What is the Difference Between a Safety Relay & Common Relay?

Relays are critical components in a circuit, found in the electronic equipment ranging from smartphones to airplanes. While sharing the same name, the various types of relays perform vastly different functions. Therefore, having an understanding of the differences between the most prevalent types of relays is critical to understanding electrical currents as a whole. In this blog, we will discuss the difference between the most common relays and a safety relay.

Relays are generally classified as "common" or "safety" depending on their inherent function. The broad role of a common relay is to control a circuit by means of an electrically operated switch . Specifically, they are used in applications in which several circuits need to be controlled by a single signal or when a signal must be repeated when traveling long distances. They consist of a coil wrapped around an iron core, which produces a magnetic field when an electric current is introduced. The magnetic field then rotates an armature that attaches to a fixed contact in a certain position. Therefore, the movement and subsequent position of the armature acts as a switch on the points of contact, allowing or blocking current.

Common relays are further subdivided into two different categories based on their resting function. Normally Open (NO) relays contain contacts that are closed off when the magnetic field current is on, pulling the armature away. Conversely, in a Normally Closed (NC) system, the contacts will be closed off when there is no current traveling through the coil. Finally, in a multiposition relay, the armature makes and breaks the connection with the various stationary contacts as it moves.

Safety relays help reduce the risk of damage to current and personnel in the case of a hazard. Large facilities typically employ multiple safety relays in their system to increase redundancy and to achieve maximal risk reduction. This in turn saves money by extending the longevity of the equipment. The safety relay works by monitoring current and checking for variables like speed and voltage level. If there are any interruptions or abrupt changes, the safety relays shut down a system before harm occurs.

There are several differences between a safety relay and a normal relay and are thus highlighted:

Contact Types

Safety relays are built with the unique ability to recognize the current contact state and thus, react to any changes if necessary. For example, if a Normally Open (NO) contact remains closed after the magnetic field is turned off, the subsequent Normally Closed (NC) contact will not reclose.

Size

On average, a safety relay is much larger than a normal relay, owing to the complex architecture of circuits found in a safety relay. Common relays, on the other hand, are one of the most simply designed circuits in electronics.

Color

While not mandated, safety relays will generally have a bright and obvious color, such as yellow, red, or blue. In comparison, common relays are typically a more muted color, such as white, silver, or black.

Applications

While safety relays are robust in their ability to protect a system and lives, they are usually not required in most applications that employ common relays. For example, the horn of a car is unlikely to experience an overvoltage or other hazardous scenario that could potentially harm the electrical system or driver. As such, only a common relay would need to be used. Conversely, a high voltage substation would require multiple safety relays as the risk of such an event is much higher.

Cost

Depending on the application and parameter requirements, a common relay will typically cost between $20-$200, with outliers on the higher and lower end. In contrast, safety relays usually start at $250 and can run up to $1000 for a more complex system.

Cold Welding

Cold welding occurs when two like contacts become fused without any external heat being applied. While rare in the context of current, the two types of relays have different approaches to the problem. A common relay will continue to operate until an emergency-stop button is pushed, whereas safety relays immediately cease once cold welding occurs.

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1. nicholas smith
2. Posted on November 23, 2021
airborn connectors

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