Neutral Current Diversion (NCD)
Neutral current diversion (NCD) describes ‘stray’ neutral currents flowing in continuous and intact metallic structures, services and pipes in an installation that would otherwise return via the system neutral conductor. In particular neutral current diversion (NCD) may be an issue in multi-occupancy type buildings where the metallic services pass between properties. This return path would be part of a multiple return system and could be complicated by potentially having multiple parallel routes. NCD may consist of a part, or all, of the neutral return current in an installation. It may flow via one or more routes at the installation and can occur for various reasons that will described in this technical article.
Typical Arrangements of Earthing and Bonding.
In an electrical installation all the simultaneously accessible exposed conductive parts and all the extraneous conductive parts should be connected to the main earth terminal (MET). Where PME conditions apply, the connection between the LV distribution system combined neutral/earth terminal and the customers installation may allow for stray neutral currents to flow between properties that share services or their structures extend beyond each properties equipotential zone.
At one time, separate neutral earth (SNE) networks were universal which meant that after the introduction of PME networks in the early 1970s, many legacy SNE networks and services have been modified to PME or have been made PME-enabled. For older installations there is a likelihood that they may have metallic services, such as water or gas, that can provide a more supportive environment for neutral current diversion. As a result of this, the cross sectional area of bonding conductors in older installations may be smaller and therefore may not be rated to withstand the levels of NCD expected in a PME network. Therefore, there can be a greater risk of neutral current diversion in these older installations which should be considered when assessing these properties.
Typical UK Configurations of Earthing of LV Systems
TN-S Earthing System
The TN-S system does not combine the neutral and earth function at any point on the network other than at the source of voltage. In a TN-S design, construction and operation, neutral current diversion (NCD) is unlikely to occur due to the separation of the neutral and earth functions.
The neutral function of the TN-S system is to carry load return current back to the voltage source, which can be described as an electrically active role. The earthing conductor should not normally carry load current as its role is to carry fault current until the protective device operates and will remain electrically passive until a fault occurs.
TN-C-S Earthing System
The TN-C-S system combines the neutral and earth functions into a single conductor, known as a PEN conductor, up to the point at which the customer connects to the network. This combines the load current return requirements of a neutral conductor, with the fault current requirements of an earth conductor.
TN-C-S PME Earthing System
PME is an example of a TN-C-S system. While the earth conductor of a TN-S system is electrically passive by design, in a TN-C-S system the PEN conductor has an electrically active function carrying return load currents.
In a TN-C-S (PME) system, the neutral and earth functions are combined on the LV distribution network in a PEN, but the neutral and earth functions remain separate within the customers installation in order to comply with BS 7671. The customers protective bonding conductors connect extraneous conductive parts such as metal service pipes (water and/or gas) to the main earthing terminal (MET) and thereon to the PEN conductor.
Neutral Current Diversion (NCD) as a result of a broken PEN conductor
Neutral current diversion (NCD) is most likely to occur where the neutral and earth functions are combined, as found in a combined neutral/earth (CNE) network, e.g. TN-C-S installations. Some relatively low NCD can be considered a ‘normal’ condition as it has been seen on networks for several decades and is only a concern where there is a broken or high impedance neutral or PEN conductor on PME networks. One broken PEN incident may affect in the region of 50 properties and tens of thousands of UK properties could therefore be affected each year. If there is a suspected broken PEN, immediately contact and report this to the relevant DNO on the 105 emergency number.
Testing for the exporting or importing of NCD
An important factor when considering the behaviour of neutral current diversion (NCD) is whether the installation is importing neutral current from the network through extraneous metallic paths or exporting neutral current to the network through extraneous metallic paths. The point of reference when considering if the installation is exporting or importing NCD is the flow of NCD on extraneous metal work. Where a property is exporting neutral current into extraneous metallic paths, this will be limited by the capacity of the installation, i.e. the service fuse is a limiting factor and the connected load at the property, therefore there is some control of how much current can flow.
However, where a property is importing NCD from the network via extraneous metallic paths, this could be from a single exporting property or from multiple exporting properties. As the load consumption is likely to vary to a greater extent from several sources, a higher NCD may be expected at the importing properties.
To test for NCD and its potential source, measurements at the L and E points are necessary to gain the required information. These measurements are made to confirm that the current on the L and N is balanced, i.e. are the same value, which is what would be expected in a typical customers installation.
If these measurements do not balance, this may indicate a problem within the customers network. For example, if there is leakage to earth from NCD due to a high impedance neutral in the supply to a gas boiler with substandard protective equipotential bonding. Tests should be carried out using a clampmeter such as the Sonel CMP-100 or Sonel CMP-200.
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Thank you to SELECT for helping to provide the information used to write this technical article.