Fuse - a circuit protection element against short circuit, the device operating principle based on overheating and melting the full heat of the electric current of a thin paste special wire. The process is irreversible, after the operation you need to buy a new product to the specified nominal value used for the project.
Justification ability to select the fuse
Most people think, the thicker the wire in the fuse, the better. And make the "bugs". Wrong calculation becomes easy to cause a fire, heated, not only fuse, but other conductors in the circuit. If you take too thin hair, circuit resistance to the electric current will be significant, and the purpose of his assembly will not stand.
As a result, people are made unreasonable costs, security is broken. About the choice of circuit breakers good says Alexei Zemskov, and the material presented here will help you learn more about the fuse.
Alexei tips are not useless. It is recommended to transfer to see the estimated current values for specific premises lay in design house wiring and fuses to choose, avoiding known not to cause damage.
Requirements for fuses described in ISO 17242. There rows permissible current values is given, not beyond the capacity of holders (for these values are indicated simultaneously).
The protective element
General guidelines for choosing the fuse
Mounting home fuse system is logical to start with the selection of the installation location. Under the standard plate used DIN-rail. Logically holders mounted to the same. Number of fuse is selected similarly to the number of machines (as recommended by A. Zemskov):
- According to the machine outlet in every room.
- According to the machine room lighting.
- One machine on the balcony. It is advisable to take the differential, if pleases pull the extension cable down and something to saw grinder.
- For wet areas are placed differential machines. The amount of the request, the total rating according to the needs.
The electrical equipment is better to be safe. Assume kitchen not all consider placing wet, but being able to take the hand of the housing (e.g., an electric plate), and the second - for steel kitchen sink... there is believed to put it without hesitation differential automatic, in case of emergency, immediately unplug branch. Similar concerns refrigerator adjacent to a steel radiator. Requirements are particularly stringent in the presence of young children in the apartment (for obvious reasons).
As becomes clear from the above, machines sometimes possible to replace the fuses. The recipe is not suitable:
- bathroom and dressing room, a bathroom;
Those interested can read on their own on the subject of the SAE (best sixth version), plus the GOST R 50571.11. Upset we hasten to reassure machines today is not too expensive. Automatic protection against excessive rise of instant and current monitors long-term overload, causing overheating of wires and fissuring of the insulation. For more details read the section Electric Machine breaker.
In addition to the buildings under the DIN-rail holder fuse-links are performed in other formats. Pattern fuse is selected according to the set (or sets) the holder of the desired denomination.
Conductor - any physical body, through which a current can be passed, is not always an ordinary wire or cable. In DC transmission occurs on a channel, angled profiles and steel rails. As a result of accidents, under the current turn any metal parts, cables, screens and even the soil (see. step voltage).
Heating conductor temperature improves the product. The process follows the law Joule. Moreover, the heat quantity is determined entirely by the square of the current and the resistance portion. Although there are other records, it is not recommended to use them to calculate the fuse.
Exceeding the heating conductor is called the temperature difference between the conductor and the environment. The value indicates how fast will begin to give off heat to the environment through the insulation.
Install heat (excess heat) is called the mode, when a further increase in temperature occurs so slowly that the time factor can be neglected.
Current current gradually heats the wire. Energy is used to increase the temperature, dangerous case when the value exceeds the operating parameters of the wire. Each of the cable insulation, as a rule, rubber or polymer. Both materials are relatively easy to break down, crack, crack, melt if overheated. Further operation of the wire becomes dangerous, therefore, inappropriate. Steady heating temperature depends on two factors:
- The current in the circuit.
- The intensity of the exchange with the environment.
The first value enables the available data to determine the amount of cable liberated heat per unit time:
- By law Joule electrical power current defined as the current square (rms) multiplied by the resistance: I x I x R.
- For the amount of power multiplied by time of energy. Further, knowing the heat capacity of the wire, it is possible to calculate the temperature to which it will heat up in a specific period.
