Circuit breakers are an integral part of any electrical circuit, be it a home network or a car's on-board electrical system. Their main task is to protect wiring from overloads and short circuits, which can lead to fire or failure of expensive equipment. But how exactly does the machine “understand” when it needs to turn off? There are two key elements responsible for this: thermal and electromagnetic releases.
If you have ever encountered a situation where, when you turned on a powerful device (for example, a welding machine or a car inverter), the automatic circuit breaker was triggered, then the electromagnetic release was most likely to blame. But if the switch turns off a few minutes after connecting the load, this is the “work” of the thermal mechanism. In this article we will analyze in detail how both types of releases are designed, what are their fundamental differences and why in modern machines they always work in pairs.
It is especially important to understand these nuances for car owners who are tuning the car’s electrical system or installing additional equipment (for example, powerful audio systems, winches or refrigerators). Choosing the wrong circuit breaker can result in false trips or, worse, no protection in a critical situation.
What is a trip unit in a circuit breaker?
The release is a mechanism inside the machine that opens the circuit when the permissible current parameters are exceeded. In fact, this is a “trigger” that works in two cases:
- 🔥 Overcurrent — when consumption exceeds the machine’s rating (for example, too many devices are connected to one line). Responsible for this thermal release.
- ⚡ Short circuit (SC) - when the current in the circuit increases tens of times in a fraction of a second. Works here electromagnetic release.
In most modern circuit breakers (including those used in cars), both releases combined in one housing. This allows you to provide reliable protection both from long-term overloads and from instantaneous current surges. For example, in the slot machines of the series ABB S200 or IEK BA47-29 (popular in auto electrics) this type of circuit is used.
It is interesting that in some specialized circuit breakers (for example, to protect electric motors) only a thermal release can be used, since a short circuit in such circuits is unlikely. But in automotive networks, where the risk of a short circuit is high due to vibrations and mechanical damage to the wiring, an electromagnetic release is required.
How does a thermal release work: principle of operation
The thermal release is bimetallic strip, which heats up when current passes through it. The higher the current, the greater the heating. If the permissible value is exceeded, the plate is deformed and opens the contacts of the machine.
The main feature of the thermal release is inertia. It does not work instantly, but reacts to long-term overload. For example, if the machine's nominal value 16A, then at current 20A it may turn off after a few minutes, and when 30A - within tens of seconds. It depends time-current characteristics (indicated by the letters B, C, D on the machine body).
- ⏳ Dependence on ambient temperature: In hot weather, the thermal release will operate faster than in cold weather.
- 🔄 Self-healing: after cooling, the plate returns to its original state and the machine can be turned on again.
- ⚠️ Wear and tear over time: The bimetallic strip may become tired after repeated trips, resulting in nuisance trips.
In automotive electrical applications, the thermal release is especially important for protecting circuits with impulse loads, for example, when operating a starter or powerful audio amplifiers. However, it is useless in case of a short circuit - this is where the electromagnetic mechanism comes to the rescue.
Electromagnetic release: instantaneous short circuit protection
If the thermal release operates slowly, then the electromagnetic release will instantly (in a fraction of a second). Its main element is moving core solenoid, which is attracted when the current increases sharply (for example, during a short circuit). The core hits the release mechanism and the circuit opens.
The main difference from a thermal release is response threshold. The electromagnetic mechanism is configured for current, in 5–10 times exceeding the nominal value of the machine. For example, for a machine C16 the response threshold of the electromagnetic release will be 80–160A. This allows you to ignore short-term starting currents (for example, when starting an engine), but reliably disconnect the circuit during a short circuit.
| Machine type | Rated current (A) | Electromagnetic release threshold (A) | Example of application in a car |
|---|---|---|---|
B |
16 | 48–80 | Lighting circuits, alarm |
C |
25 | 125–250 | Main power circuits, audio system |
D |
32 | 160–320 | Starter, winch, inverter |
In cars, an electromagnetic release often saves from the consequences insulation breakdown or short circuit to body. For example, if a bare wire touches a metal body, the current increases sharply, and the machine is triggered before the wiring has time to melt.
When choosing a machine for a car, pay attention to the markings: for circuits with high starting currents (for example, a starter), the type D, and for lighting - B or C.
Differences between thermal and electromagnetic releases: comparison table
To clearly understand when each of the releases trips, let's compare their key characteristics:
| Parameter | Thermal release | Electromagnetic release |
|---|---|---|
| Type of protection | From overloads | From short circuits |
| Response speed | From a few seconds to minutes | Fractions of a second |
| Operation threshold | Exceeding the nominal value by 10–50% | Exceeding the nominal value by 5–10 times |
| Temperature dependence | Yes (works faster in hot weather) | No |
| Possibility of reactivation | After cooling | Immediately after eliminating the short circuit |
In automotive electrics, both releases must work synchronously, since overloads and short circuits are not uncommon here. For example, when installing additional equipment (for example, compressor for inflating wheels) it is important to consider both types of protection in order to avoid false alarms or, conversely, lack of response to an accident.
Typical faults of releases and their symptoms
Like any mechanical element, releases wear out or fail over time. Let's look at the most common problems:
- 🔥 False trips of thermal release: The machine turns off for no apparent reason. Reason - wear of the bimetallic plate or installation in a hot location (for example, near an engine).
- ⚡ Failure to operate during short circuit: The electromagnetic release does not respond to a current surge. It's dangerous! Reason - core jamming or manufacturing defect.
