Neutral to earth faults

[Eddiesparks]

So since the advent of rcds, these have become more highlighted.

This has been bugging me lately. In my mind the reason why neutral to earth faults are a problem is because the cpc has a smaller csa than the neutral

Also if you lost the neutral at the supply then mains voltage would be present (minus load resistance's volt drop) at all exposed conductive parts. What other reasons are there? Thanks for help in advance.

 

[Hawkmoon]

So since the advent of rcds, these have become more highlighted.

This has been bugging me lately. In my mind the reason why neutral to earth faults are a problem is because the cpc has a smaller csa than the neutral

Also if you lost the neutral at the supply then mains voltage would be present (minus load resistance's volt drop) at all exposed conductive parts. What other reasons are there? Thanks for help in advance.

Not quite sure which element of this you are not understanding but - in the event of a neutral to earth fault in a lighting circuit:

1) The returning current (neutral) of all other circuits in your installation now have an alternate path to return to the supply (through the earthing system). At the neutral bar most current will return through the main neutral but some will return through your N-E fault path. The level of current through your fault will be determined by its resistance compared to that of the ‘normal’ route - this current can be high enough to overload those 1.5/1.0mm cables. Most often the resistance of your fault path is high enough to prevent excessive current so the fault can go undetected for years (assuming not protected by RCD)


2) If, in addition to the N-E fault, you lose your supply neutral then your fault will be the only route of return and the 1.5/1.0mm cables will attempt to carry all the neutral current of your installation - obviously this will seriously overload the cables and could result in fire. Lost supply neutrals are not common but can happen particularly with overhead supplies - also a lost supply neutral can occur in your own consumer unit by loose connection (come across this a number of times).


3) As Darkwood mentioned you can also have floating voltages present - eg if TN-C-S supply then a lost neutral will mean also a lost earth so returning current will have nowhere to go, resulting in potential difference between exposed conductive parts and GM of earth - though this will likely be reduced by main bonding connections.


Note: I have only just got up and haven’t had my porridge yet so all of the above could be absolute sh1te - if anyone wants to correct/add anything, feel free!

 

[Eddiesparks]

Yeah i think it's number one i dont get. If there is a N/E short why would current 'choose' to go down the cpc which has a smaller csa. And if it dis, as soon as cpc warmed up it would increase in resistance so current would flow back down the neutral. This is what i meant by saying it is like a rfc parallel path.

 

[Eddiesparks]

Ps i understand if you lost supply neutral then all current would go down fault path but number one i dont get so much. Thanks people

 

[Hawkmoon]

Yeah i think it's number one i dont get. If there is a N/E short why would current 'choose' to go down the cpc which has a smaller csa. And if it dis, as soon as cpc warmed up it would increase in resistance so current would flow back down the neutral. This is what i meant by saying it is like a rfc parallel path.

Think of Ohms law - how current divides among resistors in parallel - the main neutral and the fault path are just 2 resistors in parallel, one high value, one low...

Draw it out on paper and you may see what I mean

 

[Knobhead]

The stray earth current is directly proportional the resistance of N & E from the point of the fault to ether the transformer N→E link for TN-S & TT or the nearest CNE connection for TN-C-S & PME.

For Christ’s sake don’t ask about IT systems.
(Inductive Terra not Information Technology)

 

[dazza1981]

The stray earth current is directly proportional the resistance of N & E from the point of the fault to ether the transformer N→E link for TN-S & TT or the nearest CNE connection for TN-C-S & PME.

For Christ’s sake don’t ask about IT systems.

So Tony talk to me....... IT systems lol

 

[Knobhead]

I've worked on 164MVA of them.

 

[Eddiesparks]

That is very useful thank you.

One more question. When the fault path's conductors heat up their impedance would increase right so surely then the rest of the install's current would flow down the main neutral? Rendering the danger less dangerous?! Or am I still not getting it?!

Perhaps i will go and draw a pic as suggested...

 

[Knobhead]

Do the drawing and post it.

Without the drawing you're on your own.

 

[Eddiesparks]

I did a drawing Tony

 

[Hawkmoon]

I did a drawing Tony

That's not Tony, he's always a happy chappy

I take it the arrow at the top is the main incomer and the legs and arms are outgoing parallel circuits??

 

[Darkwood]

Can't believe you drawn him with a double chin....or is he a matchstick hoodie!

Surely a contender for his new Avatar

 

[Knobhead]

My avatar took a bit of skill to draw.

 

[Eddiesparks]

that's not tony, he's always a happy chappy

i take it the arrow at the top is the main incomer and the legs and arms are outgoing parallel circuits??

looool!!