When designing or upgrading a home power grid, as well as servicing the electrical equipment of a car or garage workshops, the question often arises as to why the machine knocks out when a powerful device is turned on. Many people mistakenly believe that it is enough to simply replace the circuit breaker with a more powerful one, but this is a gross mistake that leads to a fire. The key parameter here is not only the nominal current, but also the temporary characteristic of the operation, which is visually displayed as current-curve. This graph shows how quickly the chain will break under various overloads.
Understanding how the shutdown curve works avoids false protections at the time of engine start-up or, conversely, provides reliable protection of the wiring from short circuit. In this article, we will examine in detail the physical meaning of the graph, the differences between types B, C and D, and also learn how to choose the right protective equipment for specific tasks. Automatic switches - these are complex devices that combine thermal and electromagnetic cleavers.
Ignoring the performance of the trigger can cause expensive equipment to burn before the protection works. Or the situation will be that the light will go out every time you turn on the compressor or welding machine. We will understand how to read these graphs and what the mysterious Latin letters on the device body mean.
Physical meaning of the trigger schedule
The current curve is a visual representation of the dependence of the time of disconnection of the machine on the magnitude of the current flowing through it. The X axis usually displays the rated current multiplicity (I/In), and the Y axis displays the time in seconds. It is important to understand that the graph is not a single line, but a rangelimited by two curves. This is due to the fact that in production a certain range of parameters is allowed, as well as the effect of ambient temperature on the operation of the mechanism.
The top curve on the graph shows the time of operation of the “cold” machine, which has just been turned on the network. The lower line corresponds to the "hot" state when the device has already warmed up with the load current. The difference in response time between cold and hot can be as high as 50% or more.This is crucial to consider when calculating the protection selectivity.
The schedule is divided into two main areas of operation of the uncouples. The first zone is the overload area, where the bimetallic plate (thermal sever) is responsible for the circuit opening. The second zone is the short circuit area, where the electromagnetic coil is instantly triggered. Smooth transition between these zones and provides reliable protection of the cable from overheating without false shutdowns during short-term jumps.
⚠️ Attention: Never use a D-characterized machine in a regular household outlet unless there are specific industrial equipment connected. This can cause the wiring to burn out before the protection works.
Curve analysis allows an engineer or electrician to predict the behavior of a protection system in an emergency. If the current exceeds the nominal value slightly, the machine may not shut down for hours, which is normal for thermal processes. But when the multiple exceeds the count goes on milliseconds. That is why it is important not only to look at the digit of the denomination (for example, 16A), but also at the letter designation in front of it.
The heat-disconnector area
The first part of the graph, located on the left, describes the operation of the thermal disengagement. This mechanism reacts to a prolonged excess of current over the nominal value. The main element here is a bimetallic plate, which bends when heated and mechanically pushes the decoupling lever. Time of operation in this zone depends on the degree of overload: the more current, the faster the plate is heated.
At a current of 1.13 of the nominal value (In), a modern circuit breaker should not be turned off for an hour (for small denominations) or two hours (for large ones). This allows the device to withstand short-term voltage fluctuations and initiation currents without breaking the circuit unnecessarily. However, if the current rises to 1.45 In, the heat releaser must operate within an hour, preventing the insulation of the wires from overheating.
The ambient temperature has a significant impact on the operation of the thermal disengagement. In a hot garage or shield in the summer, the machine can work at a lower current than indicated in the passport. Conversely, in winter, at an unheated facility, its throughput capacity will temporarily increase. This phenomenon is called temperature compensation, although it does not completely eliminate dependence.
It is important to note that the thermal cleavage has the property of "memory". If the machine is disconnected due to overload, the bimetallic plate takes time to cool down to return to its original position. Trying to turn it on right away may be unsuccessful or result in a quick shutdown again.
Electromagnetic sever and the KZ zone
The second, cooler part of the graph relates to the operation of the electromagnetic cleavage. This node is designed to instantly react to supercurrents that occur during a short circuit. Unlike thermal, there are no delays in heating: the magnetic field of the coil instantly draws the core and opens the contacts. The response time is calculated here. tenth-second Or faster.
The boundary of operation of the electromagnetic cleavage is determined by the letter characteristic of the machine (B, C, D). It is this parameter that determines how many times the current must exceed the nominal value for the protection to work instantly. For different types of equipment, this threshold varies, which avoids false shutdowns when starting engines with large initiating currents.
The curve in this zone also has a scattering. Standards allow for instantaneous shutdown to occur in the range of the lower to upper multiplicity limit. For example, for a C characteristic, it can be from 5 to 10 denominations. Engineers should always focus on the worst-case scenario when calculating to ensure safety.
Why doesn't the initiation current turn off the machine?
At the time of switching on the electric motor or transformer, the current can briefly exceed the nominal 5-10 times. However, the duration of this pulse (tenths of a second) is too small to heat the bimetallic plate. The electromagnetic sever also fails if the initiation current does not exceed the lower threshold of its sensitivity for this characteristic.
If the short circuit current is large, the contacts break so quickly that the current does not have time to reach its maximum theoretical value. This phenomenon is called current restriction and is a critical function of modern switch-off in a modular design.
Differences in characteristics B, C and D
The most common in operation are three types of time-current characteristics: B, C and D. Understanding their differences is the key to the correct selection of protection. Each type is designed for its own load category and has strictly defined ranges of electromagnetic scatter.
Characteristics B It assumes instantaneous operation when the current is exceeded by 3-5 times. This is the most sensitive type, ideal for protecting lighting lines, active load socket groups and long cable tracks where short circuit currents can be small. Characteristics C The most common in the household (range 5-10 In). It is designed for mixed load, where there are devices with moderate starting currents: refrigerators, air conditioners, washing machines.
