We’ve all come to expect connectivity for our phones and other devices almost everywhere we go. But once you go underground there is little or no connectivity. This lack of signal is inconvenient for millions of people every day. More importantly, it makes the work of the emergency services much more difficult.
So, the customer decided to make 5G available throughout hundreds of kilometres of underground tunnels. As well as improving communication for the emergency services, business users will be more productive and millions of people will enjoy greater internet access.
The customer’s choice to deliver the signal effectively was to run a ‘leaky feeder’ along the tunnels. A leaky feeder is a cable that emits and receives radio waves through gaps in the outer conductor.
The leaky feeder consists of two cables that have to be 300mm apart to work effectively. They have to be at least 80mm clear of any other cables. And the cables have to run at the level of the carriage windows.
Site challenges include the vibration and violent air pressure changes in the tunnels and the need to avoid disturbing existing asbestos.
The customer identified a cable clip that would hold the leaky feeder, but how to fix it to the tunnel wall precisely and firmly? It explored using flanges between concrete panels but these are too far apart. This would cause problems on curves and anyway, they are not always at the right height. And the older tunnels are made from brick.
The only solution was to use existing brackets in the tunnels that already carry cables. But the new cables had to be attached to the outside of the bracket to keep them apart from existing cables. The customer came up with some initial prototypes but each one had drawbacks. They were all relatively expensive and took operatives quite a long time to install.
A few facts about these tunnels and the practicalities of the project. Throughout the hundreds of kilometres of underground tunnels, the brackets are placed every 1.24m, with each bracket needing to have two fasteners attached. That’s a lot of brackets and fasteners.
In most cases the operatives can only work between midnight and 4am, mainly at weekends. They have to finish a complete run from one point to the next in a single shift. The conditions in the tunnel are poorly lit, so fiddly work is very difficult. Unless the fastener is easy to use, the installation will take far too long.
The fastener that Jet Press designed and built for the customer cost half as much as the original prototype. But that wasn’t the biggest cost saving. It pre-assembled the fastener so all the operative has to do is offer it up to the bracket and twist the cable clip through 180°. Then the cable can be inserted and the whole assembly is held firmly in place, quickly and easily.
The nut, bolt and two brackets are the bespoke elements of the custom fastener. The nut and bolt are designed so that a 180° turn pulls the brackets into place. In use it became apparent that the cast iron brackets used in the older tunnels required different brackets. So Jet Press designed special brackets to use on the original parts of the tube.
It’s possible for an operative working alone to install the cables. However, the customer found that two people working together, one attaching the fasteners and the other putting the cable in was quicker.
