LV Switchgear Selection: GGD vs MNS vs GCK and Form of Separation

Most switchgear arguments start at the breaker brand and end at the unit price. The decisions that cause pain later are the assembly type, the form of separation and whether the enclosure meets the tolerances draw-out units need. This guide covers GGD, MNS and GCK, fixed versus draw-out, Form 1–4, Icw/Ipk verification and enclosure fabrication.

What GGD, MNS and GCK actually mean

GGD is a fixed-mounted assembly to GB/T 7251.12 (IEC 61439-2): devices bolted to internal mounting plates, few feeders per cubicle (1–4) at high current (630–3,150 A). It is the most common distribution cubicle in Chinese industrial and utility projects — simple, cheap, easy to maintain.

MNS is a draw-out assembly on a 250 mm module (1E), in sizes of 8E (2,000 mm), 4E, 2E and 1E. One drawer is one complete functional unit with its own switching device, secondary plug and interlock, withdrawable without disturbing neighbouring circuits. It is the default for new industrial plants and data centre LV rooms.

GCK is the earlier draw-out design: busbar compartment at the top rear, drawers at the front, fed from horizontal busbars across the cubicle width. Drawer entry, plug position and interlock scheme all differ from MNS. New projects rarely specify it, but legacy plants still run it and retrofits need care.

Fixed versus draw-out: the differences that matter

• Outage scope: servicing one breaker in a fixed cubicle drops the cubicle or a busbar section; a draw-out unit goes to isolated and the rest keeps running.
• Fault containment: drawer faults stay inside the sheet-steel-bounded compartment; fixed designs share one compartment for all primary devices, so the blast radius is bigger.
• Interchangeability: same-size MNS drawers are interchangeable in principle — identical plug geometry, envelope and interlock travel — so one spare drawer covers a feeder class.
• Cost and footprint: draw-out assemblies cost 30–60% more at the same current rating, and MNS runs about 1,000 mm deep versus 600–800 mm for GGD, plus withdrawal clearance in front. Measure the room first.
• Thermal penalty: drawers are more enclosed and ventilate less, so the rise margin at the same busbar section is smaller. Never copy fixed-cubicle ampacity tables onto a draw-out design.

Form of separation 1–4 (IEC 61439-1)

Form describes how the internal volume is divided. It sets the fault containment boundary and the safety basis for maintenance, and it is the item most often left vague in specifications — then argued about at handover.

• Form 1: no internal separation; one fault can involve the whole cubicle.
• Form 2: busbars separated from functional units, so a busbar fault does not reach the feeders directly.
• Form 3: busbars, functional units and outgoing terminals each in their own compartment; cable work without de-energising the busbars.
• Form 4: busbars and every unit separated, units separated from each other, so one short circuit does not affect the next feeder. Draw-out assemblies get there naturally; fixed assemblies can too, with added partitions and cost.

Specify "Form 4" or "Form 3b plus busbar separation". "Reliably separated" cannot be inspected against anything.

Busbar layout and short-circuit withstand

• Phase order and neutral: horizontal busbars run A/B/C front-to-back, neutral first or last, PE along the lower rear or side; mismatched phase order between drawings and cubicles is the most common site rework item. TN-S uses five conductors, and where triplen harmonics are present (large IT or VFD load) the neutral goes to 100% of the phase bar, not 50%.
• Vertical busbar: in an MNS line-up it carries the full dynamic stress during a short circuit. Support spacing is set by peak withstand, not ampacity — the same bar that passes ampacity can fail dynamically.
• Joints: joint faces must be cleaned and tinned, torqued to the bolt size (roughly 25–30 N·m for M10, 40–45 N·m for M12). Oxidised or under-torqued joints are the top cause of heating in service.
• Clearances: main-compartment clearances follow GB/T 7251.1 for the rated insulation voltage and pollution degree; impulse-rated assemblies expect at least 20 mm to earthed structure.

Icw and Ipk: what the rating really depends on

The rated short-time withstand current Icw (for example 50 kA for 1 s) is not the sum of device ratings. It is set by the weakest link: busbar section, support strength, drawer contact pressure and frame stiffness. If any one is weaker than the tested configuration, the nameplate value is fiction. The rated peak withstand Ipk is typically 1.7–2.2× Icw.

• Verification order: establish the prospective short-circuit current at the point of supply, then check the frame's type-test report for Icw/Ipk, then confirm the as-built busbar section and support span are not weaker than the tested configuration.
• The drawer-specific item: drawer plug contact pressure must not release under the electrodynamic force of a short circuit. That can only be shown by type test, so ask for the report, not a promise.
• Time matters: Icw is meaningless without duration (1 s is default; 0.4 s or 3 s must be agreed). Upstream protection must clear within the withstand time, otherwise raise the frame rating.

