Schneider RM17TG20 Phase Loss Monitoring Relay

Harmony Zelio Control RM17-T Modular Monitoring Relay

Type: Three-phase power monitoring relay, self-powered (no external auxiliary power supply required), DIN rail mounting, 17.5 mm width

✅ Monitoring Functions

1. Phase Sequence Detection: Reverse phase protection (motor reverse rotation protection)
2. Phase Loss Detection: Triggers a fault when ≥2 phases are lost
3. Three-phase Voltage Asymmetry (Unbalance) Detection

This model has no adjustable over/undervoltage thresholds, no delay knob, no test button, fast response upon fault.

📋 Core Electrical Specifications

Monitoring voltage range: 208～480V AC, operating limit 183～484V AC

Frequency: 50/60Hz (±10%)

Output contacts: 2 changeover contacts (2C/O), rated 5A; max switching voltage 250V AC/DC

Fault response time: ≤130 ms

Power-on start delay: ≤650 ms

Power consumption: Max. 22VA (400V/50Hz)

Mechanical life: 30 million operations; Electrical life: 100,000 operations

Mounting: DIN35 rail, IP20 (front side)

Operating temperature: -20℃～+50℃

📌 Terminal Definition

L1, L2, L3: Three-phase input (directly connected to the monitored three-phase power supply; the relay draws power here)

11, 12, 14: 1st changeover contact (11 common, 12 NC normally closed, 14 NO normally open)

21, 22, 24: 2nd changeover contact (21 common, 22 NC normally closed, 24 NO normally open)

Operating Logic

Normal three-phase condition & correct phase sequence: Relay energizes and pulls in, 11-14 and 21-24 conduct; 11-12 and 21-22 open.

Reverse phase / phase loss / severe three-phase unbalance: Relay de-energizes and drops out, normally closed contacts close, normally open contacts open, used to cut off contactor circuit or trigger alarm signals.

🛠️ Typical Application Scenarios

Three-phase motors, water pumps, fans, hoisting equipment, air compressors, conveyors; prevents motor reverse rotation and burnout due to phase loss, widely used for 380V three-phase equipment protection.

⚠️ Model Selection Comparison (RM17 Series)

RM17TG20: Phase sequence + phase loss + unbalance, no upper/lower voltage limits, no adjustable delay

RM17TU: With overvoltage and undervoltage monitoring

RM17TE: Multi-function (phase sequence + phase loss + unbalance + over/undervoltage)

RM17TT: Version with adjustable delay

🧩 Troubleshooting Guidelines

1. Wiring: L1/L2/L3 three phases must all be connected; relay is self-powered. Loss of one single phase will not necessarily trigger alarm, protection activates only when two or more phases are lost.
2. Phase sequence swap: Swap any two incoming phases, relay will act immediately.
3. Contacts: Normally open contacts close under normal condition; normally open contacts open and normally closed contacts close under fault condition.
4. Minor three-phase unbalance will not trigger alarm; only asymmetry exceeding internal threshold will activate protection.

Schneider RM17TG20 Application Scenarios

Core protection capability: Phase sequence reversal, phase loss (≥2 phases lost), three-phase voltage unbalance monitoring. No over/undervoltage thresholds, no adjustable delay, narrow DIN rail module, self-powered, 2 sets of changeover contacts. Applied for 380V-class three-phase equipment power protection to prevent motor reverse rotation, burnout caused by phase loss and load damage due to unbalanced voltage.

🏗️ Hoisting & Conveying Industry (Most Typical)

1. Overhead cranes, gantry cranes, tower cranes, port hoists: Prevent reversed phase sequence after cable replacement or switching operation, which may cause reverse movement of suspended loads and safety accidents; cut off main contactor circuit upon phase loss to protect motors.
2. Lifts, elevators, escalators: Phase sequence protection for elevator main traction motors; protection against phase sequence disorder after power maintenance.
3. Belt conveyors, material transport machines: Motor reversal on production lines causes material piling and mechanical collision damage.

💧 Water, HVAC, Pumps & Fans

1. Water supply/sewage pumps, submersible pumps, circulating pumps: Reversal of pump motors leads to no water output and impeller damage; phase loss easily burns motors.
2. Chillers for central air conditioning, cooling tower fans, boiler induced draft fans: Large three-phase fans, phase sequence protection during power maintenance or mains power switching.
3. Pumping stations, water treatment complete control cabinets, centralized three-phase power monitoring inside low-voltage cabinets.

📦 Packaging, Textile & Automation Production Lines

1. Packaging machinery, filling equipment, textile machines (carding, spinning machines), protection for three-phase main drive motors.
2. Automated production lines, external 3-axis drive units of robots; avoid mechanical collision caused by wrong phase sequence of power supply.
3. Retrofit projects for old equipment: Hardwired directly into contactor control loop without PLC, simple modification.

