Merlin Models
we have a 800A merlin gerin CB........but it trips when the load current is 510A?
Model of the CB is NS800N.
Ir is set to .98
Isd is set to 10
tr is set to 20
Can anybody answer me?
What is the reason of tripping the CB at load current 510 A.
Without knowing much of the details I am just offering some suggestions or reminders, hopefully helpful.
It could be due to too much heating due to a bad connection. You can measure the voltage drop across the connections, or use an IR thermometer (non contact) to measure the temperature.
For example, if there is 0.1V dropped across a connection or the breaker itself on any phase, the power at 500A = 50W, which can cause localised heating that transfers to the thermal trip measurement.
Other possibilities are brownout (<20ms I saw somewhere), ambient temperature (<50C), insufficient spacing between breakers (group heating).
I assume all phases are the same, and there are no other trips for the breaker, like under/overvoltage or phase rotation. These signal could be in error for example. I can't see how power factor could come into it, but have heard weird things can happen with apparent phase rotation and power factor. Check the ammeter with a different one like a clamp meter.
I don't claim to fully understand the settings, but to my mind...
Ir Adjustable overload setting current = 98% of rated current, so waits for a while.
Isd or Im short time tripping setting current at intermediate currents = 10KA, waits a much shorter time, etc.
I agree this could be below the breaker rated setting, it may need to be 20KA. Don't take my word for it though, as there may be some sort of cascade scheme in place here..
Ii instantaneous tripping at even greater currents.
tr, don't know, maybe the time delay for Ir like 20 seconds.
Eventually, call the manufacturer, a faulty unit is always possible.
Maybe you can get something from the application guide.
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