Anybody got ideas about how the time/current curves on currently available fuses interact with some of the currently available circuit breakers. (SqD,Siemens,Cutler/Hammer, etc) Only asking below 600V, and up to only about 1000A. It seems selectivity is normally not taken into consideration by designers, or the info just isn't available to them. Deadshort a 20, you'll blow a 100 ahead first. Short a 100, you'll blow an 800 first.
electure, this is what i've dug outta the handbook... it falls short, however, on the method of 'selectivity'
240.2 Definitions. Coordination. The proper localization of a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault-protective devices. Commentary..... The main goal of overcurrent protection coordination is to isolate the faulted portion of the electrical circuit quickly while at the same time maintaining normal service for the remainder of the electrical system. The electrical system overcurrent protection must guard against short circuits and ground faults to ensure that the resulting damage is minimized while other parts of the system not directly involved with the fault are kept on until other protective devices clear the fault. Overcurrent protective devices, such as fuses and circuit breakers, have time/current characteristics that determine the time it takes to clear the fault for a given value of fault current. Selectivity occurs when the device closest to the fault opens before the next device upstream operates. For example, any fault on a branch circuit should open the branch circuit breaker rather than the feeder overcurrent protection. All faults on a feeder should open the feeder overcurrent protection rather than the service overcurrent protection. When selectivity occurs, the electrical system is considered to be coordinated. With coordinated overcurrent protection, the faulted or overloaded circuit is isolated by the selective operation of only the overcurrent protective device closest to the overcurrent condition. This isolation prevents power loss to unaffected loads.
[This message has been edited by sparky (edited 03-12-2002).]
Ahh... Selective coordination. Does it become even more complicated when using series-rated components? Sounds like a question for a FUSE EXPERT. I wish there were one around to answer some of these questions?
Sparky, most Main ground fault systems are shipped from the factory set at the lowest possible level for sensitivity and time. It is the responsibilty of the owner/engineer to decide what is an acceptable level of coordination.
For Square D main breakers from the factory, are usually set about 200A and .1 sec. A Square D 20A QO breaker will hold 200A for about 1 sec. There is no ground fault coordination unless the main breaker is adjusted.
Redsy, Non-electronic trip circuit breakers are almost impossible to selectively coordinate, for short circuits above 10,000A regardless of whether they are series rated or not. Fuses, on the other hand are relatively easy to coordinate, if they are drastically different case sizes (400A can coordinate easily with 60A, but a 100A will almost never work with a 60A).
From an engineer's point of view, selective coordination is not usually feasible, if there are no strict controls on component substitution. If I plan a job using Shawmut Amp-trap fuses there is nothing to prevent a future substitution of Bussmann Fusetrons, resulting in the loss of coordination.
Selective coordination can be a very complex discussion especially with fuses and circuit breakers. When ground fault relays are added to the equation, the discussion becomes even more complex. The degree of selective coordination is also dependent upon the available fault current.
With ground fault relays, often only the main is equipped with this device. What can happen is that a ground fault condition on a downstream device may trip the ground fault on the main before the overcurrent protective device nearest the fault opens. A way to make this less of an issue is to add a second level of ground fault protection on the feeders. This is what is done for health care facilities (see NEC 517.17). This will prevent the loss of the main provided the ground fault relays and overcurrent devices are selectively coordinated. To see this concept more clearly, refer to the time-current curves.
With molded case circuit breakers and fuses, the degree of selective coordination depends upon the arrangement. If a fuse is upstream of a molded case circuit breaker, selective coordination is unlikely. If the molded case circuit breaker is upstream of a fuse, selective coordination may be possible, but may not be able to be determined for all fault levels (for higher fault currents that are "off the curve"). A good starting point is to check the time-current curves to see where an overlap in the devices can occur.
With molded case circuit breakers. Fault currents above the upstream instantaneous setting can cause a lack of coordination. For molded case circuit breakers, typically the maximum instanteous setting is 10 times (whether thermal magnetic or electronic trip). Thus if you have a 100A MCCB and an upstream 400A MCCB, a fault on the load side of the 100A MCCB of approximately 4,000A (instantaneous setting of the 400A) or higher can cause a lack of coordination. This would be true even if the MCCB had an electronic trip with short-time delay. The only way you can improve this situation is to explore options with more expensive insulated case and low voltage power circuit breakers.
With fuses, selective coordination is relatively easy...just follow the selective coordination ratios by the manufacturer. For Bussmann Low-Peak (Yellow) Fuses this ratio is 2:1 or greater. Thus if you had a 100A Bussmann Low-Peak Fuse and 200A (or larger) Bussmann Low-Peak Fuse upstream, selective coordination can be achieved. As pointed out, replacement with other fuses or other manufacturers, may adversely affect the selective coordination. This is why it is always a good practice to replace devices with the exact same device.
By the way, selective coordination for multiple elevator circuits fed from a single feeder is required by NEC 620.62. See the NEC for more details.
For more information on this topic, see the Bussmann SPD, which is available on the website at www.bussmann.com (under application info, publications and articles).
Just noticed series ratings was mentioned also. With a series rated sytems, selective coordination is not possible.
The concept of series rated systems require the upstream device to open in conjunction with the downstream device...that is how the upstream device can protect the under-rated downstream device.
In addition, be aware of limitations and addition requirements in regards to motor contribution and required labeling. See NEC 240.86 and 110.22 for more information. Also, Bussmann has additional information on series rated systems. See www.bussmann.com and go to Application Info, Publications and Articles to see the information on Series Rated Systems.
Coordination is possible with series rated devices. It is an old wives tale that the upstream device must operate in order to protect a down stream device, in a NRTL tested series connection.
This is from Square D’s publication, Bulletin No. 0600DB0108 Applying Short Circuit Current and Series Connected Ratings
“As a result of the potential simultaneous opening, some designers of electrical distribution systems believe that all circuit breaker coordination is lost when using series-connected ratings. In reality, using series-connected ratings provides virtually the same system coordination as a fully-rated system…”
The example given is for a 480V system with 30,000A available, the upstream device is an 800A breaker and the down stream device is a 125 device.
“…In both the series-rated and fully-rated systems, coordination is achieved to the same degree. In fact, when a UL Recognized series-rated combination is used, the branch circuit breaker is more likely to be a smaller frame size than in the case of a fully-rated system. The smaller frame size branch circuit breaker will have a different characteristic trip curve and may result in better overall coordination…”
The total available fault current must be taken into account when conducting any coordination study. For high fault levels and similar sized OCPDs coordination may not be possible at all. This is especially true for all circuit breakers, but is even a concern with fuses.
From the Bussmann publication, SPD Electrical Protection Handbook. “Selectivity between two fuses operating under short-circuit conditions exists [only] when the total clearing energy of the load side fuse is less than the melting energy of the line side fuse.”
This is similar to the Square D statement of “In both the fully-rated and series-rated combinations, coordination exists for all values of current below the magnetic instantaneous pickup point of the main (MAL36800) circuit breaker.”