It is clear that the process lasts forever can not. But do not necessarily have any negative consequences. To assess the possibility of damage, you need to know the ambient temperature and insulation resistance to heat transfer cable. For these parameters is calculated intensity of the cooling zone due to energy transfer air, walls, etc. If the cable is in the channel, the calculation is carried out for the two resistances:
- Between the insulation and the air channel.
- Between the channel walls and the room.
The sample is calculated by the operating conditions of any product. easy to check mathematical solution experimentally. Full algorithm for calculating the exchange with the environment:
- Heat losses through any obstacle calculated by a simple formula. Necessary area of the material multiplied by the temperature difference (on both sides), divide that number by the resistance to heat transfer material.
- The area of the conductor insulation is of a simple formula which is known from the geometry. Requires circumference (2 pi R) multiplied with the length of the calculated area. Or to perform calculations for linear meter.
- The temperature difference is here desired value, since the calculation is carried out with the search of the thermal mode. In the future, the environment baud rate is assumed to equate to power, found the law Joule, and hence expressing the difference in temperature. State environment assumed to be known, therefore, are simple addition of a finite value.
- Resistance to heat transfer material is taken from handbooks. The consultant selling plastic windows, comes to know the facts, what is the limit of losses through the PVC profile.
Take a guide ...
In the presence of a handbook on the insulating materials the task is greatly simplified. In this case, start with the construction of definitions:
- the amount of heat - energy given or accepted for a certain period of time (see. above). The building measured in watts (as in physics - in joules).
- Conductivity is the value showing how much energy (watts) passes per unit time (s) through one square meter of surface (= sq mm) overlap through unit thickness (m) at a single temperature difference (K or degree Celsius). The unit looks like: W s m / s m K = W / m K.
- Coefficient (thermal conductivity) heat transfer is calculated as a thermal conductivity divided by the thickness of the overlap. Unit: W / m K.
- Thermal resistance - the inverse of the coefficient of heat transfer material. Sometimes value can appear in the context of the insulation. Then the ratio of the values can be determined by measuring the unit above.
- The drag coefficient of heat transfer - the inverse of the coefficient of heat transfer.
- Heat transfer coefficient indicates how much power (watts) per second per square meter exchanges (Square meter) of the surface temperature difference between the surface material 1 and the air in K (K degrees or Celsius). Unit: W / m K.
- Resistance (surface) of heat transfer - the inverse of the coefficient of heat transfer.
- material density indicates how many weighs 1 kg cubic meter of material. An important parameter, PVC foam insulation is much easier to wire PV-1. In the absence of data on the last attempt is made to approximate the value.
- The moisture content of the material in this particular case, is irrelevant. Is that around the damp walls, whereas the fact is taken into account according to the situation.
The overall heat transfer coefficient of the sum, thus three components:
- Impact fence heat from the source (wires).
- Passage through the heat insulating material cambric.
- Release of heat from the outer surface cambric surrounding space.
Unit of measure is still W / m K. Said value must be calculated according to reference data found. Then divide it cambric area from the land. The resulting figure is multiplied by the temperature difference between the unknown and is equivalent to the power found from the law of Joule. The climatic conditions of the premises rely known. Thus obtained estimated temperature of the cable core at a known consumed electric current.
The resulting value is compared with operational data wires. If the temperature is above normal, the current must be reduced, or should choose a wire (with a large residential or other type of insulation).
Where can I get the value of resistance heat transfer materials
If you could not find a specialized building directory or a suitable table, there exists a solution. It is easy to see on the Internet a lot of data for the window profile, but finding the necessary data on PVC can hardly succeed. It is proposed to find the net a good online calculator to calculate the heat loss through the ceiling.
The program is written on a network language (JAVA, PHP) allows the calculation of thermal modes for complex designs. And in many cases, give a ready answer. Check the result must be in at least three locations, chosen calculators openly lie. Moreover, testing the reliability should be conducted by a simple example: wood or brick. In a complex and very easy to make a mistake.
This calculator has a built-in reference, which shows the number of resistance heaters. And it will certainly help in the calculations. The selection of materials should take into account the density. For example, PVC-foam 1 detects (according smartcalc.ru) density and 10, and 125 kg per cubic meter. It is clear that the characteristics of the materials are very different.