Electrical System Protection Relay Selections IEEE ANSI Codes
Selecting the correct protection relays based on ANSI codes is critical for ensuring electrical system safety. Protection relays are responsible for detecting faults in the system and isolating faulty equipment to prevent damage to the system and minimize safety risks to personnel. If the wrong type of protection relay is used or if the settings are incorrect, the system may not be adequately protected, leading to equipment damage, system failures, and potentially dangerous situations. For example, if an instantaneous overcurrent relay is used instead of a time overcurrent relay, the system may not be able to withstand temporary overloads, leading to false tripping and unnecessary downtime. Conversely, if a time overcurrent relay is used instead of an instantaneous overcurrent relay, the system may be at risk of catastrophic failure due to delayed tripping. Therefore, it is important to select the correct protection relay based on the system’s requirements and to ensure that it is installed and configured properly to provide the necessary level of protection.
- IEEE Standards Association (IEEE-SA): Develops standards for various aspects of power system protection, including protective relays. Examples of relevant IEEE standards include IEEE C37.2, IEEE C37.90, and IEEE C37.111.
- National Electrical Manufacturers Association (NEMA): Develops standards and guidelines for various electrical products, including protective relays. Examples of relevant NEMA standards include NEMA SG-4 and NEMA TR 1.
- International Organization for Standardization (ISO): Develops international standards for various industries and applications, including power systems. Examples of relevant ISO standards include ISO 8528, ISO 13849, and ISO 61850.
It’s important to consult relevant standards and guidelines when selecting and implementing protective relays in a power system to ensure safe and reliable operation.
Here is a table that lists most of the ANSI codes for electrical protection relays, along with their description and corresponding IEC standards, highlighted in bold are the most common protections:
| ANSI Code | Name | Description | IEC Standard No. |
| 24 | Overexcitation Relay | A relay used to protect synchronous machines against overexcitation by disconnecting them from the network when the excitation current exceeds a predetermined threshold. | IEC 60255-27 |
| 25 | Synchronizing or Synchronism-Check Relay | A relay used to check the synchronism of two alternating-current circuits before closing a circuit breaker or connecting a generator to the power system. It checks the voltage, frequency, and phase angle of the two circuits to ensure they are within acceptable limits. | IEC 60255-118 |
| 27 | Undervoltage Relay | A relay used to trip a circuit breaker or initiate other protective actions when the voltage of a power system falls below a predetermined threshold. It is designed to prevent damage to equipment and ensure the safety of personnel. | IEC 60255-27 |
| 32 | Directional Power Relay | A relay used to detect the direction of power flow in a power system and trip a circuit breaker or initiate other protective actions when the power flow exceeds a predetermined threshold. It is used to protect transmission lines, transformers, and generators. | IEC 60255-149 |
| 37 | Undercurrent or Underpower Relay | A relay used to trip a circuit breaker or initiate other protective actions when the current or power in a power system falls below a predetermined threshold. It is used to prevent damage to equipment and ensure the stability of the system. | IEC 60255-149 |
| 40 | Loss of Field Relay | A relay used to protect synchronous generators against loss of excitation by disconnecting them from the network when the field current falls below a predetermined threshold. | IEC 60255-27 |
| 46 | Negative-Sequence Relay or Reverse Power Relay | A relay used to protect generators, transformers, and transmission lines against unbalanced faults caused by negative-sequence currents. It detects the presence of negative-sequence currents and trips a circuit breaker or initiates other protective actions. | IEC 60255-27 |
| 47 | Phase-Sequence or Negative Sequence or Phase-Balance Voltage Relay | A relay used to protect generators, transformers, and motors against phase sequence and phase imbalance faults by detecting the sequence of the applied voltage and tripping a circuit breaker or initiating other protective actions if the sequence is incorrect. | IEC 60255-7 |
| 48 | Incomplete Sequence protection | Protective function used in motor protection relays that monitors the power sequence of the three-phase supply to the motor. During normal operation, the power sequence to the motor should be ABC. If there is a fault in the power supply, such as a phase loss, the power sequence can be disrupted and the motor can be damaged. In such cases, the Incomplete Sequence Protection function will detect the disrupted power sequence and trip the motor circuit to prevent damage. The Incomplete Sequence Protection function is designed to detect power sequence faults that could damage the motor, such as phase loss or phase reversal. It does this by monitoring the voltages of the three-phase supply and comparing them to the expected power sequence. If the sequence is incomplete or incorrect, the relay will trip the motor circuit to prevent damage. | IEC 60255-24 |
