Continuity for Neutral and cpc in TNCS due to PEN conductor?

[King84]

Hi,

I am struggling to understand one concept if anyone could shed some light.

as TNCS has combined neutral and Cpc so if on the consumer side hypothetically if you check continuity on N+Cpc without disconnecting any conductor, do we get continuity of Neutral and CPC as on the supply side, its PEN so they are combined as I am thinking during fault finding, this could be a problem showing short circuit even though its PEN.
thanks everyone in advance

 

[pc1966]

Other than TT where the earth rod is typically in the tens or hunderd-or-so ohms, both TN-S and TN-C-S will always have a low N-E impedance as they are common at some point, either at the substation transformer (true TN-S) or at the supplier (i.e. DNO) cut-out (TN-C-S case).

If you want to verify N-E insulation you have to disconnect the N (but not the E/CPC) so they are (or should be) isolated. In the UK at least for domestic single-phase supplies the incoming switch has to be double-pole, so simply switching off the supply allows you to test everything downstream of the isolator.

Verification of the short section from cut-out to CU should not be necessary, as it should be possible by inspection without disconnecting anything. Where that is not true (i.e. long/hidden cables) there really ought to be a switch-fuse feeding it so it can be isolated and controlled by an unskilled person (i.e. not getting the DNO to pull the fuse, etc).

You will see various threads on here about the "3m rule" for supply tails, etc. that allow more reading than is sensible at this time of the evening.

 

[Lucien Nunes]

As PC1966 says, there will always be continuity of some sort between N & E in a normal installation unless it is isolated from the supply.

• TN-C-S: N & E are connected at the incoming supply terminals, effectively zero ohms.
• TN-S: N & E are connected at the DNO's substation. Resistance if measured at the CU would be typically <1Ω but it's not a reliable measurement to make with an ordinary meter unless the entire substation is isolated.
• TT: N & E are connected at the substation, but the installation's connection to E is via the rod rather than a conductor in the supply cable, so the resistance of the rod dominates, perhaps 20-200Ω.

In a single-phase installation, the N-E link somewhere upstream is really what makes the neutral a neutral. I.e. of the two live conductors, neutral is the one that is connected to earth, and line is the one that is not.

 

[pc1966]

In a single-phase installation, the N-E link somewhere upstream is really what makes the neutral a neutral. I.e. of the two live conductors, neutral is the one that is connected to earth, and line is the one that is not.

That is a good point to make!

Sometimes we fixate on the small details and miss the larger points, so having it come back to "that defines neutral" is a useful perspective.

 

[R-fur]

DNOs introduced a system in the early seventies called MEN multiple earthed neutral. As well as connecting the neutral to earth at the fuse board in the substation the neutral is connected to earth at most joints and pot ends. In addition neutrals of adjoining networks are permanently connected together.

 

[King84]

Thanks everyone for reply and sharing your knowledge with someone who is quite new like me.

As you all stated Neutral is linked at somepoint with earth so there will always be continuity but if we switch off the mainswitch as it is doublepole switch and it would cut neutral aswell so do we still get continuity?
As I was baffled once there was this premises, I switched off the mainswitch in the consumer unit and it was locked, but when I went to one ceiling rose and check continuity between earth and netural without removing the conductors, it gave me continuity and I couldnt figure out as if this was a shortcircuit, it wouldnt have worked, but when I turned the electricity back on it was working fine yet there was continuity between netural and earth

 

[Lucien Nunes]

it wouldnt have worked, but when I turned the electricity back on it was working fine yet there was continuity between netural and earth

Why would it not have worked? The two conductors are at nominally the same potential. The only reason an N-E fault sometimes reveals itself during normal operation is by causing enough earth-leakage to trip an RCD, which in turn depends on the voltage drop in the neutral and the earth loop impedance.

I switched off the mainswitch in the consumer unit and it was locked,

Was it a 3-phase DB? If so, the neutral is often unswitched.

 

[King84]

Why would it not have worked? The two conductors are at nominally the same potential. The only reason an N-E fault sometimes reveals itself during normal operation is by causing enough earth-leakage to trip an RCD, which in turn depends on the voltage drop in the neutral and the earth loop impedance.



Was it a 3-phase DB? If so, the neutral is often unswitched.

No it was single phase DB and my understanding was there was earth fault as there was continuity between neutral and earth as even after switching off main switch there was continuity

 

[pc1966]

No it was single phase DB and my understanding was there was earth fault as there was continuity between neutral and earth as even after switching off main switch there was continuity

Almost certainly an earth fault and it should be fixed as the next fault could well result in the "earth return" going open and any attached metalwork becoming live.

As above, that would normally result in the RCD tripping and it was not , most likely there is no additional protection against shock so it is not to be ignored. Often N-E faults are not obvious as it can take a high current to divert enough current to cause the RCD to trip, so you end up with the situation of a fault on, say, lights only becoming apparent when someone switches on the shower, etc, and then a common RCD trips making it look like the shower circuit has the fault. A good reason for all-RCBO boards if at all affordable!

But more generally, often the first check you might make on an installation is to switch off the incoming supply, verify it is now safely isolated, temporarily link L-N (ideally after a MCB just in case...) and do a global IR test to the main earth terminal. That should be high, over 0.5M at the very least (but that low is not a good sign), if all is well. If not you know there is at least one fault, if not more.

Doing a L+N to E test is safe (so long as no one is touching it!) as any electronics must be insulated to well over 500V that way. However, if there is any surge protection it will show a low value (typically tens of kOhm). Testing at 250V instead will reveal if that is a likely cause as suddenly it will be in the tens of M ohm range once more, and you can take a look for any surge-protection devices plugged in (e.g. surge protected mutli-way block for PC or TV, etc) if it is not a case of SPD fitted to the CU where you can see them.