# Instantaneous, Inverse Time and Definite Time Lag Relays

- Link: https://studyelectrical.com/2017/01/relay-timing-characteristics-instantaneous-inverse-time-definite-time-lag-relays.html
- Published: 2017-01-07T06:47:00+00:00
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![relay%2Btime%2Bopt](https://4.bp.blogspot.com/-R7rwiaKhQzk/WHCHrEEN4_I/AAAAAAAACNk/
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In this post, we will explain Instantaneous, Inverse Time and Definite Time Lag 
Relays and their characteristics.

An important characteristic of a relay is its time of operation. By ‘the time of
operation’ is meant length of the time from the instant when the actuating element
is energized to the instant when the relay contacts are closed. 

 

Sometimes it is desirable and necessary to control the operating time of a relay.
For this purpose, mechanical accessories are used with relays.

### Instantaneous Relay

An instantaneous relay is one in which no intentional time delay is provided. In
this case, the relay contacts are closed immediately after the current in the relay
coil exceeds the minimum calibrated value.

 

[⌊instantaneous%2Brelay⌉⌊instantaneous%2Brelay⌉[Figure shows an instantaneous solenoid
type of relay. Although there will be a short time interval between the instant 
of pickup and the closing of relay contacts, no intentional time delay has been 
added. 

 

The instantaneous relays have operating time less than 0·1 second. The instantaneous
relay is effective only where the impedance between the relay and the source is 
small compared to the protected section impedance.

 

The operating time of instantaneous relay is sometimes expressed in cycles based
on the power system frequency.

e.g. one-cycle would be 1/50 second in a 50-cycle system.

### Inverse Time Relay

An inverse-time relay is one in which the operating time is approximately inversely
proportional to the magnitude of the actuating quantity. 

 

The figure shows the time-current characteristics of an inverse current relay. At
values of current less than the pickup, the relay never operates. At higher values,
the time of operation of the relay decreases steadily with the increase of current.

 

[⌊inverse time%2Brelay⌉⌊inverse time%2Brelay⌉[The inverse-time delay can be achieved
by associating mechanical accessories with relays.

 * In an induction relay, the inverse-time delay can be achieved by positioning 
   a permanent magnet (known as a drag magnet) in such a way that relay disc cuts
   the flux between the poles of the magnet. When the disc moves, currents set up
   in it produce a drag on the disc which slows its motion.

 

[⌊inverse%2Boil%2Bdashpot⌉⌊inverse%2Boil%2Bdashpot⌉[

 * In other types of relays, the inverse time delay can be introduced by oil dashpot
   or a timelimit fuse. Figure shows an inverse time solenoid relay using oil dashpot.
   The piston in the oil dashpot attached to the moving plunger slows its upward
   motion. At a current value just equal to the pickup, the plunger moves slowly
   and time delay is at a maximum. At higher values of relay current, the delay 
   time is shortened due to greater pull on the plunger.

 

The inverse-time characteristic can also be obtained by connecting a time-limit 
fuse in parallel with the trip coil terminals as shown in the figure below. 

 

[⌊time%2Blimit%2Bfuse⌉⌊time%2Blimit%2Bfuse⌉[The shunt path formed by time-limit 
fuse is of negligible impedance as compared with the relatively high impedance of
the trip coil. Therefore, so long as the fuse remains intact, it will divert practically
the whole secondary current of CT from the trip coil. 

 

When the secondary current exceeds the current carrying capacity of the fuse, the
fuse will blow and the whole current will pass through the trip coil, thus opening
the circuit breaker. The timelag between the incidence of excess current and the
tripping of the breaker is governed by the characteristics of the fuse. Careful 
selection of the fuse can give the desired inverse-time characteristics, although
the necessity for replacement after an operation is a disadvantage.

### Definite Time Lag Relay

In this type of relay, there is a definite time elapse between the instant of pickup
and the closing of relay contacts. This particular time setting is independent of
the amount of current through the relay coil ; being the same for all values of 
current in excess of the pickup value. 

 

It may be worthwhile to mention here that practically all inverse-time relays are
also provided with definite minimum time feature in order that the relay may never
become instantaneous in its action for very long overloads.

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