Trip curves
1. How can ambient temperature be accounted for correctly?
An overcurrent exceeds the normal rated current of appliances or equipment. There are two different types of overcurrent:
Overload: Overcurrent in an electrically faultless circuit
Short circuit: Overcurrent in an electrically faulty circuit
The overcurrent protection device selected must be adapted to the characteristics of the respective equipment and the load specifications. The following figure shows the thermal-magnetic trip curve of the E-T-A 2210-T2 circuit breaker. The tripping time is plotted against the multiple of the rated current.
1.1 How is the trip curve structured?
The trip curve can be divided into the thermal overload range (area 1) and the range of instantaneous magnetic tripping in the event of a short circuit (area 2).
Depending on the trip curve, the overload can reach up to four times the rated current in the case of direct current, and up to six times the rated current in the case of alternating current. The stress on the appliance or equipment is characterised by its duration and amplitude.
1.2 How should temporary fluctuations in temperature be taken into account?
The trip curve of a thermal or thermal-magnetic circuit breaker, such as the 2210-T2, is temperature-dependent. This is particularly important when a loadoperates outside the standard temperature range of -5 °C to +40 °C. The circuit breaker's trip curve takes the ambient conditions into account, enabling temperature-dependent device protection.
- If the ambient temperature is high, the circuit breaker trips sooner.
- If the ambient temperature is low, the circuit breaker’s trip time is delayed.
The figure below shows an example of an overload condition at four times the rated current for approximately 1.5 seconds.
Under normal conditions, the circuit breaker does not trip and allows overload operation. However, if the ambient temperature is higher – for example, 60 °C – the device will likely trip.
1.3 How can prolonged periods of high or low temperatures be taken into account?
A sustained high temperature can be taken into account by applying a derating factor to the rated current. This derating factor is specified directly in the E-T-A data sheets, as shown here for the 2210-T2 circuit breaker:
2. What options are available for temperature-independent overcurrent protection?
Temperature-compensated thermal circuit breakers and hydraulic-magnetic circuit breakers are used to ensure a temperature-independent trip curve. For this purpose, E-T-A offers different control units that are designed for a temperature range of -40 °C to +85 °C, such as the 8345-F or 8365-T.
The 8365-T circuit breaker is available for AC systems up to 230 V / 400 V and DC systems up to 800 V, covering a rated current range of 0.1 A to 100 A.
