22 Operating Instructions- The bp408 Safety Circuit - Circuit Board SPL-01
4.4 Safety Circuit on SPL-01
4.4.1 Functional description of safety circuit
The single channel safety circuit monitors the synchronous operation of the
external and internal zone signal. The output circuit remains open in case of a fault
or asynchronous operation. The external encoder element is connected to terminal
ZS. If the release signal (speed limit + bypass) is set by the processor unit, the
release signal of the control relay K552 may engage first, but only if relays K551,
K553 and K554 have disengaged. By engaging K552, K553 is allowed to engage,
which causes the engagement of K554 in turn. If relays K553 and K554 have
engaged, relay K551 is started. This causes relay K552 to disengage and only after
that will the output circuit be closed. Due to the closing of the output circuit, it is
only possible to bypass the door and bolt elements of the safety circuit. It must be
ensured that the SSZ terminal is only connected to terminal 11 of the query unit.
This sequence can only be verified if the external encoder element works correctly
and the assembly movement to configure the internal signal has been performed
correctly. If the input signals are present, the same switching sequence generally
applies irrespective of the timing. K552 is engaged first and prepares the activation
of relay K553, which is responsible for activating relay K554 in turn. This causes
relay K551 to be activated, which causes relay K552 to disengage, thereby closing
the output circuit of the safety circuit (SSZ connection/pilot controls of COM
connection). In parallel to this process, the condition of the zone and/or the safety
circuit is monitored by a sequence control circuit. If the input signal were to no
longer apply, the associated relay would disengage and therefore open the output
circuit.
Reactivation of the relay can only be performed after the initial switching state is
reached. To this end, the relay assigned to the other channel must be disengaged.
Only after that can the prescribed switching sequence of relays take place. In
normal operation, the described sequence is always executed in a cyclic fashion.
If an error occurs, the output circuit is opened and the circuit prohibits reactivation
even if additional errors or faults occur.
A dangerous operating condition can therefore not exist at any time with virtual
certainty.
4.4.2 Error analysis of safety circuit
If, for example, a defective encoder element on channel 1 were to cause a
continuous signal on connection ZS or the anchor of relay K553 would no longer
disengage, this error would lead to relay K552 staying disengaged. This would
mean that relay K554 could no longer be engaged and that the output circuit would
remain open continuously. During approach and readjustment with the cabin door
opened, an error therefore causes no bypassing of the door and door lock switch.
This means that the unlocking zone can only be approached with the door closed
and the door lock switch engaged. When using the safety circuit as a substitute for
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