Introduction
Heat is a critical factor in the failure of electronic components within switchboards, with a big impact on their longevity and reliability.
When the temperature inside a Switchboard exceeds the maximum permitted operating temperature by 10 Kelvin, the service life of these components is effectively halved, while their failure rate doubles.
💨 If natural cooling isn't enough, consider forced cooling with fans and filters. #CoolingTechnology #Innovation #Sparkycalc
Addressing heat dissipation in switchboards depends on the design and installed accessories.
Switchboards equipped with cooling accessories, such as filter fans and cooling units, can efficiently manage heat losses through active dissipation, protecting the components.
In comparison, switchboards with high IP ratings or without cooling accessories rely on passive heat dissipation through the walls, which may be less efficient in maintaining the right temperatures for electronic components.
This difference shows how important it is to control temperature correctly when designing and maintaining switchboards, so that the electronic parts last longer and work well.
In switchboards, heat is primarily transferred by thermal conduction and convection, with radiation playing a lesser role.
In this blog post, we will explore how heat is managed in switchboards and how to appropriately size cooling and heating equipment.
The calculation methods and formulas used in this article are taken from ‘IEC TR 60890: A Method of Temperature-Rise Verification of Low-Voltage Switchgear and Controlgear Assemblies by Calculation.’
Please refer to this standard for further reading. Additionally, it’s worth noting that our website offers a free calculator that performs all the calculations covered in this blog post.
Switchboard Natural Cooling
The first method that we will look at is the natural cooling method, where the switchboard uses its exposed total surface area to dissipate internal heat.
This method is particularly effective when there is a positive temperature difference, meaning the internal temperature of the device is higher than the external environment.
This difference in temperature drives the heat transfer, enabling the warm air inside to move out, thus cooling the internal components.
The maximum cooling output for this method is around 500W or so, depending on the size or total exposed surface area of the switchboard.
This implies that the air circulation cooling can effectively manage heat dissipation for devices generating heat up to 500 watts.
Beyond this threshold, the effectiveness of air circulation in reducing the temperature might be significantly reduced. To check if natural convection is enough to get rid of heat Qv we use the formulae;
Calculating Switchboard Surface Area
In the above equation, the total surface area of the switchboard is required, and this area depends on the installation arrangement of the switchboard and the position of the specific cubicle in question in relation to the larger switchboard suite.
The most common switchboard installation conditions are shown in the image below, and the formula for calculating the total surface areas is also given in the table below.
Once the installation arrangement has been determined and the surface area calculated, this value can then be used in the above equation.
How to determine maximum ambient temperature, Tu
To determine the maximum ambient temperature Tu, we have to make use of historical data.
For example, if the maximum ambient temperature ever reached in a particular location is 40 degrees Celsius, then we can use 40 degrees Celsius or a more conservative value as the maximum ambient temperature.
Some clients might specify the maximum ambient temperature you have to design for in their installations.
How to determine maximum internal temperature, Ti
To determine the maximum internal temperature Ti, we need to look at the equipment that will be installed inside the switchboard.
We refer to the datasheet of each piece of electronic equipment and determine, from the manufacturer’s datasheet, the maximum allowable temperature that each piece of equipment can safely be exposed to.
From those values, we consider the equipment with the lowest temperature rating; that’s the temperature Ti, that we will use as the maximum allowable switchboard internal temperature.
How to determine heat gain, Qv
To find the total heat gained inside the switchboard, we need to determine the total waste heat produced by the equipment inside the switchboard, which can be accounted for in one of two ways.
Either by adding the published power dissipation, in watts, of all the equipment in the switchboard, or by finding the total amperage draw for each electrical device, multiplying that number by the supply voltage, and then multiplying the result by one minus the rated efficiency of the equipment.
The results for each device are then added to find the total heat load for all equipment.
Calculation of heat gained due to switchboard busbars
Switchboards will most likely always contain busbars and/or panel wiring. The heat gained in switchboards due to busbars and/or panel wiring can be calculated using the following formula;
Forced Air Switchboard Cooling
If our calculations and checks show that natural cooling isn’t enough to keep the switchboard’s temperature low enough for the equipment inside, then we use fans and filters for forced cooling.
These fans move the air around inside the box, helping to spread the heat more evenly inside the switchboard and along its walls. To figure out how much air the fan needs to move, we use the following equation;
Once the volume flow rate has been calculated, an appropriate filter and fan combination should be chosen.
It is recommended to select a fan-and-filter unit with an airflow approximately 20% greater than the result of the calculation. This allows for fouling of the filter mat without reducing airflows.
Other cooling methods
The two methods we have discussed above are the most common ones in electrical LV switchboards, but there are other types of climate control methods used in temperature-sensitive applications like data centers.
We won’t cover them here, but just to broaden your knowledge, these methods include heat dissipation with air/air heat exchangers, air/water exchangers, and thermoelectric cooling.
Driving away moisture and cold in Switchboards
The longevity of electrical equipment in a switchboard can also be reduced by very low temperatures.
To protect these components, it’s important to heat the inside of the switchboard, especially to prevent moisture and frost.
To achieve this, heaters and anti-condensation heaters must be correctly sized and fitted into the switchboard. These can be sized using the following formula;
The solution to this equation will give you the size of the heater in Watts. The value of all the parameters in this equation are as previously described.
Some important notes to remember when working with heaters include: installing the heater in the floor area as close as possible to the center and placing it below the components to be protected; in cases of excessive air humidity, a hygrostat should be used to achieve accurate temperature control.
Temperature Controllers
While we can install heaters and fans to regulate temperature, we still need something to switch them on and off. That’s where electronic thermostats come in.
There are several on the market, but the most popular ones are the KTO 01140 thermostat (normally closed contact breaker for regulating heat) and the KTS 01141 thermostat (normally open contact breaker for regulating filter fans, heat exchangers, and for alarming when temperature setpoints have been exceeded).
I have used these in previous projects, and they work well. A typical wiring diagram is shown below
Rounding Up!
In conclusion, understanding and managing the heat load in switchboards is vital for the safety, efficiency, and longevity of electrical systems.
We’ve explored natural and forced air cooling methods, along with other essential considerations like ambient and internal temperatures, and the sizing of heating and cooling components.
Remember, the right approach varies depending on specific circumstances and requirements. By applying the principles and formulas discussed, you can ensure optimal performance and reliability of your switchboard installations.
We encourage you to keep exploring and stay informed about the latest in electrical engineering solutions. Please feel free to browse through our blog posts to learn more and make use of our free calculators