Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 124)

ϑ
I/I
e
t
t
Release temperature
Warning temperature
Fig. 4.1-16
Prewarning enables a disturbance to be rectified before a protective shutdown is required
The possibilities are manyfold and extend the function of the protective device to an integrated
component for an optimum process control. Integration in the communication network of the
control systems supports integration and the minimization of costs.
4.1.3 Protection against high overcurrents, short-circuit protection
See also Section 2.3.4 .
4.1.3.1 Definition and characteristic of a short-circuit
In accordance with IEC 60947-1, a short-circuit is a conductive path between two or more
conductive parts forcing the electric potential differences between these conductive parts to be
equal to or close to zero. In this section, a short-circuit is understood as a connection with very
low impedance between a pole conductor and a second pole conductor, the neutral or the
protective earth conductor or ground bypassing the load impedance and leading to the devel-
opment of a very high overcurrent (> overload current of the circuit).
The characteristic and magnitude of the short-circuit current in a circuit are determined by the
impedances of the components in the circuit. These are:
ƒ Impedance of the power supply (transformer, connecting lines)
ƒ Impedances of connecting points, any components (for example fuses, disconnect switches,
circuit breakers) and lines in the circuit
ƒ Impedance of the location of fault (frequently electric arc)
The magnitude of the prospective short-circuit current (symmetrical component) is a function of
the driving voltage and the impedances of the short-circuit loop. For the purpose of estimation it
is useful to determine the short-circuit current of the supplying transformer and the damping of
the short-circuit current by the lines up to the fault location. For the short-circuit current of the
transformer with a short-circuit directly at the terminals the following applies approximately:
I
ccT2
= I
2e
· 1/u
k
= (P
T
/(U
2e
· √3)) · 1/u
k
I
ccT2
Prospective short-circuit current on secondary (r.m.s. value)
I
2e
Rated secondary current
u
k
Short-circuit voltage
P
T
Rated power of the transformer
U
2e
Rated secondary voltage (pole-pole)
For an estimate of the damping effect of lines on the short-circuit current see RALVET [13].
If large motors are running on a supply, then their contribution to the entire short-circuit current
should be taken into account. Their locked-rotor current can approximately serve for this.
Because of the lack of a load impedance in the short-circuit loop, short-circuit currents are
strongly inductive. This has an effect on the peak value of the prospective short-circuit current,
as depending on the time of occurrence of a short-circuit within a supply half cycle, a more or
less high overshoot (prospective peak short-circuit current) is produced (
Fig. 4.1-17 ).
LVSAM-WP001A-EN-P - April 2009 4-16