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Differences between ESD and F&G Safety Relays

Across the industry, there are many terms used to describe these 2 applications. In this article, we will have a look at the different terms and understand the uniqueness of each application.

Emergency Shutdown Application

Under normal operating conditions, a load like an electrical motor, must be continuously powered. The ESD safety relay which is used to power up a load must also be continuously powered and is described as Normally Energized (NE).

During a safety incident (e.g. the electrical motor is overheated), the load needs to be switched off to achieve a safe state. As the power status of the safety relay and load has a direct relationship, switching off (de-energizing) an ESD safety relay, will result in the load being switched off as well. Thus, the terms, De-energized To Safe (DTS) and De-energize To Trip (DTT) are used to describe this phenomenon.

Fire & Gas Application

Under normal operating conditions, a load like a sounder and a beacon is not powered. The F&G safety relay is only required to be powered up during a safety incident and to allow the sounders and beacons to sound off the alarm and provide light signals. Under normal operating conditions, the F&G safety relay is Normally De-energized (ND).

During a safety incident (e.g. a gas leak or fire incident), the load needs to be switched on to achieve a safe state. As the power status of the safety relay and load has a direct relationship, switching on (Energizing) an F&G safety relay will result in the load being switched on as well. Thus, the terms, Energized To Safe (ETS) and Energize To Trip (ETT) are used to describe this phenomenon.

Some people might be confused by the term, Energize To Trip (ETT). This is because the word “Trip” is commonly used on circuit breakers, where “Trip” here refers to power down. However, the word “Trip” has a deeper meaning and can be used to describe the powering up and down of a load. A similar word for “Trip” is “Trigger”, and you can think of it as “Energize To Trigger”.

Summary

The key differences between the ESD and F&G safety relays are the type of load they are connected to, the definition of a safe state and the energy state under normal operating conditions. For ESD, the loads are usually machines used in production; the definition of the safe state is to switch off the relay / load and is normally energized under normal operating conditions. For F&G, the load are usually sounders and beacons, the definition of the safe state is to switch on the relay / load and is normally de-energized under normal operating conditions.