Delivering clarity on a current issue with new suite of resources

In line with ongoing concerns about neutral current diversion, we’ve been on the case to create some handy guides to help you identify the issue and take appropriate action

There is no doubt that neutral current diversion (NCD) continues to be a hot topic across the electrical industry and that there is a certain degree of uncertainty and confusion around the issue.

However, following the recent publication of Engineering Report 141 by the Energy Network Association (ENA) and discussions with Members and the wider electrical industry, SELECT’s Technical team have put their heads together to try and provide some clarity on the subject.

The result is the creation of three new practical resources, which offer easy-to-follow advice and are free for all Members:

A1 posters, for use in workshops and training centres
A4 flowchart, for free download, printout and digital distribution
Pocket-sized foldout, designed to be kept in toolboxes.

The material has been produced following close collaboration with experts from a range of industry bodies, including the Institution of Engineering and Technology (IET), ScottishPower Energy Networks (SPEN) and the Scottish Electrical Charitable Training Trust (SECTT).

Easy to follow with a clear step-by-step process, all the new resources are designed to keep contractors and their operatives safe.

SELECT is also planning to make NCD a key part of next year’s Toolbox Talks, with further explanation and advice for Members who want to find out more.

So what is NCD?

NCD is a term used to describe ‘stray’ neutral currents that take an alternative or diverted route back to the earth connection of the supply transformer, e.g. via continuous metallic water or gas pipes entering a building.

Normally, load current flowing in a circuit will return via the circuit neutral conductor to the supply transformer via the neutral conductor of the network supply cable.

However, NCD can occur under load conditions due to a parallel path that may exist in the network or if a break in the neutral conductor of the supply cable occurs.

Where NCD occurs, the current that’s flowing may consist of all or part of the neutral current that would normally return in the network supply cable.

Where is it found?

NCD may be a particular issue inmulti-occupancy type buildings wheremetallic services pipes are common to all properties.

The return path that NCD could take in such a property could potentially have multiple parallel routes.

NCD is most likely to occur in an earthing system where the neutral and earth functions are combined, such as where a TN-C-S earthing system has been provided by the DNO.

It can also exist where the neutral and earth functions are considered to be separate, as in a TN-S earthing system.

NCD can also occur where a main protective bonding conductor connection is made between the main earthing terminal and extraneous-conductive-parts that may exist within the consumers installation.

TN-C-S earthing systems, also known PME, were first introduced in the early 1970s and are known as Combined Neutral and Earth (CNE) systems by DNO network operators.

Prior to this, Separate Neutral and Earth (SNE) DNO networks were universal. However, over time, many legacy SNE networks have been modified to PME or made PME-enabled.

What about older properties?

Older properties are more likely to have metallic water services and therefore provide a more supportive environment for NCD. Protective equipotential bonding in these properties may be specified to an older version of BS 7671, therefore may not have been upgraded as BS 7671 has evolved.

Consequently, the smaller cross-sectional area (CSA) of protective bonding conductors in older properties may not be suitably sized for PME conditions, and therefore not rated to withstand the levels of NCD that can occur in a PME network.

Some relatively low NCD is a ‘normal’ condition that has been on networks for several decades.

It is only really of concern when a broken neutral or high impedance neutral fault occurs on the PEN conductor on PME networks.

A free PDF of the flowchart can be downloaded by going to bit.ly/ncd-poster

For technical queries about NCD, call the SELECT Technical Helpline on 0131 445 9218.