12 kV Ring Main Unit

The ratings are the same — 12 kV, 630 A, 20 kA short-time withstand for 4 s, 50 kA peak — so the choice is not made on parameters. The SF6 load-break switch in an XGN15-12(L) is sealed inside a tank, giving stable arc-quenching performance and long switching life, at the cost of putting tank pressure status on the inspection round, a 1% annual leakage rate, and exposure to fluorinated-gas regulation in some markets with rules for end-of-life handling. The vacuum interrupter in an HXGN□-12Z needs no gas top-up and raises no gas disposal question, at IP2X against the other IP3X, with an earthing-switch short-circuit duration of 2 s against 4 s. Projects with limited maintenance resource or in markets restricting fluorinated gases go to vacuum; projects wanting a sealed-tank solution with high switching endurance go to SF6.

They sit at different points in the network. MV switchgear stands in a substation, one circuit per cubicle, with a circuit breaker as the main device clearing short circuits automatically through protection relays, which suits large incoming and outgoing circuits. A ring main unit stands on a distribution ring, several ways side by side on one busbar section, with a load-break switch as the main device — it makes and breaks load current but does not break short-circuit current — and the transformer feeder relies on a fuse to clear a short circuit once. So a ring main unit is smaller, cheaper and simpler, with limited protection capability, and is used at the distribution transformer and terminal distribution layer. A common arrangement in a distribution room is a ring main unit taking the incoming supply and feeding the transformer, with a low-voltage line-up on the transformer secondary.

Because a distribution transformer circuit carries a modest load current against a large prospective short-circuit current, and a fuse clears that short circuit both faster and more cheaply than a breaker. The load-break switch handles routine making and breaking of transformer load and no-load current, and the fuse operates once on a short circuit, after which the fuse-link is replaced. The limits of the combination are that a fuse operation needs someone on site to replace the link rather than a remote reclose, and that a fuse has a rated breaking capacity ceiling, so a circuit rating or system short-circuit capacity above a certain level moves the design to a breaker. Transformer rating, system short-circuit capacity and whether remote operation is required are the three inputs that decide which way to go.

It follows the ring connection, not the load. The basic composition is three ways: a load-break switch way each for ring incomer and ring outgoing, plus a transformer feeder. A second transformer on site adds a transformer way; metering or sectionalising adds the corresponding way; a terminal site with no ring outgoing keeps a single incoming way. Send the ring connection diagram, the transformer rating and number on site, and whether metering and distribution automation are needed, and the way composition and scheme number can be settled. The ring is extended one way at a time, without replacing the whole line-up.

The annual leakage rate is the proportion of gas the sealed tank is permitted to lose per year under normal operating conditions. It is a sealing performance figure, not an instruction to top up 1% every year — at that rate the tank normally needs no top-up within its design life. Routine maintenance means putting tank pressure status on the inspection round: rated pressure is 0.04 MPa (20 C gauge), and a pressure indication falling into the alarm band is referred for handling rather than opened on site. At end of life the SF6 is recovered and handled under local fluorinated-gas regulation and must not be released. These clauses are written into the technical agreement against the rules of the destination market.

Three things give a preliminary scheme: the ring connection and where the site sits on the ring, rated voltage and rated current, and the system short-circuit capacity. For a contract-grade quotation, add the transformer rating and number on site, the intended way composition and scheme number, the preferred arc-quenching medium, cable sizes and entry direction, control supply voltage, whether the unit joins a distribution automation system and under which protocol, the installation environment, and the standards and document list for the destination market.

Yes, and this is one of the most common ways the family ships as a complete delivery. Switchgear is supplied as equipment while a prefabricated substation or power module is supplied as an enclosure: the first is the panel itself, the second covers the enclosure, equipment layout, auxiliary systems and interface integration. Where the line-up goes inside a module as the medium-voltage primary equipment, ETENZ details the way arrangement, busbar connections, cable entry and in-module interfaces, and the delivery boundary is settled once at contract stage.

Standard documents travelling with the panels are the approved project drawings, the ring scheme diagram, component list, wiring records and packing list. Inspection records are issued to the scope agreed in the order technical agreement; where witnessed tests or a specific report format are needed, raise it during quotation and technical clarification so test and witness slots can be arranged.