Mitigating Electrical Accidents Using Smart Energy Meters – Part 2

Let us now consider a fault condition. Suppose a single line to ground fault takes place in C phase as shown in figure 9.

The fault current this case will complete its circuit as shown in figure 9. Fault current IC will be equal to the vector some of IA and IB. Therefore,

IC = IA + IB

Please note that, IC in C phase will flow toward the neutral. Therefore, we can say that, phase voltage of C phase has reversed its direction. This in turn means that, the voltage of neutral point has shifted from ground potential to phase voltage.

Because of this shifting of neutral voltage, the voltage of healthy phase will become equal to the line voltage. Due to this raised voltage of healthy phases, charging currents will increase i.e., charging current in faulted phase is three times that of the normal charging current. Due to this, heavy arcing will take place in the faulted phase. This phenomenon of arcing is known as Arcing Ground.

Role of adequate earthing/grounding

Grounding/ Earthing means making a connection to the general mass of earth. The use of grounding is so widespread in an electric system that at practically every point in the system, from the generators to the consumers’ equipment, earth connections are made.

There are two types of grounding (Refer figure 10):

• Neutral Grounding
• General (Equipment) Grounding

Objectives of general grounding system include:

• To provide a low resistance return path for fault current which further protects both working staff and equipment installed in the premises (Refer figure 11).
• To prevent dangerous GPR with respect to remote ground during fault condition.
• To provide a low resistance path for power system transients such as lightning and over voltages in the system.
• To provide uniform potential bonding /zone of conductive objects within substation to the grounding system to avoid development of any dangerous potential between objects (and earth).
• To prevent building up of electrostatic charge and discharge within the substation, which may result in sparks.
• To allow sufficient current to flow safely for satisfactory operation of protection system.

The main objective of grounding electrical systems is to provide a suitably low resistance path for the discharge of fault current which ultimately provides safety to working personnel and costly installed equipment by providing sufficient current to safety devices.

In India, the use of earthing terminals in switchboards is often inconsistent, and this has serious safety implications. Earthing, or grounding, is a crucial electrical safety measure designed to prevent electric shocks and fires by directing stray currents safely into the ground. Despite its importance, many people in India neglect or fail to implement proper earthing in their electrical systems. Understanding the reasons behind this and the associated dangers can shed light on why this issue is so critical. (Refer figure 12)

• Lack of awareness and understanding
• Inadequate implementation and enforcement of standards
• Economic constraints
• Unqualified or Undertrained Electricians

Dangers associated with lack of earthing: The dangers of neglecting earthing are severe and multifaceted. Without proper earthing, electrical faults such as short circuits or insulation failures can lead to electric shocks, which can cause injury or even death. Electrical appliances and wiring can become live with stray currents, posing a constant risk to anyone in contact with them. Additionally, the absence of earthing increases the risk of electrical fires, which can result in property damage, loss of life, and financial losses.

Basic concept of smart meter 

A smart meter is an AC static watt-hour meter with time of use registers, internal connect and disconnect switches with two-way communication capability. It is designed to measure flow of forward (import) or both forward (import) and reverse (export), store and communicate the same along with other parameters defined in the standard. It shall be remotely accessed for collecting data/events, programming for select parameters.

The smart meter is a component of Advanced Metering Infrastructure. For the purpose of this standard the smart meter is conceived as single unit comprising of following functional zones:

• Metering,
• Load switch,
• Metering protocol, and
• Communication modules.

The smart meters may have wide usage and the buyer may like to choose desired features to meet the objectives of their overall system and site conditions. In order to facilitate such a flexible approach, the Smart Meter architectures are categorized into two variants. The two variants are diagrammatically represented in Fig. 13 and Fig. 14 respectively. These variants are applicable to both built in type and pluggable type of Smart Meters. Main components are described below:

Smart meter functional requirements

The smart meter is developed as per the standard required to support handling of following operational requirements:

Disconnection Mechanism: The smart meter shall support disconnection (all the switches shall operate) under the following conditions:

• Over current (minimum 105% of Imax in any phase for predefined persistence time),
• Load control limit (programmable and set by utility),
• Pre-programmed event conditions (factory set),
• Disconnect signal from utility control centre, and
• In case of pre-paid facility under defined/ agreed conditions.
• It must be noted that as per relevant Indian Standard:
• Persistence time value to be provided by utility.
• List of events for disconnection to be pre-programmed shall be provided by utility.

