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Voltage, temperature, phase and current monitoring relays

There are many types of monitoring relays. These include voltage, temperature, phase and current monitoring relays. Throughout this article, we will look at what monitoring relays are and at Finder's range.


What is a monitoring relay?

A monitoring relay is a protective control device that monitors important electrical or physical parameters. There are many types of monitoring relays. These include voltage, temperature, phase and current monitoring relays. They measure if parameters are within the set limits and take action if they are outside of them. The device operates a contact that may give an alarm signal or initiates a shutdown process and there can be a time limit before the action takes place (short term variations that can be tolerated must be allowed to prevent ‘nuisance tripping’).


A monitoring relay may also have a ‘Fault Memory‘. If there is NO ‘Fault Memory’ when normal parameters return, the alarm condition will be cleared automatically, and the system may restart automatically. If there is ‘Fault Memory’ when normal parameters return, the alarm condition will NOT be cleared automatically, and manual intervention will be required to restart the system.

This is useful:

  • As it can allow detection that an alarm condition has occurred even though nominal conditions have been restored. The cause and consequence may then be investigated.
  • If the equipment requires checking before restarting because an automatic restart might cause damage or a hazard.

Parameters that are monitored by monitoring relays include voltage, current, phase rotation, phase loss, neutral loss, current and temperature.

List of monitoring relay parameters and Finder’s models

70 Series

Voltage (Type 70.11)
Voltage and phase (Type 70.31)
Voltage, phase and neutral (Type 70.41 and Type 70.42)
Phase (Type 70.61 and Type 70.62)

Current (Type 70.51)

Temperature (Type 70.92)

  • Voltage monitoring relays are checking supply conditions.
  • Phase monitoring relays are checking supply conditions.
  • Current monitoring relays are checking the load (often motors)
  • Temperature monitoring relays are checking the load temperature (usually motor windings)

Finder’s voltage monitoring relays are Types 70.11, 70.31, 70.41, and 70.42. Voltage monitoring relays can detect not only under-voltages and over-voltages, but also voltage-related issues such as phase imbalances, phase loss, and phase sequence.

Finder’s Current monitoring relay is the flexible Type 70.51. It is extremely versatile, with adjustable Start Delay, Switch Off Delay and Switch On Hysteresis. It also has undercurrent, overcurrent and window detection modes. The Type 70.51 current monitoring relay also has independently settable high and low current limits and a selectable fault memory mode and measures currents up to 10A AC/DC (Direct connection) and 600A AC (current transformer).

Finder’s phase monitoring relays include Types 70.41, 70.42, 70.61, and 70.62. Phase monitoring relays monitor your system and safeguard it against phase sequence, phase failure and asymmetry due to any wear and tear within applications. They also monitor any drops or increases in voltage to help prevent any damage to machinery or injury to personnel.

Finder’s thermistor temperature monitoring relays include Type 70.92. These protect against overheating, overload, and insufficient cooling. For use with PTC temperature sensor, with inbuilt PTC short circuit and open circuit detection.

How does a current monitoring relay work?

Current monitoring relays are components within industrial applications to prevent issues due to overcurrent or undercurrent. In the event of overcurrent within the application, the current monitoring relay will activate, indicating the problem or switching off the flow of energy through the circuits. The same process applies in the event of an undercurrent.

Current monitoring relays utilise a direct connection or a current transformer to measure the current flowings through the system being monitored. A typical overcurrent situation might be an overloaded or stalled motor. The use of a current monitoring relay allows the stall condition to be detected and the motor can be shut down quickly to prevent any damage. An undercurrent situation could occur in a kiln with multiple heating elements. Failure of one element may not prevent heating from continuing but would affect the temperature distribution within the kiln, resulting in incorrect treatment of the items being processed.


Finder also has current sensing types in the 60 Series relays. Current sensing coils are available with models covering DC (0.1A to 2A) and AC (0.1A to 2.5A) currents. A coil of the correct range is simply connected in series with the load. These are useful in medium current applications, for example detecting the failure of warning or signal lamps.

Unique features of Finder’s 70 Series monitoring relays

All Models (70 Series)
•    Positive logic contacts
•    Clear multicolour LED status indication

Types 70.11 / 70.31 / 70.41 / 70.42
•    Multifunction with selection dial
•    Over and undervoltage levels independently selectable
•    Adjustable Off delay time

Types 70.31 / 70.41 / 70.42
•    Integrated phase loss / sequence detection

Types 70.61 / 70.62
•    Dedicated phase loss/sequence detection
•    Ideal retrofit solution for phase loss and sequence protection

Finder’s full range of monitoring relays

70 Series

The 70 Series comprises a range of multifunction devices that allow monitoring of undervoltage and overvoltage, phase sequence and phase failure.

Features include (according to Type):

  • Multifunctional types, providing the flexibility of monitoring Undervoltage, Overvoltage, Window Mode, Phase rotation, Phase loss, Asymmetry and Neutral loss
  • Current monitoring and control
  • Thermistor temperature monitoring and control
  • Coloured LEDs for clear & immediate visual indication
  • Modular housing, 17.5 or 35 mm wide

72 Series Level control relays

Finder also has the 72 Series level control relays for conductive liquids.

Here the datasheet of the 72 Series.

Features include (according to Type):

  • Sensitivity fixed or adjustable (5…150kΩ)
  • 35mm rail (EN 60715) mount

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