Ohio Change Location

Tech Tip 04

Coordinate Instantaneous Relays for Power Quality


  • Customer Complaints About Momentary Interruptions.
  • Frequent Instantaneous Operations on Circuit.
  • Many Instantaneous Breaker Operations Associated with Blown Fuses.
  • Larger Substation Transformer With Higher Short Circuit Current Capacity.


  • Instantaneous Relay Trying to Clear Temporary Faults Before Fuse Blows.
  • High Short Circuit Current Blows Fuse Before Breaker Can Open.


  • Reduce Causes for Insulation Failures.
  • Coordinate Fuses with Breaker Clearing Time.
  • Remove Instantaneous Trip from Service.


A warehouse and distribution center complained about too many momentary interruptions. The power would go off and then back on often for no apparent reason. This caused problems for computer inventory systems, file servers, voice mail and etc.

The 12.47kV circuit feeding this customer was short, just two or three miles long. The supply substation had a 30 MVA transformer with about 8,000 amperes of short circuit current available. Most tap line fuses were no larger than about 125 amperes.

The substation log and interruption reports showed that nearly every instantaneous operation at the substation was associated with a blown tap line fuse. There were very few occasions where the instantaneous breaker operation cleared temporary faults without blowing a fuse. A disturbance analyzer confirmed one event where the tap line fuse blew just before the circuit breaker opened. Figure 1 shows the sequence of events.

 Fuse Blows Before Breaker Opens

Figure 1: Fuse Blows Before Breaker Opens


Analysis of the circuit showed it was very difficult for the substation breaker to open before short circuit current damaged tap line fuses. The higher current blew the fuse faster than the breaker's 0.05-0.08 second minimum clearing time. Tap lines near the substation had larger fuses, but even large fuses would blow first. This case suggested removing the instantaneous relay from service and changing the reclosing.


One job for the instantaneous relay is to prevent permanent tap line interruptions for temporary faults such as lightning. The price for this feature is a momentary interruption for all customers on the feeder. Most industrial and large commercial customers today have electronic equipment that shuts down for momentary outages. It has become more desirable to allow tap fuses to blow before the substation circuit breaker trips by removing the instantaneous trip on substation circuit breakers.

It may make sense to retain instantaneous tripping on certain circuits such as circuits supplying mostly residential customers. If an instantaneous trip is kept on a circuit, remember to include the breaker clearing time when coordinating fuses with instantaneous relays. Consider how many permanent tap line interruptions will be prevented against momentary interruptions for the entire circuit. The better solutions consider line protection, fuse coordination, and the number of temporary tap line insulation failures.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.