Monitoring Relays

Choosing Monitoring Relays components

Monitoring relay selection comes down to what condition is being detected, the setpoint and hysteresis, the response time, and how the output is wired into the control system. A monitoring relay that triggers reliably on the wrong condition is worse than no monitoring at all.

Specs to confirm before ordering:

Monitored parameter: voltage (single-phase, three-phase), current, phase sequence, phase loss, asymmetry, level, temperature, insulation resistance
Setpoint range and adjustability (fixed-setpoint vs. user-adjustable via dials or digital config)
Hysteresis (deadband between trip and release)
Trip delay to prevent nuisance trips from transient conditions
Output type: SPDT or DPDT relay contacts, transistor output, analog retransmit
Output behavior: energize-on-fault vs. de-energize-on-fault (fail-safe behavior under power loss)
Supply voltage — typically 24 VDC or 100–240 VAC universal
Mounting: DIN-rail width (17.5 mm, 22.5 mm, 35 mm) or panel-mount

Common gotchas: setpoint hysteresis is often misunderstood — a relay that trips at 9 V and resets at 9.5 V will not protect a load that fluctuates between 8.5 V and 9.2 V (the contacts will chatter). Three-phase monitoring relays need correct phase wiring on installation; a phase sequence relay wired with two phases reversed will report a fault on a known-good supply. Level monitoring relays for conductive liquids use a low-voltage AC signal between probes; using them on insulating fluids (oil, DI water) gives no signal. Insulation monitors are intended for IT-grounded systems (where neutral is not earthed); they do not work on TN-grounded systems where they false-trip on the bonded neutral.

Typical applications: motor protection from phase loss and imbalance, transformer secondary voltage monitoring, sump pump level detection, motor winding temperature monitoring via PTC thermistors, and IT-system ground-fault detection on critical loads. On legacy installations, in-kind monitoring relay replacement preserves the setpoints and control logic.

For obsolete monitoring relays, send the OEM part number for a sourcing quote.

Do you stock obsolete monitoring relays?

Yes. Discontinued Allen-Bradley 813S/814S, retired Carlo Gavazzi DPA/DPB, end-of-life Siemens 3UG3, and earlier Phoenix Contact EMD codes are sourced through our supplier network.

What does "fail-safe" mean for a monitoring relay?

A fail-safe (de-energize-on-fault) relay drops its output on a detected fault AND on loss of supply power — so power failure looks the same as a fault, ensuring the protected load is stopped. Energize-on-fault relays do not give this behavior; choose per the application's safety logic.

Can I use a phase monitoring relay on a single-phase supply?

No. Phase sequence and asymmetry detection require all three phases connected. Use a single-phase voltage monitoring relay instead.

How do I set the hysteresis?

For most relays, the hysteresis is either fixed (e.g., 5% of setpoint) or adjustable via a separate dial. Set it wider than the expected normal fluctuation to avoid chatter, narrower than the fault condition you want to catch.

Do I need a transformer for high-voltage monitoring?

For voltages above the relay's direct input range (typically 600 V), yes — use voltage transformers (VTs) appropriately rated and apply the burden requirements to the relay's input.

What is the warranty?

12-month functional warranty. Damage from over-voltage, mis-wiring, or operating outside rated input range is not covered.

Monitoring Relays

Pilz 326701 Fieldbus Module

Pilz 774260 PLID d1 Safety Relays

Pilz 784260 PLID d1 C Safety Relays

Pilz 827250 Voltage Monitoring Relay

Pilz 827260 Voltage Monitoring Relay

Pilz 828010 Current Monitoring Relay

Pilz 828035 Current Monitoring Relay

Pilz 828050 Current Monitoring Relay

Pilz 837271 Three-Phase Voltage Monitoring Relay

Pilz 837380 Three-Phase Voltage Monitoring Relay

Pilz 838000 Insulation Monitoring Relay

Pilz 840400 Phase Monitoring Relay

Pilz 840410 Phase Monitoring Relay

Pilz 840415 Phase Monitoring Relay

Pilz 840775 Phase Sequence Monitoring Relay

Pilz 841000 Voltage Monitoring Relay

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