Short circuits will destroy unprotected power semiconductors. Learn about protective devices that can help safeguard your system.

Solid-state controls can be protected against short-circuit damage.
Photo courtesy of Payne Engineering

If it is a given that your cables are sized properly for both steady-state currents and predictable short-term surges, you have two choices for protection from short circuits and overloads: fuses and circuit breakers.

Fuses. Common practice is to put in a fuse rated for 125 percent of the maximum load current -- or the next standard size up. The voltage rating is important too because it affects arc extinction time and energy let-through. If the load is subject to high inrush currents -- such as that on motors, transformers and heaters that have low cold resistance -- note their duration. They could, upon startup, take some six times the steady-state current, but usually only for a few seconds.

This pattern usually is no threat to the wiring or the loads, so you would choose a normal fuse, not a high-speed fuse. It would survive most common surges. You would use a slow-blow fuse if your inrush were of unusually long duration, threatening nuisance trips. If you are on a high-capacity/low-impedance supply with a high prospective short circuit current, you will need a high rupturing capacity (HRC) fuse designed to break that current. Some fuses come with a red flag that pops up when the fuse blows. This can save time when you are checking why the power went off.

Power semiconductors such as diodes and silicon controlled rectifiers (SCRs) have a low thermal mass at their junctions compared with metal contacts, so a relatively small dose of energy can destroy them when overload occurs. This led to the design of special fast fuses having very low mass elements and a length that varies with rated voltage. I2Rt represents the dose of heat delivered to a device, and the energy tolerance of a semiconductor came to be expressed as its I2t rating. This rating varies according to the prospective short circuit current. This in turn depends on the line voltage and the short circuit loop impedance, which includes that of the AC power source.

When selecting a matching semiconductor fuse (also known as an I2t fuse), use the I2t rating corresponding to your prospective current and make sure that its corresponding energy let-through (I2t) rating is less than that of your semiconductor device.

If the I2t rating of your SCRs does not sufficiently exceed that of your fuse, one option is to use bigger SCRs -- rated for say three or four times your steady-state load current. You do not need to uprate the heat sink because it will take brief overloads.

You will find curves and more detailed procedures in manufacturers’ data sheets. However, this is a topic where I highly recommend a dialog with the fuse manufacturer.

Circuit Breakers. Often, there is not a lot to choose between a basic circuit breaker and a fuse, except that you can reset a breaker many times and buy it only once.

When you get down to applications, a look through your catalog will tell you that breakers are offered in many designs and options. The simplest and cheapest is the thermal breaker, in which a built-in heater carries the load current and trips a bimetal switch in series with the load. Like the fuse, the higher the current, the faster they act. They come with a manual reset button or automatic reset that closes the contact when the bimetal cools. Magnetic breakers do not offer automatic reset. Note that unlike a magnetic breaker, a thermal type needs a cool-off time before reset. The current to trip is higher and less precise than the 1.25 times rated current of the typical magnetic breaker, and it is somewhat ambient temperature dependent.

A magnetic breaker is operated by a coil that carries the load current. Actuation may be combined with a heater and bimetal (thermal/magnetic) to add an element of time delay. Alternatively, delay may be achieved by a shunt plate on the coil to retard growth of the magnetic field.

A more versatile design (hydraulic/magnetic) uses a damping-fluid-filled cylinder connected to the actuator. This design provides a range of speeds. Like most breakers, its operating time is inversely proportional to the square of the current magnitude. Variations of this law are offered. Some simple magnetic breakers can act in a few milliseconds -- fast enough for solid-state relay protection in selected applications.

Other features of magnetic circuit breakers include manual reset, auxiliary contacts and a shunt trip breaker. Manual reset is the standard option on magnetic breakers, and you normally reset only after you have diagnosed and cleared the overcurrent problem that tripped it. Like all breakers, holding in the reset will not prevent a re-trip if the overcurrent condition persists. Auxiliary contacts can be provided to indicate the condition of the breaker. The coil’s actuating current can be from a different source than that in the interrupting contacts.

A shunt trip breaker has a volt coil that, when energized, operates the trip mechanism. It can act alone from a voltage source commonly signaling some condition that calls for a shutdown -- usually not a current overload. A volt coil also can be fitted on the same core as the overcurrent coil, enabling a trip to be initiated for some other reason.

Semiconductor fuses have low mass elements and a length that varies with rated voltage.

Other Devices

Thermostats. Common types include bimetallic and liquid-filled expansion type. Use them on motor windings; SCR heat sinks; and inside control enclosures, ovens, plastic machinery, gearboxes, hot air ducts and in heat transfer fluids. Speed often is not a requirement because of the slow changes in most thermal processes.

Electronic Thermostats.For response speed and ease of installation, consider a thermocouple-actuated electronic thermostat. This is virtually an on/off temperature controller with added features such as it stays tripped after the temperature has returned to normal. It can have a manual-acknowledge-and-reset capability that you cannot cheat, and its action can be validated quickly and frequently without taking the temperature up to the trip point.

Thermal Cutoffs or Thermal Fuses.These are small non-resettable temperature-sensitive devices that interrupt the circuit when its environment temperature exceeds its temperature rating. A disadvantage of these devices is that, unlike the electronic thermostat, you can only verify their action by heating them to their trip point -- after which they are not reusable.

Arc Fault Circuit Breakers.In addition to tripping on overcurrent, arc fault circuit breakers will trip on sensing the noisy signature of an arc fault current caused by a bad connection. Home wiring is a first-priority market for breaker manufacturers because there is a lot of combustible material around, teamed up with a lot of unsupervised and ill-maintained wiring, appliances and connections. Though industrial plants, aircraft and ships are more strictly regulated than homes, they can benefit from arc fault protection as well.