Electrical switchboards — design basics

A switchboard is an important asset in a power distribution network and provides the base for circuit, equipment and user protection.

The Q & A below explains the basis of switchboard design.

Q: What is the Australian standard for main switchboards?

The current Australian standard for main switchboards is AS/NZS 3439.1: 2002, which is based on IEC 60439. Once published, the new series AS/NZS 61439 standard will supersede the current standards five years from the initial publication. However, when the new standard comes into effect, the specifiers and end customers in Australia may request the board builders manufacture the switchboards in accordance with the new standard. 

Q: What are the type tests that need to be carried out to verify standard compliance?

• Verification of temperature-rise limits
• Verification of the dielectric properties
• Verification of the short-circuit withstand strength
• Verification of the effectiveness of the protective circuit
• Verification of clearances and creepage distances
• Verification of mechanical operation
• Verification of the degree of protection and internal separation

Q: Can you claim type tested assembly if the switchboard construction is different to the tested arrangement or modifications are made to suit different applications?

No. If modifications are made to the components of the switchboard, new type tests have to be carried out, but only in so far as such modifications are likely to adversely affect the results of the seven type tests listed above.

Q: What is a PTTA?

A partially type tested assembly (PTTA) is a low-voltage switchgear and control gear assembly, containing both type tested and non-type tested arrangements. In this construction, non-type tested arrangements need to be derived (eg, by calculation etc) from type tested arrangements. This switchboard construction is mostly used as it is not possible to cover all the possible configurations in your type tested arrangements and therefore customised switchboard arrangements derived from type tested arrangements are more practical in real-world applications.

Q: Do you need to perform any other tests even if you have type test certificates?

Yes. Every switchboard needs to be routine tested (four tests) by the manufacturer. Routine tests are intended to detect faults in materials and workmanship as follows:

1. Inspection of the switchboard including inspection of wiring and, if necessary, electrical operation test.
2. Dielectric test.
3. Checking of protective measures and of the electrical continuity of the protective circuits.
4. Verification of insulation resistance.

Q: How do you calculate the distribution busbar size of 10 circuits in the absence of actual currents of those circuits?

In the absence of actual currents, rated diversity factor is used to calculate the minimum busbar sizing. Below conventional values are used as per table 1 of AS/NZS 3439.1:2002.

(eg, If 10 x 100 A MCCBs are fitted in a distribution chassis the diversity factor is 0.6 allowing a minimum busbar size of 600 A.)

Q: What is the minimum clearance distance in a low-voltage installation?

Table 14 of AS/NZS 3439.1:2002 is referred in obtaining this information and if you consider a maximum of 12 kV rated impulse withstand voltage with pollution degree of 4 (worst case), the clearance distance should be more than 14 mm between phases and neutral/earth.

Q: What is the minimum creepage distance in a low-voltage installation?

If pollution degree is 1 or 2 (normally non-conductive pollution occurs; occasionally, however, a temporary conductivity based on condensation may occur), the creepage distance should not be less than the associated clearance distance. This leaves the creepage distance at 14 mm.

Q: What is the minimum IP rating allowed for indoor switchboards?

IP2X, considering there is no need for protection against ingress of water.

Q: What is the minimum IP rating allowed for outdoor switchboards?

IP23. For assemblies for outdoor use having no supplementary protection (protective roofing of the like), the second characteristic numeral shall be at least 3.

Q: Are higher IP-rated (eg, IP66) enclosures better for switchboards?

This is not necessarily true unless the switchboard is installed in a location where there could be more damage due to water or presence of dust and gases that could increase the pollution degree inside a switchboard.

Switchboards need to be properly ventilated to enable the switchgear to operate within its tested conditions (normally 35–40°C maximum temperature) and the heat generated during the operation (through watt loss of conductors and switchgear) should have passage to escape. If the switchboards are constructed with higher IP rating, the switchgear will have to be derated and the conductors shall be upsized to make the switchboard run cooler. Also, you need to consider pressure release valves to enable the release of pressure build-up during an arc.

Q: Apart from a properly enclosed switchboard, what is another important environment factor to consider for outdoor switchboards?

Where the switchboards are intended to be installed in a location with high humidity and temperature varying within wider limits, suitable arrangements (ventilation and/or internal heating, drain holes, etc) shall be made to prevent harmful condensation within the switchboards.

Q: What are the temperature rise limits for components inside a switchboard?

Temperature rise limits given in this standard or calculated apply for mean ambient air temperature less than or equal to 35°C. (Therefore, this does not apply for outdoor switchboards where the ambient temperature reach above 35°C and the effect of solar irradiation on the switchboard operating temperature is unknown.)

