Tn-C-S Pme Earthing System

[Doc Hudson]

Ok, so why on the installation side when earth and neutral connect there is an earth fault?

A traditional fuse or circuit breaker would not detect a neutral and earth touching. Though an RCD will detect an imbalance if current can flow to earth.

I am currently doing an apprenticeship, just was trying to understand how you can have 230v down a neutral that is directly connected to earth and have no fault

Have you done anything about electricity generation yet, such as the three phases coming from a supply and how the star point is physically connected to earth?

Doc H.

 

[apprentice87]

AC is all on the L and returns down the neutral same as DC!!!! Yeah right!!!!! Course it is!!!!!

If AC is all "lopsided" then why does a full wave rectifier need 4 diodes....

john...

 

[SPECIAL LOCATION]

Hi everyone I need some help understanding how a TN-C-S earthing system works, I am struggling to understand how the earth can be connected to neutral and no neutral to earth fault occur?

Wouldn't the earth become live if connected to neutral?

Please help

Thanks

Have you got a test meter that you can read voltages on?

Try putting a meter between:

a)  Live and Neutral

b)  Live and Earth

c)  Neutral and Earth

see what voltage you actually have with respect the each of the two points...

If you had 230v on neutral as you imply... 

there would be a dead short and a fuse would blow somewhere!

Unless there is a fault, neutral and earth are, (or should be), both at 0v with respect to the physical earth we stand on.  

so as they are both at the same potential they can be joined at various parts along the supply network.

 

[LtMcClane]

No I haven't done a great deal on three phase yet, thanks for all your help guys things are starting to make more sense now, was just wondering if any current flows down the neutral? The way I see it is that it would be impossible for current to flow down the neutral as there us 0v on the neutral but correct me if I'm wrong

Hi doc H, would an RCD be able to detect a neutral to earth fault? Because if there is 0v on the neutral no current would flow to earth so therefore the RCD wouldn't trip?

 

[SPECIAL LOCATION]

Neutral is part of the circuit...

without it no current flows..

whatever current is flowing up the live flows back along the neutral...

Consider a simple light circuit...

Supply -> Permanent Live -> Switch -> Switched Live -> Load (light bulb) -> Neutral -> Supply

The current flows around the circuit...

try making a battery and lamp work with only 1 wire..

a return (neutral) wire is needed to complete the circuit to carry the current back to the supply..

An RCD measures the current up the live & neutral which should be equal..

if it detects an imbalance then it trips off.

In your case where the neutral is also touching earth.

There is now a parallel return path so some current flows to earth and less down the neutral..

thus thew RCD trips!

 

[apprentice87]

Deary deary me......

Hi LtMcClane,

I will try to explain for you.., one concept you NEED to understand, is that when you are measuring voltage, what are you ACTUALLY measuring?? Answer; The "potential difference" [difference in potential] between the two points where you are taking your readings

Now, BOTH "live" [it is properly called "line" or "phase"] and the neutral, are BOTH, yes BOTH, "live conductors" Make no mistake about this at all.

The line and the neutral "take turns" with their polarity. At one moment "line" is positive, and "neutral" is negative, and then, a short time later, they swap around, and "line" is now negative, and "neutral" is now positive. Each "swap" and back, is termed one cycle, and there are 50 cycles a second as you know. [so the polarity changes 100 times a second]

So, at one moment in time, the power comes up the "line" and returns down the "neutral" and then, at the next moment, it changes around, and the power comes up the "NEUTRAL" and returns down the "line" .It alternates to and fro, this is why it is called alternating power.

Now, why then do you not get a shock from the neutral then???

Well, the answer to this, is that at the transformer that powers your house, [and at the power station that powers the transformer] the neutral is connected to earth, literally to the earth, by means of a spike or similar banged into the ground. This is ALSO what the mains cable earth conductor is connected to, or, in the case of TNCS, the combined neutral/earth conductor.

So, if you connect your meter between earth and neutral, just because the reading is 0V [or close to this] does NOT mean the neutral is not "live" IT IS, 50 times a second, but the reason your meter stays on 0V [and you do not get a shock if you stick your finger in the neutral] is because the neutral is connected to earth at the "other" end of the cable, the power station end if you like, and so, THERE IS NO "POTENTIAL DIFFERENCE" BETWEEN THE TWO.

To put things in simple terms, [terminology is wrong, but i am trying to help you understand here] imagine if you connected a large square metal plate to the "Live" in your house and then jumped onto it. Then imagine you grabbed hold of the "live" wire connected to the plate with your hand. Would you receive a shock?? NO, because there would be no "potential difference" between the plate you are standing on, and the live wire you are holding, there cannot be, they are connected together.

So it is with the earth conductor and the neutral....

john

 

[LtMcClane]

Thank you for your answer john I finally! Understand