Relay Characteristics

Relay manufactures design them for special applications to full fill industries needed and coupe with advanced technologies. This time i want share information about relays characteristics most often used in typical automation and instrumentation work.

As i explained in my last posted,basically principal for small DC and AC relays,sensitive relays, miniature and sealed relays, and some small power relays is common.

To get a suitable relay for our application,we should understand about basic relay characteristic and general specification about operation and relay design because manufacture designs many types of relay for different application.

How to select Rating, Size, and Other Criteria for relay?

When we selecting relays for particular applications such as automation or instrumentation project, the first thing we need to consider is about the actuating energy that is available or needed to operate a relay. It mean for a rough classification,associating coil requirement with power-handling ability.

The values in this table are only approximate.Relays can also be classified by operating function. Available energy often dictates relay choice.

 Types of relays application

1) Meter relays

Meter relays and ultra sensitive relays are actuated by very small energies (perhaps as small as a fraction of a microwatt). They are used in applications where only a very weak signal is available, such as at the output of a transducer or bridge.

2) General purpose

General-purpose, or small control, relays are used in cases in which no great amount of power need be handled but flexibility in application and reliability in operation are both essential.

Depending on the number of contacts, they usually require coil power of 200 to 800 mW, which may be either AC or DC. Typically they will control perhaps 5 A at 120 V per contact.It common used for control panel machinery or motor control application.

3) Small relay

Small power relays are used in cases in which more power must be controlled. These usually have coils operating on 120
or 240 VAC, which can be controlled by sensitive relays.

These small power relays have contact capacities up to 30 A, at voltages up to 600 V. Much larger relays are of course
available or it often called magnetic contactors or controllers.


  1. Dear sir,
    Great pleasure to read about protection relay.I am working in hydro power station of 75 MW, 25MW of each. we have static type protection relay9 Siemens) and need to replace digital numerical type. let me know of reliable manufacture/company. also want to know about synchronizing check relay for 1 MW generator. THANKS

    Rambijul mandal

  2. we have 200kw motor for centrifugal pump, how to set the relay settings from motor protection relay make: EE

  3. We are have a 200KW motor for centrifugal pump,it was connected in delta connection,we have one motor protection relay English Electric make I will asking to you how to set the relay settings we are using 500/1a Current transformers 3No’s for relay
    Relay details are: Model : CTMM505DF 2A
    i th :70% -130%
    i th (A): 70%-90%
    i (inst) :600%-1200%
    i2 : 30%-630%
    i0 10ma-100ma:
    vv raju

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