N-E joined at the incomer of your house

[Kev2632]

I was just wondering with the N-E being joined at the incomer, if i was to go downstream of a circuit which was "NOT" Rcd protected and tested between Neutral and Earth that would appear as a dead short between N-E is that correct? but if i was to go downstream of a RCD protected it wont be there is at that correct? as it cant get passed the RCD is that right?

Thanks.

 

[Rockingit]

If I'm following your thinking correctly, yes, it just relies on the MCB for overload.

 

[Kev2632]

If I'm following your thinking correctly, yes, it just relies on the MCB for overload.

Yea thats correct, as i was saying there on the other page, when there is no RCD protection the Neutral to Earth is present through out the installation to give everything a low enough fault path for the current to flow and trip the MCB very quick should a L-N or L-E Happen is that correct there?

And on a RCD protected circuit, the N-E " joined " is not present throught out the installation, so the MCB will just work as normal detecting L-N, L-E but it cant rely on the N-E joined for its path is that correct now yea? as its downstream of the RCD

 

[Kev2632]

If I'm following your thinking correctly, yes, it just relies on the MCB for overload.

Have i got it right there Rockingit?

Cheers i think i have?

 

[Engineer54]

Also, the RCD Will if there is a N-E fault, down stream of that RCD see that fault and trip!!

 

[WPD Lad]

At no point in a domestic installation should the neutral and earth be joined beyond the DNO's cutout

(apart from exported PME)

 

[Geoffsd]

Yea thats correct, as i was saying there on the other page, when there is no RCD protection the Neutral to Earth is present through out the installation to give everything a low enough fault path for the current to flow and trip the MCB very quick should a L-N or L-E Happen is that correct there?

When you say "Neutral to Earth" - I get the impression you think they are connected somewhere
They are not.

And on a RCD protected circuit, the N-E " joined " is not present throught out the installation, so the MCB will just work as normal detecting L-N, L-E but it cant rely on the N-E joined for its path is that correct now yea? as its downstream of the RCD

There is no difference in the wiring (other than the CU) of an installation whether an RCD is fitted or not.

If you draw a single circuit and follow the current path it will seem clearer.

 

[Spudnik]

Are you playing devils advocate?

Not at all.

Makes one think for a minute.

 

[Spudnik]

I'M sorry but this thread reads like a child asking an adult WHY? WHY? WHY?

And your explanation for the OP is??

 

[Darkwood]

For the O/P....

In TNCS and TNS as mentioned the Neutral/Earth is combined on the DNO's side either at the cutout or centre tap of transformer respectively, it is seperate within the consumers installation and except for exceptional circumstances must remain seperated throughout the installation.

If you install is from the 15th edition its possible to have no rcd's thus measuring resistance between the N/E at any point on the installation should show a very low resistance. The main switch is double pole and switching this off will stop you seeing this N/E low resistance as you can only see the consumers side and not the DNO's side.

Any board fitted under the 16th or 17th is likely to have rcd's fitted, if the rcd is duel modular (takes up 2 spaces) then it is double pole and operating it will hide the N/E link from the DNO, if it is single modular (like an rcbo) then it is single pole and operating it wont hide the DNO's neutral earth link.

As mentioned before modern rcd's/rcbo's only look at the imbalance of the load passing through the live and returning on the neutral if the difference is beyond the set value then it will trip, it only monitors the load side (and heres where your confused) its thus blind to the supply side. So here's the thing - the N/E will show up as shorted if you test on the load side (if rcd DP then only when switched on) but it wont trip the rcd as the DNO link is prior to the rcd so if circuit is fine no current will leak across on the load side keeping the rcd balanced.

Remember ... the rcd isn't looking for a link between N/E it is looking for a difference in the load going down the L and returning on the N - so if the short N/E is not on the load side then current cant leak to it in a position where the rcd can see it.

 

[Darkwood]

View attachment Rcd load side earth fault.pdf
View attachment Rcd supply side earth fault.pdf

This might make it easier for you to understand, although the leakage on the supply side is theorectical as it wouldn't happen like that as its commoned up on DNO side, and i know there isn't an overload device its just to explain in pictorial form an rcd with various N/E faults.

Just click each pdf attachment and click open when prompted at bottom of screen, i found this best way for clear A4 size drawing. Ps i drafted these up myself so do hope they help but if not im sure they will be handy another day

 

[Kev2632]

View attachment 12090
View attachment 12091

This might make it easier for you to understand, although the leakage on the supply side is theorectical as it wouldn't happen like that as its commoned up on DNO side, and i know there isn't an overload device its just to explain in pictorial form an rcd with various N/E faults.

Just click each pdf attachment and click open when prompted at bottom of screen, i found this best way for clear A4 size drawing. Ps i drafted these up myself so do hope they help but if not im sure they will be handy another day

Thanks very much Darkwood!! I find your Help very helpful thanks very much!

 

[GLENNSPARK]

So that was the way to protect circuits in the days before an RCD came into play is that correct ?

fuses & MCBs protect cable/s and installations...RCDs/RCBOs protect persons and/or livestock....in the case of a TT system...an RCD will often be the only form of fault protection you have.....