Can any one explain this?

Joined
30 Jan 2014
Messages
342
Reaction score
2
Location
Essex
Country
United Kingdom
I removed all my sockets in the living room, with the wires stuffed back in to the wall, and even though the power to them had been isolated, the RCD kept tripping from using a socket elsewhere in the house.

In the end I separated all the wires so none were touching and this solved the problem.

Can someone explain why this is in the simplest possible terms, or provide a link for me to read to try and understand it?

:oops: Thanks
 
Sponsored Links
Sponsored Links
Because the neutral is at earth potential, the same as you are. With no difference in potential, no current will flow, so you don't get a shock.

The RCD trips because some electricity is completing the circuit via the earth wire rather than the neutral wire. As there is more electricity leaving on the live wire than is returning on the neutral, the same as would happen when you get a shock (the electricity completes the circuit through you rather than the neutral wire).
 
To isolate you need to switch off All live conductors that includes line and neutral with a domestic consumer unit this means using the Isolator or RCD the MCB or RCBO will not isolate.

The RCD is operated by current not voltage so with the neutral to earth fault there is likely not enough voltage to feel but there is enough current to trip a RCD. With a TN earth system specially a TN-C-S the neutral and earth are bonded very close to the house so very little current will flow
the circuit shows what is happening when the RCD trips open the switch and although fault is still there no current flows.

Oddly we use the same consumer unit for TN and TT supplies however reading the regulations it would seem with a TT supply really we should have twin pole MCB's but this all comes down to history with a fuse we could not link two fuses and we found fusing the neutral was dangerous so we did not fuse the neutral and when we moved to the MCB we followed the same idea and did not use twin pole MCB's however looking at the regulations it may be that to follow to letter with a TT supply we should use twin pole.

In fact when one reads the regulations it does become interesting as it raises the question of what is a circuit which was academic until the Part P regulations said you need to notify the LABC when a new circuit is created.

Our regulations make interesting reading with a TN system each MCB forms a new circuit but with a TT system because the neutral is not switched the MCB does not form a new circuit. However the regulations to the common man are like gobbledegook I have listed some of them below. Can you understand them?

Regulations 314.1 (vi) prevent the indirect energizing of a circuit intended to be isolated, and 521.8.2 The line and neutral conductors of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated. Table 53.2 Note (5) In an installation forming part of a TT or IT system, isolation requires disconnection of all the live conductors. See Regulation 537.2.2.1.

However 537.1.2 Combined protective and neutral (PEN) conductors shall not be isolated or switched except as permitted by Regulation 543.3.4.
Except as required by Regulation 537.1.4, in a TN-S or TN-C-S system the neutral conductor need not be isolated or switched where it can be regarded as being reliably connected to Earth by a suitably low impedance. For supplies which are provided in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, the supply neutral conductor (PEN or N) is considered to be connected to Earth by a suitably low impedance.

And 537.2.1.1 Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliable connected to Earth by a suitably low impedance.
Provision may be made for isolation of a group of circuits by a common means. if the service conditions allow this.

537.4.1.2 Except as provided in Regulation 537.1.2. where a risk of electric shock is involved the emergency switching device shall be an isolating device and shall interrupt all live conductors.
Except as required by Regulation 537.1.4, where the neutral conductor can be regarded as being reliably connected to Earth in a TN-S or TN-C-S system the neutral conductor need not be isolated or switched.

551.6.2 For a TN-S system where the neutral is not isolated. any RCD shall be positioned to avoid incorrect operation due to the existence of any parallel neutral-earth path.
NOTE: It may be desirable in a TN system to disconnect the neural of the installation from the neutral or PEN of the system for distribution of electricity to the public to avoid disturbances such as induced voltage surges caused by lightning
 
... when we moved to the MCB we followed the same idea and did not use twin pole MCB's however looking at the regulations it may be that to follow to letter with a TT supply we should use twin pole.
I suspect not. As you have quoted, 537.2.1.1 says:
Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliable connected to Earth by a suitably low impedance. Provision may be made for isolation of a group of circuits by a common means. if the service conditions allow this.
Note that, at the start, although it says that it should be possible to isolate (in TT) both live conductors of every circuit, it does not say that it has to be possible to isolate each circuit independently (i.e. isolate just the one circuit) - and the final sentence, which I have emboldened, seems to underline the fact that double-pole (in TT) isolation of multiple circuits is acceptable. Hence the (2-pole) RCD(s), or even Main switch, in (or around!) the CU probably satisfies the requirement of 537.2.1.1 for there to be a provision of 2-pole isolation of every circuit in a TT installation (given that every circuit in a TT installation requires RCD protection, and all single-phase RCDs are 2-pole).

Kind Regards, John
 
As it's Friday, let's take this up a gear :)

Why is there a potential difference between the neutral conductor and the CPC?
 
As it's Friday, let's take this up a gear :) Why is there a potential difference between the neutral conductor and the CPC?
Depends partially on the earthing system - if its TN-S or TT, there might well be a neutral-CPC potential difference even at the origin of the installation. However, even with TN-C-S (where CPCs are neutral are joined at the cutout), voltage drop in neutral conductors (assuming there are some loads connected) will lead to a neutral-CPC potential difference.

If, in a TN-C-S system, there are NO loads connected, hence no neutral currents or VD, then there will be no neutral-CPC pd, so a N-CPC fault will NOT cause an RCD to trip.

Kind Regards, John
 
The current from the load flowing along the neutral creates a voltage drop along the neutral. So there is a potential difference between the neutral at the socket and the neutral at the cutout where the CPC is connected to the neutral.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top