Understanding three-phase control relays: Why they matter and how they work

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A three-phase network is a type of electrical power transmission system that plays a vital role in keeping our world moving. There are countless machines, components, and systems operating with the three-phase electric power in buildings, industries, and infrastructure that greatly impact our daily lives.

A healthy three-phase network supply not only ensures the proper operation of machines or systems, but it can also help prolong their lifetime and prevent or protect them from operating in inefficient or unfavorable operating situations.

Figure 1: Ideal three-phase power network

Unfortunately, many of them struggle to operate reliably or efficiently, mainly due to the challenges of three-phase power-related issues such as:

A phase lost due to a broken wire or an open or intermittent wire termination in the field

Figure 2: Three-phase power network with one-phase loss

An imbalance between the phases (or phase asymmetry) due to a poor three-phase power network, e.g. uneven or unstable supply or distribution.

Figure 3: Three-phase power network with phase asymmetry

A three-phase control relay can effectively help in such situations by monitoring the three-phase (L1/L2/L3) operating conditions and controlling the application circuitry through the relay output (R status, e.g. from 11-12 to 11-14) when necessary. 

Figure 4: Typical three-phase control relay connection diagram

For example, the basic monitoring version (1) may only cover phase loss and phase sequence. The standard (2) or enhanced (3) version may cover more monitoring parameters, e.g. phase imbalance and under/over voltage with specific monitoring thresholds in percentage.

Figure 5: Example of the Basic, Standard, and Enhanced version of three-phase control relay.

In case of an abnormality arises in the three-phase your power network (e.g. an exceeded parameter threshold), control relays are essential for several reasons:

1. Equipment protection: Monitoring helps protect sensitive equipment from damage due to electrical faults such as overvoltage, undervoltage, phase loss, or phase imbalance. This ensures the longevity and reliability of the equipment.

• Operational efficiency: By constantly monitoring the network, you can ensure that the system operates within optimal parameters. This helps ensure optimal operation and optimized energy consumption.

• Preventative maintenance: Enable active monitoring which allows for early detection of potential issues. This reduces downtime and avoids costly repairs by addressing problems before they escalate.

Don’t hesitate to contact a Schneider representative if you would like to find out more about Harmony control relays.