METHOD TO ASSESS THE TEMPERATURE RISE INSIDE A LOW VOLTAGE PANEL BOARD
INTRODUCTION
In order to assess the temperature rise of the air inside the low voltage panel board, a method is proposed in IEC 60890 ( A method of temperature-rise assessment by extrapolation for partially type-tested assemblies ( PTTA ) of low-voltage switch gear and control gear ).
ASSUMPTIONS AND CONDITIONS
- Air temperature within the enclosure is equal to the ambient air temperature outside the enclosure plus the temperature rise of the air inside the enclosure caused by the power losses of the installed
- There is an approximately even distribution of power losses inside the
- The equipment installed is designed for ac ( 50 or 60 Hz ) and dc current up to 3150
- The influence of the enclosure material and wall thickness is
- Steady state conditions are assumed here.
- Eddy current losses are
- There are no more than three horizontal partitions in the PTTA or a section of it.
- This calculation applies only for
- For enclosure with ventilation openings, the cross-section of the air outlet openings is at least
1.1 times the cross-section of the air inlet openings.
- For enclosures having more than one vertical section, this calculation is done
CALCULATION
Necessary information
The following data are needed to calculate the temperature rise of the air inside an enclosure.
- Dimensions of the enclosure (Height, Width and Depth)
- The type of installation of the enclosure (As per graph)
- Design of enclosure (with or without ventilation openings)
- Number of internal horizontal partitions
- Effective power loss of equipment installed in the enclosure (For equipment, rated power losses provided by manufactures are For conductors, power losses are calculated)
Procedure
- Determine the effective cooling surface (Ae) of the
The effective cooling surface (Ae) of the enclosure is the sum of the individual surfaces (Ao) multiplied by the surface factor (b).
The surface factor (b) is determined by the below table.
| Type of Installation | Surface factor b |
| Exposed top surface | 1.4 |
| Covered top surface (ex – Built in enclosures ) | 0.7 |
| Exposed side faces ( ex – front, rear and side walls ) | 0.9 |
| Covered side faces ( ex – rear side of wall mounted enclosure ) | 0.5 |
| Side faces of central enclosures | 0.5 |
| Floor surfaces | N / A |
- Determine the enclosure constant (k)
The enclosure constant (k) allows for the size of the effective cooling surface for enclosures without ventilation openings and, in addition, for the cross section of the air inlet openings for enclosures with ventilating openings. We can find this by using following graphs.
Fig 01. Enclosure Constant (k) for enclosures without ventilation openings, with an effective cooling surface .
Fig 02. Enclosure Constant (k) for enclosures with ventilation openings and an effective cooling surface
Fig 03. Enclosure Constant (k) for enclosures without ventilation openings, with an effective cooling surface
- Determine the temperature rise factor for internal horizontal partitions inside enclosure (d)
The temperature rise factor for internal horizontal partitions inside enclosure (d) allows for the dependence of the temperature rise on the number of internal horizontal partitions. We can find this by using following tables.
– For enclosures without ventilation openings and
| Number of Horizontal Partitions ( n ) | 0 | 1 | 2 | 3 |
| d | 1.00 | 1.05 | 1.15 | 1.30 |
– For enclosures with ventilation openings and
| Number of Horizontal Partitions ( n ) | 0 | 1 | 2 | 3 |
| d | 1.00 | 1.05 | 1.10 | 1.15 |
- Determine the temperature distribution factor (c)
The temperature distribution factor (c) allows for the temperature distribution inside an enclosure. Its determination varies with the design and installation of the assembly as follows.
| Effective cooling surface / m2 | Enclosure factor for finding C |
| height / base factor
|
|
| height / width factor |
– For enclosures with ventilation openings
| Effective cooling surface / m2 | Enclosure factor for finding C |
| height / base factor
|
Here;
- h – Height of the enclosure in m
- w – Width of the enclosure in M
- Ab– Surface area of the enclosure base in m2
Fig 04. Temperature distribution factor (c) for enclosures without ventilation openings, with an effective cooling surface .
Fig 05. Temperature distribution factor (c) for enclosures with ventilation openings, with an effective cooling surface .
Fig 06. Temperature distribution factor (c) for enclosures without ventilation openings, with an effective cooling surface
.
- Determine the Effective power loss of equipment installed inside
The Effective power loss of equipment installed inside enclosure can find by adding the power losses of the each individual items in the panel board.
- Calculating the power loss of the switchgears and controlgears
It is specified by the manufacturer at the rated currents. The summation of those values will be the power loss of the switchgears and controlgears.
- Calculating the power loss of the bus bars
I2R losses of the bus bar are considered here. Resistivity of the relevant bus bars will be specified by the manufacturer.
- Determine the exponent for the effective power loss
The exponent for the effective power loss is expressed as follows.
| Effective cooling surface / m2 | Exponent |
| 0.804 | |
| 0.804 |
– For enclosures with ventilation openings
| Effective cooling surface / m2 | Exponent |
| 0.715 |
- Determine the Internal temperature rise of enclosure
The Internal temperature rise of enclosure can be calculated by using following equations. Temperature rise at mid height of enclosure
Temperature rise at top of enclosure
- Obtain the characteristic curve for temperature rise of air inside enclosure
By considering the temperature in horizontal level are equal, a characteristic curve is obtained by considering the temperature rise as a function of the enclosure height. The temperature rise at the bottom of the enclosure is close to zero. Following graphs can be obtained.
Fig 07. Temperature rise characteristic for enclosures with
.
Sample Calculation
Data
In order to find the temperature variation inside an electrical panel board, a case study was taken and it was designed with form 01, 2b and 3b architecture.
Fig 08. Proposed distribution system for the electrical panel board
Fig 09. Form 01 and 2b arrangements of the electrical panel board
Calculation
- The effective cooling surface (Ae) of the enclosure
| Form 01 | Form 2b | |
| Ae / m2 | 3.3615 | 3.3615 |
- The enclosure constant (k)
| Form 01 | Form 2b | |
| Factor (k) | 0.2 | 0.2 |
- The temperature rise factor for internal horizontal partitions inside enclosure (d)
| Form 01 | Form 2b | |
| Factor (d) | 1 | 1.15 |
- The temperature distribution factor (c)
| Form 01 | Form 2b | |
| Factor (c) | 1.59 | 1.59 |
- The Effective power loss of equipment installed inside enclosure
| Equipment | Power Loss / W |
| 250 A 4P 36kA MCCB | 71.2 |
| 160 A 3P 16kA MCCB | 45.0 |
| 100 A 3P 16kA MCCB | 21.0 |
| 63 A 3P 16kA MCCB | 12.9 |
| Bus bar – 20 x 4 mm | 21.25 |
| Total | 171.35 |
- The exponent for the effective power loss
| Form 01 | Form 2b | |
| Factor (x) | 0.804 | 0.804 |
- The Internal temperature rise of enclosure
| Form 01 | Form 2b | |
| 12.50 | 14.38 | |
| 19.88 | 22.86 |
- The characteristic curve for temperature rise of air inside enclosure
Fig 10. Temperature rise characterisitcs for form 01 assembly
Fig 11. Temperature rise characterisitcs for form 2b assembly