Maintaining power supplies

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Using new high-temperature connector technology allows Hindhead tunnel to combine improved electrical safety, in temperatures up to 400?C, with reduced maintenance time and costs

One of the biggest challenges of building the longest non-estuarial road tunnel in the UK is making sure people have sufficient time to escape from the twin tunnels in the event of a fire. For the Hindhead Tunnel, that meant finding a way to maintain the power supply to the extractor fans despite extremely high temperatures.
The Hindhead Tunnel is a major construction project with a total cost of over £370m. Running for 1.2 miles and with a maximum depth of 65 metres, the twin tunnels pass beneath the Devil’s Punchbowl, near Hindhead in Surrey, to complete the dual-carriageway link on the A3 bypass between London to Portsmouth. For users, the tunnel promises faster journey times; for local residents the tunnel will significantly reduce the traffic congestion that blights this area of outstanding natural beauty.

 

Work started on the project in January 2007 and the tunnel is scheduled to be open for traffic in July 2011. Among the state-of-the-art equipment being installed are safety and communication systems such as high-quality lighting and ventilation, as well as CCTV surveillance and radio re-broadcast systems. Maintaining power to these systems, and in particular to the extractor fans, is of paramount importance. Although the fans are routinely used to extract exhaust fumes from the tunnel at busy periods, they are also vital for extracting smoke in the event of a fire. Every minute these fans can keep the tunnels clear of smoke, gives users more time to be safely evacuated.

The installation of the ventilator system is governed by safety regulations which state the extractor fans must be capable of operating for a minimum of two hours even when the temperature reaches 250°C. Typically, this would mean each fan would be hard-wired.

However, hard-wiring has its down-sides: not only does it take a qualified electrician considerable time to install each fan, but it takes time, and a relatively high cost, for a qualified electrician to disconnect and reconnect the hard-wiring every time that maintenance is required. This means lanes must be closed, and fans must be taken out of operation for unacceptably long periods even for routine maintenance.

The Hindhead Tunnel needed to address these issues to find a solution which would improve safety in the event of a fire, and minimise downtime and costs during routine maintenance.

In France, industrial connector specialist, Marechal had been considering the same problem. Its  goal was to develop a connector technology which would combine very high-temperature operation with fast, safe connection and disconnection, even by operators without electrical qualifications.

For inspiration, Marechal turned to its existing plugs, outlet sockets and distribution boxes, developed for use in some of the most challenging applications. The result was wall-mounted socket-outlets and plugs capable of operating for up to two hours in temperatures of 400°C, and with full compliance to the IEC EN 12101-3 fire safety standard.
The sockets comply with other standards too: they are sealed to IP55, which protects against dust and water from compromising their electrical performance; and they are rated to IEC EN 50102 standard, level IK09, which is the second-highest level of protection against shocks, allowing the sockets and plugs to withstand an impact energy of 10 joules from an object with a radius up to 50mm. This new high-temperature connector technology also allows the extractor fans in the Hindhead Tunnel to meet the IEC EN 60204-1 standard for electrical machinery, and the AC22 load-break requirements of the EN 60947-3 switch standard.

The original installation of mounting the sockets onto the tunnel wall, and fitting plugs onto the power cables to the fans, is carried out by a qualified electrical contractor. The power cable is wired onto steel terminals mounted on a fire-resistant ceramic base, and the socket-outlet is pre-wired to the same terminals using fire-resistant cable.

However, unlike the conventional alternative of hard-wiring the extractor fans, once the socket-outlets and plugs have been wired-in, the fans can be connected and disconnected safely in seconds, even by operators with no electrical qualifications. This is due to a number of safety features which are built into the Marechal technology.

An automatic locking mechanism ensures a secure connection as soon as the plug is inserted into the outlet socket, whilst a quick-release, load-break button on the top of the socket triggers springs in the socket casing, and on the contacts, to break the connection at speeds which prevent arcing. This allows the plug to be disconnected from the socket in seconds, even by operators with no electrical qualifications. The load-break switch also eliminates the need for a pilot cable or for additional switches. When the plug is disconnected, the socket’s safety shutter shields the live contacts preventing operators from accidentally touching them.

In addition to the considerable maintenance advantages the new connector technology provides, there is the further advantage of an extended operational lifetime. Inside each socket, spring-assisted conductor terminals provide resistance to shock whilst the braided butt contacts are tipped with solid alloy of 85% silver and 15% nickel to prevent corrosion. These features can extend the expected lifetime of the connectors to 15 years or more, regardless of how many times that it is connected and disconnected, even in the harsh environment of the tunnel. They also ensure the connection can be broken when the plugs have not been removed for a long time. This is a common problem with force-fit contacts which can become cold-welded over time, making it difficult to remove the plug from the socket.

By breaking with tradition and replacing conventional hard-wiring with the latest high-temperature connector technology, Hindhead Tunnel has gained greater flexibility and a significant cost-saving during maintenance as well as giving users valuable time in which to escape from the tunnel in the event of a fire.