• Switchgear and controlgear:

1. Temperature rise should be as per the manufacturer’s recommendations.
2. Normally switchgear is calibrated at 35–40°C operating temperature and higher operating temperature would mean derating the circuit breakers as per the manufacturer data. Also, you may need to consider the maximum operating conditions in order to limit the temperature rise within the assembly.

• Terminals for external conductors

1. 70k rise is based on conventional tests

• Busbars and conductors

1. Generally considered to be compliant if the temperature rises do not exceed 70k for H.C copper busbars and 55k for H.C aluminium busbars. This is based on 105°C and 90°C maximum temperature within an assembly.

Q: What is the maximum length of unprotected cable that can be installed in a switchboard where a circuit breaker is fitted at the end of it?

Three metres. There are conditions that need to be met with this and are available in table 5 of AS/NZS 3439.1:2002 (eg, single insulated cables with operating temperature above 90°C where no applied external pressure on them or sheathed, double insulation cable).

Q: What is the recommended minimum height from ground/platform for the terminals of a circuit breaker?

200 mm as per section 7.6 of AS/NZS 3439.1.

Q: What is the maximum height for the operating devices such as handles, push-buttons etc from ground/floor?

For floor-mounted switchboards, indicating instruments which need to be read by the operator should not be located higher than 2 m above the base of the switchboards. Applicable for operating devices, such as handles, push-buttons, etc.

Q: Will internal separation guarantee the integrity of the assembly (switchboard) in the event of an arcing fault?

No, this will only limit the probability of an arcing fault and extra measure to be taken as per section 2.5.5 of AS/NZS 3000:2007 and Annex ZC of AS/NZS 3439.1.

How is internal separation achieved?

There are two methods of achieving this:

• Standard construction

1. By barriers or partitions (these can be metallic or non-metallic)
2. Form 1, 2a, 2b, 3a, 3b, 4a, 4b   

• Alternative construction

1. By using integral housing (IP2x) of functional unit denoted by ‘h’. Applicable forms are 3ah, 3bh, 4ah and 4bh
2. Insulation of busbars or shrouds denoted by ‘I’. Applicable forms are 2bi, 3bi and 4bi
3. The combination of housing (h) and insulation (i). Applicable forms are 3bih, 4aih and 4bih

Q: Why are there different methods of internal separation of switchboards and who decides which one to use?

Internal separation is agreed on by the manufacturer and the user as per section 7.7 of AS/NZS 3439.1.

Following points are considered in determining the appropriate internal separation by the user, or in absence of specification by the switchboard manufacturer.

• Is the rated current of the switchboard less than 800 A? If YES, you are free to select any form of separation (eg, Form 1, 2a, 2b, 3a, 3b, 4a, 4b, etc.)
• Is the rated current of the switchboard more than 800 A? If YES, you are restricted to use one of the prescribed forms of separations as per section 2.5.5.2 of AS3000:2007 to reduce the probability of initiating an arcing fault (eg, Form 3b, 3bi, 3bih, 4a, 4ah, 4aih, 4b, 4bi, 4bh and 4bih.)
• Can the assembly be isolated elsewhere before removing covers, etc? (If the answer is YES, you may decide to go with the lowest form of separation [form 1] without adding pressure on the switchboard price.)
• Do you require additional integrity by having separation between functional units and busbars where the access of busbars in live conditions results in risk of contact with live busbars? (If the answer is YES, you will need to go with minimum form 2 construction.)
• Do you require access to functional units (eg, circuit breakers) for limited maintenance (eg, changing the settings) with adjacent circuits live? (If the answer is YES, you will need to go with minimum form 3 construction providing also separation between functional units.)
• Do you require access to cable terminals (eg, connecting cables to a spare circuit breaker in future while the switchboard is live) of a functional unit with adjacent functional units live? (If the answer is YES, you will need to go with the maximum form of separation — form 4.)

Q: What measures need to be taken for increasing security against the occurrence or the effect of internal arcing faults?

• By insulation of all live conductors.
• By the arrangement of busbars and functional units in a switchboard where there are vented compartments designed to promote rapid extinction of the arc and to prevent the arc or arc products affecting other parts of the switchboard.
• By the use of devices (eg, fuses or circuit breakers) designed to limit the magnitude and duration of the arcing current by interruption of the fault.
• Combinations of above items, or other methods designed to either prevent the initiation of an arc, or to reduce the damage or risk of injury resulting from an arc, by sensing of the fault followed by interruption.

Q: Can a successful internal arcing fault test guarantee to withstand all arcing faults that may occur in service?

No. It is not possible to simulate all the conditions that can produce arcing faults in service and that the arc does not always behave in a repeatable manner.

Reference: AS/NZS 3439.1:2002 and AS/NZS 3000:2007.

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