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Preserving past and present

SELECT Member Oliver McCrone reveals the challenges of protecting a historic building from the risk of fire

Gladstone’s Land, one of the oldest buildings in Edinburgh’s Royal Mile, has undergone a £1.5 million restoration and is now equipped with the newest electrical fire prevention technology: arc fault detection devices (AFDDs). The 500-year-old six-storey townhouse reopened in the summer with a new coffee shop and ice cream parlour – in keeping with its long historic commercial use – and, in addition to a revamped three-storey museum, it now also boasts four luxury holiday flats for rent on the top floors.

The building is owned by the National Trust for Scotland and the main contractor Laurence McIntosh, a bespoke architectural joinery firm, subcontracted J. K. McCrone Electrical Services for the electrical overhaul, which included the installation of AFDDs in fuse boards throughout the building.

Started by James McCrone in 1974, J.K. McCrone is based in Edinburgh and has built up a reputation for electrical work on high end and historic properties. It is now run by his sons Oliver and Paul with support from eight electricians and two apprentices.

The requirement for AFDDs was a ‘first’ for Oliver, who oversaw the electrical contracting work for the Gladstone’s Land project. He said: “We had to rewire and install distribution boards for the downstairs commercial units, which included the new café and ice cream parlour, plus three floors of the museum section of the building and the four new apartments at the top, and all distribution boards stipulated the use of AFDDs.

“I’ve heard about AFDDs before but until now had never been instructed to fit them on a job. They were required by the client, the National Trust for Scotland, and, in this case, you can understand why because a potential fire caused by a spark from a faulty appliance or wall socket could do untold damage to this historic building. However, it was also important to protect the safety of people working and visiting the building, particularly guests staying overnight in the new apartments.

“Guests often bring their own appliances like phone chargers, laptops or tablet devices as well as things like curling tongs and hair dryers, which can cause sparks if they are faulty. The whole point of the AFDD is to detect fluctuations in the electrical current which indicate an arc fault and cut off the electricity to that circuit.”

Unlike circuit breakers or RCDs, the AFDD does not have an electromechanical trigger but uses a micro-processor to analyse current flow and voltage curves to identify any changes that indicate an arc fault, and then it instantaneously trips the circuit. Manufacturers claim that AFDDs can reliably differentiate between an arc fault – both series and parallel –and the waveform signature in normal switching and control events, thus preventing false tripping.

AFDDs have been used in the USA for the past 20 years to reduce the risk of fire due to faulty conductors and connections, and have been used in Germany since 2016. In the UK, the 18th edition of BS 7671 recommends the use of AFDDs on 230 volts AC final circuits that include ring circuits, radials, spurs etc, to provide additional protection against fire.

While they provide extra fire protection against arc faults, the main practical issue with working with AFDDs is their size and the consequences this has on the size of the consumer unit, as Oliver explained: “When you have a one-bedroom flat with a kitchen then you are probably dealing with around six circuits, such as a couple of rings for power, lighting, a boiler, fridge etc, In this case, the normal RCD consumer unit might be 400mm wide by 250mm deep but if you need to fit additional AFDD units then you are going to need a bigger consumer unit as the AFDDs are almost double the size of normal protective devices. And this is an issue as it takes more time to install and, in some cases, is a problem of where to ‘hide’ the unit in a room so that it is not an eyesore.

“So, when you’ve got a one-bedroom flat with stone walls and a spiral staircase that’s 500 years old, as we had to contend with at Gladstone’s Land, then the space for fitting a larger consumer unit is limited. It’s not a modern property with places to put these boxes in cupboards or utility areas – there’s just no space.

“However, we worked with the joiners to mount them within nice cabinets to help them blend in with the 16th century style of the apartments.”

Helpfully, for the Gladstone’s Land project, the AFDDs were integrated into the new distribution boards which were designed and manufactured by Hager, who also delivered them to the site prefabricated.

Oliver’s team had already removed the existing distribution boards for the museum and other rooms and rerouted the electric mains to a new location where a new large board for both the café and museum could be located.

The main three phase distribution board was a 24-way unit, fed with a 125 amp main switch and a surge protective device. It had multiple miniature circuit breakers and Residual Current Breakers with Over-Current (RCBOs) fitted for final circuits. Adjacent to the main board, attached, was an extension enclosure housing 17 AFDDs serving ring circuits and appliance radial circuits. The board came prewired, linking the final circuit fault devices to the AFDDs, all final connections were made to the AFDDs. The main board serves all final circuits within the basement, toilets, café and ice cream parlour and the museum areas.

Oliver added: “The distribution board for the café and museum was enormous. We’re talking about a three-phase installation that already has a fairly big distribution board but when you add the AFDDs it’s huge, twice the size of normal. AFDDs were fitted on most of the circuits for fixed appliances and socket circuits – around 20 circuits in all – to protect against the risk of fire, but this meant we also had weight issues. It was probably in the region of 35kg, but as it was all prewired, we could not take it to bits so we had to find a way to safely mount it on a wall. This was installed by prefabricating wall brackets and fixings so the board could be lifted into place in one motion, taking the weight prior to finally fixing it to the structure.”

The renovation started in January 2020 and, because of COVID-19 interruptions, was not fully finished until August 2021, but Oliver does not think this is the last he has heard about AFDDs.

