Downstream Circuit breaker

X-to-R ratio of generators is high, e.g. between 20 (for LV generators) and 100 (for HV generators). However, the value of armature resistance is of most importance when considering the downstream circuit-breaker fault clearance capabilities. This aspect is described in sub-sections 7.2.7 and 7.2.11. The calculation of current magnitudes may be carried out in several ways depending upon the amount and accuracy of the data available. 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

When fuses or moulded case circuit breakers are applied to a circuit it is necessary to ensure that their /-squared-t characteristics coordinate properly with the thermal capabilities of the downstream equipment, especially the cables. In order to determine the /-squared-t characteristics of a protective device it is assumed that the current in the device suddenly changes from a normal load value to the fault value in a very short period of time, i.e. similar to a step change in a control system. Hence for each value of current along the x-axis of the device s time-current characteristic the value of the current squared multiplied by the corresponding time can be plotted. For cables and busbars the /-squared-t function equals a constant (k) for each cross-sectional area of conductor, as explained... 

If a particular type or model is chosen from a manufacturer it can be seen that this low horizontal part may be similar or the same for aU ratings of circuit breakers within the range. Supposing a 2 1 or 3 1 ratio of upstream rating to downstream rating is chosen for a particular circuit. Selective tripping of the downstream unit can only be relied upon for fault currents beyond the magnetic vertical part of the curve for the downstream unit, but less than the vertical part of the upstream unit. For faults beyond the vertical part of the upstream unit there will be a race between both units and the upstream unit may trip before the downstream unit. This is not a satisfactory... 

In the case of a system that has 200,000 A fault current available, a 200,000-A main circuit breaker might be used, and downstream from the 200,000-A main circuit breaker, there may be a 100,000-A feeder breaker, then a 65,000-A breaker, down to a 42,000-A breaker, protecting branch circuits, to the minimum circuit breaker available today, which is a 10,000-A interrupting capacity (AIC) breaker. All circuit breakers are labeled on the face with their AIC rating—the amount of fault current that can safely be interrupted by the breaker. 

Care should be taken in the design of systems such that critical circuits are not used with series-rated systems. In the case of our 200,000-AIC main, if there were a high current fault at a downstream sight, the 100,000-AIC circuit breaker would interrupt and it would also cause the 200,000-AIC main circuit breaker to interrupt simultaneously, which would terminate power to the entire distribution system. If this were used in, say, a hospital, you would not want a short circuit in a panel board or a downstream distribution switchboard to also cause the main circuit breaker to trip, taking aU power out of the hospital. In those types of critical situations, the minimum circuit breaker AIC rating should be no less than the available system fault current delivered by the utility. 

Analyze and calculate hazards for equipment on the plant floor used to make your product or provide your service. Do not focus solely on large distribution equipment like switchgear, bus ducts, and large circuit-breaker panels. OSHA requires a hazard assessment must be com pleted wherever a hazard might exist. Based on research from all arc flash analyses performed by Lewellyn Technology, we have concluded that over 60 percent of arc flash hazards were found to be in the downstream equipment on the plant floor, not in the distribution equipment. 

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