🏭 Construction Machinery, Mobile Equipment & Backup Power Switching

1. Construction site mobile equipment, large agricultural three-phase equipment, three-phase units for refrigerated trucks.
2. Mains/generator dual power switching circuit: Monitor phase sequence matching when generator is switched on, prevent reverse rotation during paralleling or on-load operation; output fault alarm signals upon power abnormality to interlock ATS switching.
3. Power supply monitoring for sensitive three-phase loads, provide power fault DI alarm signal to PLC (two contact sets: one for shutdown protection, one for alarm).

⚙️ Air Compressors, Refrigeration & General Industrial Motors

1. Screw air compressors, scroll air compressors, main motor protection with phase loss/reverse phase protection.
2. Industrial refrigeration units, three-phase compressors for cold storage.
3. Complete electric control cabinets for all types of general three-phase asynchronous motors, suitable for compact control boxes (only 17.5 mm width).

✅ Application Prerequisites & Selection Limitations (Must Read Before Selection)

Supply voltage range 208～480V AC, commonly used for domestic 380V three-phase systems

⚠️ No single-phase phase loss protection: Alarm only when two or more phases are lost; no over/undervoltage monitoring. Select RM17TE series if over/undervoltage protection is required.

Installation location: PLC control cabinet, local motor control box, DIN35 rail mounting

Contact usage: One set wired in series with contactor coil for shutdown protection; the other set connected to PLC or sound-light alarm for fault alarm.

❌ Not Recommended Applications

Applications requiring single phase loss protection (alarm when only one phase is disconnected)

Applications requiring overvoltage or undervoltage threshold monitoring

Applications requiring custom fault delay (this model has no delay knob)

RM17TG20 Phase Sequence / Reverse Phase Protection Working Principle

RM17TG20 is a self-powered three-phase monitoring relay. L1/L2/L3 serve both as sampling inputs and the power supply for the relay itself. Built-in analog phase detection circuit detects the phase timing of three-phase voltages to judge whether it follows the forward sequence L1-L2-L3.

1. Normal Condition (Correct Phase Sequence L1→L2→L3)

Peak values of three-phase voltages arrive sequentially following the standard timing L1, L2, L3. Internal phase logic confirms correct phase sequence:

Internal output electromagnetic relay pulls in

2 changeover contacts: 11-14, 21-24 (NO normally open) conduct; 11-12, 21-22 (NC normally closed) open

Panel indicator light is ON

Control circuit: Normally open contact wired in series to contactor coil circuit, motor start is permitted.

2. Reverse Phase Fault Condition (Any two phases swapped, reversed sequence L1-L3-L2)

When any two incoming phase wires are interchanged, the peak sequence of three-phase voltages is reversed (reverse phase sequence):

Internal phase detection circuit immediately identifies phase reversal

Output relay de-energizes and drops out (response ≤130 ms, no delay)

Contact state flips: NO normally open opens, NC normally closed closes

Panel indicator light turns OFF

Control effect: Normally open contact wired in series with contactor coil opens, main motor contactor is cut off, motor start is prohibited to prevent reverse rotation; normally closed contact can be connected to PLC or sound-light alarm.

3. Key Points
4. Only detects phase timing, not motor rotation direction: It only checks the voltage phase sequence of incoming L1/L2/L3 power supply, not the actual rotation direction of the motor.
5. No external auxiliary power required, powered directly from L1/L2/L3, simple wiring.
6. Phase sequence, phase loss and unbalance share the same output: Reverse phase, phase loss (≥2 phases lost) and severe three-phase unbalance all trigger the same action: relay dropout and contact state flip.
7. This model has no adjustable thresholds and no adjustable delay; it acts instantly once reverse phase sequence is detected.
8. Typical Wiring Logic

11/14 (NO normally open) wired in series with motor contactor KM coil circuit: When phase sequence is correct, circuit conducts and startup is allowed; upon reverse phase, circuit opens and startup is prohibited.

21/22 (NC normally closed) connected to PLC digital input or sound-light alarm: Closes on fault to output fault alarm signal.

5. Commissioning & Verification Method

Field test: Under power-off condition, swap any two phase wires connected to the relay’s L1 and L2. Power on, relay drops out immediately, indicator light goes off, contactor cannot pull in. Restore wiring to correct phase sequence, relay pulls in again and equipment startup is permitted.

⚠️Note: VFD-driven motors generally do not require this protection. The VFD rectifies power to DC bus, so incoming phase sequence will not affect motor rotation direction. However, this protection must be used for direct line-frequency driven equipment such as hoists and water pumps, for the bypass line-frequency circuit of VFDs, to avoid reverse rotation accidents after cable replacement during maintenance.

