Electrical components and Altitude issue

Temperature Rise Test / Heat-Run Test for LT control panels (as per IEC 61439-1&2)

"The verification of temperature rise on a low voltage assembly is a key element in confirming it is constructed for use within its designed operating capability, as defined in the IEC 61439 standard. It finds maximum operating temperature of the system, on which the system works satisfactorily."

With a temperature-rise test, the object will be tested up to reaching the point of steady temperature condition, according to the rated current, specified by the manufacturer/customer. The examination, significant for all equipment, is considered as passed, when the high temperature, determined at different measuring stations, does not exceed the fixed level specified in the test requirements.

How temperature rise effect Switchboard operation?

Remember Ohm's Law: V=IR. This can be re-arranged to I=V/R. This states that the current (I) through a circuit is equal to the voltage across the circuit (V) divided by the resistance in the circuit (R). Since ‘I’ is inversely proportional to R, then for a fixed V, the current will decrease as the resistance increases. One of the things which increase electrical resistance is the increased motion of the molecules and free electrons of a conductor caused by heating. This is due to their random collisions with the electrons making up the current which tends to interfere with their forward progress. So, it is true that heating of the power line causes the free electrons to move faster, but in random directions. As a result, the net electron current will be diminished due to the increased frequency of random collisions.
That means, increase in temperature rise will increase the resistance of the current flow of copper conductor. See below formula

 

Temperature rise depend on the circulating electric current, the materials involved, and the contact resistances, the ambient air temperature, the air velocity and the geometry of conductors and compartment components.

Test procedure:
All circuit within the assembly shall be individually capable of carrying their rated current (sec: IEC61439-1/sec.5.3.2). However, the current carrying capacity may be influenced by adjacent circuits. So, Test shall be conducted with maximum current of each incomer and outgoer. We have chosen two combinations to match incomer value.

1. Bring all external connections of Breaker 2 meters outside the panel, by using sufficient cables or busbars as recommended in IEC 61439-1, Sec: 10.10.2.3.2, table 11 & 12.

2. Connect incomer external terminal (2 x 50 x 5 sq.mm. 2 meters, tinned copper busbars) to Test transformer. (Pls see GA attached).

3. Switch ON incomer and outgoers, which are participating in test (i.e., in test-1, Feeders IC, OG-1, 3, 5, 6). Make sure that other outgoers, which do not use in this test is in OFF condition.

4. In each outgoer, connect 2 meter extended external terminals of all phases together (make star point) as shown in figure1.

5. Inject the current of 800A to the incomer. Read the current in each phase of the extended terminals of outgoers, which is shorted.

6. If the outgoers load current is not matching with customer requirement (in our case, Values as shown in Table 1, Test-1/2). Balance the load current to requirement by adding/removing the cables.

That means, to increase the load current we can either add the cross sectional area of cable or reduce the cable length and vice-versa.
Switch OFF the transformer main supply after load balancing.

7. After balancing the load, Place the temperature sensor probes (thermocouples) in the different place where temperature may vary and rise gradually. IEC standard suggests to place thermocouple at

• a. Busbar joints
• b. Incoming and outgoing terminals of each breaker which are participating in testing. (OGs & IC)
• c. Terminal blocks of each outgoer which is participating in test.
• d. Body of each breaker which is participating in test.
• e. Extended rotary handle/external knobs of breakers (which is testing)
• f. Body of the panel.
• g. At least two ambient temperature points.

8. These probes shall be connected to a local monitoring system. And we can read the temperature at every point from here.

9. Switch ON Transformer supply which is feeding our system/panel.