The IRDH275-427 is a high-performance insulation monitoring device from Bender, part of the ISOMETER® IRDH275 series. It is designed for unearthed AC, AC/DC, and DC power systems (IT systems) and continuously monitors the insulation resistance of the main circuit. The device is particularly suitable for modern power supply systems that contain rectifiers, frequency converters, thyristor-controlled DC drives, and directly connected DC components.
The device complies with IEC 61557‑8 and also meets or exceeds the requirements of NEC 250.21(B), CEC 10‑106(2) for ground detectors in ungrounded AC systems, as well as NEC 250.167(A) for ungrounded DC systems.
The IRDH275-427 is the standard version of the IRDH275 series, powered by a low-voltage AC/DC supply (AC 19.2…55 V / DC 19.2…72 V), making it particularly suitable for installations where only low-voltage AC or DC auxiliary power is available. It utilizes Bender‘s patented AMPPlus measuring technology, which automatically adapts to modern power system conditions including pure DC systems, systems with rectifiers, and those with variable-frequency drives (VFDs).
Key Features
· Patented AMPPlus Measuring Principle – Employs Bender’s patented AMPPlus measuring technology, delivering accurate insulation monitoring even in systems with extensive DC components and high leakage capacitances – including pure DC systems, systems with rectifiers, and those with VFDs. The 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 IRDH275-427 automatically adapts to the existing system leakage capacitance (maximum permissible ≤ 500 μF), optimising measurement time and ensuring dependable operation without manual adjustments. In systems with high leakage capacitance, the device automatically adjusts to maintain reading accuracy.
· Two Independently Adjustable Response Values – 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, offering flexible configuration to meet different system safety requirements.
· Two-Line LC Display – Equipped with a clear two-line LC display with backlight, showing measured insulation resistance values, system leakage capacitance readings, and other operational parameters. On-device operation is facilitated by front-panel keys (INFO, TEST, RESET, MENU, ESC, Arrow keys, Enter), enabling easy on-site status checks and parameter adjustments.
· Alarm Indicators – Front panel equipped with three LED indicators: AL1 LED (pre-warning), AL2 LED (critical alarm), and System Fault LED (lights on connection or device error).
· 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 supervises the integrity of the measuring leads. If any fault is detected, the system fault LED provides immediate indication. The function can be tested by pressing the TEST button.
· External Test and Reset Functions – External push-buttons can be connected to perform device and connection testing (normally open contact), or to clear stored fault messages (normally closed contact). When the reset button terminals are open, the fault message will not be stored, helping to identify intermittent faults.
· Standby Contact – Includes a standby contact. When the contact is closed, no insulation measurements take place. This allows the device to be temporarily disconnected from the monitored system, which is particularly useful when multiple devices are used in coupled IT systems.
· Fault Memory (Selectable) – Includes a fault memory function that can be activated (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.
· Analog Output for External kΩ Display – 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.
· De‑Energised System Monitoring – Because the device operates on an independent external power 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 – 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 0…793 V; DC 0…650 V
· Frequency range (fₙ): DC, 0.2…460 Hz
· Supply voltage (Uₛ): AC 19.2…55 V (47…63 Hz); 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 for device‑internal fault reporting
· Analog output: 0…400 μA (for external kΩ indicating instrument)
· Standby contact: Yes
· External functions: Supports connection of external test (N/O) and reset (N/C) push‑buttons
· Communication interface: None (standard version; no RS‑485)
· Display: Two-line LC display with backlight
· Operating temperature: –10 °C … +55 °C
· Storage temperature: –40 °C … +70 °C
· Relative humidity: 5…95% (non‑condensing)
· Protection class: Terminals IP30, enclosure IP20
· Mounting: DIN rail according to IEC 60715 (35 mm standard rail)
· Weight: Approx. 0.55 kg
· Standards compliance: IEC 61557‑8, DIN EN 61557‑8; meets or exceeds NEC 250.21(B), CEC 10‑106(2), and NEC 250.167(A)
· Order number: B91065104
Model Number Explanation
· IRDH275 – Product family designation for Bender 275 series insulation monitoring devices
· 427 – Hardware variant configured for a low‑voltage AC/DC supply: AC 19.2…55 V / DC 19.2…72 V
Typical Applications
· Industrial IT systems – Insulation monitoring for AC, DC, and combined AC/DC unearthed systems containing rectifiers, frequency converters, thyristor-controlled DC drives, variable-frequency drives (VFDs), and other power electronic loads.
· Low‑voltage DC powered environments – Applications where only low‑voltage AC or DC auxiliary power is available, avoiding the need for an AC supply conversion.
· UPS systems and battery systems – Continuous monitoring of insulation integrity in uninterruptible power supplies and battery backup installations.
· 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).
· Pure DC and mixed‑voltage networks – Including systems with VFDs and other DC components.
· 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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