What is a ring main unit in an HV connection?

A ring main unit, or RMU, is high-voltage switchgear used to connect and protect distribution transformers [S1]. It controls electrical circuits rather than changing voltage. SSEN lists installation instructions for 11 kV RMUs in secondary substations [S2]. If your proposal includes one, confirm its owner, operating authority and maintenance arrangements before commissioning. Checked October 2026.

Last checked 19 October 2026

What is a ring main unit?

A ring main unit (RMU) is a high-voltage switchgear assembly used to connect and protect distribution transformers. It belongs to the switching and protection part of a substation, rather than being the transformer itself [S1].

Why it matters for your site: an RMU in your connection proposal means you need clear equipment, access and operating responsibilities. HSE’s switchgear guidance stresses competent operation and maintenance records [S3].

High voltage (HV) describes the electrical voltage class. SSEN’s document register includes instructions for 11 kilovolt (kV) RMUs used in secondary substations [S2]. That is a network equipment rating, not the power available to a charger.

Where does an RMU sit in a connection design?

Think of the RMU as the switching assembly associated with the transformer connection. The transformer changes the voltage; the RMU connects and protects the circuit supplying it [S1]. Your single-line diagram, a simplified electrical drawing, should identify both items rather than combining them into an unexplained substation price.

Ask the designer to mark the network cables, RMU, transformer, metering position and ownership boundary. SSEN lists separate documents for these components and for safety and ownership signs [S2]. Use when a site needs a new substation for the wider connection question.

What maintenance information should you request?

HSE recommends an asset register and maintenance records that identify the equipment and work completed. The maintenance approach should reflect its condition and operating duty [S3]. Request:

• The equipment identifier, ratings and commissioning records.
• The owner and authorised operating contact.
• The inspection, testing and maintenance programme.
• Arrangements for outages and emergency access.

Discuss the boundary with your distribution network operator (DNO), which manages the local network. Read LV versus HV supply and the grid upgrade cost guide before accepting the scope.

Next step

Consider how your project could earn more from your site. Bring the equipment schedule to the site check, then get matched with an installer that covers your area.

Piggy Pineapple Ltd operates this referral service. We refer enquiries to installer partners and may be paid a fee. Our owner holds shares in at least one partner. Before sharing your details, we name the proposed partner and disclose any shareholding in it.