What is LSI”G” and is it required?
- Introduction
- What is LSIG
- Where is LSIG mentioned?
- IEEE 242-2001: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
- IEEE 3004.5-2025: IEEE Recommended Practice for the Application of Low-Voltage Circuit Breakers in Industrial and Commercial Power Systems.
- IEEE C37.17™-2022: IEEE Standard for Trip Systems for Low-Voltage (1000 V and below) AC and General Purpose (1500 V and below) DC Power Circuit Breakers.
- Design philosophy of earth fault protection
- Automatic Disconnection of Supply (ADS):
- Measuring the fault current (Overcurrent): The disconnection time is decided, and based on the TCC of the breaker, the Zs (max) is calculated.
- Measuring the differential current: Zs (max) is found based on the IΔn (A) and the touch voltage limit.
- Earth fault protection in NFPA 70:
- Conclusion and Discussion:
- Check whether ADS can be achieved by the prospective fault current.
- If the installation is in the design stage, software such as EOM-NEC(IN), which is compliant with the requirements of IS 732 and NEC 2023, can be used.
- If the installation is constructed, specialised electrical inspection can be sought out to verify the ADS as per IS 732 and NEC 2023.
- Software (EOM-NEC(IN)) capable of calculating the cumulative leakage current from all connected loads and ping errors to avoid unnecessary tripping.
What is LSI”G” and is it required?
Circuit breakers are designed to protect against earth faults, short circuits, and overloads. They (MCCB and ACBs) introduce settings which affect the time-current curves (TCC). The circuit breakers introduce “L S I G” in different configurations, such as LI/LS/LI/LSIG. This blog is to discuss about LSIG setting, and is “G” required as per the fault protection concept of IEC and Indian Standards.
Note: In this blog, we will focus only on the automatic disconnection of supply (ADS) and protective equipotential bonding as the fault protective measure in a TN-S system. For other measures, refer to IEC 60364 or IS 732. Additional protective measures, such as RCD, are not discussed in the blog
Long Time (L): Disconnection time in minutes, to 1 or 2 hours. Provided for overcurrent protection for cables, conductors and equipment.
Short Time (S): Disconnection time in seconds to milliseconds. Provided for overcurrent and earth fault protection. They provide usability during the starting current and inrush current requirement for the equipment by providing a time delay or a bigger pickup.
Instantaneous (I or INST): Disconnection time in milliseconds. Provided for earth fault and short circuit protection.
Earth Fault (G): Disconnection time in Seconds and milliseconds. Only for earth fault protection. Generally, works by sensing the differential current of the connected circuit.
Figure 1 Typical LSIG TCC
As far as IS or IEC is concerned, LSIG is specified in IEC 60947-2 as part of the informative annexe, which is a product standard. However, no installation standards from IEC or IS bring LSIG into the books for design, erection and testing.
LSIG can be linked to multiple IEEE standards, namely
In IEC and IS, earth fault protection is achieved by ADS and protective equipotential bonding. ADS can be achieved through 2 methods, as mentioned in 4.1.
It is common sense that during a fault, the supply must be cut off (the circuit breaker should trip), preferably the closest upstream breaker to the fault location (selectivity). This can be achieved by making sure that fault loop impedance is within limits and selectivity is achieved in the circuit.
Generally, for circuit breakers, there are two principles used to calculate the Zs(max) to achieve ADS, which are related to how earth fault protection is achieved.
Generally, all OCPDs follow point 1, and RCDs follow point 2. ADS for the Earth fault protection module “G” in OCPDs can follow point 2.
It can be noted that the US also follows the IEC for ground fault protection, with some exceptions. Below are some of the clauses from NFPA 70:
“250.4 (3) Bonding of Electrical Equipment: Normally non-current carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
250.4 (4) Bonding of Electrically Conductive Materials and Other Equipment: Normally, non-current-carrying electrically conductive materials that arc likely to become energised shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
250.4 (5) Effective Ground-Fault Current Path: Electrical equipment and wiring and other electrically conductive material likely to become energised shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault occurs to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.”
A special ground fault protection for the equipment is also to be provided for circuits above 1000 amps. However, this clause doesn’t provide information on how ground fault protection is to be achieved.
“210.13 Ground-Fault Protection of Equipment: Each branch circuit disconnecting means rated 1000 amperes or more and installed on solidly grounded wyc electrical systems of more than 150 volts to ground, but not exceeding 1000 volts phase-to-phase, shall be provided with ground-fault protection of equipment in accordance with 230.95.
215.10 Ground-Fault Protection of Equipment: Each feeder disconnect rated 1000 amperes or more and installed on solidly grounded wyc electrical systems of more than 150 volts to ground, but not exceeding 1000 volts phase-to-phase, shall be provided with ground-fault protection of equipment in accordance with 230.95.
230.95 Ground-Fault Protection of Equipment: Ground-fault protection of equipment shall he provided for solidly grounded wyc electric services of more than 150 volts to ground but not exceeding 1000 volts phase-to-phase for each service disconnect rated 1000 amperes or more. The grounded conductor for the solidly grounded wyc system shall be connected directly to ground through a grounding electrode system, as specified in 250.50, without inserting any resistor or impedance device.
The rating of the service disconnect shall be considered to be the rating of the Largest fuse that can be installed or the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted.”
IEEE recommendations use ground fault protection module (measuring the differential current to disconnect, point 2 of 4.1) in MCCBs and ACBs (LVCBs) to achieve the above requirement from NFPA 70. However, there is no direct requirement from NFPA 70 that specifies such a use.
All MCCBs and ACBs are capable of achieving ADS by measuring the fault current (overcurrent). Where the differential current method is used for ADS, then MCCBs and ACBs require an earth fault protection module “G”. However, they tend to trip frequently due to the combined leakage current of all the loads, especially applicable for commercial buildings used for office purposes and data centre applications. In the long run, in such locations where continuity of supply is needed, the G module is not suitable, and it may be disabled by the user.
One way to overcome the issue of “G” module safely is by:
On average, adding the earth fault module (G) in MCCB costs anywhere between 10,000 and 20,000 INR. Is it required if a proper design can be made?.
Hint: The same question can be used for other functions of microprocessor-based MCCB, ACB.
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