It is a given that your cables are sized properly for both steady-state currents and predictable short-term surges. You have a few choices for protection from short circuits and overloads.

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


In previous columns, I have dealt with protection of the process and the product. In this column, I'll turn to protection of wiring, control panel components and various plant items.

Wiring and Loads

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. 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. Check if the load is subject to high inrush currents and note their duration. Examples are motors, transformers and heaters that have low cold resistance. 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 under overload. 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.

The 2 Millisecond Fuse. It is common practice to protect power semiconductor devices using standard semiconductor fuses using data straight out of the catalog. That fact notwithstanding, at least one controls manufacturer states that such fuses cannot guarantee the limited current let-through and clearing times required for protection. One example cited is the delay time between chop off and the next zero crossing. Specially selected and tested fuses with a guaranteed 2 ms clearing time are therefore offered.



Circuit breakers come with a manual reset button or automatic reset that closes the contact when the bimetal cools.

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 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.

Some breakers are small enough to fit in a 0.375" inch hole in a panel and may incorporate a reset button. Sizes increase with the current-breaking capacity and voltage rating.

Magnetic Breaker. This 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 wide 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. Because shunt tripping needs the presence of a signal to trip, it does not have the fail-safe integrity of a magnetic contactor whose operation depends on the absence of a voltage on the hold-in coil.

There are too many combinations of the various features to mention here, so refer to your manufacturers' catalogs.



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