their use is beneficial, but not as good as a device that disconnects all live conductors.

 

[davesparks]

Take one bit of 2 core flex, connect both cores at one end to a neutral, then connect the other ends of the two cores together.
What current will flow?
Where will it flow to.
That is essentially what you have with the N & E linked on the distribution side, and your N-E fault in the installation.

No it isn't, the whole of the installation is connected into it too.

In your example you would have to connect the neutral side of a load to one of the cores halfway along its length to simulate this. The neutral current from the load will flow to neutral directly down the shorter path and through the longer path going to the far end of the flex and back to neutral.

The ratio will be determined by resistance of each path

 

[davesparks]

I would agree with this.

However, how many circuits are there in existence that are not RCD protected but have had neutral to earth faults for years with no apparent effects?

A single pole RCBO would hopefully provide protection to somebody receiving an electric shock, so I think their use is beneficial, but not as good as a device that disconnects all live conductors.

And how many have had disastrous effects? How many times have fires started from N-E faults?

You have only seen the ones which haven't caused a problem because the ones which have caused a problem have been fixed or burned out already.

Obviously you rarely see the ill effects of them in domestics because there is naff all neutral current to start with !

 

[ImpededLoop]

And how many have had disastrous effects? How many times have fires started from N-E faults?

I doubt anybody really knows.

Obviously you rarely see the ill effects of them in domestics because there is naff all neutral current to start with !

I've never seen it in commercial either, but perhaps I've been lucky.

 

[davesparks]

I doubt anybody really knows.



I've never seen it in commercial either, but perhaps I've been lucky.

Nobody will know, but that's no excuse for not understanding the simple theory and choosing the best protection based on that.

 

[Deleted member 26818]

No it isn't, the whole of the installation is connected into it too.

In your example you would have to connect the neutral side of a load to one of the cores halfway along its length to simulate this. The neutral current from the load will flow to neutral directly down the shorter path and through the longer path going to the far end of the flex and back to neutral.

The ratio will be determined by resistance of each path

What does this diverted neutral current do when it meets the diverted neutral current coming from the N & E link up the CPC to the N-E fault?

 

[davesparks]

What does this diverted neutral current do when it meets the diverted neutral current coming from the N & E link up the CPC to the N-E fault?

Wtf?

Current doesn't flow from the cutout up the cpc to the fault!
It flows via the fault then through the cpc back to the cutout

 

[Deleted member 26818]

Why doesn't it?
If there was no fault it would, so why doesn't now that there is a fault?

 

[davesparks]

Why doesn't it?
If there was no fault it would, so why doesn't now that there is a fault?

Current flowing through the cpc from the cutout in normal operation ???
Where did you get that one from?

The only thing flowing in a cpc under normal conditions will be leakage current from smps's and similar.

 

[Engineer54]

You’ve changed your tune.

The trouble is the SP RCBO just don’t match what it says on the tin. For this reason I think many have accepted them as an instant cure all in every situation. This can only be attributed to lack of knowledge on the part of the electrician. (The ones that bite my head off now will be the ones in the know.)

Some manufacturers do point out the short comings of the SP RCBO in handling a N→E fault. To my mind it should be made clear to everyone. As we’ve often discussed it seems to be the doctrine of colleges to preach fitting RCD’s to everything.

How have i changed my tune??

 

[Deleted member 26818]

Current flowing through the cpc from the cutout in normal operation ???
Where did you get that one from?

The only thing flowing in a cpc under normal conditions will be leakage current from smps's and similar.

I give up.

 

[Geoffsd]

It's similar to this-

 

[davesparks]

I give up.

Have a look at the picture Geoff posted, it's not that clear but does show the diverted current

 

[Deleted member 26818]

Have a look at the picture Geoff posted, it's not that clear but does show the diverted current

I know how it works, just having difficulty making you understand.