Plug Setting Multiplier (P.S.M) of a Relay

The Plug Setting Multiplier (P.S.M) is a term used in protective relaying to indicate the severity of the fault. It is a dimensionless quantity that represents how many times the fault current is greater than the relay's pick-up current.

Let's break down the components:

• Pick-up Current: This is the minimum current at which the relay coil operates or "picks up". It is usually set by adjusting the plug setting of the relay, often as a percentage of the rated secondary current of the current transformer (CT).
• Fault Current in Relay Coil: This is the actual current flowing through the relay coil under fault conditions. This current is proportional to the primary fault current scaled down by the CT ratio.

The formula for the Plug Setting Multiplier (P.S.M) is given by:

\( \text{P.S.M.} = \frac{\text{Fault current in relay coil}}{\text{Relay pick-up current}} \)

Examining the given options:

• Option 1: Ratio of pick up current to normal current. This is incorrect. Normal current is the load current, not directly used in the P.S.M. calculation in this ratio format.
• Option 2: Ratio of rated secondary current of CT to rated current of CT. This refers to the CT ratio itself, not the P.S.M. of the relay.
• Option 3: Ratio of fault current in circuit breaker to normal current. The fault current in the circuit breaker (primary current) is related to the fault current in the relay coil (secondary current via CT), but the ratio is specifically defined with respect to the relay's pick-up current, not the normal current.
• Option 4: Ratio of fault current in relay coil to pick up current. This matches the definition of Plug Setting Multiplier.

Therefore, the Plug Setting Multiplier (P.S.M) of a relay is correctly defined as the ratio of the fault current flowing through the relay coil to the relay's pick-up current.