| 49 | Thermal Overload | A protection element that provides protection against thermal overloading of equipment, such as motors, generators, and transformers. The thermal overload protection element works by measuring the current flowing through the equipment and comparing it to a reference value. If the current exceeds the reference value for a certain period of time, the 49 element will trip the circuit breaker to isolate the equipment and protect it from damage due to thermal overloading. | IEC 60255-8 |
| 50 / 51 | Instantaneous Overcurrent/Time Overcurrent Relay | A relay used to detect overcurrents in a power system and trip a circuit breaker or initiate other protective actions when the current exceeds a predetermined threshold. The 50 variant operates instantaneously, while the 51 variant is time-delayed. This combination of elements provides both fast-acting and time-delayed protection against overcurrents, which is important for protecting equipment and maintaining the stability of the power system. | IEC 60255-151 |
| 50N | Instantaneous Neutral Overcurrent Relay | Relay designed specifically for neutral current protection. This relay is used to detect and protect against ground faults in the electrical system. The relay is typically set to trip when the current flowing through the neutral conductor exceeds a certain threshold value. | IEC 60255-151 |
| 51N | Time-Delay Neutral Overcurrent Relay | Relay designed specifically for neutral current protection. This relay is used to provide protection against phase-to-neutral faults in the electrical system. The relay is typically set to trip after a specified time delay when the current flowing through the neutral conductor exceeds a certain threshold value. | IEC 60255-151 |
| 51H | High-set Overcurrent Relay | overcurrent relay that provides protection against phase-to-phase faults. The “H” in the code indicates that this element is a high-set overcurrent relay, which means that it operates at a higher current level than the standard overcurrent relay elements. Typically used for protection against severe faults that could damage equipment or disrupt the power system. When the current exceeds a certain level, the 51H element will trip the circuit breaker to isolate the fault and protect the system. | IEC 60255-151 |
| 52 | AC Circuit Breaker | A relay used to control the opening and closing of an AC circuit breaker. It detects the current flow through the breaker and activates the trip mechanism when a fault is detected. | IEC 60255-131 |
| 59 | Overvoltage Relay | A relay used to trip a circuit breaker or initiate other protective actions when the voltage in a power system exceeds a predetermined threshold. It is designed to prevent damage to equipment and ensure the safety of personnel. | IEC 60255-27 |
| 64 | Ground Detector Relay or Earth Fault Relay | A relay used to detect ground faults in power systems and initiate protective actions to clear the fault. It detects the presence of current in the grounding system and trips a circuit breaker or initiates other protective actions. | IEC 60255-131 |
| 64R | Restricted Earth Fault Relay | protection relay that is specifically designed to detect earth faults in power systems where there is a neutral grounding resistor. This relay measures the current flowing through the neutral grounding resistor and compares it to the current flowing through the protected circuit, and it trips the circuit breaker if the difference between these two currents exceeds a predetermined value. | IEC 60255-131 |
| 67 | AC Directional Overcurrent Relay | A relay used to detect overcurrents in a specific direction in a power system and trip a circuit breaker or initiate other protective actions when the current exceeds a predetermined threshold. It is used to protect transmission lines, transformers, and generators. | IEC 60255-131 |
| 68 | Blocking or Out-Of-Step Relay | A relay used to prevent tripping of a circuit breaker by other protective relays or external devices. It is used to ensure the selective operation of protective relays and prevent unnecessary interruption of power. | IEC 60255-27 |
| 70 | Voltage-Transformer Supervision Relay | A relay used to monitor the performance of voltage transformers and trip a circuit breaker or initiate other protective actions when a fault or abnormal condition is detected. | IEC 60255-27 |
| 74 | Alarm Relay | A relay used to indicate an abnormal condition or fault in a power system. It is used to alert operators and initiate corrective actions. | IEC 60255-8 |
