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Query on support of MV cables sweeping into RMU gland plate

seansasta
Hi Guys,

I'm hoping someone can give me some guidance; we have a project where we have 2 parallel trefoil 20kv MV cables entering a RMU from a concrete cable chamber directly under the switchroom. The cables are cleated horizontally on cable ladder every 600mm and sweep upwards into the RMU gland plate, approx 1000mm from last cleat to each gland (see attached pic). Fault current at RMU is 20kA for 3 sec. Cables are around 40mm OD and separate to approx 100mm apart from each other where they enter the gland plate. Given this vertical rise to gland plate is relatively short and within the recommended spacing requirements for horizontal cables in trefoil, is there a need to install additional short circuit rated supports and cable cleats? If there is we would obviously have to design a frame that surrounded the group of 6 single core cables and cleat each one separately....it just seems a bit OTT?....Thoughts / Comments?

31673a0005ec0c091f2584c02e16e30e-huge-image.png



Thanks

Seansasta
  • 0
    The magnetic forces from a 20KA fault are unlikely to tear the wiring out if that is the concern here (!) There is a bit of a refresher on how to estimate the forces  here  

    The key thing to note here is that the force is less as the cable spacing increases, so although your cables have a longer un-tied span , they also have a lower force pushing them.


    However, 20kV is not a common UK level and your use of 'MV' suggests you are not local - where is this installation ?

    There may be local codes that require something specific to be done regardless of the physics.


    Also for amusement, here are some nice videos (at far higher currents) of how far it can go when it comes unzipped.

    Mike
  • 0
    Thanks Mike
    Looking at T&D website, I've decided to go with the recommended cleat spacing of 300mm from bend and add a supplemental support frame, ensuring no ferrous bracings between conductors.

    Thanks for the advice.

    Seansasta

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