Molly Ayala
A separately derived system is a wiring system that is powered by a source of electrical energy or equipment other than a service. This means that the system has no direct electrical connection to circuit conductors of any other electrical source, except for grounding and bonding connections. Separately derived systems include most transformers, as well as generators, batteries, converter windings, solar photovoltaic systems, and wind turbine generators. These systems offer advantages such as improved power quality due to reduced common-mode noise and simplified ground fault sensing. However, they also raise safety questions due to the lack of a direct connection, which results in a voltage difference from other systems.
| Characteristics | Values |
|---|---|
| Definition | A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. |
| Connection to other systems | No direct electrical connection to circuit conductors of any other electrical source other than those established by grounding and bonding connections. |
| Examples | Transformers, generators, batteries, converter windings, solar PV systems, wind turbine generators. |
| Grounding rules | Refer to NEC 250.30. |
| Ground fault sensing | Simplified compared to non-separately derived systems. |
| Common mode noise | Reduced compared to non-separately derived systems. |
| Parallel operation | Possible if there is no interconnected neutral. |
| Safety considerations | Lack of direct connection raises safety questions. |
| Voltage | Will be at a different potential from other systems. |
| Grounding procedure | Connect structural steel and metal piping to the neutral conductor at the SDS per 250.104(D). |
| System bonding jumper | Bonds metal parts of an SDS to the system neutral point. Provides a low-impedance fault current path to clear ground faults. |
| Objectionable neutral current | Can flow on conductive metal parts if more than one system bonding jumper is installed. |
| Transfer switches | Used with generators to open the neutral conductor and ensure grounding in both transfer switch positions. |
| Overcurrent protection | A supply-side bonding jumper is required between the generator and transfer equipment if the system lacks an overcurrent protective device (OCPD). |
| Wye-wye transformers | Not considered separately derived systems due to direct connection between neutral conductors of primary and secondary. |
| Autotransformers | Not considered separately derived systems as one conductor is common to primary and secondary. |
| Advantages and disadvantages | SDS and non-SDS both have pros and cons. SDS typically chosen when ground fault relays are used or sensitive electronics are operated. |
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What You'll Learn
Grounding and bonding
A separately derived system is an electrical source that is derived from something other than a service and has no direct electrical connection to circuit conductors of any other electrical source. Most transformers are separately derived systems, as are generators or UPS supplying a transfer switch that opens the neutral conductor.
The National Electrical Code (NEC) provides requirements for SDSs in Article 250.30. A grounding electrode is a device that establishes an electrical connection to the earth, and it is important to install the grounding electrode as close as possible to the system bonding jumper. Per 250.52(A), a metal water pipe electrode or structural metal electrode can be used as a grounding electrode. If neither of these electrodes is present, a concrete-encased electrode can be used, either vertically or horizontally.
The system bonding jumper is a conductor, screw, or strap that bonds the metal parts of an SDS to the system neutral point. This provides a low-impedance fault current path to the power supply, facilitating the clearing of a ground fault by opening the circuit overcurrent device. The system bonding jumper is essential for safety, as during a ground fault, metal parts of electrical equipment, metal piping, and structural steel will become energized, creating the potential for electric shock and fire.
It is important to note that installing more than one system bonding jumper will result in dangerous objectionable neutral current flowing on conductive metal parts of electrical equipment, metal piping, and structural steel. Therefore, a neutral-to-case connection on the load side of the system bonding jumper is not permitted, except as outlined in 250.142(B). The GEC should terminate at the neutral conductor either at the SDS or the system disconnecting means, but not both.
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Safety and power quality
Safety is a key consideration when it comes to separately derived electrical systems. These systems are defined as electrical sources that have no direct connection to the circuit conductors of any other electrical source, except through grounding and bonding connections.
One of the critical safety aspects of these systems is managing voltage differences. Separately derived systems (SDSs) will inherently have a different potential (voltage) compared to other systems. This voltage difference can create hazards for people and equipment, so it is essential to eliminate these differences while still keeping systems separate. For example, connecting the structural steel and metal piping of an area served by an SDS to the neutral conductor can help ensure the quick removal of dangerous voltage from a ground fault.
Additionally, the use of system bonding jumpers is crucial in preventing electric shock and fire hazards during a ground fault. A ground fault occurs when metal parts of electrical equipment, metal piping, and structural steel become energised. System bonding jumpers provide a low-impedance fault current path to the power supply, allowing overcurrent devices to operate and remove the dangerous condition. However, installing more than one system bonding jumper can lead to dangerous objectionable neutral current flowing on conductive metal parts. Therefore, careful consideration and adherence to guidelines, such as those provided by the National Electrical Code (NEC), are vital to ensure safety.
Power quality is another important aspect of separately derived systems. One advantage of these systems is their ability to reduce common-mode noise, which improves power quality. Common-mode noise on the neutral-ground conductor will not propagate to the load, enhancing the quality of the power supply. However, it is important to note that differential noise (phase-neutral) will not be blocked.
Separately derived systems also offer benefits in ground fault sensing. Unlike non-separately derived systems, which often have complicated ground fault detection schemes due to interconnected neutrals, separately derived systems simplify this process as they have very little circulating current within the switchboard.
In summary, safety and power quality are essential considerations in the design and operation of separately derived electrical systems. These systems help improve power quality by reducing common-mode noise and simplifying ground fault sensing. Additionally, they enhance safety by eliminating voltage differences, utilising system bonding jumpers to prevent electric shock and fire hazards, and adhering to guidelines provided by regulatory bodies such as the NEC.
