SIEMENS
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Siemens IC 10 · 2017
Solid-State Switching Devices for Resistive/Inductive Loads
Solid-State Relays
General data
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Overview
Solid-state relays (without heat sink)
SIRIUS solid-state relays are suitable for surface mounting on
existing cooling surfaces. Mounting is quick and easy, involving
just two screws. The special technology of the power
semiconductor ensures there is excellent thermal contact with
the heat sink. Depending on the nature of the heat sink, the
capacity reaches up to 88 A on resistive loads.
The solid-state relays are available in three different versions:
• 3RF21 single-phase solid-state relay with a width of 22.5 mm
• 3RF20 single-phase solid-state relay with a width of 45 mm
• 3RF22 three-phase solid-state relay with a width of 45 mm
The 3RF21 and 3RF22 solid-state relays can be expanded with
various function modules to adapt them to individual
applications.
Version for resistive loads "zero-point switching"
This standard version is often used for switching space heaters
on and off.
Version for inductive loads "instantaneous switching"
In this version the solid-state relay is specifically matched to
inductive loads. Whether it is a matter of frequent actuation of the
valves in a filling plant or starting and stopping small operating
mechanisms in packet distribution systems, operation is carried
out safely and noiselessly.
Special "Low noise" version
Thanks to a special control circuit, this special version can be
used in public networks up to 16 A without any additional
measures such as interference suppressor filters. As a result, in
terms of emitted interference, it conforms to limit value curve
class B according to IEC 60947-4-3.
Single-phase solid-state relays with a width of 22.5 mm
With its compact design and a width of just 22.5 mm, which is
not exceeded even for currents of up to 88 A, the 3RF21 solid-
state relay offers an ultra-small footprint. The logical connection
method, with the power infeed from above and load connection
from below, ensures tidy installation in the control cabinet.
Single-phase solid-state relays with a width of 45 mm
The solid-state relays with a width of 45 mm provide for
connection of the power supply lead and the load from above.
This makes it easy to replace existing solid-state relays in
existing arrangements. The connection of the control cable is as
space-saving as the 22.5 mm design, as it is simply plugged on.
Three-phase solid-state relays with a width of 45 mm
With its compact design and a width of just 45 m, which stays the
same even at currents of up to 55 A, the 3RF22 solid-state relay
offers an ultra small footprint. The logical connection method,
with the power infeed from above and load connection from
below, ensures tidy installation in the control cabinet.
The three-phase solid-state relays are available with
•Two-phase control (suitable in particular for circuits without
connection to the neutral conductor) and
•Three-phase control (suitable for star circuits with connection
to the neutral conductor or for applications in which the system
requires all phases to be switched)
Selection notes
When selecting solid-state relays, in addition to information
about the network, the load and the ambient conditions it is also
necessary to know details of the planned design. The solid-state
relays can only conform to their specific technical specifications
if they are mounted with appropriate care on an adequately
dimensioned heat sink.
Mounting solid-state relays directly on a mounting plate made of
sheet steel is inadequate in terms of heat dissipation.
The following procedure is recommended:
•Determine the rated current of the load and the mains voltage
•Select the relay design and choose a solid-state relay with
higher rated current than the load
•Determine the thermal resistance of the proposed heat sink
•Check the correct relay size with the aid of the diagrams
© Siemens AG 2016