Trystan Casey
Single-line diagrams (SLDs) are a key tool in developing and documenting an electrical power system. They are used to communicate power system design requirements through a combination of drawings, schedules, and technical specifications. The numbers in parenthesis on an SLD define the quantity of each specific element. For example, the 50/51N function is shown as a quantity of (1). The 50/51 functions are protective elements that can be used in both relays to back each other up in the event of a failure.
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What You'll Learn
- /51N function: a transition from three individual phase elements to a single residual neutral protective element
- /51 functions: protective elements used in both relays to back each other up in case of failure
- and 51 are part of the most commonly used numbering for power circuits
- and 51 are part of the ANSI/IEEE standard for labelling protection devices
- Single Line Diagrams (SLDs) are a key tool in developing and documenting an electrical power system
50/51N function: a transition from three individual phase elements to a single residual neutral protective element
In electrical drawings, the numbers 50 and 51 refer to types of overcurrent relays, which are the most commonly used protective relay type. Type 50 is an "instantaneous off" relay, while type 51 has an inbuilt time delay. The N, G, etc. depicts not what the relay is, but how it is wired up. In the case of 50/51N, the N is wired to a current transformer around the neutral (earth-connected) point of a star (Y) 3-phase system.
The 50/51N function, therefore, refers to a transition from three individual phase elements to a single residual neutral protective element. This is shown in electrical drawings with the symbol to the right of the relay, which represents the transition from three individual phase elements to a single residual neutral protective element. The output of each protective function is indicated with a dashed line and arrow, showing the action to be taken if the relay determines that the monitored values exceed the preset thresholds.
In a solidly-grounded medium voltage system, the most common choice for ground fault protection is to add a fourth relay in the residual connection of the CTs to monitor the sum of all three phase currents. This relay is referred to as a residual ground overcurrent or 51N (or 50/51N) relay. The CT arrangement for 50/51 and 50/51N relays for a solidly-grounded system is shown in Figure 2. For a low-resistance-grounded system, the best option is usually to use an overcurrent relay connected to a CT in the service transformer or generator neutral. This CT should have a ratio smaller than the phase CTs, and the relay pickup range in conjunction with the neutral CT should allow a pickup as low as 10% of the neutral resistor rating.
In an MV system with an isolated neutral, the relay has 50N/51N protection set. It senses three-phase currents and vectorally sums them, and it is set to trip at a certain percentage of the nominal load. However, this is not sufficient for ground fault protection, as an isolated or ungrounded system requires a voltage-based detection system, such as a 59N.
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50/51 functions: protective elements used in both relays to back each other up in case of failure
Protective relays are critical components in power systems, providing essential protection for various elements such as generator sets, outgoing feeder and load networks, and incoming utility sources. They are indispensable in maintaining the safety and reliability of power systems. The 50/51 protective relay is one such relay that includes multiple set points.
The 50/51 overcurrent relay's instantaneous function (50) provides an instant trip signal when the current flow exceeds the setting of the relay's 50 element. This function is electromechanical and consists of a coil, armature, and contact assembly. The 50 function can be added as an instantaneous attachment to a 51 time-overcurrent relay.
The time function (51) of the 50/51 relay provides a trip signal when the current flow exceeds the setting of the relay's 51 element for a given period. This allows for significant overcurrent magnitudes, provided the overcurrent events are brief enough to avoid heat damage to power equipment. The 51C relay operates on overcurrent when the voltage is below a preset value, and the 51 V relay pickup current shifts with voltage changes, responding only to overcurrents at reduced voltage.
The 50 and 51 functions can be used in both relays to back each other up in case of failure. This is possible because the 50 function has no inherent time delay, so it can act as a backup to the 51 function, which has a time-overcurrent protection function. In practice, three different protective relay circuits would be connected together to the circuit breaker's trip coil, so that if any of the 51 relays detect a timed overcurrent, the breaker will trip.
