NCDs can cause gas explosions when the current is diverted through gas pipes

Industry must acknowledge dangerous network fault, warn experts

Image credit: Firefighters tackling a gas explosion. Photograph: Gerard Koudenburg |

The network operators and the government must publicly recognise the dangers of neutral current diversion, so that potentially lifesaving work can be carried out on the electricity network, E&T has learned.

Electrical experts, including a former employee of one the UK’s major power networks, have told E&T that there is a real risk of deadly gas explosions and fires in the UK due to an increasingly common fault on the electricity network. 

They argue that the Distribution Network Operators (DNOs) and the Health and Safety Executive (HSE) must acknowledge the risks of neutral current diversion (NCD) so that a programme of testing can be implemented - and lifesaving upgrades made to the grid.

An NCD can occur on the network when the combined protective earthing and neutral (PEN) conductor fails. The current is then diverted by making a circuit via exposed metalwork on buildings including gas, water and oil pipes. This can lead to a significant build-up of heat, leading to fires and gas explosions.

PEN conductors are particularly susceptible to damage, corrosion and general wear and tear across an ageing PME network – and given the huge projected load growth on the network, incidents are expected to continue to rise.

Last year, E&T revealed that the reports of broken PEN conductors had increased by more than eight times over the last 18 years in the UK from 57 in 2003 to 474 reports in 2021.

Broken PEN graph

Image credit: The number of reported broken PEN incidents has increased by more than eight times since 2003 *not including Northern Ireland

One broken PEN incident may affect in the region of 50 properties, meaning tens of thousands of properties could be affected each year in the UK.

The trade body for the DNOs, the Energy Networks Association (ENA), told E&T that “there have been a comparatively small number of incidents associated with PEN conductors”.

But former SP Energy Networks (SPEN) employee Gordon Mackenzie says this is because testing is not being carried out routinely to measure NCD. 

Last year, the Institution of Engineering and Technology (IET) published a standardised way of testing for NCD in its Guidance Note 3 publication, but the “DNOs need to acknowledge the problem,” says Mackenzie. During the 38 years Mackenzie worked at SPEN, he tried to draw attention to the dangers of NCD. Since retiring in 2021, he has continued to do so.

The fault is not confined to Scotland.

Paul Meenan, asset manager (M&E) at train operator C2C, which serves parts of London and Essex, is also trying to raise awareness about the dangers of NCD.

Meenan says NCDs “are a problem on the railways at the moment” and part of his job is to find solutions. C2C issued a safety alert in 2020 to its supply chain, as well as to Network Rail and the DNO UK Power Networks, after excessive unknown transient energy levels were measured flowing into the Low Voltage earthing system. The alert, obtained by E&T, says the readings indicated NCD was present, and puts extra safety measures in place.

Meenan, is also chairman for the subcommittee of the Electrical Safety Roundtable (ESR) covering electrical safety in the workplace. The ESR is due to publish a code of practice, “to educate people on different scenarios where [NCD] could happen, so electricians are better informed as how to work with it, around it and how identify it.

“It is something we are going to have to live with to a certain extent as the networks get older but we don’t want to carry out work in peoples’ homes or businesses where we exacerbate the conditions. This is where we need the DNOs to come to the table and acknowledge the problem.”

Mackenzie, the former SPEN employee, first became aware of a potential public safety issue in 2014 when he was alerted to an incident in a property in Galashiels, Scotland. A resident’s coat had fallen onto the property’s gas meter and had caught fire. Readings taken at the scene detected 35 amps of current flowing through the metallic gas service pipe entering the property. Mackenzie said it was fortunate the resident was at home at the time.

“There were 72 houses fed through a cable that runs between two substations without a neutral meaning there was the entire neutral current of 72 houses going through that one gas pipe,” recalls the former senior engineer.

Worryingly, there was nothing to indicate the problem he says, with “not one single flickering light bulb in the 72 houses. If that had happened during the night, that cupboard would have caught fire, the gas would have exploded and that would have been blamed on a gas leak,” he says.

At the time, SPEN produced a safety alert which said it was “working together with the ENA to agree an appropriate process for highlighting and responding to incidents of this type and the agreed process will be communicated in due course”.

Mackenzie notes that NCDs are so prevalent on gas pipes that the gas industry attaches jump leads between gas pipes before changing them, because they generate sparks. “Make no mistake about it, NCDs can and almost certainly are causing gas explosions across the country,” he says.

The cause of the majority of gas explosions are recorded as gas leaks, but Mackenzie says this is just because the ignition point is so badly destroyed by the fire, that “you can’t prove it was not a gas leak”.

