What is Selective Coordination?

Selective Coordination

The two one-line diagrams above illustrates the concept of selective coordination during electrical design. The system represented by the one-line diagram to the left is a system without selective coordination. A fault on the load side of one overcurrent protective device unnecessarily opens other upstream overcurrent protective device(s). The result is unnecessary power loss to loads that should not be affected by the fault. This is commonly known as a "cascading effect" or lack of coordination.

 

"Selective Coordination is the localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings". 

The system represented by the one-line diagram to the right is a system with selective coordination. For the full range of overload or fault currents possible for this system, only the nearest upstream overcurrent protective device opens. 

All the other upstream overcurrent protective devices do not open. Therefore, only the circuit with the fault is removed and the remainder of the power system is unaffected. The power for other loads in the system continues uninterrupted. 

Related Article: How to Interpret the Different Region of Circuit Breaker Curves?

The overcurrent could occur on a feeder circuit, too, and a selectively coordinated circuit would only have the immediate upstream feeder overcurrent protective device open. Selective coordination is an easy concept to understand. 

However, quite often in the design or equipment selection phase, it is ignored or overlooked. And when it is evaluated, many people misinterpret the information thinking that selective coordination has been achieved, when in fact, it has not. The following sections explain how to evaluate whether overcurrent protective devices provide selective coordination for the full range of overcurrents.

Currently, three methods are most often used to perform a coordination study:

1. For fuse systems, 600V or less, use the published selectivity ratios of the fuse being used. The data is provided by the manufacturer. The ratios apply for all overcurrent conditions including overloads and short-circuit currents. Using the fuse selectivity ratio method is easy and quick. There is no need to use time-current curves. 
2. Computer programs allow the designer to select time-current curves published by manufacturers and place curves of all OCPDs of a circuit on one graph. However, simply plotting the curves does not prove selective coordination. The curves must be analyzed and interpreted properly in relation to the available fault currents at various points in the system. 
3. Overlays of time-current curves, with the manufacturers’ published data are hand traced on log-log paper. Proper analysis and interpretation is important in this case, also

Related Article: What are the General Guidelines in Electrical Design?

Coordination Analysis 

The next several pages cover selective coordination from various perspectives. The major areas include:

• Fuses 
• Circuit breakers 
• Systems with fuse and circuit breaker mixture 
• Mandatory selective coordination requirements 
• Why selective coordination is mandatory 
• Selective coordination system considerations 
• Ensuring compliance 
• Requirements inspection checklist 
• Fuse and circuit breaker choice considerations table 
• Objections and misunderstandings 
• Ground fault protection relays

Reference: 

• Selective Coordination | Download
• Publisher: Cooper Bussman