Ring Main Unit (RMU) and the switch-fuse combination — IEC 62271-105

The guide on vacuum and SF6 circuit breakers and F-gas regulation covers the arc-quenching technique of individual circuit breakers. This article covers a specific, widely used application of that technology in public medium-voltage distribution: the Ring Main Unit (RMU), and in particular the switch-fuse combination per IEC 62271-105 that often forms its transformer feeder bay.

What an RMU is

A Ring Main Unit is a compact, factory-assembled and sealed medium-voltage switchgear assembly, typically consisting of three bays in a single enclosure:

Two ring bays — load-break switches (per IEC 62271-103) that loop the cable ring through on either side of the substation. These switches can make and break normal load current, but cannot interrupt a short circuit.
One transformer bay — intended to feed and protect the connected distribution transformer.

RMUs are widely used in urban and rural medium-voltage ring networks because of their compact, often gas-insulated (or, increasingly, solid-dielectric) design, which takes up little space in a street-side or building substation.

Why the transformer bay needs a switch-fuse combination

A load-break switch alone cannot interrupt short-circuit current — that requires either a circuit breaker with relay protection, or, for smaller transformer ratings, the combination of a load-break switch with medium-voltage fuses, standardised in IEC 62271-105 ("Alternating current switch-fuse combinations"). In this combination:

The load-break switch makes and breaks the transformer's normal operating current (including the inrush/magnetising current, see also the guide on transformer inrush current and selectivity).
The fuses interrupt the short-circuit current in the event of an internal fault in the transformer or the outgoing cable section, faster and with a higher breaking capacity than the switch itself could achieve.

The tripping mechanism: why one blown fuse is not enough

A medium-voltage fuse interrupts only one phase when it blows. Without a further provision, the transformer would then continue running on the two remaining, healthy phases — single-phase operation, which for a three-phase transformer causes a severely disturbed voltage relationship between windings and can lead to overheating and irreparable damage, without any visible or audible alarm occurring at that moment.

To prevent this, IEC 62271-105 requires a mechanical coupling between the fuse and the switch: each fuse is fitted with a striker pin that is mechanically ejected when that specific fuse blows. This striker pin triggers a tripping mechanism that opens all three phases of the load-break switch simultaneously — even if the other two fuses remain completely intact. This prevents the transformer from ever continuing to run single-phased or two-phased after only one fuse has blown.

Note: this tripping mechanism is a purely mechanical fuse-blown detection function — it does not replace an electrical protection relay and only detects that a fuse has blown, not the cause or extent of the underlying fault.

Coordination between switch and fuse

IEC 62271-105 also sets requirements for the coordination between the load-break switch and the fuses: the fuse's minimum breaking current must be lower than the switch's making capacity, so that no "coordination gap" exists — a current range in which neither component handles the fault reliably. For a fault within that theoretical gap, neither the switch (not designed to make or break a fault of that magnitude) nor the fuse (which does not blow fast enough at too low a current) would safely terminate the situation.

SF6, vacuum or solid dielectric

Historically, many RMUs have been built with SF6 gas as the insulation and quenching medium, because of the compact dimensions that allows. As covered in the guide on vacuum and SF6 switching technology and F-gas regulation, the EU F-gas regulation is nonetheless pushing the market toward SF6-free alternatives — vacuum interrupters combined with air insulation or a solid-dielectric insulation (for example epoxy) — also for compact RMU enclosures, which affects the mechanical implementation of the switch-fuse combination but does not change the principle of the fuse-blown tripping coupling itself.

Practical relevance

When assessing or specifying an RMU for a distribution transformer, it is important to verify that the transformer bay is actually implemented as a switch-fuse combination per IEC 62271-105 — including a functioning striker-pin tripping mechanism — rather than a bare load-break switch without fuses, and that the fuse selection (rated current, breaking capacity) is matched to both the inrush current and the short-circuit level at that network location.

Common mistakes

Assuming a load-break switch alone provides adequate short-circuit protection for a transformer bay — without fuses (or a circuit breaker with relaying) the switch cannot interrupt an internal transformer fault.
Not periodically checking the striker-pin tripping mechanism — a mechanically jammed or decoupled tripping mechanism reintroduces the single-phase-operation risk, even if the fuses themselves are correctly rated.
Replacing fuses with a different make or rated current without re-checking coordination with the switch and tripping mechanism.
Ignoring the "coordination gap" between fuse and switch when assessing protection for a specific network location, when that very fault-current range is precisely what neither component handles reliably.