- 🛠️ The machine does not turn on after triggering: The contacts may be burnt or the release mechanism may be broken.
- ❄️ Slow response in cold weather: The thermal release may freeze, especially in vehicles driven in northern regions.
Problems with releases occur especially often in cars, where the circuit breakers are exposed to vibrations, temperature and humidity changes. For example, in SUVs or commercial vehicles it is recommended to use machines with a reinforced design (for example, series ETI Polaris or KEAZ OptiMat).
How to check the functionality of releases without special equipment?
1. Thermal release test: Connect a load close to the rated value of the circuit breaker (for example, 15A for 16A machine), and note the shutdown time. If it operates too quickly or too slowly, the release is faulty.
2. Electromagnetic release test: Briefly connect a load 5 to 10 times the rated load (for example, 100A for 16A machine). The machine should work instantly. Attention: This test is hazardous to wiring - perform this test only using a laboratory power supply!
How to choose a machine with the right releases for your car?
When choosing a circuit breaker for your car, there are several key parameters to consider:
- Rated current: Should exceed the maximum load current by 20-30%. For example, if your audio system consumes
20A, select the machine at25A. - Type of time-current characteristic:
B— for circuits with low starting currents (lighting, alarm).C- a universal option for most automotive chains.D— for equipment with high starting currents (starter, winch).
- Single- or double-pole circuit breakers - for protecting individual circuits (for example, radios or additional headlights).
- Four-pole - for protecting three-phase circuits (for example, in trucks or motorhomes).
- Vibration resistance: in cars it is better to use automatic machines with shockproof housing (for example, series ABB SH200L or Schneider Electric Multi9).
Also note climatic version. For vehicles operated in conditions of high humidity or extreme temperatures, machines with markings are suitable UHL (temperate and cold climate) or T (tropical version).
Is the rated current suitable for the circuit load?|Does the type (B, C, D) match the nature of the equipment?|Is there a current reserve (at least 20%)?|Is the housing resistant to vibration?|Is the climatic version suitable for your region?-->
Common mistakes when working with circuit breakers in cars
Even experienced auto electricians sometimes make mistakes when choosing or installing machines. Here are the most common ones:
⚠️ Attention: Never replace the machine with a more powerful one “so as not to knock it out”! This may lead to overheating of wiring and fire. If the machine operates too often, look for the cause (overload or short circuit), and do not increase its rating.
- 🔌 Using household machines in a car: They are not designed to withstand vibration and humidity. Use only specialized series (for example, Blue Sea Systems or Bussmann).
- 🔧 Incorrect installation: Loose terminals lead to heating and false alarms. Always check the reliability of the contacts.
- 📉 Ignoring inrush currents: if machine type
Binstalled in a circuit with the starter, it will operate every time the engine starts. In such cases, you need a typeD. - 🔄 Repeated activation after triggering: If the circuit breaker is knocked out due to overload, allow it to cool down (especially the thermal release) before turning it on again.
Another common mistake is parallel connection of machines to increase the rated current. This violates the principles of selectivity and can lead to the fact that during a short circuit the machine will not operate as it should. If you need protection for a high current circuit, use one circuit breaker of the appropriate rating.
When connecting powerful equipment (for example, a 2000 W inverter), always use a separate line with a circuit breaker whose rating is calculated with a reserve. For inverter on 2000 W (current ~17A) automatic will do 25A type C or D.
FAQ: Answers to frequently asked questions about circuit breakers
Can the release be repaired if it is broken?
No, trip units cannot be repaired. These are precision mechanisms, and any attempt to restore them (for example, straightening a bimetallic strip) will lead to unpredictable operation. If there is a malfunction, the machine must be replaced.
Why does the machine operate without load?
This can be caused by several reasons:
- Short circuit in the circuit (even if the load is disconnected, there may be a short circuit in the wiring).
- Malfunction of the machine itself (for example, sticking of the electromagnetic release).
- External factors: high temperature, vibration or humidity.
It is necessary to diagnose the circuit with a multimeter or megohmmeter.
Which machine to choose to protect the battery from deep discharge?
To protect the battery from discharge through leaks (for example, an alarm or radio), use circuit breaker with thermal release type B and a rating corresponding to the leakage current. For example, if the total leakage is 50 mA, you can install the machine on 1A. There are also specialized discharge relays, which disconnect the load when the voltage drops below critical.
What is the difference between vending machines for home and for cars?
Main differences:
- Housing: car automatic machines have a reinforced design that is resistant to vibration and moisture.
- Operating temperature range: automatic machines are designed for
-40…+85°C, while household ones - on-25…+40°C. - Terminals: Automotive machines use terminals for wires with automotive lugs (for example, ring or fork).
- Certification: car machines often have certificates ISO 8846 (for marine and motor transport) or SAE J1171.
The use of household machines in a car is unacceptable!
Is it possible to use fuses instead of circuit breakers?
Fuses and circuit breakers perform similar functions, but there are key differences:
- 🔹 fuse operates once (the fuse-link burns out), after which it requires replacement.
- 🔹 Automatic can be re-enabled after the cause of the trigger has been eliminated.
- 🔹 Fuses are cheaper, but do not provide protection against overloads (only against short circuits).
- 🔹 In automobiles, fuses are often used to protect low voltage circuits (e.g. cigarette lighter or USB ports), and the machines are for powerful consumers.
For circuits with high currents (for example, a starter or winch), an automatic machine is preferable, since it does not need to be changed after each operation.