“Gas meters are not designed to carry electricity. If there is a high resistance with a high current, it generates a lot of heat. Every time I see a gas explosion I would at least like the HSE at least having some decent understanding at what they are looking at, so that when they are doing their investigations, they can rule or rule out NCD.

The HSE failed to respond to questions from the E&T. However, documents seem by E&T show that the HSE has been alerted to the of dangers NCD on more than one occasion. In one piece of correspondence seen by the E&T, Steve Critchlow, the principal gas engineer at the HSE said checks for NCDs would not be carried out routinely after gas explosions but only if he had reason to suspect an electrical current had contributed to the explosion.

Neutral diverted currents can cause gas explosions, fires and electric shocks

An ammeter reading showing a neutral diverted current. Photograph: Paul Meenan

Image credit: An ammeter reading showing a neutral diverted current. Photograph: Paul Meenan

Mackenzie says the HSE does not want to acknowledge the extent of the problem as it wants to avoid “a nationwide panic” but a number of councils have published safety alerts for electricians.

In 2019, West Lothian Council published a safety alert in which it stated that over the past few years it had received “a number of reports from gas suppliers of live pipework when they had been replacing gas meters”. The alert mistakenly requests that suppliers use voltage pens to check for NCDs. Mackenzie says clip-on ammeters must be used for testing as “NCD testing is about testing for current first, not voltage”.

A West Lothian Council spokesperson said: “NCD is a rare but potentially dangerous issue that can affect any property. “We have continued to keep awareness of NCD high amongst our staff and sub-contractors since then and have had two further incidents in the last four years, both of which were identified, recorded and reported to Scottish Power for investigation.”

“Regular discussions on the issue have taken place between Scottish Power and representatives from councils across Scotland via the Local Authority Best Practice Forum. 

The council said updated draft guidance for council staff and sub-contractors have been prepared, which includes adding regular checks for NCDs as part of annual gas service and five-yearly electrical inspection programmes.

It said it would circulate this to all staff concerned, as soon as final advice and information has been received from Scottish Power via the Local Authority best practice forum, an organisation established to promote a shared approach to excellence in electrical standards.

Scottish Power Energy Networks failed to respond despite repeated requests from E&T. The network operator released a training video on NCDs in April 2021 but it has been since taken down.

Another incident occurred in Tillicoultry, Clackmannanshire during the Christmas of 2021, when a power surge cut electricity to more than 200 homes, blowing fuses and damaging electrical appliances. The Local Authority Forum for Electrical Best Practice says SPEN had asked LAF members for assistance in testing for and reporting any deterioration of aluminium cable heads in properties that might lead to NCD.

Speaking to Cable Talk Magazine, Sandy Mackintosh, electrical project manager at Clackmannanshire Council, said: “We had an emergency meeting with our service leaders and organised toolbox talks with all our plumbing and gas engineers to explain the issues, and how the use of continuity clips would help them work safely. We even arranged for them to have training at Scottish Power’s training centre.”

When the reports came through of the power surge in Tillicoultry I knew what had happened and what to do,” he added

Out of 5,000 Clackmannanshire Council homes his team identified around 2,000 that had aluminium TNCS cables and developed a programme of upgrades to their electrical boards with surge protection.

Bob Cairney, director of technical services at SELECT, the trade association for the electrical contracting industry in Scotland, which runs the LAF, says: “Following on from the release of the SPEN YouTube video, several Scottish local authorities picked upon this and started to check for NCDs…on their properties and this led to a number of possible problems being highlighted. These were reported back to SPEN however at that stage they did not have a procedure in place to deal with the reports coming in – also in many instances their own operatives lacked a knowledge of the problem to be able to deal with the problems.” 

Cairney says SPEN has organised at least two industry workshops to explain this process to local authority technical staff. The issue is also now being highlighted to electricians through industry update training courses.

“I would highlight that initially there were a significant number of NCD incidents - in the hundreds - being reported, however with general industry lack of awareness of the issue this number has dropped off.”

Meenan says the DNOs do not want to acknowledge the phenomena as this would involve added costs. "The contracts DNO have mean they must make repairs when there is a failure and invest in monitoring but they do take a large profit from their funding allocations - and they are a private, for-profit business".

“I also think they haven’t got the bandwidth to manage the network. It is easier to be reactive than preventative,” he adds.

A spokesperson for ENA said: “DNOS placed the safety of their network and customers as their top priority as they strive to meet the highest safety standards.

“As with any piece of infrastructure exposed to the weather or hazards like failing trees, a small number of malfunctions occur – any incident is resolved as quickly as possible and managed within their overall approach to risk management. In the next investment cycle, DNOs will invest £235m specifically in safety measures to continue to protect their teams and the general public,” they added.

Sign up to the E&T News e-mail to get great stories like this delivered to your inbox every day.

Recent articles