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Ground fault detection
A separately derived electrical system is defined by the National Electrical Code (NEC) as "a premises wiring system whose power is derived from a source of electrical energy or equipment other than a service". In other words, it is an electrical source that is not directly connected to any other electrical source, except through grounding and bonding connections. Most transformers are separately derived systems, as are generators with 3-pole automatic transfer switches (ATS) and no interconnected neutral.
To detect ground faults in a separately derived system, several methods can be employed:
- Utilize a ground fault detection system: This system can monitor the electrical system for any voltage differences and alert operators to potential ground faults.
- Ground one of the phase conductors: By grounding one of the phase conductors, you can transform an ungrounded separately derived system into a grounded one. However, this may introduce complexities, such as the need for specific circuit breakers.
- Employ a system bonding jumper: As per NEC 250.2, a system bonding jumper connects the metal parts of an SDS to the system neutral point. This provides a low-impedance fault current path, allowing the circuit's overcurrent device to clear the ground fault by opening the circuit.
- Ensure compliance with NEC requirements: The NEC provides specific guidelines for grounding and bonding of separately derived systems, such as NEC 250.30, which outlines the requirements for SDSs, and NEC 250.104(D), which instructs connecting structural steel and metal piping to the neutral conductor to prevent voltage differences.
- Consider the use of fuses: Appropriately selected fuses can serve as protection against ground faults, although they may be expensive to replace and require professional handling.
It is important to note that the specific ground fault detection methods employed will depend on the unique characteristics of the separately derived system in question. Consulting with qualified electrical professionals and adhering to local regulations is essential to ensure the safe and effective detection and mitigation of ground faults.
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Common mode noise
Common-mode noise is a fundamental concept in electronics engineering. It is a type of noise that occurs in electrical systems when noise currents flow in the same direction on both the signal and return lines, completing the circuit through the ground path. This results in electromagnetic interference, which can cause issues such as radio frequency interference (RFI) and electromagnetic compatibility (EMC) problems.
In a separately derived electrical system, the benefit of reducing common-mode noise and improving power quality is achieved. This is because common-mode noise on the neutral-ground conductor will not propagate to the load. However, it is important to note that differential noise (phase-neutral) will not be blocked in such a system.
The path taken by the common-mode current is often unpredictable and can flow through any nearby conductive objects, such as a metal table or the ground itself. This makes it challenging to measure and control. To address this issue, solid metal plates are grounded to Earth and placed near the device under test, creating a predictable path for the common-mode current and enabling more reliable and repeatable measurements.
Engineers can employ various strategies to minimise common-mode noise and its impact on electronic systems. These include the use of common-mode chokes, chip bead ferrites, and proper grounding practices to reduce noise and improve electromagnetic compatibility. Additionally, shielding sensitive electronic devices and circuits from noise sources can help minimise Radiated Electromagnetic Interference (EMI) issues.
Common-mode noise is often contrasted with differential-mode noise. Differential-mode noise occurs when noise currents flow in opposite directions through the source and return paths, and it can be suppressed using a Class-X capacitor. On the other hand, common-mode noise occurs when noise currents flow in the same direction, and it can be addressed using common-mode chokes and proper shielding.
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Grounding rules
A separately derived system is a premises wiring system that derives its power from a source of electrical energy or equipment other than a service. It is a separate power source, such as a battery, generator, photovoltaic (PV) system, transformer, wind turbine, or other electrical power-producing source.
- Grounding a separately derived system means connecting the system to the ground (earth) through a grounding electrode.
- Grounding requirements for separately derived alternating-current systems are based on whether they are grounded or ungrounded. Grounded systems must follow 250.30(A) guidelines, while ungrounded systems must comply with 250.30(B).
- Sections 250.20, 250.21, and 250.26 are directly related to the requirement for grounding a separately derived system.
- The system bonding jumper is a critical component that bonds the metal parts of an SDS to the system's neutral point. It provides a low-impedance fault current path to the power supply, facilitating the clearing of a ground fault by opening the circuit overcurrent device.
- To prevent dangerous voltage buildup during a ground fault, connect the structural steel and metal piping in the area served by the SDS to the neutral conductor at the SDS per 250.104(D).
- Ensure that there is only one system bonding jumper to avoid dangerous objectionable neutral current flow on conductive metal parts of equipment, metal piping, and structural steel.
- The grounding electrode conductor (GEC) should terminate at the neutral conductor either at the SDS or the system disconnecting means, but not both locations per 250.30(A)(3).
- The bonding jumper on the supply side of the first system OCPD is sized according to 250.102(C) and Table 250.102(C)(1), while the conductor installed on the load side is an equipment grounding conductor (EGC) sized using Table 250.122.
- Generators grounded as separately derived systems must meet the requirements outlined in 250.30(A) and 250.35 for permanently installed generators.
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Frequently asked questions
A separately derived electrical system is a wiring system that is powered by an electrical energy source or equipment other than a service. In other words, it has no direct electrical connection to circuit conductors of any other electrical source.
Examples of separately derived electrical systems include generators, batteries, converter windings, transformers, solar PV systems, and wind turbine generators.
One benefit of a separately derived electrical system is that it simplifies ground fault sensing. Additionally, it can improve power quality by reducing common mode noise.
