Omron K8DS-PU2 — 3-Phase Undervoltage Monitor, 380–480 VAC
SKU: K8DS-PU2USD 23.00
17.5 mm slim 3-phase undervoltage, phase-sequence and phase-loss monitoring relay for 380–480 VAC supplies, one SPDT 5 A output, rotary switch setting, DIN-rail mount.
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Product Overview
The K8DS-PU2 is the 380–480 VAC variant of Omron's K8DS-PU slim monitoring-relay family, designed to protect three-phase loads from undervoltage, phase loss, and phase-sequence (incorrect phase rotation) faults on industrial control panels. In a 17.5 mm DIN-rail housing — the same width as a single MCB — it integrates a three-in-one monitor that, in the past, would have required three separate relays or a custom PLC logic block. Its single SPDT output (5 A at 250 VAC / 5 A at 30 VDC resistive) trips a contactor, sounds a horn, or signals the PLC when any of the three fault conditions is detected.
What makes K8DS-PU2 different from siblings
The K8DS family splits along two axes: input voltage (K8DS-PU1 for 200–240 VAC, K8DS-PU2 for 380–480 VAC, this SKU) and monitored function. Within the PU family (undervoltage + phase-loss + phase-sequence), the only difference between -PU1 and -PU2 is the rated input voltage range. If your mains is 400 VAC nominal (Europe, most of Asia), pick the K8DS-PU2. If your mains is 230 VAC (older European or North-American single-phase stepped up with a transformer), pick the K8DS-PU1.
Setting and wiring the K8DS-PU2
The K8DS-PU2 uses rotary switches on the front for configuration. The undervoltage setpoint dial covers -30 % to +25 % of the nominal rating — turn the dial to the undervoltage trip point you want (e.g., on a 400 VAC supply, set -20 % to trip at 320 VAC). The reset differential is fixed at 5 % of the operating value, so on a 400 VAC supply with -20 % undervoltage setpoint, the relay picks up at 320 VAC and resets at 340 VAC — this 20 VAC hysteresis prevents output chattering on borderline supplies.
Use cases and integration with PLCs
Wire the K8DS-PU2 SPDT output to a 24 VDC PLC digital input: route the COM to the 24 VDC supply and the NO contact to the input. A fault causes the relay to drop out, the input to read low, and the PLC to take action (e-stop the line, page the operator, log the fault). Alternatively, wire the K8DS-PU2 in series with a contactor's coil: when the relay trips, the contactor drops out and the motor stops. This is the classic wiring for a hard-wired protection circuit that operates even if the PLC is dead.
Key Features
- Slim 17.5 mm DIN-rail housing — same width as one MCB
- 3-in-1 monitor: undervoltage, phase loss, phase sequence in one unit
- 380–480 VAC rated input
- Rotary-switch setpoint — -30 % to +25 % of nominal
- Adjustable undervoltage operating time 0.1 to 30 s
- Phase-sequence trip time 0.1 s, phase-loss 0.1 s, power-ON lock 1 s
- One SPDT output relay, 5 A at 250 VAC / 5 A at 30 VDC resistive
- Higher inverter-noise immunity than older K8AK series
- SKU: K8DS-PU2
- Series: K8DS (Slim 17.5 mm monitoring relay)
- Voltage: 480 VAC
- I/O: 380 I/O
Best-Fit Applications
- Phase-sequence and undervoltage protection on a 400 V conveyor motor in a bottling plant
- Undervoltage trip on a 400 V chiller compressor to prevent low-voltage stall
- Phase-loss alarm on a pump station where a missing phase would damage the impeller
- Protection of a 400 V hydraulic power unit downstream of a soft-starter or VFD
Buying Checklist
- Pick K8DS-PU2 for 380–480 VAC supplies; pick K8DS-PU1 for 200–240 VAC supplies
- Pick K8DS-PU2 for 3-in-1 monitoring; pick K8DS-PH1 if you only need phase-sequence / phase-loss
- Pick K8DS-AS1 if you need a dedicated asymmetry setpoint instead of undervoltage
- Set the undervoltage dial before powering up — a wrong setting will trip on normal operation
- Allow 1 second power-ON lock to ride through transformer inrush
- Set operating time to 0.5–2 s for typical motors; ride through motor starting transients
Technical Specifications
| Model | K8DS-PU2 |
|---|---|
| Series | K8DS (Slim 17.5 mm monitoring relay) |
| Functions | Undervoltage, Phase loss, Phase sequence |
| Rated Input Voltage | 380 / 400 / 415 / 480 VAC, 3-wire 3-phase |
| Input Load | Approx. 2.8 VA |
| Undervoltage Setting Range | -30 % to +25 % of rated input voltage |
| Reset Value | 5 % of operating value (fixed) |
| Reset Method | Automatic |
| Undervoltage Operating Time | 0.1 to 30 s (adjustable) |
| Output Relay | 1 × SPDT, 5 A at 250 VAC / 5 A at 30 VDC resistive |
| Terminal Type | M3 screw |
| Mounting | DIN rail |
| Operating Temperature | -20 to +60 °C |
| Certifications | UL, CSA, CE, CCC |
Resources & Downloads
FAQ for K8DS-PU2
What's the difference between K8DS-PU1 and K8DS-PU2?
Only the rated input voltage range. K8DS-PU1 covers 200/220/230/240 VAC three-phase supplies; K8DS-PU2 covers 380/400/415/480 VAC three-phase supplies. The functions (undervoltage, phase-loss, phase-sequence), output relay, and dimensions are identical. If your mains is 400 VAC nominal (Europe, most of Asia), pick the K8DS-PU2.
What's the difference between K8DS-PU2 and K8DS-PH1?
K8DS-PU2 monitors undervoltage + phase-loss + phase-sequence; K8DS-PH1 monitors only phase-loss + phase-sequence (no undervoltage threshold). If you only need to detect a missing or reversed phase without an undervoltage trip, the PH1 is cheaper and has a simpler setting. If you need full 3-phase protection including brownout detection, pick the PU2.
Can the K8DS-PU2 be used on a single-phase 230 VAC supply?
No — the K8DS-PU2 requires a three-phase 3-wire 380–480 VAC supply. For single-phase monitoring, use the K8AK-PA or K8AK-PW single-phase voltage monitor instead. For three-phase 200–240 VAC, use K8DS-PU1. Connecting the K8DS-PU2 to a single-phase supply will leave one terminal floating and the relay will not function correctly.
How long does the K8DS-PU2 take to detect phase loss?
0.1 s max. This is fast enough to protect most motors against the destructive heating that builds up within 1–2 seconds of running on two phases. The phase-sequence detection is 0.1 s ± 0.05 s. Power-ON lock time is 1 s ± 0.5 s, which means the relay ignores faults for the first second after power-on so it does not trip on the inrush of a large transformer or capacitor bank.
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