In a medium-voltage ring main unit, the load break switch way carries the through-current of the ring; 400 A is typically used for lightly loaded radial feeders or transformer tee-offs, while 630 A is the default for ring busbars and heavily loaded loop segments [S1][S5].
Most published RMU datasheets list LBS current ratings as 400 A or 630 A per way, with the 630 A figure treated as the practical upper limit for gas-insulated ring switchgear at 12-24 kV [S2][S6].
Where 400 A and 630 A Actually Sit in the Network
Ring main units are compact, gas-insulated switchgear modules used to connect, isolate, and sectionalize medium-voltage feeders in 6-36 kV distribution networks, and a typical 3-function unit pairs two LBS ways with a fused or breaker tee-off [S3][S5]. On a true ring feeder, the LBS does not have to break load beyond what is downstream of it, so the through-current it carries is the combined load of the ring segment it feeds, often close to the transformer cluster rating, which is why 630 A appears as the de-facto maximum full-load rating on the ring ways [S4][S5]. A 400 A LBS way sits naturally on radial spurs, lower-density rings, and transformer tee-offs where the connected load is well below 630 A; the alfanar product datasheet explicitly lists 400 A as a normal LBS rating alongside 630 A in the same RMU family [S6].
For the transformer tee-off (the so-called T-way), the current is far smaller: rated current of the transformer cabinet is generally no more than 125 A, and a fuse-switch combination takes care of the short-circuit duty, not a 400 A or 630 A LBS [S2]. So the 400 A vs 630 A choice is not about transformer size, it is about how much through-current the ring busbar itself must carry continuously.
Selection Criteria Beyond the Nameplate Ampere
Rated current is only the first filter; the next three are short-time withstand current (Ik), short-circuit making capacity, and busbar rating, and they usually force a 630 A selection even when the continuous load looks modest [S1][S7]. For example, a circuit breaker function in the same RMU is often specified at 630 A with a 21 kA interrupting rating so that the unit can sit anywhere on the ring without coordination headaches, and the LBS ways in that same unit are then matched to the same 630 A busbar to keep the through-fault duty coherent [S7].
For purely switching duty, a 400 A LBS way is acceptable when the connected transformer cluster does not exceed roughly 0.7 MVA at 11 kV, three-phase (0.7 MVA / (sqrt(3) x 11 kV) is approximately 37 A, but the ring segment in front of it may still carry several hundred amps from neighbouring loads), and when the upstream protection clears faults before the LBS short-time withstand rating is exceeded [S1]. The alfanar RMU datasheet confirms that 400 A and 630 A LBS ratings ship in the same product line, with the choice driven by the rating plate of each function, not the enclosure [S6].
On cable charging, transformer no-load, and parallel-loop switching, the 630 A LBS way gives more headroom because the same contact system handles the making and breaking of load currents up to its rating, plus the mechanical endurance required by frequent sectionalizing [S2][S4].
Decision Matrix: 400 A vs 630 A LBS for Ring Feeders
The table below lines the two options up against the four criteria that actually decide the specification on a real ring main unit bill of material. [S1]
Continuous through-current on the ring: 400 A LBS is sufficient for radial spurs and lightly loaded rings where measured peak load stays below roughly 320 A (a common 80% design margin); 630 A LBS is required for any ring segment that may carry the full combined load of the loop or a paralleled feed during contingency [S1][S4][S5]. Short-time withstand (1 s or 3 s Ik): both ratings are usually available at the same 20-25 kA short-time level when supplied in the same RMU family, so Ik alone does not force 630 A; the alfanar line confirms 400 A and 630 A share the same assembly short-time rating in many builds [S1][S6]. Coordination with the breaker function: 630 A is the safer default when the RMU also contains a 630 A / 21 kA circuit breaker way, because matching bus ratings simplifies coordination studies and lets the breaker way sit anywhere on the ring without re-engineering [S7]. Future load growth and motorization: 630 A is the right call for any ring way that may be motorized for remote sectionalizing or that may pick up additional transformer capacity, because retrofitting a 400 A way to 630 A later means a full LBS module swap, not a wiring change [S2][S4].
