Insulation monitoring devices (IMDs) are used in ungrounded high-voltage applications to ensure electrical safety, prevent accidents, and ensure reliable operation by continuously monitoring insulation resistance for early fault prediction.
A residual current monitor (RCM) is the corresponding device used in grounded systems. In contrast, residual current devices (RCDs) protect against hazards like fires or electrocution in ac-powered applications like electric vehicle (EV) chargers.
In EVs, IMDs ensure safety by continuously measuring the insulation resistance between the HV system and the vehicle chassis. The foundational IEC standard for IMDs is IEC 61557-8 for electrical safety in low-voltage distribution systems up to 1 and 1.5 kVdc.
While the standard was developed for information technology (IT) equipment in applications like data centers, the voltage limits apply to EV drivetrains, renewable energy, battery energy storage systems, and low-voltage industrial motors.
For example, standards UL 2232 for EV chargers and IEC 61851 for EV dc fast chargers explicitly refer to IEC 61557. IEC 60204-1, which deals with the safety of electrical equipment in machines, also refers to IEC 61557.
How does that work?
IMDs apply a low-voltage signal to the HV system and measure the resulting current flow through the insulation to the ground. In addition to ensuring safety, IMDs enhance system reliability.
For example, excessive voltage can damage the winding insulation and reduce the lifespan of motors. An IMD alerts operators to potentially harmful conditions, allowing the system to be shut down before a critical value is reached (Figure 1).
Figure 1. An IMD can issue a warning alarm before the critical shutdown value is reached. (Image: Aktif Elektroteknik)
Insulation monitoring is also vital to the safe and reliable operation of marine systems, renewable energy installations, aerospace systems, and more.
Monitoring current
RCMs are designed to protect against excessive leakage currents in grounded electrical systems. They continuously monitor the difference between the current flowing in and out of a circuit to identify leakage current.
They provide data for system monitoring, potentially triggering alarms or other actions without immediately shutting down the power. If the leakage current reaches a critical level, the RCM will trip and disconnect the circuit (Figure 2). RCMs can be designed to monitor ac or dc flows.
Figure 2. An RCM monitors current flow and can issue multiple alarms before reaching shutdown. (Image: Dold)
The primary standard for residual current monitors (RCMs) is IEC 62020, which applies to applications up to 440 Vac and rated currents not exceeding 125 A. The standard details the requirements for two types of RCMs. Type A RCMs detect ac and pulsating dc residual currents for use in general-purpose applications.
Type B RCMs are designed for use in applications with electronic loads, variable-speed drives, and other systems where smooth dc currents need to be monitored. They can also monitor ac and pulsating dc currents.
Pure protection
An RCD, also known as a ground fault circuit interrupter (GFCI), quickly shuts off power in case of an earth fault or leakage current. It doesn’t provide any monitoring. It simply detects imbalances in the current flowing through the circuit and trips if a predetermined amount of current leaks to the ground, preventing electric shock and protecting from fires.
In residential settings, RCDs are often used as standalone protection devices. In industrial and commercial applications, they can be used with RCMs to provide more reliable system operation.
IEC 60755 covers general safety requirements for RCDs, including those intended for ac systems (IEC 60755:2017) and dc systems (IEC 60755-1:2022). RCDs are classified by their sensitivity to different types of residual currents, including:
- AC is for general-purpose use and detects sinusoidal ac residual currents.
- A for equipment with electronic components detects sinusoidal ac and pulsating dc residual currents.
- F is used with frequency-controlled speed drives and detects sinusoidal ac residual currents, pulsating dc residual currents, and smooth dc residual currents.
- B is used for specific three-phase applications, including electric vehicle chargers and solar photovoltaic systems. It detects smooth dc residual currents in addition to ac and pulsating dc.
- S has a time delay to ensure selectivity between devices and cannot be used for life protection.
Summary
IMD, RCM, and RCD support electrical safety and system reliability. Their applications include EV drivetrains and chargers, industrial drives, and renewable installations. Each type of device is governed by a different series of IEC standards.
References
- Enhancing the safety of electric and hybrid vehicles, Bender
- How solid-state relays simplify insulation monitoring designs in high-voltage applications, Texas Instruments
- Insulation monitoring: The concept, Bender
- Insulation monitoring device, Wikipedia
- Insulation monitors, Dold
- Residual current monitors, Residual current monitoring, Dold
- The Importance of Insulation Monitoring Devices, Aktif Elektroteknik
- What Is Insulation Monitoring Device Working Principle? Blue Jay Technology
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