The IRDH375B-427 is a high‑performance insulation monitoring device from Bender, designed for unearthed AC, AC/DC, and DC power systems (IT systems). It continuously monitors the insulation resistance of the main circuit, providing early warning of insulation faults to ensure electrical safety and prevent system downtime. The device is particularly suitable for modern power supply systems that incorporate variable‑frequency drives (VFDs), rectifiers, inverters, and other power electronic components.
The IRDH375B-427 operates on a low‑voltage DC supply (19.2…72 V DC), making it ideal for installations where only low‑voltage DC auxiliary power is available. It uses Bender‘s patented AMP Plus measuring principle (patent EP 0 709 679 B1), which accurately distinguishes between actual insulation faults and the system‘s natural leakage capacitance – an essential capability in systems with large rectifiers, inverters, or DC components. Compared to the standard IRDH375-427, the “B” version adds four key features: a memory with real‑time clock for date‑ and time‑stamped alarm storage, an electrically isolated RS‑485 interface (BMS protocol) for data exchange, an ISOMETER® disconnecting relay for use in coupled IT systems, and an isolated 0(4)…20 mA analog current output.
The device complies with IEC 61557‑8 and DIN EN 61557‑8, and also meets or exceeds the requirements of NEC 250.21(B) and CEC 10‑106(2) for ground detectors in ungrounded AC systems, as well as NEC 250.167(A) for ungrounded DC systems.
Key Features & Functions
· Patented AMP Plus Measuring Principle – The IRDH375B-427 employs Bender‘s patented AMP Plus measuring technology (patent EP 0 709 679 B1). Unlike standard methods, it reliably distinguishes between real insulation faults and the system‘s inherent leakage capacitance, making it highly effective in systems with large rectifiers, inverters, VFDs, or significant DC components. In systems with high leakage capacitance, the device automatically adjusts to maintain accurate readings.
· Automatic Adaptation to System Leakage Capacitance – Modern power systems often generate high leakage capacitances to earth due to interference suppression measures (e.g., EMI filters). The IRDH375B-427 automatically adapts to the existing system leakage capacitance (maximum permissible ≤ 500 µF), ensuring dependable operation without manual adjustments.
· Two Independently Adjustable Response Values – The device features two separately adjustable alarm relays (K1 and K2), allowing users to distinguish between pre‑warning and critical alarm. Each response value can be independently set from 1 kΩ to 10 MΩ, supporting a two‑stage warning system (e.g., “Maintenance Required” at a higher threshold and “System Shutdown” at a lower threshold).
· Backlit LC Display – A clear backlit LC display shows the measured insulation resistance value in real time, enabling quick on‑site status checks without external tools.
· Electrically Isolated RS‑485 Interface with BMS Protocol – The device includes an isolated RS‑485 interface that supports the Bender Measuring Device Interface (BMS) protocol. This enables seamless data exchange with other Bender devices, central monitoring systems, and PLCs. Maximum transmission distance is 1200 metres.
· Isolated 0(4)…20 mA Analog Current Output – A galvanically isolated analog current output (configurable to 0…20 mA or 4…20 mA) provides a signal proportional to the measured insulation resistance. It can be connected to external instruments, PLCs, or SCADA systems for remote monitoring.
· Memory with Real‑Time Clock – Built‑in alarm storage with a real‑time clock records all alarm messages together with precise date and time stamps, facilitating troubleshooting, maintenance planning, and event analysis.
· ISOMETER® Disconnecting Relay – An integrated disconnecting relay, controlled via external terminals, allows the ISOMETER® to be temporarily disconnected from the monitored IT system. This is essential when operating multiple devices in coupled IT systems, ensuring that only one device remains active at any given time.
· Continuous Self‑Test and Connection Monitoring – An automatic self‑test function continuously verifies the internal measuring circuits and system connections. If an internal fault or a broken connection is detected, a dedicated system fault LED provides immediate indication, ensuring high functional reliability.
· External Test and Reset Functions – External push‑buttons can be connected to initiate a device self‑test and to reset stored fault messages, enhancing convenience for remote or cabinet‑mounted installations.
· De‑Energised System Monitoring – Since the device is powered by an independent external supply, it can monitor the system even when the main circuit is de‑energised, ensuring continuous safety oversight during maintenance or system shutdown periods.
· Voltage Extension via Coupling Devices – Using appropriate Bender coupling devices (e.g., AGH series), the nominal system voltage range can be extended to higher levels such as DC 0…1760 V or 3/N/AC 0…7200 V, making the device suitable for medium‑ and high‑voltage IT systems.
