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 X 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:

oIz: 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).

oIfault min: minimum electrical fault current at the end of the circuit depending on earthing system

oTmax short-circuit: maximum time for maximum short-circuit current

Guidelines are given for setting the following protections:

oLong-time overcurrent protection

oShort-time overcurrent protection

oInstantaneous overcurrent protection

oIDMTL overcurrent protection

oDirectional overcurrent protection

Overcurrent Protection Setting Guidelines by Application

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

Application

MicroLogic 2.0 X

MicroLogic 3.0 X

MicroLogic 5.0 X, 6.0 X, 7.0 X(1)

Secondary side of MV/LV transformer (switchboard main incomer) with other MasterPact, 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

Ii ≤ Ifault min

Ii tripping mode: Standard

Selectivity possible with PowerPact B-, H-, J-, and L-frame feeders only.

Ir = Iz

tr ≤ 24 s

Isd ≤ Ifault min

tsd  tsd of downstream MasterPact, 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, 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

Ii ≤ Ifault min

Ii tripping mode: Standard

Ir = Iz

tr ≤ 24 s

Isd ≤ Ifault min

tsd = 0

Ii mode: ON

Ii tripping mode: Standard

Ii = Isd

Generator output

with other MasterPact, 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

Ii ≤ Ifault min

Ii tripping mode: Standard

Selectivity possible with PowerPact B-, H-, J-, and L-frame feeders only.

Ir = Iz

tr ≤ 1 s

Isd ≤ Ifault min

tsd > tsd of downstream MasterPact, Compact NS, or PowerPact P- and R-frame circuit breaker(2)

Ii mode: OFF

Generator output

without other MasterPact, 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

Ii ≤ Ifault min

Ii tripping mode: Standard

Ir = Iz

tr ≤ 1 s

Isd ≤ Ifault min

tsd = 0

Ii mode: ON

Ii tripping mode: Standard

Ii = Isd

Feeder with other MasterPact, 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

Ii ≤ Ifault min

Ii tripping mode: Standard

Selectivity possible with PowerPact B-, H-, J-, and L-frame feeders only.

Ir = Iz

tr ≤ 16 s

Isd ≤ Ifault min

tsd > tsd of downstream circuit breaker(2)

Ii mode: OFF

Feeder without other MasterPact, 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

Ii ≤ Ifault min

Ii tripping mode: Standard

Ir = Iz

tr ≤ 16 s

Isd ≤ Ifault min

tsd = 0

Ii mode: ON

Ii tripping mode: Standard

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 ≤ 8 s

Ii = 2-3 x In≤ Ifault min

Ii tripping mode: Fast

Ir = Iz

tr ≤ 16 s

Isd = 1.5-2 x In≤ Ifault min

tsd = 0

Ii mode: ON

Ii tripping mode: Fast

Ii = 2–3 x In

ERMS settings with ERMS Digital Module only

Ir_ERMS = Iz

tr_ERMS ≤ 1 s

Isd_ERMS = 1.5 x Ir

Ir_ERMS = Iz

tr_ERMS ≤ 1 s

Ii_ERMS = 1.5 x In

Ii_ERMS  tripping mode: Fast

Ir_ERMS = Iz

tr_ERMS ≤ 1 s

Isd_ERMS = 1.5 x Ir

tsd_ERMS = 0

Ii_ERMS = 2 x In

Ii_ERMS  tripping mode: Fast

(1) Ground-fault protection and earth-leakage protection depend on the earthing system and local regulations. As a rule ground-fault and earth-leakage sensitivity should be as low as possible without being disturbed by permanent or transient leakage current. The ground-fault and earth-leakage 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.