Basic Function Principles - Beckhoff EL922 Series Documentation

Basic function principles

4 Basic function principles
The functional principle of the electronic overcurrent protection terminals is based on measurement and
evaluation of the current flow. Depending on the result of the evaluation, the corresponding action is then
executed. One possible action is shutdown.
There are several shutdown options: once according to typical tripping times (characteristic curve), according
to hardware parameters or after exceeding the limit load integral..
Shutdown based on characteristic curve
Shutdown based on the characteristic curve can take place if the outputs were previously switched on
successfully. How long the respective overcurrent may flow until the shutdown is triggered depends on the
tripping times specified in the characteristic curve. Typical tripping times can be found under Technical data
[} 19].
Shutdown based on hardware parameters
The electronic overcurrent protection terminals EL9221, EL9222 and EL9227 have an internal current and
power limitation based on hardware parameters. The shut-down according to hardware parameters (power
limitation / current limitation) occurs when the outputs are already overloaded when switched on and could
not be switched on. If, for example, an existing short-circuit is switched on. However, the hardware shut-
down also takes effect in characteristic curve operation if the power limit and/or current limit are exceeded. If
one of them is exceeded, the switch-off takes place after a fixed time.
The specification of the power dissipation limitation refers to the internal power dissipation in the terminal.
The value of the power dissipation limitation is fixed, whereas the current flow depends on the voltage drop
in the terminal. For example, if a 0 Ohm short circuit was present, the 24 V DC would drop completely in the
terminal. A power limitation of 400 W would result in a current of approx. 16.67 A until shutdown. Depending
on the voltage distribution, the current increases, but only to a maximum limiting value. The respective
limitation values can be found under Technical data [} 16].
The power loss limitation including current limitation is always active. This means that this function is always
available regardless of whether the outputs could already be switched on or not.
Shutdown after exceeding the limit load integral
The tripping time can vary if, for example, the limit load integral is exceeded.
Example: In pulsed operation, an overcurrent occurs several times in succession. Shortly before the switch-
off, the overcurrent remains off, and then returns after a short time. As a result, the limit load integral has
built up and finally a switch-off occurs.
Shutdown during the switch-on process
Electrical loads can be switched on in two different ways. Once directly via the output at the overcurrent
protection terminal or via other switching products. When the load is switched on via the channel at the
terminal, the power dissipation and current limitation values are decisive for a possible switch-off at the
switch-on time. When a load is switched on via additional switching terminals (output of the overcurrent
protection terminal is already switched on), the data of the characteristic curve are decisive.
20 Version: 1.0 EL922x