A low voltage switchgear Blokset specification is a parts list with a rating attached. The name belongs to a modular low voltage assembly system developed by Schneider Electric and built by partner factories under licence. It appears in tender documents because the format is standard enough to be copied from project to project. What the buyer actually signs is an assembly with a rated current, a short-circuit withstand, a separation form and a set of type-test certificates.
Read the four together and most of the arguments in a switchgear enquiry disappear. Read only the current and the arguments start at the drawing stage, when the compartments no longer fit the cables.
THE RATINGS THAT DECIDE EVERYTHING ELSE
The published range of the system sets the ceiling. An individual assembly sits somewhere below it, and the offer has to state where.
| Parameter | Published range | Why it matters on site |
|---|---|---|
| Rated current In | up to 6300 A | sets the busbar and the incoming device |
| Rated operational voltage Ue | 690 V AC | 400 V networks run well inside it |
| Rated insulation voltage Ui | 1000 V AC | clearance and creepage distances |
| Rated impulse withstand Uimp | 12 kV | overvoltage category IV |
| Short-time withstand Icw | up to 100 kA for 1 s | sets the fault level the assembly survives |
| Peak withstand Ipk | up to 176 kA | the dynamic load on the busbar |
| Degree of protection | IP20 to IP55 | dust, water and the room it sits in |
The trap is the relationship between current and fault level. A 6300 A assembly with a 100 kA withstand is a different object from a 6300 A assembly with 50 kA. The difference shows up in busbar section, support spacing and the bill of materials rather than on the front sheet. Ask for both numbers on the same line of the offer.
FORM 1 TO 4B, AND WHAT THE FORM BUYS
The separation form describes how much metal sits between live parts and the things a person can reach. Form 1 has no internal separation. Form 2b separates the busbar from the functional units. Form 3b adds separation between the functional units and the terminals. Form 4 separates the terminals as well, including the terminals of adjacent units.
What the buyer gets for the higher form is maintenance with the busbar live. A form 4 assembly lets a technician work on one feeder while the rest of the board stays energised. That is the whole point on a data centre or a hospital, where the load cannot be switched off for a cable change.
That is also where the arc flash question enters. Motor control centres built for arc flash incidents are designed around compartment barriers and pressure relief, and the form is the part of that design a buyer can actually specify. The wider question of arc flash protection starts with the form but does not end there.
FIXED OR WITHDRAWABLE, AND THE ROOM IT NEEDS
Functional units come fixed, withdrawable, or fixed and separated. A withdrawable unit can be racked out and replaced without touching the busbar, which cuts the mean time to restore a feeder. A fixed unit costs less height in the cubicle and less maintenance training.
The room is decided by three dimensions. The cubicle height is 2200 mm. The width runs from 700 mm on a U-type to 1300 mm on the widest, and the depth is 600 or 1000 mm depending on whether the frame carries one or two depths of equipment. A double busbar arrangement is what takes the assembly to 6300 A; a single copper busbar tops out around 4000 A in the same family.
Cable entry belongs in the same conversation. Top entry eats height above the cubicle, bottom entry eats trench depth, and both change the layout of the room rather than the layout of the board.
IP rating and heat travel together. A sealed IP54 enclosure keeps dust out and keeps heat in, so the temperature-rise verification has to be read against the enclosure that will actually be supplied. A board verified at IP20 and offered at IP54 with the same current runs hotter than the test says, and the derating has to be applied before the offer, not after commissioning.
THE FAULT LEVEL ARRIVES FROM UPSTREAM
Icw is not chosen from a catalogue. It comes from the impedance of the transformer feeding the board and from the length and size of the cable between them. A 2000 kVA unit at 400 V with a 6 % impedance delivers around 48 kA of prospective fault current, which leaves a 50 kA board with almost no margin. That calculation is the first page of the switchgear file.
Two habits cause trouble here. The first is to size the board from the load list, which says nothing about fault current. The second is to take the transformer's rated current as the design figure, when the fault level is several times larger and arrives in the first half cycle.
Cable impedance helps, and it is why a board at the end of a long run can carry a lower Icw than the same board beside the transformer. A supplier who asks for the cable length is doing the calculation properly, while one who quotes Icw without asking has assumed a number.
THE CERTIFICATE CHAIN BEHIND THE NAMEPLATE
An assembly to IEC 61439-1 and 61439-2 is verified by design, not by opinion. The manufacturer holds type-test or design-verification documents for the busbar system, the functional units, the enclosure and the temperature rise. The routine test at the end of the line confirms that the particular assembly matches the verified design.
The routine test is the other half of the chain. Every assembly leaves the line with an insulation resistance test, a dielectric test, a wiring check and a functional check of the interlocks, and the report travels with the board. A file with design verification but no routine test report describes a design, not the unit in the crate.
Three documents decide whether the offer is real. The first is the design verification for the busbar arrangement at the stated Icw. The second is the temperature-rise verification at the stated In, because a 6300 A assembly that was verified at 5000 A is a 5000 A assembly. The third is the internal arc classification where the project asks for it, tested to IEC 61641 rather than assumed from the form.
Monitoring sits on top of that. Temperature monitoring of low voltage assets turns a loose joint into an alarm instead of a failure, and a virtual walk-through lets a maintenance team rehearse the board before it is energised.
WHAT TO CHECK WHEN THE SPEC NAMES BLOKSET
A low voltage switchgear Blokset clause in a tender usually arrives with six blanks to fill: In, Icw, form, IP, cable entry and the type-test certificate list. Fill them in this order and the offer becomes comparable between suppliers.
Start with the fault level from the transformer, not with the load list. A 2000 kVA transformer at 400 V delivers a fault current that decides Icw before anything else does. Then set the form from the maintenance regime rather than from habit. Then set IP from the room, because IP54 in a clean switch room buys nothing but heat. Cable entry follows the building. Certificates come last and are the only part that cannot be changed after the order.
Two standards references are worth writing into the clause: IEC 61439-1 and 61439-2 for the assembly, and IEC 60529 for the IP rating. Where the project also asks for UL 508A or GB/T 7251.1, say so at the enquiry stage, because the two routes lead to different internal layouts.
WHERE THE NAME SITS IN A PROJECT
The board does not arrive alone. It sits downstream of a transformer and upstream of the distribution, and the devices at both ends set constraints that the switchgear has to accept. The 35 kV oil-immersed transformer sets the fault level through its impedance, and the incoming breaker sets the discrimination margin at the top of the board.
Data centres are the clearest case for the higher form, where rugged switchgear is judged on continuity of the load rather than on the room it sits in.
That sequence is why the enquiry works better when one party holds the transformer, the breaker and the low voltage switchgear assembly. XIAOPAI designs, manufactures and integrates the power distribution equipment on its own line. The fault level, the form and the cable entry are settled against the same single line diagram instead of three separate ones.
Where a project names Blokset, the assembly is quoted against that clause list, and the offer states which type-test certificates apply to the build. That statement is the difference between a compliant board and a board that looks compliant on the day of delivery.
None of this is exotic. A low voltage assembly is a set of ratings, a separation form and a certificate, and the buyer who reads all three before signing gets a board that can be maintained with the lights on.
