Machine Design

Circuit Breakers

Circuit breakers are manual switches that open under overload conditions. Circuit breakers discussed here are general-purpose types that meet the minimum Underwriters' Laboratories Inc. (UL) standards. Heavy-duty types used in power distribution systems such as oil-filled types, are not included.

Thermal, compensated-thermal, and thermal-magnetic breakers depend on heat for tripping. Therefore, their characteristics differ only slightly when used on ac or dc.

Operating qualities of hydraulic-magnetic breakers vary because of the magnetic pulses on ac, as compared to the constant pull of dc. On ac, the impedance changes as an iron core moves into the coil. Hydraulic-magnetic breakers have a different ampere-turn ratio for ac than for dc; thus, they should be used on their respective types of current. This is also true of 400 and 60-Hz breakers.

An instantaneous circuit breaker responds to a predetermined value of overload without any purposely delayed action. Any small surge or inrush of current causes the instantaneous breaker to trip and open the circuit. The tripping range is usually 101 to 120% of the current rating.

Most circuit breakers have an inverse time-delay characteristic. With an inverse time delay, there is a point where overload is great enough to operate the breaker instantaneously. In a hydraulic-magnetic breaker, this point can be varied.

In trip-free breakers, the contact arm is independent of the operating handle or mechanism, making it impossible to manually hold the contacts closed during an overcurrent or short-circuit fault.

There are two primary ways of controlling contact arcing. One is by bending the arc so that damage occurs on special surfaces and not on the contact faces. The other is by breaking up the arc into smaller, harmless arcs. Arc protection is mandatory where circuit breakers also are used as on-off switches.

Remote tripping can be accomplished by using a two-pole rather than a single-pole breaker. The second tripping element is connected to a remote switch to trip both poles when the switch is closed. Some breakers employ two tripping elements in a single pole. Most remote tripping elements are voltage or low-current sensing elements.

In breakers with an undervoltage release option, the trip circuit is a normally closed circuit. When the voltage in the supply line drops to a predetermined level, the circuit opens and trips the circuit breaker, protecting equipment from current overload because of a low-voltage condition. The normal range for undervoltage operation is 30 to 60% of line voltage.

Generally, the voltage must be at least 80% before the circuit breaker can be reset. Most units are instantaneous trip types, but some undervoltage units have a time delay to prevent momentary low voltages from causing circuit-breaker nuisance tripping.

Breakers containing auxiliary contacts usually have a snap-action switch attached to the circuit breaker so that some portion of the breaker mechanism actuates the switch. The switch is usually single-pole, double-throw, and is electrically isolated from the circuit breaker. Some breakers are available with internal auxiliary contacts, but these breakers are hardly ever used.

Overvoltage and overcurrent protectors are available that combine a high-speed voltage-sensing element with a current-sensing circuit-breaker mechanism. If the voltage-sensing element detects the presence of a dangerous transient or overvoltage, it fires and causes the protector to open the line. A third lead for reference must be provided. Installation and reset techniques are otherwise the same as ordinary circuit breakers.

Two-pole breakers are generally used where service requirements are beyond the capability of single-pole units. Circuit breakers with ratings up to 2,500 A and voltages up to 600 Vac or 250 Vdc are generally available. The ac types are normally for single-phase operation.

Three-pole breakers are generally used for multiphase operation. Most two and three-pole breakers are the common-trip type to prevent single phasing of equipment if an overload occurs in only one pole. Generally, a three-pole breaker has a 240-V minimum rating. Circuit breakers can have ratings up to 2,500 A and 600 Vac. Some manufacturers also make three-pole breakers for 250 Vdc applications. Four, five, and six-pole circuit breakers for special applications are also available. Single-pole types are generally available in current ratings up to 100 A at up to 227 Vac (60 Hz) and up to 250 Vdc.

Sizes: Alphabetical frame sizes have been resolved into ampere-designated frame sizes with their respective interrupting capacities. These are listed in NEMA Standard AB 1-1069. Smaller sizes are also available for lower-capacity circuits with voltages of 120 and 120/240 Vac.

All circuit breakers are UL rated at 5,000-A interrupting capacity when the voltage is 250 V or less, or the current is 100 A or less. Circuit breakers rated over these values have an interrupting capacity of 10,000 A. Dc interrupting ratings are available from circuit-breaker suppliers. Higher interrupting ratings are listed in UL Standard 489.

Circuit breakers can have built-in current-limiting fuses in each pole linked mechanically to the circuit-breaker common trip mechanism. The fuses are normally rated higher than the circuit-breaker poles.

Ground-fault interrupters: Ground-fault interrupters (GFIs) remove power when they sense a current imbalance between the hot and neutral conductors supplying operating power. A ground fault results when a current-carrying part of a circuit accidentally contacts any grounded conducting material. The resistance of the path to ground may be high or low, depending on the nature of the contact.

Low-resistance faults draw heavy current which can trip circuit breakers and open fuses, thereby preventing equipment damage and fire. But when ground faults have relatively high resistance, such as that of a human body, fault current, though it may be lethal, is not large enough to open circuit breakers and fuses. Furthermore, if the unintentional contact from a live wire to a high-resistance ground is intermittent (such as through vibration) the resultant sparking could trigger a fire -- still without opening overcurrent protectors.

Another protection circuit, immersion detection circuit interrupters (IDCI), are smaller and less costly than GFIs. IDCIs can be built into the power cords of appliances. If the appliance falls into water, the IDCI instantly interrupts power. IDCI circuits are less sophisticated than GFI circuits making them more compact and less expensive.

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