· “W” Version for Harsh Environments (optional) – For applications requiring enhanced mechanical durability, devices with the “W” suffix (e.g., IRDH375BW-427) provide improved shock and vibration resistance. A special varnish coating on the electronics offers higher resistance against mechanical stress and moisture, making them suitable for shipboard installations, railway rolling stock, and seismic environments.
Technical Specifications
· Nominal system voltage (Uₙ): AC 0…793 V / DC 0…1100 V
· Frequency range (fₙ): DC, 0…460 Hz
· Supply voltage (Uₛ): DC 19.2…72 V
· Response value range (Rₐₙ₁ / Rₐₙ₂): 1 kΩ … 10 MΩ, two thresholds independently adjustable
· Maximum permissible system leakage capacitance (Cₑ): ≤ 500 µF
· Measuring principle: AMP Plus (patented adaptive measuring pulse method)
· Response time: ≤ 5 s (at R_F = 0.5 × R_an and C_e = 1 µF)
· Measurement accuracy: 1 kΩ … 1 MΩ: ±15% ±1 kΩ; 1 kΩ … 10 MΩ: ±15% ±0.1 MΩ
· Display: Backlit LC display
· Alarm relays: Two independent changeover contacts (K1, K2), configurable for N/O or N/C operation
· System fault relay: One changeover contact for device‑internal fault reporting
· Communication interface: RS‑485, electrically isolated, supporting BMS protocol (Bender Measuring Device Interface)
· Analog current output: 0(4)…20 mA, galvanically isolated, configurable
· Fault memory: Built‑in storage with real‑time clock for date‑ and time‑stamped alarm messages
· Disconnecting relay: Internal ISOMETER® disconnecting relay for coupled IT system operation
· External inputs: Supports external test and reset push‑buttons
· Operating temperature: –10 °C … +55 °C
· Storage temperature: –40 °C … +70 °C
· Climate class: According to IEC 60721‑3‑3 (class 3K5; 5…95% relative humidity, no condensation)
· Protection class: Terminals IP30, enclosure IP20
· Mounting: DIN rail (35 mm standard rail)
· Dimensions (W × H × D): 144 × 133 × 72 mm (panel cutout 138 × 68 mm when panel‑mounted)
· Weight: ≤ 510 g
· Standards compliance: IEC 61557‑8, DIN EN 61557‑8; also meets or exceeds NEC 250.21(B), CEC 10‑106(2) for ungrounded AC systems, and NEC 250.167(A) for ungrounded DC systems
· EMC: IEC 61326‑2‑4 (shock and vibration resistant)
· Order number: B91065006
Model Number Explanation
· IRDH375 – Product family designation for Bender 375 series insulation monitoring devices.
· B – Enhanced version with additional features: real‑time clock and alarm history storage; electrically isolated RS‑485 interface (BMS protocol); ISOMETER® disconnecting relay for coupled IT systems; isolated 0(4)…20 mA analog current output.
· 427 – Hardware variant configured for a low‑voltage DC supply (19.2…72 V DC).
Typical Applications
· Industrial IT systems – Insulation monitoring for AC, DC, and combined AC/DC unearthed systems incorporating rectifiers, converters, thyristor‑controlled DC drives, VFDs, and other power electronic loads.
· Renewable energy systems – Monitoring insulation in large‑scale photovoltaic (PV) arrays and wind turbine control systems, where high leakage capacitance due to long cable runs and EMI filters is common.
· Marine and offshore applications – Essential for ships and oil rigs where a single ground fault must not cause a power outage. The optional “W” version provides enhanced vibration, shock, and moisture resistance for shipboard installations.
· Industrial automation – Protecting equipment with large VFDs and extensive motor cabling.
· Power plants – Monitoring generator excitation circuits and auxiliary DC systems.
· UPS systems and battery systems – Continuous monitoring of insulation integrity in uninterruptible power supplies and battery backup installations.
· Coupled IT systems – Installations with multiple interconnected IT subsystems, requiring coordinated insulation monitoring via the integrated disconnecting relay so that only one ISOMETER® remains active at any given time.
· De‑energised system monitoring – Applications where continuous insulation monitoring is required even when the main circuit is de‑energised, such as during maintenance or system idle states.
· Extended‑voltage applications – Through coupling devices (e.g., AGH series), the monitoring range can be extended to DC 0…1760 V or 3/N/AC 0…7200 V for medium‑ and high‑voltage IT systems.
Translation missing: en.general.search.loading