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50 and 51 are part of the most commonly used numbering for power circuits
To comply with the standards of the American National Standard Institute/Institute of Electrical and Electronics Engineers (ANSI/IEEE), electrical drawings use symbols, numbers, and letter prefixes and suffixes. The most commonly used numbers for power circuits are 88, 42, 6, 51, and 52. For example, in a Star-Delta starter numbering system, 88 is used for the Main Contactor, 6 for the Starting Contactor, and 42 for the Running Contactor.
Numbers in parentheses in electrical drawings define the quantity of each specific element. In many cases, this quantity is (3), representing the three phases. However, in the 50/51N function, the quantity is shown as (1), indicating a transition from three individual phase elements to a single residual neutral protective element.
The 50/51 functions are essential protective elements in power circuits. They represent the 50/51 Elements of the protection relay, which include Instantaneous Overcurrent and Time Overcurrent. These protective functions are designed to take specific actions if the monitored values exceed preset thresholds. For instance, in the Eaton's ETR-5000-T1 Transformer Differential Relay, the 50/51 Elements are shown to trip a high-speed 86-M1 Lockout Relay.
Understanding electrical drawings is crucial for electricians in all branches of electrical work. These drawings provide essential information about the location of outlets, the routing of circuits, the placement and size of panel boards, and other critical electrical details. Electrical drawings are a key tool in developing and documenting electrical power systems, ensuring effective communication between power system designers, engineers, and other stakeholders.
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50 and 51 are part of the ANSI/IEEE standard for labelling protection devices
In electric power systems and industrial automation, 50 and 51 are part of the ANSI/IEEE standard for labelling protection devices. The ANSI/IEEE standard uses device numbers to identify equipment and devices in a system, such as relays, circuit breakers, or instruments. These devices protect electrical systems and their individual components from damage when an unwanted event occurs, such as an electrical fault.
Historically, a single protective function was performed by one or more distinct electromechanical devices, with each device receiving its own number. Today, microprocessor-based relays can perform many protective functions in one device. When a device performs several protective functions, it is typically denoted as "11" by the standard as a "Multifunction Device".
The numbers 50 and 51 are used to denote an Instantaneous/Time-delay Overcurrent relay. In the case of the 50/51N function, the number (1) indicates a transition from three individual phase elements to a single residual neutral protective element. The output of each protective function is shown with a dashed line and arrow, indicating the action to be taken if the relay determines that the monitored values exceed the preset thresholds.
The 50/51 functions can be used in both relays to back each other up in the event of a failure. For example, the protection relay's 50/51 Elements (Instantaneous Overcurrent and Time Overcurrent respectively) can be shown tripping a high-speed 86-M1 Lockout Relay.
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Single Line Diagrams (SLDs) are a key tool in developing and documenting an electrical power system
SLDs are used to communicate design requirements and provide a framework for incorporating different types of information, such as incoming and utilisation voltages. They are essential for power system designers to visualise and document the power distribution system, ensuring efficient planning and troubleshooting, as well as reducing potential outages. The diagrams start with the incoming power source and follow the power flow through various conductors and voltage transformations to feed distribution equipment buses.
Standard electrical symbols are used in SLDs to represent different components and their relationships within the system. These symbols include transformers, fuel tanks, switchgears, switchboards, automatic transfer switches, and uninterruptible power supply (UPS) units. By using these symbols, SLDs can show all or part of a system, from simple DC circuits to complex three-phase systems.
In addition to visualising the system, SLDs can also provide key information on installation details, such as voltage and current. This makes them valuable for communicating with installers about how the system will be connected. SLDs are also useful for protection schemes, where certain protective elements (such as the 50/51 functions) can be used in backup relays to ensure system functionality in the event of a failure. The 50/51 functions refer to the protection relay's instantaneous overcurrent and time overcurrent elements, respectively, which can trigger a high-speed lockout relay if the monitored values exceed preset thresholds.
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Frequently asked questions
50/51 are the protective functions of the power relay.
The 50/51 protective functions are used to back each other up in the event of a failure.
The 50 function is an Instantaneous Overcurrent protective function, while the 51 function is a Time Overcurrent protective function.
Numbering is done as per ANSI/IEEE (American National Standard Institute/Institute of Electrical and Electronics Engineers) standards.
Other commonly used numbers for power circuits include 88, 42, 6, and 52.
