Pick Up Current | Current Setting | Plug Setting Multiplier and Time Setting Multiplier of Relay

Overcurrent protection uses several related settings. The terms used for older induction-disc relays also appear in modern numerical electrical protective relays, but their implementation can differ. The four terms below describe when protective relays pick up and how an inverse-time element determines its operating time:

1. Pickup current.
2. Current setting.
3. Plug setting multiplier (PSM).
4. Time setting multiplier (TSM).

Pick Up Current of Relay

In electromechanical electrical relays, a spring, gravity or magnetic restraint holds the moving contacts in their normal position. Current in the operating coil produces torque or force in the opposite direction.
The pickup value is the lowest actuating current at which the relay element starts or asserts under its specified conditions. Pickup does not necessarily mean that a trip contact has closed or that a circuit breaker has opened. Modern numerical relays compare a measured current with a configured threshold, but the threshold has the same protective purpose. The configured threshold is the pickup current of the relay.

Current Setting of Relay

An induction-disc relay needs a fixed operating ampere-turn value. Changing the active turns on its current coil therefore changes the current needed for pickup.
Fewer active turns require more coil current to produce the pickup torque. More active turns require less current. This explains the plug-bridge or tap arrangement used on many older relays, but it is not how every relay implements the setting.

A current setting selects the pickup threshold for the protected circuit and the required coordination. An electromechanical relay may use coil taps and a plug bridge. A numerical relay normally stores the value as a setting.
The current setting of relay may be entered in primary amperes, CT secondary amperes or per unit of rated current. On a legacy plug-setting relay, it is often expressed as pickup current divided by the rated CT secondary current, multiplied by 100%.

For that percentage convention:

Suppose an over current relay has a 125% plug setting and a 1 A input from a 1 A secondary current transformer. Its nominal pickup is 1 A × 1.25 = 1.25 A secondary.
The element should pick up when its measured current reaches the applicable operating threshold, subject to the relay’s published pickup tolerance and reset ratio.
Using the definition above:

Current setting is also called current plug setting on this type of relay.
Ranges such as 50% to 200% for phase overcurrent or 10% to 70% for earth fault appear on some legacy relays. They are not universal. Use the available range, step size and accuracy stated in the manual for the installed relay.

Plug Setting Multiplier of Relay

Plug setting multiplier of relay is the current measured by the relay divided by its pickup current. Both values must use the same primary or secondary basis.

Consider a 200/1 A protection CT and a 150% setting on a 1 A relay.
Nominal pickup is 1 × 1.50 = 1.5 A secondary.
If the primary fault current is 1000 A, the ideal CT secondary current is 1000 × 1/200 = 5 A.
The PSM is therefore 5/1.5 = 3.33. In modern relay manuals, the same quantity is often called a multiple of pickup, M or I/I>.

Time Setting Multiplier of Relay

The operating time of an inverse-time overcurrent element depends mainly on two settings:

1. The selected time-current curve and the current multiple.
2. The time multiplier, time dial or equivalent time-scale setting.

On an induction-disc relay, moving the contact changes the disc travel needed to close the trip contact. This mechanical adjustment is the origin of the time-dial explanation. A numerical relay applies a selected curve equation or stored characteristic instead. Time setting multiplier of relay is normally a dimensionless scale, not a setting in seconds. A scale marked 0 to 1 in 0.05 steps applies only to relays designed that way.
TSM alone does not give the operating time. The relay first determines a base time from its selected inverse curve and the multiple of pickup. IEC, IEEE and manufacturer-specific curves use different equations and constants. The manufacturer’s manual or verified coordination software is therefore the source for the actual time.
For a curve whose time is directly scaled by TSM, actual operate time equals the curve time at TSM 1 multiplied by the selected TSM.
For example, assume the selected relay curve gives 3 seconds at PSM 10 when TSM is 1.
With TSM 0.1, the calculation is 3 × 0.1 = 0.3 seconds.
This result belongs to that curve and setting combination. It cannot be used for another relay without checking its curve equation, tolerances and any minimum operating time.

Time vs. PSM Curve of Relay

A time-versus-PSM curve plots operating time on the vertical axis and current as a multiple of pickup on the horizontal axis. The curve label must state the curve family and time setting used. A curve drawn at TSM 1 gives the base time for that specific characteristic.
If the illustrated curve gives 3 seconds at PSM 10 and TSM 1, the relay’s calculated base operating time is 3 seconds. Other normal inverse, very inverse, extremely inverse, long-time inverse or manufacturer curves can give different times at the same PSM.
For an inverse-time curve, operating time generally falls as current multiple rises. The mathematical relationship is set by the chosen curve rather than a universal inverse-proportion rule.
Some relays impose a definite minimum time or change to definite-time operation above a stated current multiple. The transition point is product-specific. A general claim that every relay becomes constant above PSM 20 is incorrect. Coordination must use the actual curve, breaker clearing time and required grading margin.

Calculation of Relay Operation Time

To calculate relay operating time, use the following data from the protection study and relay manual.

1. Pickup or current setting.
2. Fault current at the relay location.
3. Current-transformer ratio and connection.
4. Selected time-current curve or equation.
5. Time multiplier or time dial.

Step-1
Identify the CT ratio and relay input basis. For a 100/1 A CT, 100 A primary ideally produces 1 A secondary.

Step-2
Calculate pickup on the same basis. At a 150% plug setting with a 1 A input, nominal pickup is 1 × 1.50 = 1.5 A secondary.

Step-3
Convert the specified fault current to the relay basis, then divide it by pickup. With 1500 A primary through a 100/1 A CT, ideal relay current is 1500/(100/1) = 15 A. The PSM is 15/1.5 = 10.

Step-4
Read the base operating time from the selected curve or calculate it with the relay’s documented equation. If that curve gives 3 seconds at PSM 10 and TSM 1, use 3 seconds as the base time.

Step-5
Apply the time multiplier in the way specified for that curve. If the base time is 3 seconds and TSM is 0.1, actual operating time is 3 × 0.1 = 0.3 seconds, or 300 ms.
Check the relay’s timing tolerance and add breaker operating time when calculating total fault-clearing time.