He explained: “I can see a time when AFDDs will be specified as standard because there has been a trend in recent years of introducing fire protection technologies in buildings, particularly after the Grenfell Tower tragedy.”

However, while AFDDs may be effective at dealing with potentially hazardous arc faults they come at a price. They can be anywhere between £75-£100 a unit and, as you need one AFDD for each circuit, particularly for fixed appliances and sockets, the cost soon adds up.

He added: “I remember when RCDs came on the scene around 15 years ago and they too were astronomical in cost at that time. They weren’t a requirement before 2008 on most jobs but now they are, and while they are still not cheap – costing around £30 compared to a £7 circuit breaker – they are now part of our electrical regulations and need to be installed.

“I’ve never known any new technology that’s been introduced over the past 30 years, like AFDDs, that hasn’t ended up becoming a legal standard. Risk assessment is a big issue in the electrical industry today and there’s a lot of onuses on the designers of electrical systems, so while AFDDs are currently a ‘recommendation’, I can see them become a standard requirement in the near future.”

One of the slight worries that Oliver has about AFDDs is their ability to differentiate between real arc faults and arcs created as part of an appliance’s normal function. He explained: “You have to be careful how you design and test an installation with AFDDs as I believe that nuisance tripping could be a problem with these sensitive sensors, particularly with fixed appliances like boilers, freezers and ovens etc.

“In addition to arcing because of bad wiring or poor connections, you also get natural arcing caused by components within an appliance ‘kicking in’, like a washing machine starting or a thermostat instructing a compressor to kick in to cool a warming freezer.”

Oliver suggests spending time testing each fixed appliance to make sure nuisance arcing is not happening and tripping the AFDDs. If there is an issue of that happening, then the appliance should be put on its own circuit – you don’t want a faulty kettle tripping out a commercial freezer.

Oliver thinks that the uptake of AFDDs will take some time but he recommends that his fellow electricians get familiar with the new technology, so they are able to advise their clients if applicable. He said: “I don’t think electricians will be installing AFDDs when they are not specified in a job but they should be aware of what they do and how they can increase fire protection so they can advise their clients where necessary as part of their professional service.

“For example, it’s the same with surge protection: the new wave of devices on the market which obviously protect against overvoltages coming into the installation. I have a client who is a musician and has thousands of pounds worth of electrical equipment in his house that I am refurbishing. Although the work has not stipulated surge protection devices, I have talked about the benefits of having this technology installed because of the value of his equipment. The cost to install surge protection is a fraction of the cost to replace any damaged equipment if a major surge was to occur.

“Therefore, in the same vein, I think electricians should find out more about AFDDs and the benefits they bring to added fire protection, particularly for buildings used for accommodation, from youth hostels to hotels and especially properties used as Airbnb.”

AFDDs are also recommended in commercial premises that house combustible materials and where there are fire propagating structures in buildings like timber frame houses and thatched roofs. They also provide strong fire protection in places that house irreplaceable items and collections, such as museums, libraries or historic buildings, such as Gladstone’s Land.

Summing up his experience with AFDDs, Oliver has some advice for his electrician colleagues: “Just bear in mind that distribution boards housing these AFDDs are twice as big and twice as heavy as normal boards, and they will probably take twice as long to install.

“So, do I think AFDDs are a good thing? Yes, I do, but remember somebody has to be willing to pay the extra for them.”


Quick facts: J. K. McCrone Electrical Services

BASED: Edinburgh

PHONE: 0131 339 1842



When to fit arc fault detection devices

BS7671:2018 18th Edition Wiring Regulations recommends that they be fitted in the following locations:

  • Premises with sleeping accommodation (e.g. a youth hostel)

  • Locations with a risk of fire due to the nature of stored or processed materials (e.g. barns)

  • Locations with combustible construction materials (e.g. a building constructed primarily of timber)

  • Fire propagating structures (e.g. chimney-like spaces, lift shafts)

  • Locations with endangering of irreplaceable goods (e.g. museums).

AFDDs are available in a number of different configurations for use in a domestic installation:

  • They can be installed as a standalone device in series with a suitable protective device, i.e. your circuit breakers or RCBOs to the relevant standards

  • They can be installed as one single device that consists of an AFDD combined with a protective device such as a circuit breaker or RCBO

  • Some manufacturers also offer to add on AFDD modules for their other protective devices, these are however only compatible with that manufacturer’s protective devices.



By Darren Sweeney, Technical Adviser

It was great to meet Oliver for a technical assessment and see for myself the project he’d carried out

at Gladstone’s Land.

Given its nature, location and historical importance, the installation was never going to be straightforward and certainly threw up some interesting technical challenges.

Among them was the pre-wired and configured distribution and protective devices provided by Hager. Seeing the narrow staircases and 17th-century corridors for myself, it must have been a mammoth task to manoeuvre such a precious cargo into its final resting place.

After taking a good look around, the pièce de résistance for me was the main distribution board and consumer units in the residential property, complete with AFDDs installed on all socket outlet final circuits.

Understandably for such an important project, providing arc fault, residual current, overload and short circuit protection will ensure this unique piece of Scottish culture is preserved for everyone’s future.

Well done to Oliver and the guys at McCrone for doing such an excellent job.


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