Three separate circuits are operated in a safe switching mode by ensuring a specific sequence of operation. One of the three circuits is used to interlock the operation of the other two. Two separate circuits are to provide relay actuation (c11, c12) by manual activation of switches (b11, b12). Contacts of a third relay (c13) are included as interlocks. Only when the third relay (c13) is energised is the first relay (c11) capable of energisation. This is latched (c11/2) and simultaneously the second relay (c12) put into a state of readiness by two contacts (c11/3, c13/3). When both of the interlocked relays are latched the third relay drops out due to a contact opening (c12/4).
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