Why Bother with Protective Device Coordination?
Protective Device Settings...
Your facility's electrical power distribution system is equipped with protective devices, such as breakers or fuses. A circuit in a typical manufacturing setting will include several protective devices, each located to protect the circuit's individual branches, sub-branches, and equipment from the excessive amounts of current generated during an electrical fault.
A protective device setting or rating that is too low will lead to unnecessary and possibly frequent interruptions to the electrical supply of the plant equipment that it protects, resulting in potentially costly breaks to the production process. A protective device with a setting or rating that is too high for the application, however, will experience another problem. In this case, when a fault occurs, a protective device farther upstream in the circuit might trip before the more local protective device, potentially disrupting power to a larger branch of the circuit and a larger area of the plant, possibly interrupting production processes. Or, the setting or rating may not protect the equipment it was intended to protect.
An electrical system with protective devices that are set or rated incorrectly is considered to be uncoordinated. An uncoordinated condition can lead to unnecessary lost production and downtime. Protective Device Coordination is the solution to this problem and, along with the Short Circuit Analysis, is a second, key component of the Arc Flash Study.
Protective Device Coordination...
Protective Device Coordination ensures that protective equipment in the electrical circuit, such as relays, breakers, and fuses, are rated and set so that each device will activate in the appropriate order. Time-Current Curves for each device are produced and evaluated to determine if an electrical fault in an electrical branch will result in the correct isolation of that branch’s fault or an undesired trip of an upstream protective device, resulting in the de-energizing of several branches of the system.
The evaluation of the Time-Current Curves produces recommended settings (or ratings) for each protective device and, in many cases, results in an action item to update protective device settings in a facility.
Ideally, a protective devices settings or ratings are within a range that allows for normal, coordinated operation of the facility's electrical protective devices. The upstream protective device closest to the fault will isolate the incident area and interrupt the fault without disrupting the power provided to other areas of the circuit. The electrical components in that part of the circuit will be protected and other parts of the circuit and other areas of the plant will continue to operate. In this state, the protective devices are considered to be coordinated.
To determine whether your facility's protective devices are coordinated and to minimize the risk of production downtime, a Protective Device Coordination Analysis should be performed periodically. A PDC Analysis is one more way to reduce the risk of production downtime, as well as to ensure the equipment supplied by your facility's electrical system are adequately protected.