Merits of type testing

Be it domestic, commercial or industrial applications, safe reliable power distribution is more than ever a critical requirement for modern-day living. Electrical faults and power disruption through faulty equipment, poor design, installation or lack of maintenance services can be catastrophic on life, property and enterprises. In this article, David Clark discusses the merits of type-testing switchboards.

A well designed, implemented and maintained electrical system will provide years of safe reliable service. Various national and international standards are applied to products to ensure they are safe and perform reliably and consistently for the intended purpose. Low-voltage switchboards built and operated in Australia and New Zealand are subject to AS/NZS 3439.1: 2002 standard. Applying the tests nominated in the standard to the switchboards is to verify that the design complies with the standards relevant to the application. While the AS/NZS 3439.4:2002 standard details requirements for enclosures used for construction sites, another standard applies to busbar ducting systems and there are other standards for other applications. Switchgear used in the switchboard or a motor control centre assembly must apply to the relevant standards pertaining to that type of equipment.

The standard allows assembly manufacturers scope for innovative design to meet clients’ requirements. The phrase “by agreement between manufacturer and user” is used throughout the standard, covering elements such as Form of Separation (covered later). Designers and specifiers need to have a thorough understanding of the standard and its association to the other standards and codes that apply across their project.

Modern switchboard and switchgear designs are allowing for a smaller footprint, better operator protection with faster coordinated fault isolation and protection times, with optional access to a range of communication protocols for remote data acquisition, monitoring, control and security.

Switchgear enclosures can be offered as a type-tested assembly (TTA) or partially type-tested assembly (PTTA). TTA is offered as conforming to the standard without deviation that would significantly affect performance. The PTTA is a combination of type-tested and non-type-tested arrangements, provided that the non-type-tested assemblies elements are derived, by calculation, from type-tested arrangements that comply with the tests. As a proof of calculation, original documents for type-tested assembly design should be available for verification.^1,3

Below are the type tests that are required for TTAs and PTTAs

• Temperature rise
• Dielectric properties
• Short-circuit withstand strength
• Effectiveness of the protective circuit
• Clearances and creepage distances
• Mechanical operation
• Degree of protection

The type tests will prove design capability - testing is undertaken when the product is being developed or being modified. The costs associated with type testing, ie, to produce the design and test sample of product, would usually be undertaken by the manufacturer. Testing can be carried out at various laboratories around the world. Test results are recorded and the manufacturer is issued a report and certificate that can be presented to the clients for inspection.^1,3

Temperature rise testing

Temperature rise testing is performed to provide assurance on the assembly of its operating current rating. According to the standard, the switchgear must not exceed the temperature rise (K) limits, based on an ambient temperature of up to 35°C.

A temperature limit of 70° rise applies for terminals for external insulated conductors. Various elements, such as switchgear arrangement, ventilation systems and components, affect the design and test results. Variations in switchboard arrangements and components should be retested for compliance to the standard.^1,3

Dielectric type testing

Dielectric type testing confirms the level of voltage the assembly will withstand, for the main and auxiliary circuits. Main circuit of an assembly relates to the conductive parts included in a circuit that is intended to transmit electricity. The auxiliary circuit of an assembly includes all the conductive parts, other than the main circuit, intended to control, measure, signal, process data, etc. This testing is not required on parts that are type tested to the relevant parts standards for dielectric strength, and when through the installation process the unit’s compliance to the standard has been altered.^1,3

Short-circuit testing

Short-circuit testing is required to be conducted on all elements of an assembly. A short-circuit withstand test is required to verify the capability of the complete assembly, including busbars, all interconnections and, where appropriate, mounting arrangements. This is covered by short-circuit tests on the incoming circuit(s) and busbar system and through fault tests on the outgoing circuits. Main switchboards are frequently installed adjacent or close to the main incoming supply point and therefore potentially high fault currents could occur in the event of a failure. Such applications may call for, eg, 50 kA for 3 seconds or 80 kA for 1 second. The dynamic and thermal stresses which such faults subject to an assembly can only really be verified through adequate testing.

The magnetic effects on busbars, support systems and enclosures during such high fault will impose oscillating forces measured in tonnes. The thermal effects on the copper busbars themselves may reach a level which would anneal the copper, thereby rendering them unfit for further service.

In the case of insulated busbars, thermal characteristics must be taken into account.^1,3

Protective circuit

The protective circuit with an assembly serves two functions: to ensure all exposed conductive parts are effectively bonded to the main earth terminal, providing personal protection; and provide a safe earth return for earth faults downstream.

Doors and other exposed conductive parts which don’t have electrical equipment attached to them do not need additional bonding above their normal fixing means. For doors, panels, etc, with equipment attached rated above extra low voltage, it is recommended a protective conductor be attached.^1,3

Clearance and creepage distances

Clearance and creepage distances for devices within an assembly are determined in accordance with their own product standards. Minimum distances for other parts within the assembly are established from understanding the environment the assembly is intended to be used in and the insulating materials being used. Elements that need to be considered in calculating clearance and creepage distances include the pollution level of the environment, impulse withstand, insulating materials and electric field within an assembly.

