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).
