‘Neutral to Earth’ faults
Bill Allan from NAPIT looks into ‘Neutral to Earth’ faults.
According to the National Grid, up to 20% of homes in the UK, and probably more, have accidental neutral to Earth connections. This statistic was arrived at following measurements taken on a sample of homes in the UK with measurements taken ‘as found’. But when scientists actively looked for such connections they were found in 70%. This is an alarming claim. Could it be true?
Neutral to Earth faults, especially how they can affect RCDs, are among the most baffling and the most time-consuming to find. This article will discuss how neutral to Earth faults occur, what the symptoms are and how to locate them.
Neutral to Earth faults
Neutral to Earth faults can happen where:
- the neutral conductor at an accessory has been 'pinched' by a fixing screw so that it comes into contact with an earthed metal box
- a concealed cable has been penetrated by a nail, screw or the like causing the neutral conductor and the circuit protective conductor (cpc) to be bridged
- the neutral conductor and the cpc have been connected to the wrong terminals (ie. the neutral conductor has been connected to the earthing terminal and vice versa)
Do neutral to Earth faults trip overcurrent devices?
Neutral to Earth faults on final circuits will not cause single-pole overcurrent devices to trip. If a fault is causing such an overcurrent device to trip, it is not a neutral to Earth fault.
RCDs and neutral to Earth faults
RCDs operate on a principle of imbalance. The search coil in the RCD monitors the outgoing line and neutral conductors to ensure that the same current that flows out to the load through the line conductor flows back again through the neutral conductor.
Imbalance occurs when some or all of the returning current flows to Earth rather than through the neutral conductor (see above). This imbalance can be caused by line to Earth faults and it can also be caused by neutral to Earth faults.
If the resulting imbalance is such that it causes the rated residual operating current (I∆n) of the RCD to be exceeded, this will be detected by the trip coil, causing the RCD to trip.
However, neutral to Earth faults do not always cause RCDs to trip. This depends on two main factors - the potential difference between the neutral conductor and Earth and the resistance of the earth path.
Potential difference between N and E
If the RCD is to trip due to a neutral to Earth fault, the potential difference between the neutral conductor and Earth must be sufficient to cause at least some of the returning neutral current to return to the origin via the cpc rather than via the neutral conductor.
In TN-C-S systems, the neutral conductor and Earth are likely to be at substantially the same potential due to the N-E link in the cut-out. In such a situation, no current will flow and the RCD will not trip.
However, when the load of the circuit with the N-E fault is energised, the current flow causes a voltage drop in the neutral conductor and this voltage drop results in a potential difference between it and Earth. The greater the load, the greater the current flow and hence the greater will be the voltage drop. If the voltage drop is sufficient, some of the returning neutral current will flow through the cpc. Should enough current flow through the cpc, the RCD's search coil will 'see' an imbalance and cause the RCD to operate. Therefore, in a TN-C-S system with no load or with a very light load, a N-E fault may remain undetected.
The resistance of the earth path
The resistance of the earth path is also an important factor in determining whether an RCD will trip. In TT systems, the resistance of the earth path is usually significant and the current flow may be insufficient to trip the RCD. Hence, in TT systems, N-E fault can remain undetected.
However, at higher loads, the resultant potential difference between neutral and Earth may be sufficient to cause the current to flow in the cpc. Hence, the nature of the fault is load-dependent. This may cause the RCD to trip randomly at times and the fault may then be described as unwanted tripping.
Let us consider a TT system with an earth rod resistance of 100 ohms. Ohm's Law tells us that to get 30mA to flow to Earth will require a voltage difference between the neutral conductor and Earth of 3 volts (0.03 x 100). Such a voltage difference will usually occur only at higher loads.
A similar situation with a significant neutral to Earth resistance can also exist in TN-S systems.
Therefore, in TT and in TN-S systems, N-E faults can remain undetected.
The insulation resistance test
N-E faults can go unnoticed unless an insulation resistance test is carried out, whether the circuit with the N-E fault is protected by an RCD or not.
Regulation 612.3.1 requires that the insulation resistance be measured, 'between live conductors and the protective conductor connected to the earthing arrangement'. This means that the insulation resistance test must be carried out between the neutral conductor and Earth as well as between the line conductor and Earth. Any 'short' should be picked up at this stage.
Are N-E faults dangerous?
If sufficient current flows along the cpc due to a neutral to Earth fault condition in a final circuit which is protected by an RCD, the RCD will trip. If a final circuit with an N-E fault is not protected by an RCD, the cpc may become overheated, with a potential risk of fire. But even where the final circuit is RCD-protected, the possibility remains that, where the level of current flowing in the cpc is insufficient to trip the RCD, the cpc may become overheated. A circuit with a N-E fault clearly does not comply with BS 7671 and must be rectified.
If an N-E fault is discovered during a periodic inspection and testing of an installation, a Code 2 is recommended.
This article could not hope to cover every issue that could arise with neutral to Earth faults. No doubt there will be other questions and some of the material in this article may need to be clarified or qualified to suit certain situations.