The local reconnection due to disconnection under over current and load control limit shall be as follows:

• The switch re-connection is decided by meter locally. It will try to re-connect the load up to predefined time, with predefined interval (time and interval is programmable by utility). If the consumption is within limits meter shall remain in normal connect mode.
• If the consumption is still more than the programmed limits, it will lock out and wait for 30 min (lock out period). After this period the meter shall reconnect the load and if the consumption is still above the limit, the procedure as defined above in (a) shall be repeated with status update to HES.
• In all conditions other than ‘over current and load control limit’ reconnection shall normally be done from HES. In case of failure of communication with HES, reconnection shall be possible through optical port locally with specified security.

Reconnection mechanism for prepayment meter

As per agreed prepayment structure with utility.

• Status of Load Switch: Indication of status of load switch (that is connected/ disconnected) shall be available on display as well as at HES.

Load switches for protection against fire and electrocution

Smart meters are sophisticated devices that play a crucial role in modern energy management, offering enhanced capabilities beyond traditional meters. One of their most critical functions is improving safety, particularly in preventing fire and electrocution risks. A key component in achieving this is the load switch, an integral feature in many smart meters. This section explores how load switches in smart meters contribute to safety by preventing fire and electrocution, and provide guidelines for their effective use.

Understanding load switches in smart meters

A load switch in a smart meter is essentially a relay or circuit breaker that can control the flow of electricity through the meter. It can be remotely operated and is designed to disconnect the electrical load if certain conditions are met. This capability is crucial for enhancing safety by enabling immediate response to dangerous situations.

Protection against fire

• Overcurrent Protection: One of the primary functions of a load switch is to provide overcurrent protection. Excessive current can cause overheating in electrical wiring, potentially leading to fires. Smart meters equipped with load switches can detect when the current exceeds a safe threshold. When this happens, the load switch can automatically disconnect the circuit, preventing overheating and reducing the risk of fire.
• Overvoltage Protection: In addition to overcurrent, overvoltage conditions can also pose a fire risk. Smart meters with load switches can protect against this by disconnecting the circuit when voltage levels exceed safe limits. Overvoltage conditions may result from issues such as lightning strikes or power surges. By disconnecting the load, the smart meter helps to prevent potential damage and fire hazards associated with these conditions.

Protection against electrocution

• Ground Fault Detection: Ground faults occur when electrical current unintentionally flows to the ground, often through a person or conductive material. This can result in electric shock or electrocution. Smart meters with load switches can incorporate ground fault detection features that monitor for abnormal current paths. If a ground fault is detected, the load switch can quickly disconnect the circuit, significantly reducing the risk of electrocution.
• Detection Mechanisms: The load switch in a smart meter can be equipped with ground fault sensors that measure the difference in current between the live and neutral wires. Any imbalance, indicating a potential ground fault, triggers the switch to disconnect the load, protecting individuals from electric shock.
• Residual Current Protection: Residual Current Devices (RCDs) are crucial for protecting against electrocution. Some smart meters integrate RCD functionality into their load switches. These devices detect residual currents that may indicate leakage or faults in the system. When an unsafe residual current is detected, the load switch disconnects the circuit, thus preventing potential electric shock.
• Sensitivity Settings: Load switches with RCD capabilities can be adjusted for sensitivity, ensuring they react appropriately to varying levels of residual current. Proper calibration is essential to balance sensitivity and minimize false tripping while still providing effective protection.

      To be continued…

Dr. Rajesh Kumar Arora obtained his B. Tech. and M.E. degrees in Electrical Engineering from Delhi College of Engineering, University of Delhi. He completed his PhD in grounding system design from UPES, Dehradun. He is also a certified Energy Manager and Auditor and has worked in 400kV and 220kV Substations for more than 14 years in Delhi Transco Limited (DTL). He has also worked as Deputy Director (Transmission and Distribution) in Delhi Electricity Regulatory Commission (DERC). Presently he is working in D&E (Design and Engineering) department of DTL.