MCCB PROBLEM

Sabah al khair, Mr Mohammed,

Your problem is very basic. MCCBs responds to many other quantities you can find from your type installed and manfacturer. select reputed brands of HRC fuses or MCBs of correct rating to protect down stream circuits. Analyse your loading at main breaker, if critically loaded and ambient is above 50 degrees C. in the location, Remove defects if noticed.If your system study is correctly done and original settings are not tempered then 250 Amps main breaker is doing its job correctly. you need to take close look and remove causes before doubting system integrity. In short you need a check for good understanding of problem,then you will be able to find answer for your self.Have a good day.

Mohammad Fawad,

You are dealing with a problem of coordination between the MCCB and the downstream fuses. It is easy to confuse short-circuit and overload protection.

Overload protection is the ability of the fuse or CB to interrupt the current flow when a long-term (minutes to hours) overload exists. The greater the overload the shorter the time before the CB trips or the fuse blows. For overloads, the only one that will react is the smallest rated fuse or CB. Therefore, a 20 A load downstream of the 250A CB, 32A fuse, and 16A fuse will blow only the 16A fuse (ignoring any additional loads on the other fuse or the CB).

However, under short circuit conditions, the current flowing into the short is limited by the available short circuit current of the supply and the circuit's impedance. If transformers are large and feeders are short (not long), this current can be as high as thousands of amps (and is the "available fault current"). Each fuse type and size has a characteristic curve of how much current it will "let through" and how quickly it will blow to extinguish the short. "Current-limiting" and "fast-acting" fuses can keep this short circuit current quite low and the time quite short. General use fuses, however, allow more through for a longer time. While the fuse is in the process of blowing, the amount of current flowing through the circuit can be high enough to "unlatch" the instantaneous trip function of the CB. Although the fuse will most likely finish extinguishing the short before the CB has actually opened its contacts, the CB's mechanical linkage is already acting.

You are seeing a large current flowing for a brief time into the short circuit. This is understandably blowing both fuses and tripping the MCCB.

This is what you can do:

1. Get time and current curves for the MCCB and both fuses. Draw the curves for all three on a single graph; you will see that although the fuses limit the current, enough is flowing to trip the CB. (If you need a value for available short-circuit current, assume it is 10,000 A.)

2. Look at the load and determine if a different fuse, with lower let-through current, can be used on this load. The fuse needs to be able to handle any inrush current when the load is first energized. Usually you can select a replacement type of fuse that will have a peak let-through current below the tripping current for the CB.

This is the meaning of "coordination" between the upstream and downstream protective devices, whether CB's or fuses in any combination. In general, you can achieve coordination between fuses if the larger one is 3-times the size of the smaller (and sometimes even if their sizes are closer together). For coordination between a fuse and an upstream CB, the size difference often must be at least 7-times. Since the ratio between the two fuses is only 2-times, you will be unlikely to get good coordination. However, since the ratio between either fuse and the CB is over 7-times, you should be able to get good coordination so the CB does not blow.

For further information, I suggest a look at the Bussman web site because they have posted many good references on fuse selection and coordination. Also you can search on line for additional information on this topic. I believe a thread about 1-year ago discussed short circuit current calculations, but Bussman treats this topic well also.

Peace be with you--JMM