Technical Article

Understanding the Differences Between Protection Relays vs Control Relays

October 31, 2022 by Muhammad Asim Niazi

Protection systems are critical in today’s fast-paced industrial revolution for the safety of people and processes. This article discusses electronic protective relay types and how they differ from standard relays.

Relays are an essential element of industrial control engineering, and it is impossible to design machine control without relays. Traditionally, relays had been used as ON / OFF features, but advancement in relay technology has enabled designers to implement relays beyond ON / OFF applications.

We will discuss the application of relays for the protection system, but first, we’ll start with a short comparison of general-purpose control relays and protective relays.


protections relays versus control relays

Figure 1. Protection relays are fundamentally different in construction and purpose from general-purpose control relays.


Standard Control Relay vs Protective Relay 

Some application areas in which control and protective relays differ from each other are:

  • Protective relays measure specific process variables such as current or voltage and, based on their value, switch the output. Control relays, on the other hand, do not monitor any constant. Instead, they only detect the presence of an electrical signal at its input (coil) terminal to change its output (contact) state. 

  • Protective relays require specialized wiring for their integration in machine design. Control relays are quite simple to use and do not require complex wiring.

  • The main purpose of protective relays is to prevent electrical and electronic systems from exposing hazards in an industrial system. Control relays can only be used in switching applications; i.e., to switch the output from one state to another, and do not inherently protect from hazardous supply power.

  • Protective relays can also often indicate the value of the measuring variable, such as voltage or current. Control relays cannot output variable values and can only output either ON or OFF.


Types of Protective Relays 

There are many types of protective relays, the selection of which depends on the hazard for which the protection is required. 

Here are some types of protective relays common in industrial control design:


Earth Fault Relay

Earth fault in a system occurs when current passes in an earth terminal. Since the current in the earth terminal is a significant fault within the wiring of a system, it can damage the wiring as well as connected electrical and electronic components. The earth fault relay protects the system from the hazards of current flowing in the earth terminal.


An earth fault relay and earth leakage relay monitor current passing from phases to ground

Figure 2. Earth fault (and earth leakage) relays monitor for current passing from phases to ground. Image used courtesy of Littelfuse


When the earth fault relay detects current in the earth terminal, it trips the connected circuit with an alarm to protect it from hazards. It requires a current transformer to calculate and detect current, which is connected to the external circuit that requires protection. 

For example, if moisture accumulates at the power terminals, it will short the earth and phase terminals. As a result, current will start to flow from power terminals to earth terminals, and this condition is called earth leakage.

The relay is connected to the current transformer to monitor the current in the earth terminal. When a fault is detected (i.e., current starts to flow in the earth terminal), the relay trips the connected circuit, usually a circuit breaker. It indicates the fault with an alarm state on the relay. 

Earth fault relays are used in medium- to high-voltage applications. They deal with high currents in high-voltage areas such as distribution panels, transformers, and substations.

Manufacturers of earth fault relays include:

  • ABB

  • Siemens

  • Hager

  • Littelfuse


Earth Leakage Relay

Like earth fault, earth leakage is the flow of current in the earth terminal. Earth leakage occurs due to issues within the equipment, due to gradual deterioration or degradation of the equipment.

For example, insulation may begin to break down and reduce the impedance between phase lines and grounded metal casings. As a result, current will gradually start to flow from power terminals to earth terminals, and this condition is called earth leakage.

Earth leakage relays are used to protect electrical and electronic systems from hazards of earth leakage. These relays detect the leakage current and trip the circuit. 

Earth leakage relays are used with a current transformer (CT) to detect and measure current. Sometimes a CT is externally connected, while some earth leakage relays have built-in CTs.

Typically used in high-voltage applications, earth leakage relays function in distribution systems having a voltage range greater than 500 volts, frequency range up to 400 hertz, and current greater than 1,000 amperes.

Manufacturers of earth leakage relays include:

  • ABB

  • Schneider Electric

  • Siemens

Some standards that direct the installation of earth leakage current include but are not limited to:

  • EC 60 755 and IEC 60947-2

  • UL 1053 

  • CSA C22.2


Voltage Relay: Overvoltage and Undervoltage

Voltage relays protect against voltage fluctuations. The two major voltage relay types are overvoltage and undervoltage.


a voltage protection relay to monitor for overvoltage and undervoltage

Figure 3. Voltage protection relays monitor for overvoltage (destroy equipment and harm operators) or for undervoltage (affect equipment operation). Image used courtesy of Dold


Overvoltage relays detect if a connected load has consumed more than the rated current. If the load current exceeds the rated value, it is a sign of malfunction in the connected load's electrical, electronic, or mechanical system. When an overvoltage relay detects an abnormal current consumption, it trips itself and isolates the incoming voltage source with the output circuit or system.

Undervoltage relays protect the connected load (equipment and control panels) when voltage drops below a certain level. This is necessary because underrated voltage can permanently damage the electrical and electronic components, and the system can only be put into working condition by replacing these components. 

Voltage relays can be used for voltage values from 5 volts up to 500 volts for both AC and DC. They can be used for both single- and three-phase configurations, and they monitor power supplies in industrial machines. These relays are also a protection mechanism in industrial and building control panels.

Manufacturers of these relays include:

  • ABB

  • Omron

  • Schneider Electric

  • Dold

Some standards that direct installation of voltage relays include:

  • UL 508

  • CSA C22.2 No.14


Phase Monitoring Relay/Phase Loss Relay

Phase loss relays detect the absence of a phase on one of the terminals in a three-phase system. When it detects a loss in one of the three phases, the relay trips and disconnects the circuit at its output. 

Phase monitor/loss relays are used in a system that requires three phases for normal operation. If one or more phases become absent, the entire system stops working. For this reason, these relays are effective preventive measures for electrical and electronic components. 

For example, a mixing machine uses a three-phase motor for its mixing operation. The motor will not operate if three phases are not present. Besides motor applications, phase monitor/loss relays are used in transformer and generator applications.


a phase loss relay

Figure 4. 3-phase line (phase) loss relays are wired parallel to the load along the phase lines. Image used courtesy of Omron


They are used as control components in three-phase AC, 380- to 400-volt applications. 

Manufacturers of these relays include:

  • Omron

  • Eaton

  • Schneider Electric

Some standards that direct installation of phase monitor/loss relays include:

  • UL

  • GL 

  • CSA 

  • RCM 

  • CE 

  • EAC

  • EN/IEC 60255-1


Protective Relay and Safety Relay Differences

Protective and safety relays are often confused because they are mainly used as a protection mechanism in industrial systems and machines. These two types of relays are different, and their implementation criteria are also different from each other.

Here is a brief comparison of both relays:

  • Protective relays’ functioning depends upon external variables such as voltage or current. Safety relays, on the other hand, are exclusively designed as safety devices to detect relevant hazards. 

  • Protective relays require external components, such as a current transformer, for integration into the electrical system. Safety relays can be directly integrated into the main controller for further processing or automating the process. 

  • Protective relays are generally installed in an electrical panel similar to other ordinary electrical or electronic components. Safety relay installation requires appropriate physical dimensions to cover the hazard’s footprint. 

  • Examples of protective relays include earth faults, earth leakage, and overvoltage, as explained previously. Safety relays examine for the presence of a triggered safety device, such as a light curtain, pull chain, or emergency stop (e-stop) button.


Circuit Protection

Electronic circuit protection is always essential, governed by areas of regulation, formal hazard assessment, and even common sense. 

Choosing the right relays includes consideration of the control circuit with standard control relays, the safety devices with safety relays, and monitoring for safe flow of power with electronics protective relays.


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