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Customer having nuisance tripping problem with STR Masterpact trip unit. Low loads with ground fault function.

Recommend 24Vdc on terminals F1 (negative) and F2 (positive) to external power the trip unit.

The Masterpact breaker is load powered for overcurrent protection functions and for most other options. This power is provided by the current sensors and is proportional to the load current flowing through the breaker. During very low load operations (< 20% of the sensor rating), the current from the current sensor is not sufficient to power all functions.

External 24 Vdc power is required with the following conditions and options:

Required for accurate ammeter indications for loads less than 20% of the current sensor rating.
Required for ``V`` (remote selected fault indication) option. The 24 Vdc is required to power the optical triac static switch when the breaker trips.
Required for ``T`` or ``W`` (ground fault) option for the lower settings (settings in the red zone). The 24 Vdc is required to power the trip unit for ground fault pickup settings as shown in Table 1.
Required for ``C`` (communications) option.
May be used for ``F`` (fault indication) option if memory is not powered by the PIL battery.

We have found, in a few applications with very low loading and significant harmonic content, the external 24 Vdc power to the trip unit eliminates random nuisance ground-fault tripping. Use the AD module or the AD module with battery backup to supply the 24 Vdc external power.
Table 1
Use AD module or external 24Vdc power on F1 and F2 with ground-fault pickup settings shown below:

Sensor Size Ground-fault pick-up
800/1000/1200 A 0.1, 0.2, and 0.3 - Use AD module
1600/2000/2500 A 250A to 600A - Use AD module
3000/3200/4000 A 400A to 1000A - Use AD module
5000/6300 A All settings - Use AD module

CTA-ID : 8600
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