Pick-up Current, Plug Setting Multiplier (PSM) and Time Setting Multiplier (TSM)

Plug Setting Multiplier and Time Setting Multiplier are used only for Electromechanical Relays. These terms or parameters are not so used in Numerical Relays but they are conceptually used and incorporated in Numerical Relays too but the way of their implementation is quite different than that of Electromechanical Relays. In this post we will focus on the concept and implementation of Plug Setting Multiplier and Time Setting Multiplier for Electromechanical Relays.

As we know that an Electromechanical Relay has a coil which when energized, operates the Relay to have contact changeover. But there shall be some minimum current which when flows through the Relay coil, produces enough magnetic force to pull the lever to make contact change over. Isn’t it? Yes, if you ever get a chance to see electromechanical relay, you will observe that there is a flapper kind of thing which is attached with the lever. The lever in turn is attached with contacts. Thus when a specified current flows through the relay coil, then only it will produce enough magnetic pull to attract the flapper and lever to operate the Relay. A simple picture of relay demonstrating its construction and operation is shown in figure below.


This minimum current in the Relay coil at which Relay starts to operate is called Pick-up Current. If the current through the Relay coil is less than the pick-up value then Relay won’t operate. On contrary, if the current through the Relay coil is more than the Pick-up current, Relay will operate. In industries, we normally perform Relay Pick-up and Drop-off Test to check the healthiness of relays.

Hope your concept of Pick-up current of Relay is clear now. Now we will move on to Current Setting of electromechanical relays.

Current Setting of Electromechanical Relays: Current Setting of relay is nothing but adjusting its pick-up value. Suppose we are using a CT of ratio 1000/1 A and the pick-up current needs to be set at 1.2 A. Then we will simply put the plug provided on relay coil to 120% or 1.2. Thus we can say that

Pick-up current = Plug Position x Rated CT Secondary Current.

The plug or tapping is provided on the Relay Coil so that changing the position of Plug changes the number of turns of the relay coil as shown in figure below.

 As shown in figure above, the plug is kept at 5. This means that pick-up current of relay will be 5 times of rated CT Secondary current. Likewise, if we put the plug at 8.75 then pick-up current of relay will be 8.75 times of the rated CT Secondary current.

Plug Setting Multiplier (PSM): Plug Setting Multiplier (PSM) is defined as the ratio of fault current to the pick-up current of the relay. Thus,

PSM = Fault Current / Pick-up Current

         = Fault Current / (Plug Position x Rated CT Secondary Current)

Suppose we are using CT of 100/1 A, a fault current of, say 5000 A is flowing through the network protected by the relay.

Fault current in CT secondary = (5000×1) / 100 = 50 A

Assume that Current Setting or the position of plug is at 5 then

PSM = 50 / (1×5) =10

It shall be noted here that we shall not bother about PSM for instantaneous relay rather we shall consider PSM for relays having characteristics of Inverse Time, Very Inverse Time etc.

For Detail on Relay Characteristics read Over Current Relay and Its Characteristics

Time Setting Multiplier (TSM): Again it is worthwhile to mention that we shall not bother about TSM for instantaneous relay rather we shall consider TSM for relays having characteristics of Inverse Time, Very InverseTime etc.

A Relay is generally provided with control to adjust the time of operation of the Relay. This adjustment is known as Time Setting Multiplier or TSM. Normally a Time Setting Dial is provided which is calibrated from 0 to 1 s in step of 0.05 s. For practical exposure, let us consider a relay as shown in figure below. Please Zoom the image to clearly view every part of the Relay for better understanding.

As can be seen from the figure, there is a Time Setting Dial which is rotated to set the time of operation of the relay. Lets say we want to set the time on Time Setting Dial to 0.5 s, then we need to rotate the dial till 0.5 s on the dial matches with the fixed mark provided. So in this case, our TSM is 0.5.

How to find the time of operation of Relay?

Well, assume that plug is set at 5 and TSM at 0.5 s. For finding the actual time of operation of relay, we need to refer the Graph between the Operating Time and PSM. This graph is generally provided on the Relay cover itself. Let us consider a graph between Operating Time and PSM as shown below.