For clearances and creepage distances, four levels of pollution are used to indicate the environment in the assembly.

Pollution degree 1: No pollution or only dry, non-conductive pollution occurs.

Pollution degree 2: Normally, only non-conductive pollution occurs. Occasionally, however, a temporary conductivity caused by condensation may be expected.

Pollution degree 3: Conductive pollution occurs or dry, non-conductive pollution occurs, which becomes conductive due to condensation.

Pollution degree 4: The pollution generates persistent conductivity caused, for instance, by conductive dust or by rain or snow.

The standard pollution degree for industrial applications generally used is pollution degree 3.

Minimum creepage distances can readily be determined from the standard and confirmed by measurement knowing the rated insulation voltage of the assembly, the pollution degree applicable to the environment it is intended to be installed in and the material group into which the insulating materials belong. Ribs, grooves, steps, etc, can be incorporated into a design as per reference to Annex F of the standard.^1,3

Mechanical operation

For mechanical operation tests, equipment incorporated in the assembly that can be proven to comply with its own respective standard requires no further testing. For other devices specifically designed for the assembly, a minimum test of 50 mechanical operations should be conducted by the assembly manufacturer and the mechanical efficiency should be maintained at the end of the test. Generally the assembly shall be constructed only of materials capable of withstanding the mechanical, electrical and thermal stresses as well as the effects of humidity which are likely to be encountered in normal service. Protection against corrosion shall be ensured by the use of suitable materials or by the application of equivalent protective coatings to the exposed surface, taking account of the intended conditions of use and maintenance.

All enclosures or partitions including locking means for doors, withdrawable parts etc, shall be of a mechanical strength sufficient to withstand the stresses which they may be subjected to in normal service.^1,3

The equipment and circuits in the assembly shall be arranged to allow for their operation and maintenance, and at the same time ensure the necessary degree of safety.^1,3

Degree of protection

The degree of protection provided by any assembly against contact with live parts, and ingress of solid foreign bodies and liquid is indicated by the designation IP rating, in accordance with AS 1939.

The general interpretation of the IP number is in terms of the external protection of an assembly. Any dust or moisture that is permitted to enter the enclosure shall have no harmful effect and, in normal service, personnel shall not be able to touch any dangerously live parts.

In addition to determining the external protection, the IP code is also used to define the internal degree of protection under conditions necessitating the accessibility to internal parts by authorised personnel or for assemblies with movable or withdrawable parts. The degree of protection of an enclosed assembly shall be at least IP2X after installation, in accordance with the manufacturer’s instructions. Internal barriers and shields may be required to provide adequate touch protection.^1,3

Routine tests are intended to detect faults in materials and workmanship. They are carried out on every assembly after its assembly or on each transport unit. Another routine test at the place of installation is not required.

Hence, routine tests on assemblies are undertaken at the manufacturer’s premises. These tests would usually form part of the manufacturer’s quality assurance program.^1,3

Tests include: wiring and electrical operation, dielectric test, checking protective measures and insulation resistance.^1,3

Forms of internal separation are subject to agreement between the manufacturer and user.

The internal separation is not intended to guarantee the integrity of the assembly in the event of an arcing fault. An assembly is separated to facilitate access to a part of the assembly while other parts may remain energised and in service. Where it is impractical to totally isolate an assembly prior to carrying out work within an assembly, the degree of separation, and the way in which the separation is achieved within the assembly, should be considered in a risk assessment undertaken by the client.

Separation does not generally improve the electrical performance of the assembly but it provides protection against contact with live parts belonging to the adjacent functional units and against foreign bodies from one unit of an assembly to an adjacent unit.

Within an assembly it can be acceptable and even advantageous to use more than one form of separation. For example, within a Form 4 assembly it may be acceptable to have an MCB distribution board that may be regarded as one functional unit. Alternatively, the Form 4 assembly may include several circuits of different Form 4 types with different cable termination arrangements.^1,3

Forms of separation:

Form 1: No internal separation is provided.

Form 2: Functional unit separate from the busbars

Form 3: Functional units separate from other functional units.The ‘a’ designation denotes terminals are not separate from the busbar. The ‘b’ designation denotes terminals for external conductors are in a separate compartment to the functional unit.

Form 4: Terminals for external conductors separate each other. The ‘a’ designation denotes terminals within the functional unit and the ‘b’ designation denotes terminals for external conductors are in a separate compartment to the functional unit.^1,2

Please note that this is an overview of some of the elements of switchboard testing requirements and is not offered as a technical guide to cover all requirements that go into a well-designed switchboard or motor control centre.

Any design requirements should be discussed with switchgear engineers, such as NOJA Power, who are able to design and manufacture a compliant assembly to suit your specific requirements.

References

1. AS/NZS 3439.1:2002
2. Beama Guide to Forms of Separation; July 2011
3. Beama Guide to TTTA & PTTA