Record a TT Earthing System

The tyre fitting building is a steel structure with a water supply fed via an alkathene pipe. The supply changes to a TT earthing system. The supply enters the building and connects to a time delay RCD enclosed in a metallic enclosure.

  1. In the left tree, right-click on the supply circuit (4L2), select “New” -> “Board” and in the “New Board” dialog under “Type” select “RCD (Metal Enclosure)”.

    Because this is a TT earthing system and the first item of equipment is a RCD in a metal enclosure, ETAS will recognise this fact and will offer an automatic observation regarding the exposed conductive part (metal enclosure).

  1. Select the “RCD” tab and enter the details.

    You will notice that as this is an RCD unit, there is no RCD 2 to choose and that the “Circuits Covered By…..” has been automatically entered.

  2. This RCD is a time delay type, therefore, “tick” the box and enter the time of delay.

  1. Enter the bonding details.

    If there are any exposed conductive parts i.e armoured cable glands that have not been insulated and are touchable at the load end, then the field “Is the upstream earthing system isolated….” is “No”. An automatic observation will be offered relating to the exposed conductive parts. For this exercise we will choose “Yes”.

As the CPC has been insulated at the load end of the circuit, you are not able or required to carry out any earth related tests, therefore, any CPC related fields on the Tyre Bay supply circuit will have NA (not applicable) chosen.
  1. Under the “Main” tab enter the earth cable connecting to the earth electrode.
  1. The water supply is in alkathene piping, therefore, “Is water present” = “Yes” and “Carried Out” = “NA”. Carry on entering details of the gas, oil etc.
  1. The RCD supplies a 2-way fuse board. As the RCD is a 61008 type (without overload protection) choose “Solid Link” as the protective device. Note the “Maximum Allowable Disconnection Time”, the software recognises that the sub-main is on a TT earthing system and therefore, automatically enters 1 second (shaded). Circuits will have 0.2 second automatically entered.
  1. When you arrive at the “Bonding” tab enter “No”. This fuse board is not outside of the immediate equipotential area i.e. it’s within the TT area of the Tyre Bay.
After recording the downstream circuit(s), it is advisable to check that circuits have been allocated to their RCD protective devices(s). Here you can see that the 100mA RCD (500 Ω) and the local 30mA RCBO (1667 Ω) have been recorded successfully. Again, note the “Maximum Allowable Disconnection Time”; the software recognises that as the circuits are on a TT earthing system the software automatically enters 0.2 second (shaded).

That’s the TT system section now complete.

➲ Next: Review Observations.