Safety combos have certifiable benefits

Siemens believes it is the first company to offer safety relays pre-assembled with solid-state contactors or load feeders, and sold as certified components. It says that its Siguard combinations offer the advantages of solid-state safety switching - such as reliability, resistance to vibration, continuous monitoring and an ability to withstand crossover and earthing faults - as well as a reduction in wiring costs and errors.

By combining solid-state switches with a circuit-breaker and two series-connected contactors, Siemens has created a type-approved safe load feeder that can switch three-phase currents up to 25A, or DC currents up to 35A. For a machine that has one motor and an emergency stop circuit, a single safe load feeder combines the functions of a motor starter and an emergency stop circuit. If several motors are fitted to a machine, the safe load feeders can be linked together with a single emergency stop function.

The combination modules are cascadable and expandable. Safety logic functions can be created, for example, to protect a series of linked machines in such a way that operating any emergency stop switch will halt all of the machines, but opening a gate will stop just one section of the machine.

Siemens claims that this approach could result in significant savings. For example, a typical six-motor installation with two emergency stop switches and four safety switches would normally need around 160 connections between the various electrical components. Using a cascaded arrangement of combination modules, the tally is cut to around 80 connections, and benefits are introduced such as fewer wiring faults and the protection of the motors by circuit breakers.

The combinations are suitable for use with safety systems up to Category 4 (as defined in BS EN 954-1) or SIL 3 systems (as defined by IEC 61508).