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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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Plug setting multiplier (PSM) is given by
Fault current/Peak up current
Peak up current/Fault current
Fault voltage/Fault current
None of above
Fault current/Peak up current
The Plug Setting Multiplier (PSM) is a dimensionless ratio used to determine the operating time of an IDMT (Inverse Definite Minimum Time) relay. It is defined as the ratio of the actual fault current flowing through the relay coil to the relay's rated pick-up current.
The Plug Setting Multiplier (PSM) is a dimensionless ratio used to determine the operating time of an IDMT (Inverse Definite Minimum Time) relay. It is defined as the ratio of the actual fault current flowing through the relay coil to the relay's rated pick-up current.
PSM=Rated┬аPlug┬аSetting┬аCurrentSecondary┬аFault┬аCurrentтАЛ
When a fault occurs, the secondary current of the Current Transformer (CT) flows through the relay coil. The relay is set to operate at a specific 'plug setting' (pick-up current). The PSM determines the point on the inverse-time characteristic curve where the relay operates; a higher PSM results in a faster operating time for the relay.
PSM is independent of the Time Multiplier Setting (TMS).
It helps identify the operating time on the relay's standard characteristic curve.
The pick-up current is calculated as: IpтАЛ=Rated┬аSecondary┬аCurrent├ЧPlug┬аSetting%.
Typically, relays have plug settings ranging from 50% to 200% in steps.
Allows adjustment of relay sensitivity for different fault levels.
Enables coordination between series-connected relays in power systems.
Complexity in setting selection for varying load conditions.
Incorrect settings can lead to nuisance tripping or failure to clear faults.
Overcurrent protection of transmission lines.
Protection of power transformers and switchgear.
The 'Pick-up current' is also referred to as the 'Relay setting current'.
Option B is the inverse, which is not physically meaningful in this context.
Option C refers to impedance, not the relay characteristic ratio.
A is correct тАФ The PSM is defined as the ratio of the fault current in the relay coil to the relay pick-up current.
Always ensure the CT ratio is accounted for when calculating the 'Fault current' reaching the relay, as IrelayтАЛ=IfaultтАЛ├ЧCTratioтАЛ1тАЛ.