Siemens SIWAREX R Manual de Instruções - Página 7
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Edition 06/2009
1.3
System Configuration
In weighing technical systems one or more load cells are connected to a weighing
module for evaluation of the measured signal. Several load cells of a scales are
connected in parallel in a junction box to supply a common output signal.
Load cells may only be connected in parallel when they have the same
characteristic value, the same rated load and the same internal resistance.
ATTENTION
The total resistance in parallel connected load cells may not drop below the
minimum resistance specified in the technical data of the weighing module.
The maximum number of load cells that can be connected to a weighing module
depends on the total resistance of the parallel connected load cells. This must be
within limits of the weighing module specified for the load resistance. The length of
the lines and the data of other elements, e.g. Ex i-Interface, must also be taken into
account.
CAUTION
If several load cells are connected in a scales it cannot be determined whether
individual load cells are overloaded in the event of uneven load distribution.
The maximum cable length between the load cell and the weighing module can be
found in the technical data of the weighing module. For Ex applications the data of
the Ex-i interface must also be taken into account.
A shielded 6-wire cable, e.g.. Li2Y(ST)CY 6x0.75 mm
702-8AB or, for Ex applications, 7MH4 702-8AA must be used to connect the
junction box to the weighing module, to extend a load cell connecting cable or for a
cross-connection between two junction boxes.
Overloads should be avoided in the layout of load cells by creating a safety
reserve. The safety reserve for three application points should be 20%. In the case
of statically aspecific use of more than three application points, the safety reserve
must be at least 50% if it cannot be ruled out that the load is only resting on two
diagonally opposite load cells. The reason for this may be drops in the foundation
or improper assembly. Accidental overloads or overloads due to the process must
be taken into account or kept away from the load cells by overload protection
devices. Overloads are caused for example:
•
in the case of uneven load distribution as a result of attachments or tipping
mounds,
•
when rolling/pushing the load onto platform or rollway scales,
•
by hard setting down of the load,
•
by setting down the load in free fall,
•
when persons are leaning/standing on the scales,
•
by wind forces on the side of a silo facing away from the wind.
SIWAREX R Load Cells Operating Manual
A5E00159640
Technical Description
2
, Siemens order no.: 7MH4
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