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Dear all,
I saw in UPS catalog 300kA fuse value as below screen shot. I did not understand this value what is for? Is this for maximum capability of fuse to withstand?
Ok thanks Juan and steve. Just farther clarification. UPS contains internal fuse which will open in 0.1 second but down the UPS there is a breaker of 3600A which has breaking time of 25ms from schneider catalog. If the short circuit is at point B and the maximum short circuit which will flow through this line be 65kA, per my view first breaker will open before fuse. But on other hand if 65kA flow through this line and will create high energy of I2t (I square t), and can cause to meld fuse more faster and will open before air circuit breaker. Is this true?
Yes...
You should have to calculate the short circuit current at point B:
If you have a short circuit current (Ik) at point B : 3600*10 (Im, breaker) < Ik < 65 KA then the short circuit will be cut off in 25 ms by the breaker.
If you have a short circuit current (Ik) at point B: 65 KA < Ik < 300 KA then the short circuit will be cut off in 100 ms by the fuse. I think that 100ms is the time to melt the fuse when it reaches 5000x(1.6 or 1.8 depending what is in the fuse IEC) Ampers; if you reach 300KA it should melt faster.
If your Ik is more than 65KA then I will combine the branch protection in series with another fuse at point B.
You have to ensure that the breaker have a short time withstand current (Icw) enough to open the circuit without being destroyed and can be reused after a shorcircuit.
I hope that I have clarified your enquiry.
With the given information the breaker clearing time is 4x faster than the fuse with the given fault current.
Breaker is 25ms.
Fuse is 100ms.
The breaker should clear the first fault, there will be a point (number of faults) at when the fuse is weakened (melted) so it will open first.
Steve I agree with your statement and this is also my thought. As you mentioned there will be a a point at when the fuse is weakened (melted), I want to know this point?
Juan said at 300kA, fuse will be melted faster.
At 300kA the breaker will be ejected from the UPS, unless the fuse can save it !
The fuse curve would show how long 63kA would take to melt the fuse element. This would give some insight into how many faults the fuse could survive when the breaker does the clearing.
This fuse can be time delay fuse. In general a time delay fuse protects a motor from overload while allowing greater starting amperage to get the motor up to speed. During an in-rush, the fuse should not open. The specified fuse in this case should be a time-delay fuse allowing the piece of equipment to start up properly without a nuisance opening of the fuse when the overcurrent occurs
It is related to the let through energy I2t characteristics.
The selectivity limit current is the value at which the let-through energy I2t of the circuit-breaker exceeds the pre-arcing I2t of the fuse (curve or value provided by the manufacturer).
Advice:
To ensure full selectivity between the upstream fuse and the downstream circuit-breaker, we recommend replacing the upstream fuse after clearance of short-circuit by the downstream circuit-breaker.
