In both the civil and industrial fields, safety is non-negotiable
in installations, but how exactly are we protected from electrical
contacts? When we receive an electric shock, a portion of the
current disperses from the system to the ground, passing through
our bodies. This portion of the current, called “differential”, is
detected by appropriate systems (electrical, electronic) that
trigger, by means of residual current devices (RCD), the breakers
that power the system. Generally, an RCD is composed of a
detection unit (electric, electronic) and an actuator (trip) that
triggers the mechanical disconnection or “trip” of the power
breaker.
Fig. 1 – The schematic of an RCD trip switch
An RCD-trip switch (shown in the figure) activates the release
button of the circuit breaker by means of a switch-lever that is
operated by a loaded flexural spring held in static equilibrium by a
permanent magnetic circuit. Once the trip is triggered, an electrical
discharge from the capacitor powers a solenoid that depolarizes
the magnetic circuit, allowing the switch-lever to move. Since it is
equally governed by electromagnetic and mechanical phenomena,
an RCD-trip is considered to be a Multi-physical system. The aim
of this article is to describe how such a switch can be simulated.
A preliminary analysis was conducted with Cetol6σ in order to
identify the construction factors of greatest functional influence
and to generate a scale of sensitivity for the geometric and
dimensional elements on the drawings. The RCD’s kinematic
nature is clearly a 1 degree-of-failure (DoF) system, which was
studied with RecurDyn based on rigid-body modeling. The aim
of this part of the study was to obtain
the laws of total force (elastic-friction)
and of equivalent mass, both of which
were transposed to the translational
free coordinate of the plunger, which
coincided with the magnetic gap used in
the final studies with Ansys Maxwell.
Ansys Maxwell simulation software for
electromagnetic fields is used to design
and analyze 2- and 3-D electromagnetic
and electromechanical devices, including
motors, actuators, transformers, sensors
and coils. It uses the finite element method
to solve static, frequency-domain, and time-varying electromagnetic and electric fields. One of its key
benefits is its automated solution process, which only requires
the user to specify the geometry, the material properties and the
desired output, after which Maxwell automatically generates an
appropriate, efficient and accurate mesh for solving the problem.
The analyses with Maxwell resolved the coupled equations
between the dynamic and electromagnetic fields, which enabled
the governing laws in the coil, the magnetic field and the induction
in space, position and speed of the plunger to be obtained.
There are considerable differences in the RCD’s performance
depending on the set of physical-geometric factors permuted
according to the possible worst-case scenarios. We note that the
RCD’s response time can double depending on the specific worst
case, just as it can even fail if the upper limits of dimensional
and flowmetric tolerance for iron and magnets are not contained.
This result is in accordance with what can be observed from the
working point on curves B_H, which highlight the “saturation” of
the magnetic circuit (and consequently a marked increase in the
forces antagonistic to the motion, in proportion to the iron and
magnets).
Newsletter EnginSoft Year 17 n°4
By Enrico Gnata | Bticino
Emiliano D’Alessandro, Giovanni Falcitelli, Enrico Boesso, Fabiano Maggio, Davide Girardi | EnginSoft
About BTicino
Founded in 1936 and an established leader in the field of Civil
Electrical Breakers, Bticino today is an accredited part of the
Legrand Group, where it constantly aims at developing solutions
for distribution, safety, communication and control in the world
of low voltage electricity.
For further information, visit: www.bticino.it