For our case, PSM = 10 (Please see the calculation and case considered above in our discussion of PSM) and TSM = 0.5 s.

From the Graph, the time of operation of Relay for PSM = 10 is 3 s.


Actual Time of Operation of Relay = 3s x TSM

                                                       = 3 x0.5 s =1.5 s

Thus we can say that actual time of operation of Relay is equal to the time obtained from the PSM & Operating Time Graph multiplied by TSM.

Hope your concept of Pick-up current, PSM and TSM is clear. I tried to show you by my practical experience. If still you have any doubt feel free to write in comment box. Thank you!

31 thoughts on “Pick-up Current, Plug Setting Multiplier (PSM) and Time Setting Multiplier (TSM)”

  1. Suppose ct ratio is 2000/5 and current setting is 1,and current injected in relay is 6 amps then what is psm if range of current setting given is 0.75 to 2 amp

    • Since Plug Setting Multiplier is ratio of Fault Current to relay setting, therefore the PSM will be equal to (6/1) i.e. 6. Here I am assuming fault current in terms of CT secondary to be 6 A i.e. 2400 A in terms of primary and relay setting equal to 1 A in terms of secondary i.e. 2000 A in terms of primary.

    • The TMS setting basically depends on the relay coordination. In case of CDG relay, you can keep TMS setting anywhere in between 0 to 1 sec depending upon how fast you want relay to operate. In case, you keep TMS equal to 0.5 sec, then relay operating time will be as per the PSM Versus Operating time Graph. Hope it help. If you still need any help, please revert. Thank you!

    • You are correct, fault current may not be same all the time. But we don’t need to change the setting. We adopt one IDMT curve as per IEC and based on the curve the relay operating time will be governed on the basis of PSM. Did you get it or shall I further clarify?

  2. if ct ratio is not mentioned but maximum and minimum fault current at the bus is mentioned also plug setting is mentioned .then how to compute to compute psm

  3. Sir,
    I have one doubt,which one first operate Tsm or Psm ..as per u r explanation Tsm is first operate then what is need of Psm…?

    • TSM is time setting multiplier which comes into role only when pick-up value is reached. Practically PSM tell us about the magnitude of fault. The more the value of fault current, the more will be the PSM and hence the relay should clear the fault more quickly. Thus the time of operation of relay should be dependent on PSM. Actually, there is a curve between PSM and TSM which determines the relay actuation time. Did you get it? Please revert if you need further clarification.

  4. I’m so much indebted that you run this blog. I acquire great deal of knowledge from here.

    Thanks a lot sir for this wonderful platform.

    • I’m willing to do so but presently I’m little engaged in some personal pusuits. I’ll definitely come back and let you know when I’m ready.
      Will you be available here when I try to contact you?

  5. Sir, if iam testing a micom relay and injecting current by loading transformer. I have the CT ratio suppose 300/1. Now my question is what is the fault current i will consider??? . Is it the setting which i provide in relay of any fault some other.

    • I think Is is setting value for protection element.A current more than rhe setting value may be assumed to be fault current. I can explain better if you can mention the detail of protection you are testing.

  6. The fault current will be the value at which the relay start pick up.ie-just ur set value.
    Suppose u have set 0.8*In
    Then fault current will be 0.8*In,here In is 1.

  7. 13 An IDMT type over current relay is used to protect a feeder through 500/1A CT. The relay has a Plug Setting of 125% and TMS=0.3.Find the time of operation of the said relay if a fault current of 5000A flows through the feeder. Make use of the following characteristics
    PSM 2 3 5 8 10 15
    Time for unity TMS 10 6 4.5 3.2 3 2.5
    (100% current=1A)

  8. I am just confused, PSM just a ratio or its derived value is actually current value? because from the plug setting part of the relay in the picture above, the psm values are labeled as amp. thank you

  9. is the PSM just simply a ratio, or it is a value of current? say psm =50/5 = 10, does this mean 10 amperes? because in the picture above on the right side of PSM is says “amp”
    Thank you.


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