Over-voltage monitoring relay LINETRA ® VMD421H series

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Product overview
The LINETRAXX® VMD421H is a multi‑function voltage monitoring relay for 3(N)AC systems. It monitors undervoltage, overvoltage, frequency and the voltage window between two threshold values. Voltages are measured as r.m.s. values and the current measured value is shown continuously on the LC display. The device is powered from the monitored system and includes an internal energy backup.

Key features

• Undervoltage, overvoltage, frequency and window‑discriminator monitoring in 3(N)AC systems (70…500 V; variants 70…500/288 V)
• Self-powered (no external supply required) with integrated energy backup
• r.m.s. value measurement including AC+DC components
• Adjustable start‑up delay, response delay and delay on release
• Adjustable switching hysteresis
• Selectable monitoring functions: <U, >U, <f, >f
• Preset function for automatic basic parameter setting

Indicators and controls

• Multifunction LC display showing the current measured value
• LEDs: Power On, Alarm 1, Alarm 2
• Measured value memory for the trip value
• Continuous self monitoring
• Internal test / reset button
• Password protection for device settings

Outputs / Mechanical / Connections

• Two separate alarm relays, each with one changeover contact (SPDT)
• N/C or N/O operation and fault‑memory behaviour selectable
• Two‑module enclosure (36 mm)
• Push‑wire terminals (two terminals per connection); screw‑terminal variants referenced by article numbers
• RoHS compliant

Applications

• Monitoring of voltage‑sensitive machines and electrical installations
• Automatic switching of equipment at defined voltage thresholds
• Monitoring of standby and emergency supply systems
• Supply voltage monitoring for portable loads
• Protection of three‑phase motors against phase failure and phase reversal
• Transformer protection; detection of asymmetrical loading

Variants (table)
Type | Nominal system voltage U_n* | Response value | Art. No.**
VMD421H-D-3 | 3(N)AC 70…500 V, 15…460 Hz | AC 70…500 V | B73010007 / B93010007

Optional accessories (table)
Type | Art. No.
Mounting clip for screw mounting (one piece per device) | B98060008
Mounting frame XM420 | B990994

Downloads (documents)

• UKCA - VMx42x (Declaration; VMx42x family)
• VMD421 - Tender texts (RTF, EN)
• VMD421H - Manual (PDF, EN)
• VMD421H - Datasheet (PDF, EN)
• VMD421H - Quickstarts (PDF, EN/DE)
• VMx42x - CE Declarations of conformity (PDF)

Technical specifications

• Measured voltage range: AC 70…500 V (3(N)AC, 15…460 Hz)
• Monitoring functions: undervoltage (<U), overvoltage (>U), frequency (<f, >f), window discriminator
• Measurement: r.m.s. value measurement (AC + DC components)
• Display: multifuncitional LC display showing current measured value
• Energy: powered from monitored system with internal energy backup
• Delays: adjustable start‑up delay, response delay and delay on release
• Outputs: two alarm relays (each one changeover contact)
• Operation modes: N/C or N/O selectable; fault memory selectable
• Indicators: Power On, Alarm 1, Alarm 2 LEDs
• Protection: continuous self monitoring; internal test/reset
• Security: password protection for settings
• Mechanical: two‑module (36 mm) enclosure; sealable transparent cover
• Terminals: push‑wire (two terminals per connection); screw‑terminal variants available
• Compliance: RoHS; CE/UKCA documentation available for VMx42x family
• Variant article numbers: B73010007, B93010007
• Notes: * absolute values; ** B7… = push‑wire terminal / B9… = screw‑type terminal