Description
Product Overview
The BENDER VMD258 is an analog voltage monitoring relay designed for monitoring three-phase AC power systems for undervoltage and overvoltage conditions.
Unlike a conventional insulation monitoring device, the VMD258 monitors the voltage level of a 3AC system using a window discriminator function. It does not require a neutral conductor connection, so it can be used across a wide range of three-phase AC installations.
The unit uses pure analog technology without a microprocessor. The electronics are supplied directly from the monitored system, while the electronics, relay circuits, and external energy-storage connection are separated from the monitored system by double insulation. Special input transformers help attenuate interference from the monitored power system.
Product Datasheet
| Parameter | Specification |
|---|---|
| Brand | BENDER |
| Model | VMD258 |
| Product Series | LINETRAXX |
| Product Type | Three-Phase AC Voltage Monitoring Relay |
| Monitoring Function | Undervoltage / Overvoltage |
| Measurement System | 3AC |
| Maximum Nominal System Voltage | Up to 690 V AC |
| Neutral Connection | Not required |
| Technology | Pure analog |
| Undervoltage Response Range | 0.7…0.95 × Un |
| Overvoltage Response Range | 1.05…1.3 × Un |
| Adjustable Response Delay | 0…5 s |
| Alarm Outputs | Separate undervoltage and overvoltage relays |
| Relay Contacts | 2 potential-free changeover contacts |
| Status LEDs | Operation, overvoltage, undervoltage |
| Auxiliary Supply | Derived from monitored system |
| Electrical Separation | Double insulation |
| Installation | Industrial electrical/control panels |
| Product Status | Discontinued |
Bender specifies several VMD258 variants for 100, 110, 230, 400, 440, 480, 500, and 690 V three-phase systems.
Main Functions
⚡ Three-Phase Voltage Monitoring
The VMD258 continuously monitors the voltage of a three-phase AC system and detects conditions outside the configured operating window.
⚡ Adjustable Undervoltage and Overvoltage Limits
The undervoltage threshold can be adjusted from 0.7 to 0.95 × Un, while the overvoltage threshold can be adjusted from 1.05 to 1.3 × Un.
⚡ Adjustable Response Delay
The response delay can be continuously adjusted between 0 and 5 seconds. This allows the relay to distinguish voltage disturbances from conditions that require an actual switching or alarm response.
⚡ No Neutral Conductor Required
The VMD258 is designed for three-phase systems without requiring a neutral-point connection.
⚡ Analog Technology
The relay uses pure analog technology rather than a microprocessor-based design. Bender specifically describes this architecture as suitable for demanding safety-related applications.
Relay Outputs and Indication
The VMD258 provides separate alarm relays for undervoltage and overvoltage.
The relay outputs use potential-free changeover contacts, allowing the monitoring signal to be connected to external control circuits, alarms, contactors, or other control equipment.
Front-panel LEDs provide direct indication of:
- Power/operation status
- Overvoltage condition
- Undervoltage condition
This makes basic troubleshooting possible without requiring a separate diagnostic interface.
Available Voltage Variants
Bender lists the following VMD258 versions:
- VMD258 3AC 100 V — B93010060
- VMD258 3AC 110 V — B93010061
- VMD258 3AC 230 V — B93010062
- VMD258 3AC 400 V — B93010063
- VMD258 3AC 440 V — B93010064
- VMD258 3AC 480 V — B93010065
- VMD258 3AC 500 V — B93010066
- VMD258 3AC 690 V — B93010067
Important: When replacing an existing unit, verify the exact voltage variant and article number. “VMD258” alone does not identify the nominal system-voltage version.
Installation & Configuration
Before installation, verify the VMD258 voltage version against the actual three-phase system voltage.
Recommended procedure:
- Disconnect and isolate the electrical system according to the applicable safety procedure.
- Confirm the VMD258 article number and rated three-phase voltage.
- Install the relay in the designated control-panel location.
- Connect the three-phase monitoring inputs according to the Bender wiring documentation.
- Configure the undervoltage and overvoltage response values.
- Set the required response delay.
- Connect the potential-free relay contacts to the external control or alarm circuit.
- Restore power and verify the operation indication.
