Three kinds of protective devices

There are three common types of protective devices you’ll encounter in the nuclear power industry. Here I summarize their functionality . . . .

The three main kinds of protective devices most often encountered in the nuclear power industry are:

  1. Magnetic-only circuit breakers
  2. Thermal-magnetic circuit breakers
  3. Fuses

Magnetic-only breakers are typically used in motor control circuits installed inside motor control center (MCC) cubicles. Magnetic breakers interrupt high-current, instantaneous faults. It’s the magnetic component of a breaker that establishes the breaker’s curve in the instantaneous region.

The fault-clearing response is produced by the mechanical operation of the breaker mechanism, which is actuated by the electromagnetic forces generated during a fault. That’s because large, time-varying currents generate large time-varying magnetic fields, and the breakers are designed to use these fields to actuate the mechanical trip devices.


Under normal loads the currents are not strong enough to generate enough force to actuate the trip mechanism, but during a fault they generate a large force very quickly, blowing the breaker contacts apart and opening the circuit. Usually the motor will have its own thermal-overload protection (called a thermal-overload relay) that is sized to incorporate the specifics of the motor’s current characteristics which is why overload capability is omitted in the breaker itself.

Thermal-magnetic breakers contain both instantaneous and long-time protection. On a thermal-magnetic breaker, the thermal characteristic usually defines the way the breaker’s curve looks in the long-time region of the time-current curve. They’re commonly used in residential applications (houses) and in motor control centers for non-motor circuits.

The thermal-magnetic style of breaker combines a thermal element with the magnetic mechanism. The thermal element adds a “long-time” overload sensing capability. If larger currents than desired, but less than what’s generated by a ground-fault, are conducted in your cables for too long, then the thermal element (sometimes a bimetallic strip) will heat up and open the circuit. This ensures that cable insulation isn’t prematurely worn out from mild but extended overheating.

Fuses are single-use protection devices that must be replaced after they have operated (“melted”). They can be broken into two broader categories:

  • Current-limiting fuses are used to reduce bus-bracing requirements. This is because they limit the amount of electromagnetic force generated during a fault by interrupting the fault before the maximum short-circuit current values can be developed. In other words, current-limiting fuses cut the fault short. An example of a specific application is installing a current-limiting fuse in series with a molded-case circuit breaker when the available short-circuit current exceeds the breaker’s interrupting rating. This might allow the use of a smaller breaker than otherwise without the fuse. This can save time and money. Similarly, current-limiting fuses can also be used to protect cables from large fault currents. Such a fuse may have a full-load rating of several times the cable’s ampacity, but it will do the trick when needed to cut-off the short-circuit current during a fault. This protects the cable’s insulation from thermal damage.
  • Time-delay fuses are used for protecting transformers. This is to account for a transformer’s large in-rush current, which can be 12- to 18-times its rated full-load current for a short duration (typically on the order of 0.1 second). It would be an inconvenience for a protective fuse to melt upon normal transformer start-up because it confused a transformer’s in-rush current for a fault current. Similarly, time-delay fuses are good choices for protecting motors. If sized at 125% of the full-load motor current, the time-delay fuse can account for the motor’s in-rush current without melting, and it also protects against single-phasing. Single-phasing is when one phase is lost (for whatever reason), and the current increases in the remaining phases to compensate.