Use 400 A when the ring way feeds a defined, capped load (a small commercial cell, a single distribution transformer under 1 MVA, or a radial spur) and the upstream ring is already broken into smaller cells. Use 630 A on any main ring busbar, any loop where two feeders may parallel, and any cell that may be upgraded; the 630 A figure is the most common full-load rating published for ring main switches and the figure utilities default to in tender documents [S1][S4][S5].
Standards, Ratings, and What the Nameplate Must Show
RMU ratings are tested to IEC 62271-103 for the LBS itself and to IEC 62271-200 for the assembled unit, and the relevant nameplate entries for a 400 A vs 630 A choice are rated current (In), short-time withstand current (Ik for 1 s or 3 s), peak withstand (Ip), and the cable-charging / load-switching class [S1][S4]. For 12 kV class units, the typical LBS ratings are 400 A or 630 A with 20-25 kA / 1 s or 3 s short-time withstand, and a making capacity that matches the peak withstand (typically 2.5 x Ik) so the switch can close onto a fault [S1][S2].
Insulation level is given as rated power-frequency withstand voltage and rated lightning impulse withstand voltage (BIL), and these do not change between a 400 A and a 630 A LBS in the same enclosure; only the current-carrying parts differ [S1]. For transformer protection, the fuse-switch combination carries a different rating set: HRC fuse ratings up to about 125 A with appropriate striker-pin coordination to the LBS trip mechanism, and the switch itself still needs a 400 A or 630 A main current path because the LBS carries the load even after the fuse has cleared the secondary fault [S2].
Failure Modes and Common Specification Mistakes
The most common mistake is to specify a 400 A LBS way on a ring segment that, under N-1 contingency, can be fed from both ends simultaneously; the through-current then exceeds 400 A and the switch derates or the busbar runs hot [S1][S5]. The second is to mix a 400 A LBS way with a 630 A circuit-breaker way in the same RMU without checking the assembly short-time withstand, which is set by the lower-rated function unless the bus is independently rated [S6][S7].
Another frequent miss is ignoring switching endurance: a 400 A LBS used for frequent ring reconfiguration (multiple operations per day) will hit its mechanical life limit sooner than a 630 A unit of the same family, because the 630 A contact system is typically built on the same mechanism but with larger contact mass and longer maintenance intervals [S2][S4]. Earthing switch coordination is the third trap: the earthing switch must make onto the same short-time current the LBS can carry, and on a 630 A ring way with 21 kA Ik, the earthing switch must be rated for full making capacity, not just the 400 A continuous rating of a smaller LBS in a different cell [S1][S7].
Cross-Reference with Adjacent Spec Topics
The 400 A vs 630 A LBS decision is part of a wider ring-spec conversation that also covers the upstream MV cable, the LV switchboard, and the way the RMU interfaces with protection relays; for a related read on how cable specification drives ring layout, see the guide on light blue jacket shielded cable for IS circuits, and for the busbar sealing system that makes the 630 A gas-insulated cell possible, the load break switch reference page covers contact materials and arc-quenching choices that differ between 400 A and 630 A designs. The mechanical frame and busbar of a 630 A RMU cell also overlap with the wider switchgear slewing ring bearing discussion only insofar as the RMU swing frame uses similar bearing practice, but the electrical specification remains driven by the four criteria above. [S3]
Track these signals before locking the rating: (a) the connected transformer cluster MVA at 11 kV or 24 kV, which sets the continuous through-current; (b) the upstream protection grading study, which sets the short-time withstand and breaking capacity; (c) the motorization and SCADA roadmap, which sets whether remote operation must be retained for 20+ years; and (d) the utility's standard tender wording, which in many regions defaults to 630 A on all ring ways to avoid future re-specification.
Component reference pages worth checking: eddy current tester.