- Check the undervoltage and overvoltage alarm functions under controlled test conditions.
- Confirm that the connected control system receives the expected relay signals.
Do not substitute a different VMD258 voltage variant simply because the physical housing is identical.
Typical Applications
The VMD258 is intended for applications where the voltage condition of a three-phase AC supply needs to be monitored.
Typical uses include:
- Industrial machine power monitoring
- Electrical installation monitoring
- Motor and load supply monitoring
- Standby power systems
- Emergency power systems
- Portable electrical loads
- Generator and power-supply monitoring
- Switching systems based on voltage conditions
- Industrial control panels
- Power-station auxiliary systems
Bender specifically lists machine and electrical-installation power monitoring, load monitoring, standby/emergency supply monitoring, and portable-load supply monitoring among the applications.
VMD258 vs. Insulation Monitoring Devices
A useful distinction for replacement projects:
VMD258: monitors three-phase voltage, detecting undervoltage and overvoltage.
Bender ISOMETER® devices: monitor insulation resistance / insulation faults in appropriate unearthed or IT electrical systems.
Therefore, if an existing installation requires insulation-resistance monitoring, the VMD258 should not be selected as a direct functional replacement. Bender’s product portfolio separately identifies devices such as the IR425-D4 and iso685 series for insulation monitoring.
Product Status
Bender’s current product documentation identifies the VMD258 as a discontinued device. The manufacturer still provides documentation including manuals, datasheets, and conformity information for the product.
This makes the VMD258 particularly relevant for maintenance departments and operators maintaining older electrical installations where an exact replacement or compatible spare is required.
Common Questions & Answers
Q: Is the BENDER VMD258 an insulation monitoring device?
A: No. It is a three-phase AC voltage monitoring relay for undervoltage and overvoltage detection.
Q: What does the VMD258 monitor?
A: It monitors three-phase AC system voltage using an undervoltage/overvoltage window function.
Q: Does it require a neutral connection?
A: No. Bender specifies that a neutral conductor connection is not required.
Q: What is the maximum nominal system voltage?
A: Bender lists versions for nominal three-phase system voltages up to 690 V AC.
Q: Can the voltage thresholds be adjusted?
A: Yes. The undervoltage and overvoltage response values are continuously adjustable.
Q: What is the undervoltage adjustment range?
A: 0.7 to 0.95 × Un.
Q: What is the overvoltage adjustment range?
A: 1.05 to 1.3 × Un.
Q: Can the response delay be adjusted?
A: Yes. The response delay can be adjusted from 0 to 5 seconds.
Q: Does it use a microprocessor?
A: No. The VMD258 uses pure analog technology.
Q: Is the VMD258 still manufactured?
A: Bender currently identifies the VMD258 as discontinued.
Recommended BENDER Models
For customers maintaining Bender electrical safety and monitoring systems, these models may also be relevant:
- BENDER IR425-D4 — Insulation Monitoring Device
- BENDER IR420-D4 — Insulation Monitoring Device
- BENDER IRDH275 — Insulation Monitoring Device
- BENDER iso685 — Insulation Monitoring Device
- BENDER iso685-D — Insulation Monitoring Device
- BENDER iso685-S — Insulation Monitoring Device
- BENDER RCMS460 — Residual Current Monitoring System
- BENDER RCMS490-D — Residual Current Monitoring Device
- BENDER EDS440 — Insulation Fault Locator
- BENDER PEM353 — Power and Energy Monitoring Device
The correct replacement depends on whether the original application requires voltage monitoring, insulation monitoring, residual-current monitoring, or power-quality measurement.
Inspection & Quality Check
For industrial spare-part applications, check the unit before installation:
- Verify the complete VMD258 article number.
- Confirm the nominal three-phase voltage variant.
- Inspect the enclosure for cracks or mechanical damage.
- Check terminals and connection points.
- Inspect relay-contact areas for signs of overheating.
- Verify front-panel LEDs and adjustment controls.
- Check that the product identification label is readable.
- Compare the unit against the existing installation documentation.
- Perform controlled functional testing before placing the equipment back into service.
For an exact replacement, the article number and voltage variant should be matched, not just the VMD258 model name.
