What is temperature rise and how is it verified in Australian low voltage switchboards?
The temperature rise of an element or part in a switchboard is the difference between the measured value of the element or part and the ambient air temperature outside of the switchboard. This value is designated ‘K’. For example, a part installed inside an enclosure, for a specific application, has a measured value of 45 °C where the ambient air temperature outside of the enclosure has a measured value of 30 °C the difference is 15 °C which is a temperature rise of 15 K for that application.
Temperature rise verification is an important aspect in the design of low voltage switchboards, yet it is easily and often misunderstood or overlooked. High temperatures in switchboards may decrease the life of a switchboard, causing breakdown or fatigue in electrical componentry and insulation or potentially burns and injury to operators.
Switchboards are fitted with a variety of different electrical components each with different temperature characteristics. Components such as switches, circuit breakers, contactors and fuse combination units may have been temperature rise tested by the original equipment manufacturer in accordance with the AS/NZS 60947 series. These standards allow for equipment to be tested in ‘free air’ conditions if the equipment does not have an individual enclosure. This makes it more important for switchboard manufacturers to verify temperature rise inside a particular assembly type.
Table 9.2 from AS/NZS 61439.1 specifies temperature rise limits for different parts of assemblies. The switchboard and internal components shall be able to carry their rated currents without exceeding these limits. Verification of the limits is required by one or more of the following methods:
a) Testing;
b) Derivation (from a tested design) of ratings for similar variants;
c) Calculation for single compartments not exceeding 630A or for assemblies not exceeding 1600A.
The limits from Table 9.2 are defined for normal service conditions which is where the ambient air does not exceed +40 °C and its average over a period of 24 h does not exceed +35 °C. The lower limit of the ambient air temperature is - 5 °C. Outside of these conditions are deemed to be special service conditions and an agreement shall be made between the switchboard manufacturer and the user.
Some examples of defined limits for temperature rise (K) are as follows.
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Where testing is performed on a switchboard with multiple variants, the most onerous arrangement is selected for the test. The results can be used to establish ratings of similar variants through derivation. The method of test selected is considering one of the following:
1. Individual functional units, busbars and the assembly collectively
2. Individual functional units separately and the complete assembly including busbars
3. Individual functional units and the busbars separately as well as the complete assembly
Once the method has been selected, the assembly is mounted as in normal use with all cover in place and doors closed and loaded according to the rating being tested. Thermocouples are positioned to measure all points where a temperature rise limit from Table 9.2 must be observed. The ambient temperature during the test shall be between +10 °C and +40 °C.
Verification through derivation for similar variants is acceptable when functional units is of the same construction, belong to the same group as the tested unit, have the same or increased dimensions and cooling or reduced separation and power losses.
For Verification through calculation switchboard manufacturers can refer to IEC 60890 which provides a method for determining the temperature inside an enclosure. IEC 60890 can be used for single or multiple compartments with the total supply current not exceeding 1600A. For single compartments not exceeding 630A, verification can be made with consideration of all power losses internal to the compartments. For both calculation methods, the rated current of the circuits cannot exceed 80% of the rated current of the switching devices.
It should be noted that device substitution between manufacturers is acceptable under the derivation method however this cannot be performed for ratings above 3150A.
At Mayfield we are industry experts in low voltage switchboards. We can offer a large selection of motor starting and distribution arrangements, temperature rise verified through testing.
Please contact us if you would like more information on temperature rise or our Moducell and Okken low voltage switchboards offers. We pride ourselves on the ongoing development of our switchboards which supports the requirements of our clients.