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What does a Phase Monitor do? [Wiring Diagram PDF]

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Phase monitor is the device that is meant for the measurement of the technical processes to aid control. Phase monitors are either powered by alternating current (AC) or direct current (DC) and they are employed to give protection to three-phase loads.

Phase monitors are designed to prevent these loads from any impairment caused by “phase reversals” / “phase absence”.

  • If the phase loss is detected the unit will trip and cut the power.
  • If the phases are out of sequence (improper rotation), the unit will trip and cut off electricity as well.
  • If all three lines deliver less voltage (under-voltage) than its setting, the unit will trip after an adjustable trip delay had been taken into account.
  • If all three lines deliver more voltage (overvoltage) than you set it up for, the unit will trip after a certain period of time that you set up an adjustable trip delay.

For the operation of alarm circuits and for the disconnection of the load, contacts are used. The moment power is supplied, if all phases are linked and the sequence of phases is ABC, then the output relay boosts.

On the other hand, these output relays instantly de-rejuvenate if there is even one phase that suffers from disconnection or if there is any abnormality seen in phase rotation. Phase monitors are explicitly made for uneven security and engineering applications.

Here is a phase monitor overview and wiring close up:

ICM 450 phase monitor

Types of Phase Monitors

Phase Monitors can be AD driven and DC-driven:

AC driven Phase Monitors

Ac driven phase monitors shield “three-phase loads” from any harm or damage caused by “phase reversals” and “phase absence”. They are highly suitable for rugged protection & industrial applications.

Here are some characteristics of such devices:

  • Powered by the Alternative Current (AC).
  • The frequency of operation varies as 50/60/400 Hertz.
  • Trigger alarms and loads can be disconnected by contacts.
  • Highly reliable.
  • Very handy devices.

DC-driven phase monitors

These monitors work in the same fashion as the AC-derived monitors do and have the same properties except one i.e. they are derived by direct current.

Purpose of Phase Monitors

The chief purpose of phase monitors is to monitor all the central phase parameters like phase sequence, under-voltage plus overvoltage, and phase failure along with phase unbalance. Thus, these devices are designed for the maintenance of motor operation as well as systems.

They also protect the motors from operating at unapproved temperature conditions and provide protection against any disturbance caused by damaged fuses, cracked wires, or damaged contacts. They prevent the injury of personnel and the machinery as well.

So, how important it is to have a phase monitor? Here is a nice video on that:

The importance of phase monitors

Phase monitors offer supreme safety for portable machinery and unbalanced systems. They are capable of detecting the voltage and system faults earlier so they can be corrected before serious damage results,

For the operation of phase monitors, there is no need for tuning as these devices have an inbuilt power supply and operate on the “closed-circuit principle”. The output relays pick up after the stay time has passed and the LED set alight (Green on some phase monitors) if the correct phase sequence is provided at terminals.

Similarly, when the phase sequence is wrong, the output relays remain in the initial rest position.

How to Connect Phase Monitors / Phase monitor relays?

Each and every device or application has its own set of requirements, but the basic wiring technique for all phase monitors is the same. On the diagram below you can see an “ICM402 3-phase monitor” wiring with a magnetic contactor and load/motor starter:

Feel free to download this PDF file for greater detail. This is how a phase monitor gets wired:

  1. The three-phase wires are connected to the phase monitor through voltage rated 1-amp fast-blow fuses.
  2. Neutral wire and one phase wire say blue phase is also connected to the phase monitor terminals (C and 115VAC/230VAC respectively). The blue phase wire is also connected to terminal Y of the phase monitor.
  3. Phase wire from phase monitor (Y-OUT) is connected to the magnetic contactor terminal A2 through overload relay contact and thermostat switch.
  4. The neutral wire is connected directly to the magnetic contactor terminal A1.
  5. The 3-phase wires are connected to the input terminals (L1, L2, and L3) of the magnetic contactor directly.
  6. The 3-phase load is connected to the output terminals (T1, T2, and T3) of the magnetic contactor.

Working of a Three-Phase Monitor

The phase monitor continually monitors the 3-phases and detects the out of sequence phase and the missing phase faults. Under normal conditions, the phase monitor gives supply from (Y-OUT) terminal to the magnetic contactor through overload relay contact and the thermostat switch.

In technical terms, this means that the ICM402/phase monitor is energized. A Green LED is illuminated on the phase monitor and the load starts working.

If any fault (like out of sequence phase, or the missing phase) occurs, then supply from (Y-OUT) terminal cuts OFF and the magnetic contactor stops working while switching OFF the load/motor. A Red LED gets illuminated on the phase monitor. In technical terms, you can say that ICM402 is de-energized.

For the sake of extra protection, supply to the load is also switched OFF when the overload relay or thermostat switch detects a signal.

The overload relay also detects the signal when the load exceeds the rated load.

The load is measured in terms of current. If the current exceeds the rated value, the overload relay detects a signal and switches OFF the magnetic contactor, which further switches OFF the load/motor.

The thermostat switch detects the signal that load temperature is increased to the point where it’s no longer safe (or it is at a certain temperature value that you set it up to be) and shuts everything down.

For a normal load/motor working it is necessary

  • That all phases are in proper sequence.
  • No phase is missing.
  • The load current is under the rated value.
  • Load temperature is at a safe level.

Who sells phase monitors?

Following is a list of manufacturers that offer phase monitoring relays:


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