Ring Main Units

Ring main units (RMU) are widely used in industry and for distribution systems. RMUs operate independently, which means that it is possible to earth an incoming supply cable while it is still energised from the other end or operate a switch and liven a cable onto an earthed end. This will create a fault or situation that may endanger the HV operator while operating or carrying out any testing.

An example of ring main switch positions is shown below.

The key points to consider with ring main systems are:

  • Ring main units’ busbars are continuous
  • Switches opened in the middle of the board does not mean that another unit cannot be interconnected.
  • The blades of the switch are always hinged on the cable side. This allows cables to be connected to either the:
    • busbar
    • OFF position
    • earth position
  • Each unit can be operated no matter what the condition off the equipment at the other end of the cable. This means that:
    • a cable can be switched off and earthed while still being energised from the other end
    • a cable can be energised that is earthed at the other end
    • there is nothing in the normal interlocking system to prevent this happening (with the exception of a Castell keying system)

Extreme care must be taken whenever switching at an RMU to ensure that this does not occur. If a cable is to be earthed in order to allow work to be undertaken, it must be de-energised at both ends before either end can be earthed.

The use of neon indicators for verification of zero energy is acceptable. There are not always neon’s built into these systems, in these cases the HV operator must rely on drawings and correct/proven switching programs to operate these systems. You need to have access to terminations to perform a zero-energy check with a HV tester. A modiwark cannot detect voltage on a HV cable as all cores are shielded with a semiconductor.

It also needs to be noted; not all drawings indicate earths are present at the ring main units. This has caused incidents where the HV operator overlooked the presence of the earths.

An example of a typical drawing of a 2 + 2 Ring Main Unit (RMU) is shown on the right.

In reality, the RMU shown in the diagram above actually looks like this:

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