Characteristics D (10-20 In) is designed for heavy duty. It is used to protect electric motors, compressors, welding machines and pumps, which consume a huge current at the start. Using a type D machine in a normal apartment can be dangerous, as it “does not notice” a serious overload that can easily withstand wiring, but will not withstand the connection.
The correct choice of letters (B, C, D) is more important than increasing the denomination of the machine, as this provides selectivity and protection against false shutdowns when starting equipment.
There are also rare characteristics of A, K and Z that are used in specific industrial applications or to protect semiconductor electronics. For most tasks of garage maintenance, car service and home networks, it is enough to understand three main types.
Effects of Temperature and Current Multiplicity
The temperature of the environment is a factor that is often overlooked. The circuit breaker is calibrated at a certain temperature (usually +30°C or +40°C). If the shield is installed in a hot room or in the sun, the thermal protection will work sooner. Conversely, in a frosty garage, the machine can keep the overload longer than it should.
The ratio of current is the ratio of real current to nominal current. It depends on this value on which part of the schedule the disconnection process will go. With a multiplicity of 1.5, we are in the zone of thermal protection, with a multiplicity of 8 - already in the zone of electromagnetic protection. Accurate knowledge of the frequency of the equipment starting currents helps to avoid errors.
When choosing an automatic for an internal combustion engine with an electric starter or for a compressor, it is important to consider that the initiation current can last for several seconds. If the trigger curve crosses the engine start schedule, a false shutdown will occur. In such cases, sometimes you have to choose a machine with a larger letter characteristic, but strictly watching the cable cross section.
⚠️ Attention: Replacing the machine with a more powerful one (for example, from 16A to 25A) without replacing the wiring is strictly prohibited. The cable may start to warm and melt at 20A, but the new machine will not notice this, which will lead to fire.
The effect of the grouping should also be taken into account. When several machines are tightly mounted in a row in one shield, they heat each other up. This can lead to a decrease in their actual loading capacity. Manufacturers often specify load reduction coefficients during tight installation.
Response table
For ease of comparison of the main characteristics, we give a summary table. It will help to quickly navigate in the ranges of instantaneous release currents for various types of automatic machines. The data is relevant to the most common standards.
| Type of characteristics | Instant response range | Principal appointment | Example of equipment |
|---|---|---|---|
| B | 3 In — 5 In | Active loads, lighting | Lamps, TENs, sockets |
| C | 5 In — 10 In | Mixed loads (standard) | Refrigerators, vacuum cleaners, PCs |
| D | 10 In — 20 In | Inductive loads, engines | Compressors, machines, pumps |
| K | 8 In — 12 In | Inductive loads (specific.) | Transformers, engines |
From the table, it can be seen that the ranges overlap. This means that the C16 can behave like a B16 under some conditions and like a D16 under others, depending on the variation in the parameters of the particular instance. Therefore, at critical loads it is better to carry out practical measurements of initiating currents.
Practical recommendations for selection
When selecting an automatic switch, always start from the cable cross section, not from the load power. The machine protects the cable, not the device. If the cable can withstand 25A, and the load is 10A, you can put the machine on 25A, but on 10A it is safer for the device itself, although the cable will not save you from the KZ in the device itself.
Use it. electric mites to measure the actual initiation current of the equipment, if there are problems with permanent outages. It often turns out that the old engine consumes more when starting than stated in the passport due to wear. In this case, the transition from B to C will solve the problem without compromising safety.
☑️ Check before replacing the machine
Don't forget to be selective. If you have an introductory machine and several group ones, their characteristics must be coordinated. It is desirable that when a short circuit in the socket knocked out the automatic socket, and not the general input on the whole house or garage.
⚠️ Attention: Cheap machines of unknown brands often have an unpredictable trigger curve. The actual disconnect current may differ from the marking by 30-40%. For critical nodes, use products from proven manufacturers.
Frequently Asked Questions (FAQ)
Can you replace the C16 with the C25 if it is constantly knocking out?
No, you can't do it without checking the cable section. The C16 is placed on a 2.5 mm2 cable. If you put the C25, then at current 20A, the cable will start to warm and melt, and the machine will not shut down yet. It's a direct route to fire. You need to look for the cause of the overload or change the wiring.
Why does a Type B machine more often knock out when the lights are turned on than a Type C?
Because the lamps (especially LED with drivers or halogen) when turned on, there is a short-term current throw. The B characteristic is more sensitive (starts from 3 denominations) and may perceive this roll as a short circuit. Characteristics C (from 5 denominations) are more resistant to such impulses.
Does the altitude above sea level affect the machine?
Yes, it does. At high altitudes, the air is rarefied and the contacts cool down worse. Standards require operation up to 2000 meters above sea level without reducing performance. Above this level, a decrease in the nominal current of the machine may be required.
Which is better for a garage: C or D?
For a regular garage, where there are lamps, charging for an electric car and a hand tool, the C characteristic is enough. The D feature is only needed if you are constantly using powerful three-phase machines or welders that cause false shutdowns on Type C machines.
How often do I need to change the switches?
Automatic machines do not have a strict shelf life, but the mechanical resource is limited by the number of on/off cycles. If the machine often knocked out overload, its thermal disengagement could "tire" and change the characteristics. It is recommended to check their performance every 5-10 years or when changing the load.
When buying a machine, pay attention to the power of the machine (a figure in the frame, for example 6000 or 10000). For garage and home at least 6000A (6kA), as currents of KZ in modern networks can be high.