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Series relay

SIL Safety Relays

When loads, such as valves or motors, cannot be driven directly by a safety PLC or by an emergency push button, an interposing relay becomes a necessity. As an example, when the power required to switch the load is greater than what can be provided by the PLC or when multiple contacts are required to be driven by the same signal, the use of a relay is a must. In today’s high integrity applications a standard relay is no longer acceptable. Whether it is for a ESD, F&G, BMS or for any other critical application, a Safety (SIL 3) Relay must be used.

GM International SIL 3 Relays are available in various contact configurations and rating to cover the majority of applications; they are available with standard DC as well as with AC coil.
Typical applications are shutdown or relief valves control, Fire & Gas valve control, Turbine shut down, AC/DC motors control center, etc. For each application the specific Safety Function must be determined to select the correct Safety Relay. The Safety Function controls the operating condition of the SIF, therefore of the Safety Relay.

Safety Functions can be basically divided in two main groups:
  • ETS or Energize to Safety or
  • Trip and DTS De-energize to Safety or Trip.

From which four operating conditions:
  • NE Relay Coil - NC Relay Contacts ( Load Normally Energized )
  • NE Relay Coil - NO Relay Contacts ( Load Normally De-Energized )
  • ND Relay Coil - NO Relay Contacts ( Load Normally De-Energized )
  • ND Relay Coil - NC Relay Contacts ( Load Normally Energized )

By “NE Relay Coil” we mean a High condition (power on) at the relay coil terminals and by “ND Relay Coil” we mean the exact opposite, a Low condition (power off). NC or NO Relay Contacts determine whether power is available to the load under normal operating conditions or not; with NC contacts the load is Normally Energized and with NO contacts the load is Normally De-Energized.
The Safety Function is to revert the above operating conditions. For example, with NE relay coil and NC relay contacts, DTS Safety Function is to disconnect power to the load by removing power at the relay coil (From High to Low condition); This is the most typical SIL 3 relay function for a NE Load. Or, with a NE relay coil and NO relay contacts, ETS Safety Function is to power-on load by removing power at the relay coil (From High to Low condition).

Other aspects in selecting the safety relay are: the load’s contacts rating; the number of contacts required to be driven by the same control signal; if a single line or both lines of the load must be switched ON or OFF.

For each GMI Safety Relay a data sheet containing schematics of all possible applications, as well as coil and contacts rating, is available.

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