The phase sequence protection function of RM17TG20 cannot be set, disabled or threshold-modified. It is hardware fixed, with no DIP switches, potentiometers or software parameters.

Detailed Description

1. Fixed judgment logic

Factory preset: Only recognizes L1-L2-L3 forward phase sequence. Once incoming power becomes reverse sequence (L1-L3-L2), reverse phase fault is triggered immediately, relay drops out and contacts flip, response ≤130 ms, no adjustable delay.

Phase sequence protection, phase loss (≥2 phases lost) and three-phase unbalance monitoring are bound together. Any abnormal condition triggers the same output action; individual function cannot be disabled separately.

2. No adjustable components on the body

There are no potentiometers, DIP switches or test buttons on the housing. No parameter modification can be made on the relay itself.

Phase sequence detection cannot be disabled

Phase sequence judgment criteria cannot be modified

Reverse phase action delay cannot be adjusted

3. Only field workaround (NOT relay configuration)

If reverse phase sequence operation is required on site:

Option 1: Swap L1/L2/L3 wiring connected to RM17TG20, change incoming wiring to L1-L2-L3 sequence, then relay judges as normal (only external wiring changed; relay function remains unchanged).

Option 2: Bypass RM17TG20 contacts in the control circuit (electrical circuit modification, not recommended for long-term use, equivalent to abandoning protection).

📌 Model Selection Comparison

RM17TG20: Fixed functions (phase sequence + phase loss ≥2 phases + unbalance), non-configurable

RM17TE / RM17TT: Multi-function versions with adjustable unbalance thresholds, over/undervoltage; some advanced models support function selection.

Supplementary Reminder

If your project requires disableable phase sequence protection or alarm for single-phase loss, RM17TG20 is not suitable. Replace with RM17TE series or other models.

Schneider RM17T Three-phase Monitoring Relay (Harmony Zelio RM17T Series) Full Model List

All models are self-powered three-phase monitoring relays, powered from L1/L2/L3, DIN35 rail mount, 17.5 mm narrow width, monitoring voltage: 208～480V AC, commonly used for domestic 380V systems.

1. RM17TG (Fixed function type, no adjustment knobs, your target subseries)

RM17TG00: 1C/O (1 changeover contact), phase sequence + phase loss (≥2 phases) + unbalance, non-adjustable

RM17TG20: 2C/O (2 changeover contacts), phase sequence + phase loss (≥2 phases) + unbalance, non-adjustable (your current model)

✅Feature: No potentiometer on housing, fixed firmware function, phase sequence protection cannot be disabled, no adjustable delay.

2. RM17TA (Phase sequence + phase loss + three-phase unbalance, adjustable unbalance threshold)

RM17TA00: 1C/O, phase sequence, phase loss, three-phase asymmetry (unbalance), adjustable unbalance threshold, no over/undervoltage

3. RM17TU (Phase sequence + phase loss + undervoltage monitoring, adjustable undervoltage)

RM17TU00: 1C/O, phase sequence, phase loss, undervoltage protection, adjustable undervoltage threshold, no overvoltage, no unbalance

4. RM17TT (Phase sequence + phase loss + unbalance + adjustable fault delay)

RM17TT00: 1C/O, phase sequence, phase loss, unbalance; adjustable unbalance threshold + fault delay, no over/undervoltage

5. RM17TE (Full-function flagship model, maximum adjustable items)

RM17TE00: 1C/O, phase sequence, phase loss, three-phase unbalance, overvoltage + undervoltage window monitoring; adjustable unbalance threshold, over/undervoltage and delay, frequency deviation monitoring available.

6. RM17UB (With neutral wire monitoring, three-phase four-wire system)

RM17UB30: Three-phase + neutral line monitoring, neutral loss, phase over/undervoltage, suitable for distribution circuits with N line.

Model Function Comparison Table

Supplementary Notes

1. Common features of RM17T series: All are self-powered, L1/L2/L3 three phases must be connected; phase loss logic: protection triggers only when ≥2 phases are lost, single phase disconnection will not trigger alarm.
2. Contact rating: 5A, 250VAC for all models.
3. Distinguish RM17 / RM22 / RM35: RM17 is 17.5mm narrow type; RM22 is wider; RM35 is large-size multi-function relay and not part of RM17T series.
4. Quick selection guide:

Only phase sequence + phase loss required, two contact sets needed → RM17TG20

Full protection including over & undervoltage → RM17TE00

Undervoltage protection only → RM17TU00

Unbalance + delay required → RM17TT00