A facility with the correct protective coordination scheme can save you money in reducing the effect of outages.
A short circuit in a relatively remote section of a large manufacturing plant results in the entire plant shutting down. What happened? Careful investigation reveals the protective device serving the plant does not coordinate with downstream devices protecting the remote branch circuit and the connected equipment. People involved in the field of protective device coordination believe this is a problem.
What is protective coordination? It's the selection, arrangement, installation, and maintenance of protective devices to limit the effects of an overcurrent (short-circuit) situation to the smallest area.
Protective coordination requires modeling the entire electrical network. Today, this involves the use of a software program that allows you to enter information such as types of equipment, conductor characteristics, spacing, and number of phases. This modeling effort requires the skills of someone experienced in modeling electrical circuits. This person must know enough to input the information and question the output. Why? The old programming adage of "garbage in equals garbage out" applies.
After the model is complete, calculate short-circuit current values for every crucial point along the system. Correctly calculating the short-circuit current values also requires the use of symmetrical components. (See sidebar, on page 60.) (A crucial point is any point in the electrical distribution network that requires a protective device. These may be panelboards, motors, heaters, or pumps.) With this information, you can adequately select the most appropriate protective device, whether it's a molded case circuit breaker (MCCB), insulated case circuit breaker (ICCB), power circuit breaker (PCB), fuse, or relay.
With the short-current values known and the type of protective device selected, you can begin to design the exact locations for the protective devices. For this step, you'll need device time-current curves (TCC) and either a "light table" or a computer program. Begin by placing the TCC for the plant protective device and then overlying down in subsequent order the TCC of each downstream device. (Perform this per circuit.) With the placement of each TCC, compare the range of the lowest to highest short circuit current available at that location. Note the time (cycle) separation of the curves. If the TCCs have the necessary separation distances, then the devices coordinate at that location. (See the Figure, in original article).) Devices coordinating at one location do not necessarily coordinate at another.
An equally important step in protective coordination is installation and maintenance. You must correctly install and routinely test the devices. This will ensure that during a short circuit event, the outage will be limited to the smallest area possible. It seems to be a popular practice to overlook the maintenance and testing of protective devices. Remember, it is as devastating to have an inoperable protective device as it is to have the incorrect protective device. Protective devices have ratings for the highest level the device may interrupt and its housing may safely endure. With this, protective coordination requires someone experienced and trained in the use of protective devices. If you install an improperly sized device, a short-circuit condition may cause a catastrophic failure. Whatever the circumstances, an experienced individual will choose the appropriately rated device.
Protective coordination requires trial and error to achieve the desired results, because the calculations are approximations containing various assumptions. That's why it requires constant vigilance when it comes to a its scheme. If an event occurs, you must inform the protective-coordination individual of every short-circuit event so he or she may review the effects, to see if the protective devices performed as expected. A properly functioning protective-coordination scheme can offer you significant savings by reducing unnecessary downtime.