| 76 | DC Overcurrent Relay | A relay used to detect overcurrents in DC systems and trip a circuit breaker or initiate other protective actions when the current exceeds a predetermined threshold. It is used to protect DC motors, generators, and batteries. | IEC 60255-131 |
| 78 | Phase-Angle Measuring or Out-of-Step Relay | A relay used to measure the phase angle between two power system quantities and trip a circuit breaker or initiate other protective actions when the angle exceeds a predetermined threshold. It is used to protect generators and motors from damage due to out-of-step conditions. | IEC 60255-27 |
| 79 | AC Reclosing Relay | A relay used to automatically reclose a circuit breaker after a fault has been cleared. It is used to minimize the interruption of power and improve the reliability of the power system. | IEC 60255-131 |
| 81 | Frequency Relay | A relay used to trip a circuit breaker or initiate other protective actions when the frequency of a power system deviates from the nominal value. It is used to prevent damage to equipment and ensure the stability of the system. | IEC 60255-8 |
| 83 | Automatic Selective Control or Transfer Relay | A relay used to automatically switch power sources or control devices to maintain the supply of power to critical loads during a power system disturbance or fault. It is used to improve the reliability of the power system. | IEC 60255-151 |
| 85 | Carrier or Pilot-Wire Receiver Relay | A relay used to receive carrier or pilot signals transmitted over power lines and initiate protective actions based on the signals. It is used to communicate between different parts of the power system and coordinate the operation of protective relays. | IEC 60255-23 |
| 86 | Lockout Relay or Master Trip Relay | A relay used to prevent the operation of a circuit breaker or other device after a fault or abnormal condition has been detected. It is used to prevent further damage to equipment and ensure the safety of personnel. | IEC 60255-149 |
| 87 | Differential Relay | A relay used to detect and isolate internal faults in transformers, generators, motors, and other equipment by comparing the currents flowing into and out of the equipment. It is one of the most important protective relays in power systems. | IEC 60255-27 |
| 89 | Line Switch | A device used to connect or disconnect a power line to or from a power system. It is commonly used in distribution systems to isolate faults and maintain the reliability of the system. | IEC 60255-121 |
| 90 | Regulating Device | A device used to control the voltage or current in a power system. It is commonly used to regulate the output of generators and transformers and maintain the stability of the system. | IEC 60255-121 |
| 91 | Voltage Directional Relay | A relay used to detect voltage imbalances in a power system and trip a circuit breaker or initiate other protective actions when the voltage exceeds a predetermined threshold. It is commonly used to protect transformers and generators. | IEC 60255-151 |
| 92 | Voltage and Power Directional Relay | A relay used to detect voltage and power imbalances in a power system and trip a circuit breaker or initiate other protective actions when the imbalances exceed predetermined thresholds. It is commonly used to protect transformers and generators. | IEC 60255-151 |
| 93 | Field-Changing Contactor | A device used to change the field current in a generator or motor. It is commonly used to regulate the output of the machine and maintain the stability of the system. | IEC 60255-121 |
| 94 | Tripping Relay or Trip Circuit Supervision Relay | A relay used to trip a circuit breaker or other device in response to a fault or abnormal condition in a power system. It is commonly used in conjunction with other protective relays to provide selective and coordinated protection. | IEC 60255-27 |
| 95 | Out of Step Relay | A relay used to detect and isolate generators that are out of step with the power system. It is commonly used to protect against instability and loss of synchronism. | IEC 60255-118 |
| 96 | Breaker Control Relay | A relay used to control the operation of a circuit breaker. It is commonly used to coordinate the operation of multiple protective relays and ensure the proper operation of the system. | IEC 60255-149 |
| 97 | Voltage Relay | A relay used to detect voltage imbalances in a power system and trip a circuit breaker or initiate other protective actions when the voltage exceeds a predetermined threshold. It is commonly used to protect transformers and generators. | IEC 60255-151 |
| 98 | Displacement Relay | A relay used to detect the displacement of a power line from its normal position. It is commonly used to protect against damage caused by wind, ice, or other environmental factors. | IEC 60255-8 |
| 99 | Machine or Transformer Relay | A relay used to protect a generator, motor, or transformer against faults and abnormal conditions. It may include various protective functions such as overcurrent, overvoltage, undervoltage, and differential protection. | IEC 60255-27 |