Solid-state controls can be protected against short-circuit damage.

Everyone in the solid-state control industry acknowledges that short circuits will destroy unprotected power semiconductors. There is almost no agreement, however, on what constitutes protection: different manufacturers employ different methods, with very different degrees of effectiveness.

The result is confusion among users of this technology as to:

  • Whether short circuit protection is actually possible.
  • The relative effectiveness of the various means of protection.

    This article is intended to clarify these issues.

    When a short occurs in an electrical circuit, extremely high currents flow. Because these currents will be much greater than circuit design parameters, they can damage circuit components and pose fire and other safety hazards. Protection mechanisms such as fuses and circuit breakers are designed to be the weak link: They break the circuit to minimize or prevent such damage. While simple in concept, the science of short-circuit protection is a complicated one, requiring careful coordination of component withstand ratings and the interrupt capabilities of the chosen protective device(s) with the available fault current.

    When all of the components in a circuit are electromechanical devices, published technical data can be used to select equipment that will provide the required degree of protection. Introduce a device using solid-state power components -- SCRs, diodes or TRIACs -- into the circuit, and short-circuit protection becomes more difficult. In general, for a protective device to prevent damage to a circuit containing these components in a fault situation, it must both:

  • Break the circuit completely in 2 milliseconds or less.

  • Limit the peak and total let-through currents to levels that the semiconductors can tolerate.

    However, published technical data on the various protective devices do not provide accurate information on their performance within that brief time period. Therefore, short-circuit protection for power semiconductors cannot be selected from specification information alone. This fact is not widely recognized, even in the solid-state control industry, leading to confusion among users of these products over the issue of short-circuit protection.

    Some manufacturers leave short-circuit protection for their products to the user; others use a bewildering array of fuses, circuit breakers and electronic trip circuits, which may or may not actually prevent damage should a short occur. This lack of consensus may have led to the general impression among users that reliable, repeatable short-circuit protection is not possible for solid-state controls. Users, therefore, often live with a much higher incidence of short-circuit damage to their solid-state equipment than necessary simply because they do not know that there is an alternative.

    The key to selecting solid-state controls that can survive short-circuit events lies in knowing the effectiveness (or lack thereof) of the various protection schemes. The types of short-circuit protection used with SCR controls can be grouped into five general categories:

    • User-provided protection.
    • Conventional protective devices.
    • “Semiconductor,” or I2t fuses.
    • Electronic protection.
    • “2 millisecond” fuses.

    User-Provided Protection. The control manufacturer makes the user responsible for short-circuit protection. Because published technical data on the various protective mechanisms is incomplete for the brief time intervals that must be considered with solid state controls, it is impossible for the user to make an effective choice. Should a short circuit occur, semiconductor failure is almost a certainty.

    Conventional Protective Devices. These include most fuses and circuit breakers. Most fuses are simply too slow, and allow too high a let-through current, to have any chance of preventing damage. Because they are mechanical devices with moving parts, circuit breakers also break the circuit too slowly. With either of these choices, a short circuit almost always means semiconductor failure.

    Semiconductor or I2t Fuses. These fuses are marketed by fuse manufacturers specifically for use with semiconductors. However, most of these fuses may not provide reliable short-circuit protection for semiconductors. Some allow too much let-through current; others do not clear fast enough. Whatever the reason, the fact remains that even fuses that are sold for semiconductor protection do not actually provide it.

    Electronic Protection. Circuitry in the solid-state control monitors load currents and reacts to fast-rising levels (characteristic of a short circuit) by preventing further semiconductor turn-on; in effect, breaking the circuit to prevent short circuit damage. Even this method, however, does not interrupt fault currents fast enough: not because it cannot react quickly enough (it can), but because it cannot shut off a conducting semiconductor.

    Solid state AC controls are line commutated: The semiconductors only turn off when the voltage waveform goes through zero. Once triggered “on” by the control electronics for a given half-cycle of the AC waveform, a semiconductor will conduct that half-cycle of voltage and current even if the electronics are disabled. Since a half-cycle of 60 Hz power is 8.333 milliseconds long, while the nominal clearing time for effective short-circuit protection is 2 milliseconds, electronic circuits cannot and do not shut down the control before damage occurs.

    2 Millisecond Fuses. These fuses are “semiconductor” fuses that have been proven to clear within the nominal 2 millisecond time frame, and limit the peak currents and total energy let-through sufficiently to protect a semiconductor from short-circuit damage. These fuses can only be selected by empirical testing using specialized recording equipment, not from any published technical specifications. They provide the only reliable, repeatable, effective protection for power semiconductors against short-circuit damage.

    Solid-state controls can be protected against short-circuit damage, but not every control manufacturer does so. Because of the variety of protective devices employed by control manufacturers, extreme care must be taken to specify and select controls that incorporate those protective measures that have been proven to work. PH

    This article is adapted from “Technical Information Paper 102” by Payne Engineering Inc., Scott Depot, W.V., which is available on its web site.

    For more information about Payne Engineering's SCR power controls: Call (304) 757-7353. Visit http://www.payneng.com.

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