The IRDH375-427 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 IRDH375-427 is powered by a low‑voltage DC supply (DC 19.2…72 V), making it particularly suitable for installations where only low‑voltage DC auxiliary power is available. It is panel‑mounted for control cabinet door installation and utilizes Bender‘s patented AMPPlus measuring technology, which reliably distinguishes between actual insulation faults and the system’s inherent leakage capacitance – an essential capability in systems with large rectifiers, inverters, or DC components.
IRDH375-427 complies with IEC 61557‑8 and DIN EN 61557‑8, as well as NEC 250.21(B) and CEC 10‑106(2) for ground detectors in ungrounded AC systems, and NEC 250.167(A) for ungrounded DC systems.
The IRDH375-427 is the standard version of the IRDH375 series. It shares the same hardware platform as the IRDH375B-427 but does not include the enhanced features of the “B” version: alarm history storage with real‑time clock, electrically isolated RS‑485 interface (BMS protocol), ISOMETER® disconnecting relay for coupled IT systems, or isolated 0(4)…20 mA analog current output.
Key Features & Functions
· Patented AMPPlus Measuring Principle – The IRDH375-427 employs Bender‘s patented AMPPlus measuring technology, which delivers accurate insulation monitoring even in systems with extensive DC components and high leakage capacitances – including pure DC systems, systems with rectifiers, and those with variable‑frequency drives (VFDs). This patented method automatically adapts to modern power system conditions to ensure reliable readings.
· Automatic Adaptation to System Leakage Capacitance – Modern power systems often generate high leakage capacitances to earth due to interference suppression measures (such as EMI filters). The IRDH375-427 automatically adapts to the existing system leakage capacitance (maximum permissible ≤ 500 μF), optimising measurement time and ensuring dependable operation without manual adjustments.
· Two Independently Adjustable Response Values – The device features two separately adjustable alarm relays (Alarm 1 and Alarm 2), allowing users to distinguish between pre‑warning and critical alarm. The response value ranges are independently configurable from 1 kΩ to 10 MΩ (or 1…500 kΩ depending on configuration), offering flexible configuration to meet different system safety requirements.
· Backlit Two‑Line LC Display – Equipped with a clear backlit two‑line LC display, the device presents measured insulation resistance values, system leakage capacitance readings, and other operational parameters. On‑device operation is facilitated by four front‑panel keys (INFO, TEST, RESET, MENU), enabling easy on‑site status checks and parameter adjustments.
· Built‑In Self‑Test and Connection Monitoring – An automatic self‑test function continuously verifies the internal measuring circuits, system control components, and earth connections. Connection monitoring also supervises the integrity of the measuring leads. If any fault is detected, a dedicated system fault relay and an LED provide immediate indication, ensuring high functional reliability.
· External Test and Reset Functions – External push‑buttons can be connected to perform device and connection testing, or to clear stored fault messages, enhancing convenience and safety particularly when the device is installed in a control cabinet or hard‑to‑access location.
· 0…400 μA Analog Output for External kΩ Display – The device provides a 0…400 μA analog current output that can be connected to an external kΩ indicating instrument for remote display of the insulation resistance value.
· Fault Memory (Activatable) – The device includes a fault memory function that can be activated via the menu (latching mode). When enabled, alarm relays remain in the alarm condition and LEDs stay lit until an external reset button is pressed or the supply voltage is interrupted, helping to identify intermittent faults.
· De‑Energised System Monitoring – Because the device operates on an independent external power supply, it can monitor systems even when the main circuit is de‑energised, ensuring continuous safety oversight during maintenance or system shutdown periods.
· Voltage Extension via Coupling Devices – The nominal system voltage range can be extended using appropriate Bender coupling devices (e.g., AGH series). With coupling devices, the monitoring range can be extended to higher voltages such as DC 0…1760 V or 3(N)/AC 0…7200 V, making the device suitable for medium‑ and high‑voltage IT systems.
Technical Specifications
· Nominal system voltage (Uₙ): 3(N)AC 0…793 V; AC/DC 0…793 V; DC 0…650 V
· Frequency range (fₙ): DC, 0.2…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: ≤ 500 μF
· Measuring principle: AMPPlus (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
· Alarm relays: Two independent changeover contacts (Alarm 1 and Alarm 2), configurable for N/O or N/C operation
· System fault relay: One changeover contact, N/C operation, for device‑internal fault reporting
· Analog output: 0…400 μA (for external kΩ indicating instrument)
· External functions: Supports connection of external test and reset push‑buttons
· Communication interface: None (standard version; no RS‑485)
· Display: Backlit two‑line LC display
· Connection monitoring: Yes
· 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
· Electromagnetic compatibility: IEC 61326‑2‑4 (shock and vibration resistant)
· Mounting: Panel‑mounted (control cabinet door)
· Panel cutout: 138 × 68 mm
· Dimensions (W × H × D): 144 × 133 × 72 mm
· Weight: ≤ 510 g
· Standards compliance: IEC 61557‑8, DIN EN 61557‑8; meets or exceeds NEC 250.21(B), CEC 10‑106(2) for ungrounded AC systems, and NEC 250.167(A) for ungrounded DC systems
· Order number: B91065002
Model Number Explanation
· IRDH375 – Product family designation for Bender 375 series insulation monitoring devices.
· 427 – Hardware variant configured for a low‑voltage DC supply (DC 19.2…72 V) and panel-mount installation.
Typical Applications
· Industrial IT systems – Insulation monitoring for AC, DC, and combined AC/DC unearthed systems incorporating rectifiers, converters, thyristor‑controlled DC drives, variable‑frequency drives (VFDs), and other power electronic loads.
· UPS systems and battery systems – Continuous monitoring of insulation integrity in uninterruptible power supplies and battery backup installations, ensuring electrical safety for critical equipment.
· Switch‑mode power supply installations – Monitoring insulation resistance in systems with high‑frequency switched power supplies.
· IT systems with high leakage capacitance – Installations where interference suppression filters, EMI filters, or long cable runs generate significant leakage capacitance to earth (e.g., large industrial networks, data centres).
· Low‑voltage DC powered environments – Applications where only low‑voltage DC auxiliary power is available, avoiding the need for an AC supply conversion.
· De‑energised / offline system monitoring – Applications where continuous insulation monitoring is required even when the main circuit is de‑energised, such as during maintenance periods.
· 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.
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