Relay Plug Setting Multiplier

What is this formula?

The Plug Setting Multiplier (PSM) indicates how many times the actual fault current exceeds the relay pickup current setting.

When to use it

Use this formula when configuring overcurrent relays, performing protection coordination studies, evaluating relay sensitivity, and analyzing fault-clearing performance.

Example

A protection relay has:

Fault current = 2500 A

Pickup current setting = 500 A

Formula:

PSM = If / Ipickup

Substitution:

PSM = 2500 / 500

Result:

PSM = 5

The relay sees a fault current equal to five times its pickup setting.

Applications

Overcurrent relay settings

Protection coordination studies

Distribution system protection

Substation protection design

Fault analysis

Note

PSM is widely used in inverse-time overcurrent protection. A higher PSM generally results in a shorter relay operating time. The exact relationship between PSM and operating time depends on the relay characteristic curve selected, such as IEC Standard Inverse, Very Inverse, or Extremely Inverse.