11 kV MV Design for a Distribution Centre

A new distribution centre taking a 5,320 kVA supply at 11 kV. The MV design from the point of connection through the ring main unit to the LV boards, including the protection that ties them together.

The Brief

• The DNO was bringing a non-contestable supply to the site boundary. Everything beyond that point needed designing.
• A new ring main unit taking the supply into the site, with two distribution transformers downstream.
• Three 11 kV cable runs to size, the RMU specification to validate, and protection to coordinate from the incomers down to the LV breakers.

How It Was Done

• The declared fault level at the point of connection was taken from the DNO in writing rather than assumed, and cross-checked against the upstream primary substation data.
• Those two figures differ, and should: a primary busbar is always stiffer than a point out on the ring, and the ring cable impedance accounts for the difference. The declared figure governs; the upstream figure is the sanity check.
• Cable sizing, RMU validation and protection grading were carried as one exercise, so the switchgear rating, the cable rating and the settings all reconcile.

What Came Out Of It

• Cable sizes, an RMU specification confirmed fit for the duty, and a coordinated protection scheme across the incomers, the transformer feeders and the LV breakers.
• The design is built on a declared fault level rather than a guessed one, which is what makes it defensible at adoption.

Published without naming the client. The technical detail is exactly as delivered.