Temperature Rise in Electrical panel boards

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

1. 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
2. There is an approximately even distribution of power losses inside the
3. The equipment installed is designed for ac ( 50 or 60 Hz ) and dc current up to 3150
4. The influence of the enclosure material and wall thickness is
5. Steady state conditions are assumed here.
6. Eddy current losses are
7. There are no more than three horizontal partitions in the PTTA or a section of it.
8. This calculation applies only for
9. 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.

1. 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

1. 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.

1. 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

1. 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

–     For enclosures with ventilation openings and

1. 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.

–     For enclosures with ventilation openings

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

.

1. 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.

1. Determine the exponent for the effective power loss

The exponent for the effective power loss is expressed as follows.

–     For enclosures with ventilation openings

1. 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

1. 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

1. The effective cooling surface (Ae) of the enclosure

1. The enclosure constant (k)

1. The temperature rise factor for internal horizontal partitions inside enclosure (d)

1. The temperature distribution factor (c)

1. The Effective power loss of equipment installed inside enclosure

1. The exponent for the effective power loss

1. The Internal temperature rise of enclosure

1. 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