Protection Setting Guidelines

Presentation

The setting of overcurrent protection relies on installation short-circuit and electrical fault calculations. The setting guidelines cannot replace these calculations.

MasterPacT MTZ circuit breakers with MicroLogic Active control units offer flexibility to set the required overcurrent protection while maintaining selectivity and stability on transient phenomena, for example, inrush current of transformers or motors, when necessary.

For each circuit, the installation designer needs to provide the following:

Iz: continuous current capacity of the circuit according to IEC 60364-5-52 or national wiring rules. Current carrying capacity is called ampacity in the US National Electrical Code (NFPA 70).
Ifault min: minimum electrical fault current at the end of the circuit depending on earthing system
Tmax short-circuit: maximum time for maximum short-circuit current

Guidelines are given for setting the following protections:

Long-time overcurrent protection
Short-time overcurrent protection
Instantaneous overcurrent protection

Overcurrent Protection Setting Guidelines by Application

The following table gives the guidelines for overcurrent protection setting by application:

Application	MicroLogic Active 2.0	MicroLogic Active 5.0, 6.0⁽¹⁾
Secondary side of MV/LV transformer (switchboard main incomer) with other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder	Ir = Iz tr ≤ 24 s Isd ≤ Ifault min Selectivity possible with ComPacT NSXm and ComPacT NSX feeders only.	Ir = Iz tr ≤ 24 s Isd ≤ Ifault min tsd < Tmax short-circuit tsd > tsd of downstream MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker⁽²⁾ Ii mode: OFF
Secondary side of MV/LV transformer (switchboard main incomer) without other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder	Ir = Iz tr ≤ 24 s Isd ≤ Ifault min	Ir = Iz tr ≤ 24 s Isd ≤ Ifault min tsd = 0 Ii mode: ON Ii = Isd
Generator output with other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder	Ir = Iz tr ≤ 1 s Isd ≤ Ifault min Selectivity possible with ComPacT NSXm and ComPacT NSX feeders only.	Ir = Iz tr ≤ 1 s Isd ≤ Ifault min tsd > tsd of downstream MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker⁽²⁾ Ii mode: OFF
Generator output without other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder	Ir = Iz tr ≤ 1 s Isd ≤ Ifault min	Ir = Iz tr ≤ 1 s Isd ≤ Ifault min tsd = 0 Ii mode: ON Ii = Isd
Feeder with other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream	Ir = Iz tr ≤ 16 s Isd ≤ Ifault min Selectivity possible with ComPacT NSXm and ComPacT NSX feeders only.	Ir = Iz tr ≤ 16 s Isd ≤ Ifault min tsd > tsd of downstream circuit breaker⁽²⁾ Ii mode: OFF
Feeder without other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder	Ir = Iz tr ≤ 16 s Isd ≤ Ifault min	Ir = Iz tr ≤ 16 s Isd ≤ Ifault min tsd = 0 Ii mode: ON Ii = Isd
Power electronic (for example, uninterruptible power supplies, variable speed drives, photovoltaic inverters) with no other circuit breaker downstream	Ir = Iz tr ≤ 8 s Isd = 1.5-2 x In ≤ Ifault min	Ir = Iz tr ≤ 16 s Isd = 1.5-2 x In ≤ Ifault min tsd = 0 Ii mode: ON Ii = 2–3 x In
ERMS settings	Ii_ERMS = 1.5 x Ir	Ii_ERMS = 2 x In
(1) Ground-fault protection depends on the grounding system and local regulations. As a rule ground-fault sensitivity should be as low as possible without being disturbed by permanent or transient leakage current. The ground-fault time delay enables selectivity with downstream devices. (2) When tsd > 0, a fault clearance time reduction system, such as ZSI or ERMS, is required in the USA by National Electricity Code NFPA 70 (240.87) (2011 edition). Refer to the guidelines for ERMS settings.

Application

MicroLogic Active 2.0

MicroLogic Active 5.0, 6.0⁽¹⁾

Secondary side of MV/LV transformer (switchboard main incomer) with other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder

Ir = Iz

tr ≤ 24 s

Isd ≤ Ifault min

Selectivity possible with ComPacT NSXm and ComPacT NSX feeders only.

Ir = Iz

tr ≤ 24 s

Isd ≤ Ifault min

tsd < Tmax short-circuit

tsd > tsd of downstream MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker⁽²⁾

Ii mode: OFF

Secondary side of MV/LV transformer (switchboard main incomer) without other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder

Ir = Iz

tr ≤ 24 s

Isd ≤ Ifault min

Ir = Iz

tr ≤ 24 s

Isd ≤ Ifault min

tsd = 0

Ii mode: ON

Ii = Isd

Generator output with other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder

Ir = Iz

tr ≤ 1 s

Isd ≤ Ifault min

Selectivity possible with ComPacT NSXm and ComPacT NSX feeders only.

Ir = Iz

tr ≤ 1 s

Isd ≤ Ifault min

tsd > tsd of downstream MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker⁽²⁾

Ii mode: OFF

Generator output without other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder

Ir = Iz

tr ≤ 1 s

Isd ≤ Ifault min

Ir = Iz

tr ≤ 1 s

Isd ≤ Ifault min

tsd = 0

Ii mode: ON

Ii = Isd

Feeder with other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream

Ir = Iz

tr ≤ 16 s

Isd ≤ Ifault min

Selectivity possible with ComPacT NSXm and ComPacT NSX feeders only.

Ir = Iz

tr ≤ 16 s

Isd ≤ Ifault min

tsd > tsd of downstream circuit breaker⁽²⁾

Ii mode: OFF

Feeder without other MasterPacT MTZ, ComPacT NS, or PowerPacT P- and R-frame circuit breaker downstream as feeder

Ir = Iz

tr ≤ 16 s

Isd ≤ Ifault min

Ir = Iz

tr ≤ 16 s

Isd ≤ Ifault min

tsd = 0

Ii mode: ON

Ii = Isd

Power electronic (for example, uninterruptible power supplies, variable speed drives, photovoltaic inverters) with no other circuit breaker downstream

Ir = Iz

tr ≤ 8 s

Isd = 1.5-2 x In ≤ Ifault min

Ir = Iz

tr ≤ 16 s

Isd = 1.5-2 x In ≤ Ifault min

tsd = 0

Ii mode: ON

Ii = 2–3 x In

ERMS settings

Ii_ERMS = 1.5 x Ir

Ii_ERMS = 2 x In

(1) Ground-fault protection depends on the grounding system and local regulations. As a rule ground-fault sensitivity should be as low as possible without being disturbed by permanent or transient leakage current. The ground-fault time delay enables selectivity with downstream devices.

(2) When tsd > 0, a fault clearance time reduction system, such as ZSI or ERMS, is required in the USA by National Electricity Code NFPA 70 (240.87) (2011 edition). Refer to the guidelines for ERMS settings.