Setting the Short-Time Overcurrent Protection (S or ANSI 50TD/51)
The Isd and tsd settings help to ensure that the short-time withstand current of protected equipment is not exceeded.
When short-time overcurrent protection automatically disconnects the power supply in accordance with IEC 60364-4-41, the Isd setting must take into consideration the fault loop impedance of the protected circuit. For more information, refer to IEC 60364-4-41 2017 clause 411.4.4 or national low-voltage installation rules.
Alternatively this function can be performed by earth-leakage or ground-fault protection.
|
Application |
Principle |
Isd usual value |
|---|---|---|
|
Secondary side of MV/LV transformer (switchboard main incomer or tie circuit breaker between two switchboards) |
Lower than minimum short-circuit or ground-fault current at the end of the protected circuit. Selectivity with downstream circuit breakers |
10 x Ir |
|
Generators |
Lower than minimum short-circuit or ground-fault current supplied by the generator. Selectivity with downstream circuit breakers |
2–3 x Ir |
|
Feeder with other MasterPact or Compact NS or PowerPact P- and R-frame circuit breaker downstream |
Lower than minimum short-circuit or ground-fault current at the end of the protected circuit. Selectivity with downstream circuit breakers |
10 x Ir |
|
Feeder without other MasterPact or Compact NS or PowerPact P- and R-frame circuit breaker downstream |
Lower than minimum short-circuit or ground-fault current at the end of the protected circuit. Selectivity with downstream circuit breakers |
10 x Ir |
|
Primary side of LV/LV transformer |
Lower than minimum secondary short-circuit current. |
10 x Ir |
|
Power electronic (for example, uninterruptible power supplies, variable speed drives, photovoltaic inverters) |
Lower than minimum short-circuit or ground-fault current at the end of the protected circuit. Lower setting possible as no selectivity or transient current is expected. |
1.5–2 x Ir |
|
Motors |
Lower than minimum short-circuit or ground-fault current at the end of the protected circuit. Lower setting possible above starting current. |
10 x Ir |
When LV/LV transformers are switched on, very high inrush currents are produced which must be taken into account when choosing overcurrent protection devices. The peak value of the first current wave often reaches 10 to 15 times the rated rms current of the transformer and may reach values of 20 to 25 times the rated current even for transformers rated less than 50 kVA.
Example of inrush current: when transformer is switched on:
A 1st peak 10 to 25 x In
Example of inrush current of direct on line motor when started:
tsd is set according to selectivity.
Time-based selectivity is provided between two circuit breakers when the supply side circuit breaker short-time delay is at least one step higher than the load side short-time delay.
When downstream circuit breakers are Compact NSX or PowerPact H-, J-, or L-frame circuit breakers, selectivity is always provided with MasterPact MTZ circuit breakers with MicroLogic X control units, for all values of tsd.
Short-time tripping time can be definite time type (tripping time is independent of current level) or time dependent with I2t = constant curve. This function allows the curve to be smoothed for low-level overcurrent, providing fast trip at high current. This is recommended for selectivity with fuses.
|
Application |
Principle |
tsd usual value |
|---|---|---|
|
Secondary side of MV/LV transformer (switchboard main incomer or tie circuit breaker between two switchboards) |
Selectivity with downstream circuit breakers |
tsd > tsd of downstream power circuit breaker (tsd = 0.2 s if installation includes three levels of power circuit breaker) |
|
Feeder with selectivity with other MasterPact MTZ, Compact NS or PowerPact P- and R-frame circuit breaker downstream |
Selectivity with downstream circuit breakers |
tsd > tsd of downstream power circuit breaker (tsd = 0.1 s if installation includes three levels of power circuit breaker) |
|
Feeder without selectivity with other MasterPact MTZ, Compact NS or PowerPact P- and R-frame circuit breaker downstream |
No need for delayed short-time protection |
tsd = 0 s |
|
Primary side of LV/LV transformer |
Stability during inrush. Selectivity with downstream circuit breakers |
tsd = 0.1 s or tsd > tsd of downstream power circuit breaker, if any |
|
Power electronic (Uninterrupted power supplies, variable speed drives, photovoltaic inverters, etc.) |
No need for delayed short-time protection |
tsd = 0 s |
|
Motors |
Stability during inrush |
tsd = 0 s or 0.1 s |