Temperature rise, IP and hot-climate derating

IEC 61439-1 accepts two routes: a full temperature-rise test, or verification by similarity to an already tested design. Most projects use the second, and the argument rests on identical busbar section, surface finish, ventilation and device losses — which is why the envelope and busbar work cannot be value-engineered after the test reference is fixed.

• Reference ambient: 40 °C maximum, 35 °C 24-hour average. Real plant rooms in summer exceed this, so derating against the actual site ambient is mandatory.
• Limits: 70 K for copper busbars, 30 K for accessible metal surfaces, drawer plugs and terminals per device standards. One point over limit fails the assembly.
• Derating: plan on roughly 0.5–0.8% per °C above the reference ambient, plus a penalty for IP54 and above. For tropical projects ask for the derated selection table, not the standard ampacity sheet.
• IP and altitude: IP30/IP31 suits a clean dry room, IP54/IP65 outdoors; every step up costs ventilation and rise margin — sealed IP65 needs forced air or heat pipes. Above 1,000 m, confirm clearance and creepage corrections.

Interlocks: test them, do not read about them

• No accidental closing or opening of the breaker — handle blocked outside valid positions.
• No drawer insertion or withdrawal under load — main switch ON blocks extraction.
• No earthing on a live circuit, and no closing with the earthing switch on.
• No access to live compartments — doors interlocked with the disconnector.

Drawer connected, test and isolated positions must be positive, indicated, and keep the secondary plug connected in test. One full drawer stroke at handover tells you whether the mechanism and frame tolerances are right.

What the enclosure fabricator actually controls

• Frame type: bolted 8MF profile versus fully welded frame. Profile assemblies are fast and dimensionally stable, suiting batch MNS production; welded frames are stiffer, for high-current fixed cubicles and non-standard sizes.
• Frame flatness: flatness decides whether drawers stroke smoothly. Keep cumulative error within about 1 mm/m and stroke-test every drawer before dispatch.
• Door reinforcement: un-reinforced wide doors bow, breaking gasket compression and interlock alignment. Anything over 800 mm wide needs a stiffener.
• Drawer rails: rail straightness and load class must match drawer weight; heavy drawers with large frame breakers need reinforced rails and a cycle-life check.
• Coating: indoor cubicles take phosphate plus polyester powder at 60–80 μm; coastal work takes weather-resistant powder or galvanised sheet plus powder, with 500–1,000 h salt spray.
• Joint protection: factory-assembled busbar joints must be protected through shipping and installation. A joint face knocked or oxidised on site gets cleaned and re-tinned before torquing — never wipe and bolt.
• Receiving inspection: check busbar thickness and tinning with calipers on the real cubicle rather than the sample, verify partitions against the specified form, cycle every interlock three times on the actual unit, and spot-check joint torque against the table. M10 and M12 are not "feel" fasteners.

Frequently asked questions

GGD or MNS — how do I choose?

Count feeders and think about maintenance. More than six feeders, no-outage servicing or future reconfiguration argues for MNS; few feeders, high current and a tight budget argues for GGD fixed.

Is Form 4 worth the premium?

Yes where downtime is expensive — data centres, continuous process, multi-tenant distribution. For ordinary lighting and power, Form 3 is usually enough.

What does Icw 50 kA / 1 s mean?

The assembly carries 50 kA for one second without damage that impairs use. Protection must clear the fault within that second, or the frame rating must go up.

What IP for an outdoor LV cubicle?

IP54 minimum, IP65 for direct exposure, plus canopy and filtered ventilation. Sealed IP65 needs forced or heat-pipe cooling or it exceeds rise limits.

What lead time should I plan for custom LV cubicles?

Build-to-print enclosures run 7–15 days; complete assemblies with devices and type-test documentation realistically need 25–40 days.

About Xingyuan Electric (Suzhou) Co., Ltd.

Xingyuan Electric fabricates sheet metal enclosures and electrical assemblies in Lili Town, Wujiang District, Suzhou: a 30+-person team, a 3000㎡ plant and more than 20 laser cutting, CNC bending and welding machines, with standard lead times of 7–15 days and full build-to-print support. We build GGD/MNS/GCK cubicles, drawer units and busbar sets to IEC 61439 flatness and coating requirements, and can supply tinned busbars and secondary wiring as one package. Send drawings for a quotation.