Switchboards in building projects: board types and how to choose the enclosure

2. Types of boards in a building

The Schneider Electrical Installation Guide classifies low-voltage distribution switchboards by application: main low-voltage switchboard (MLVS, Main LV Switchboard, usually called the MSB in Vietnam), motor control centre (MCC), sub-distribution boards and final distribution boards. Boards for specific applications such as heating, lifts or industrial processes may be located next to the main switchboard or near the point of use. On site, boards are usually named by function as in the table below.

• Flush-mounted boards for apartments and offices: see flush-mounted distribution boards: choosing the size.
• Power factor correction panels: see reactive power compensation panels.
• Power boards: see variable frequency drives for fans and pumps and magnetic contactors and thermal overload relays.
• ATS panels and main circuit-breakers: see when to use an ACB and an ATS.
• Buildings with their own transformer substation receiving medium voltage upstream of the main switchboard: see how to choose 24 and 35 kV medium-voltage cables and medium-voltage cable accessories.

3. IP rating to IEC 60529

The IP code is a numerical classification of the protection provided by an equipment enclosure: against persons touching live parts, against ingress of solid objects and, where indicated, against ingress of water (IEC 60050, clause 426-04-02). The code consists of the letters IP and two characteristic numerals (IEC 60529). The first numeral ranges from 0 to 6, the second from 0 to 8. There is no level 7 for the first numeral, so a code such as IP77 is incorrect. A numeral that need not be specified is replaced by X, for example IPX5 or IP2X. The IP code applies to electrical equipment up to 72.5 kV (Schneider Electrical Installation Guide).

• IPX7, IPX8 (immersion) do not include IPX5, IPX6 (water jets). An enclosure that withstands both must carry a dual marking, for example IPX5/IPX7 (IEC 60529).
• Protection against access to hazardous parts also has additional letters A, B, C, D after the two numerals: B is a test finger 12 mm in diameter and 80 mm long; D is a test wire 1.0 mm in diameter and 100 mm long (IEC 60529).
• Barriers and enclosures for protection against direct contact must have a minimum degree of protection of IPXXB or IP2X, and IPXXD or IP4X on readily accessible top horizontal surfaces (QCVN 12:2014/BXD, clause 2.4.1.2).
• Where an enclosure has several mounting arrangements giving different IP ratings, the manufacturer must state the IP rating for each mounting arrangement in the instructions (IEC 60529).

4. IK impact protection rating to IEC 62262

IEC 62262 defines the IK code, which indicates the ability of an equipment enclosure to withstand mechanical impacts on all sides. The code consists of the letters IK and two numerals from 00 to 10, corresponding to the impact energy in joules (J). The AG column is the mechanical stress code in the classification of external influences of IEC 60364-5-51 (Schneider Electrical Installation Guide).

In IEC 61439, mechanical impact (clause 10.2.6) and the degree of protection of enclosures (clause 10.3) are both part of the assembly design verification list. Mechanical impact may only be verified by testing (Schneider Electrical Installation Guide, figure E34).

5. Choosing IP and IK by board location

The IP and IK ratings of the enclosure are chosen according to the external influences defined in IEC 60364-5-51: presence of solid objects (code AE), presence of water (code AD), mechanical stress and capability of persons with access (code BA). Unless national regulations state otherwise, Schneider recommends the values in the table below for indoor switchboards (Schneider Electrical Installation Guide).

• Do not install switchboards below or inside toilets, bathrooms, washing areas, laundry rooms or rooms containing chemicals (QCVN 12:2014/BXD, clause 2.1.12.8).
• In areas with high occupancy density (referred to in QCVN as KV2, KV3), switching, control and protective devices must be arranged so that only authorised persons have access. If located within walkways, they must be housed in closed boards or boxes made of non-combustible or low-combustibility material (QCVN 12:2014/BXD, clauses 2.5.3.2, 2.5.3.3).

6. Enclosure materials

IEC 61439 verifies the strength of assembly materials through the following items: resistance to corrosion (clause 10.2.2), properties of insulating materials including thermal stability and resistance to abnormal heat and fire due to internal electrical effects (10.2.3), resistance to ultraviolet (UV) radiation (10.2.4), mechanical impact (10.2.6). Resistance to corrosion may only be verified by testing, not by calculation or comparison (Schneider Electrical Installation Guide, figure E34, based on Table D.1 of Annex D of IEC 61439-1).

For small distribution boards, the Sino Vanlock enclosure catalogue (undated) shows the following material options:

• E4M flush-mounted boards: white or clear smoked cover, plastic or steel base.
• Flush-mounted boards with ABS plastic front and polycarbonate (PC) cover.
• Metal enclosures with PC protective cover, flush-mounted or surface-mounted, by number of modules (for example 28, 36, 48 modules).
• Waterproof metal enclosures, rated IP65/55 in the catalogue.
• Metal enclosures for housing MCCBs and magnetic contactors.

7. Forms of separation and functional unit types

An assembly is divided into functional units. Each unit comprises all the electrical and mechanical elements that together perform one function. An assembly has one incoming unit and one or more outgoing units. IEC 61439 specifies the forms of separation between these units, numbered from 1 to 4, with variants a or b. A higher form number includes the features of the lower numbers. The choice of form is agreed between the assembly manufacturer and the user (Schneider Electrical Installation Guide).

• For forms higher than 1, all parts in a functional unit that remain live when that unit is disconnected must be covered to at least IPXXB (IEC 61439-2:2020, abstract on webstore.iec.ch).
• Functional units come in three types: fixed (maintenance requires shutting down the whole assembly); removable (mounted on a removable base plate, with isolation on the busbar side and the outgoing side); withdrawable drawer (a faulty unit can be replaced quickly without de-energising the rest of the assembly) (Schneider Electrical Installation Guide).
• Replacing devices in the assembly with devices of another brand or range affects the temperature-rise verification and the short-circuit withstand capability. Under IEC 61439, the assembly keeps its verified performance only when devices are replaced with devices of the same brand and range with equal or better limiting characteristics (I²t, Ipk). Replacement with another brand must be verified by testing (Schneider Electrical Installation Guide).
• Selecting circuit-breakers and disconnectors for each unit: see selecting switchgear for low-voltage switchboards.
• When requesting a switchboard, state: board type, voltage, rated current, short-circuit current at the board, location, IP, IK, form, functional unit type and equipment list. For how to prepare the list, see preparing a BOQ for a fast quotation.