Withdrawable metal-clad, fixed metal-clad and ring main unit are not three versions of the same product. They are three answers to three different network questions, and picking the wrong one costs money twice.
Every switchgear selection begins with the network it will sit in. The number of incomers, the number of feeders, whether the distribution is radial or ringed, and whether the loss of one feeder may interrupt the load are all decided before a catalogue is opened.
A radial single-incomer substation and a ringed distribution network need different equipment for reasons that are structural, not commercial. If a quotation arrives before this discussion, it is a guess — and the guess usually shows up as a disposal or an expensive modification on site.
Withdrawable metal-clad (KYN28A-12 class). The circuit breaker is on a withdrawable truck, so a failed breaker can be replaced in minutes instead of hours, and the truck can be moved to a maintenance bay. The cubicle is larger than a fixed one. This is the default answer for an industrial or commercial substation where a spare breaker strategy matters and floor area is available.
Fixed metal-clad. The breaker is fixed in the cubicle. It is simpler, cheaper per bay and smaller, and it is the right answer where the duty is light, the fault level is low and the operating organisation will maintain the equipment by replacing the panel rather than the breaker.
Ring main unit (HXGN-12 class). A compact sealed unit with two or three ways, built for ringed medium-voltage networks where the objective is to keep supply alive when one section is out. It is a network device rather than a substation device, and its value is entirely in the switching scheme.
A switchgear is described by three ratings that are routinely confused.
Rated voltage is the highest voltage the equipment is designed to operate at — 12kV for a 12kV system.
Rated current is what it can carry continuously in the normal configuration. On a withdrawable arrangement this is often limited by the contact rating of the truck rather than by the busbar, which surprises people.
Rated short-circuit current is what it can interrupt. This is the number that has to be checked against the actual fault level at the installation point, and it is the number that is frequently overlooked because it is not printed on the nameplate in a way that buyers look for.
The enclosure protection class is selected for the installation, not for the building type. An indoor switchroom in a tropical coastal plant, an outdoor kiosk in a mining site and a switchroom in a temperate inland plant have three different answers. The IP class, the door sealing, the cable entry method and the roof arrangement all follow from that one decision.
Interlocking is what prevents the combination of operations that damages equipment or endangers a person: closing the earth switch on a live breaker, pushing a closed breaker into the service position, or operating the coupler while the incomer is closed.
The interlocking scheme should be written into the technical agreement as a table of permitted and forbidden operations. If it is not in the agreement, it will be resolved on site, and it is always resolved in a hurry.
Network: incomer count, feeder count, ringed or radial, and the required continuity under a single feeder outage.
Electrical: rated voltage, rated current, and the calculated fault level at the installation point.
Environment: indoor or outdoor, protection class, altitude, ambient temperature, dust and humidity.
Operation: spare breaker strategy, maintenance access, remote indication and the communication protocol.
Protection: relay type, the protection settings, the interlocking scheme, and the metering arrangement.
Per bay, usually yes, because the RMU is a compact sealed product. But an RMU is not a substitute for metal-clad — it is a network device. If your distribution is radial with one incomer, a metal-clad assembly is usually the better technical answer even at a higher unit price, because you gain the withdrawable breaker and the spare-parts option that an RMU does not offer.
The fault level at the installation point has to be calculated for the network as it will be built, including the contribution from any parallel transformer or generator. Send us the network data or the study result and we will confirm the required breaking capacity and confirm it against the equipment we supply.
Failure handling and maintenance. With a withdrawable arrangement, a failed breaker is swapped with the spare truck in minutes and the truck is taken to the workshop. With a fixed arrangement, a failed breaker means a longer outage and usually a panel-level repair or replacement.
Yes. The relay type, the protection settings, the interlocking scheme and the metering arrangement are all confirmed in the technical agreement and supplied with the equipment.
Send the technical requirement — the rating, the quantity and the destination country — and we reply with a configuration proposal and a price, normally within 24 hours.
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