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2 mars 2011 3 02 /03 /mars /2011 03:41

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city Dell Studio XPS 1640 Battery .

Origins

An early form of circuit breaker was described by Thomas Alva Edison in an 1879 patent application, although his commercial power distribution system usedfuses. Its purpose was to protect lighting circuit wiring from accidental short-circuits and overloads Dell Vostro 1710 Battery .

Operation

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker Dell KM958 battery .

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism Dell Studio 1555 battery .

The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source Sony VGP-BPS13 battery .

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself Sony VGP-BPS13/B battery .

Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit  Sony VGP-BPS13/S battery .

Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts Sony VGP-BPS13A/B battery .

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms Sony VGP-BPS13B/B battery .

Different techniques are used to extinguish the arc including:

  • Lengthening of the arc
  • Intensive cooling (in jet chambers)
  • Division into partial arcs Sony VGP-BPL9 battery
  • Zero point quenching (Contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50Hz and 120 times per second for 60Hz AC)
  • Connecting capacitors in parallel with contacts in DC circuits Sony VGP-BPS13B/B battery

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

Arc interruption

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings will have metal plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils deflect the arc into the arc chute Sony VGP-BPL15 battery .

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the stretched arc Dell Inspiron E1505 battery .

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts  Dell Latitude E6400 battery .

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device HP Pavilion dv6000 Battery .

Short-circuit current

Circuit breakers are rated both by the normal current that are expected to carry, and the maximum short-circuit current that they can safely interrupt SONY VAIO VGN-FZ Battery .

Under short-circuit conditions, a current many times greater than normal can exist (see maximum prospective short circuit current). When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue SONY VAIO VGN-FZ18 Battery .

This condition can create conductive ionized gasses and molten or vaporized metal which can cause further continuation of the arc, or creation of additional short circuits, potentially resulting in the explosion of the circuit breaker and the equipment that it is installed in SONY VAIO VGN-FZ21E Battery .

Therefore, circuit breakers must incorporate various features to divide and extinguish the arc.

In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulphur hexafluoride or have contacts immersed in oil to suppress the arc.

The maximum short-circuit current that a breaker can interrupt is determined by testing SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to explode when reset SONY VAIO VGN-FW21E Battery .

Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panelboard; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution-type circuit breakers SONY VAIO VGN-NR11S/S Battery .

Standard current ratings

International Standard IEC 60898-1 and European Standard EN 60898-1 define the rated current In of a circuit breaker for low voltage distribution applications as the current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C) SONY VAIO VGN-NR11M/S Battery .

The commonly-available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A and 100 A (Renard series, slightly modified to include current limit of British BS 1363 sockets). The circuit breaker is labeled with the rated current in amperes, but without the unit symbol "A" SONY VAIO VGN-NR11Z/S Battery .

Instead, the ampere figure is preceded by a letter "B", "C" or "D" that indicates the instantaneous tripping current, that is the minimum value of current that causes the circuit-breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of In SONY VAIO VGN-NR11Z/T Battery :

TypeInstantaneous tripping currentBabove 3 In up to and including 5 InCabove 5 In up to and including 10 InDabove 10 In up to and including 20 InKabove 8 In up to and including 12 In

For the protection of loads that cause frequent short duration (approximately 400 ms to 2 s) current peaks in normal operation Sony VAIO VGN-FZ21E Battery .

Zabove 2 In up to and including 3 In for periods in the order of tens of seconds.

For the protection of loads such as semiconductor devices or measuring circuits using current transformers.

Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features  Sony VAIO VGN-FW21E Battery .

Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, and include:

  • MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category Sony VAIO VGN-NR11S/S Battery .
  • MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

The characteristics of LV circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear Sony VAIO VGN-NR11M/S Battery .

Large low-voltage molded case and power circuit breakers may have electrical motor operators, allowing them to be tripped (opened) and closed under remote control. These may form part of an automatic transfer switch system for standby power Sony VAIO VGN-NR11Z/T Battery .

Low-voltage circuit breakers are also made for direct-current (DC) applications, for example DC supplied for subway lines. Special breakers are required for direct current because the arc does not have a natural tendency to go out on each half cycle as for alternating current  SONY VAIO VGN-FZ180E Battery .

A direct current circuit breaker will have blow-out coils which generate a magnetic field that rapidly stretches the arc when interrupting direct current.

Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel SONY VAIO VGN-FZ220E Battery.

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components SONY VAIO VGN-FZ340E Battery :

  1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation SONY VAIO VGN-FZ430E Battery .
  2. Actuator mechanism - forces the contacts together or apart.
  3. Contacts - Allow current when touching and break the current when moved apart.
  4. Terminals
  5. Bimetallic strip.      SONY VAIO VGN-FZ460E Battery
  6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
  7. Solenoid
  8. Arc divider/extinguisher      SONY VAIO VGN-FZ480E Battery

Magnetic circuit breaker

Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch SONY VAIO VGN-FZ4000 Battery .

As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid SONY VAIO VGN-FZ31E Battery .

The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc  SONY VAIO VGN-FZ31B Battery .

Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker SONY VAIO VGN-FZ31J Battery .

Thermal magnetic circuit breaker

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions SONY VAIO VGN-FZ31M Battery .

Common trip breakers

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used  SONY VAIO VGN-FZ31Z Battery .

These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120/240 volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires  SONY VAIO VGN-FZ38M Battery .

Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards SONY VGP-BPS8 Battery .

Two and four pole breakers are used when there is a need to disconnect the neutral wire, to be sure that no current can flow back through the neutral wire from other loads connected to the same network when people need to touch the wires for maintenanceSONY VGP-BPS13/S Battery .

Separate circuit breakers must never be used for disconnecting live and neutral, because if the neutral gets disconnected while the live conductor stays connected, a dangerous condition arises: the circuit will appear de-energized (appliances will not work), but wires will stay live and RCDs will not trip if someone touches the live wire  SONY VGP-BPS13A/B Battery .

This is why only common trip breakers must be used when switching of the neutral wire is needed.

Medium-voltage circuit breakers

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation  SONY VGP-BPS13B/B Battery .

Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers SONY VGP-BPS13A/S Battery .

The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc SONY VGP-BPS13AS Battery :

  • Vacuum circuit breaker—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V, which corresponds roughly to the medium-voltage range of power systems  Dell Inspiron 1320n Battery .
  • Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
  • Air circuit breaker—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit  Dell Inspiron 1464 Battery .
  • Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing the breaker to be removed without disturbing the power circuit connections  Dell Inspiron 1764 Battery ,

using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure.

High-voltage circuit breakers

Electrical power transmission networks are protected and controlled by high-voltage breakers  Dell Studio 1450 Battery .

The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly alwayssolenoid-operated, with current sensing protective relays operated through current transformers Dell Studio 1457 Battery .

In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc Dell Latitude D610 Battery .

Some of the manufacturers are ABB, GE (General Electric) , Tavrida Electric, Alstom, Mitsubishi Electric, Pennsylvania Breaker, Siemens, Toshiba, Kon?ar HVS, BHEL, CGL, Square D (Schneider Electric).

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc Toshiba Satellite M65 battery .

Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1200KV breakers are likely to come into market very soon  Toshiba Satellite M60 battery .

High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability Dell Latitude D830 Battery .

Sulfur hexafluoride (SF6) high-voltage circuit-breakers

A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas Dell Latitude D620 Battery .

Other breakers

The following types are described in separate articles.

  • Breakers for protections against earth faults too small to trip an over-current device:
    • Residual-current device (RCD, formerly known as a residual current circuit breaker) — detects current imbalance, but does not provide over-current protection Dell Inspiron Mini 10 Battery .
    • Residual current breaker with over-current protection (RCBO) — combines the functions of an RCD and an MCB in one package. In the United States and Canada, panel-mounted devices that combine ground (earth) fault detection and over-current protection are called Ground Fault Circuit Interrupter (GFCI) breakers Sony VGN-FW11S Battery ;
    • a wall mounted outlet device providing ground fault detection only is called a GFI.
    • Earth leakage circuit breaker (ELCB) — This detects earth current directly rather than detecting imbalance. They are no longer seen in new installations for various reasons  Sony VGN-FW11M Battery .
  • Autorecloser — A type of circuit breaker which closes again after a delay. These are used on overhead power distribution systems, to prevent short duration faults from causing sustained outages.
  • Polyswitch (polyfuse) — A small device commonly described as an automatically resetting fuse rather than a circuit breakerSony VGN-FW139E/H battery .

An Arc Fault Circuit Interrupter (AFCI) is a circuit breaker designed to prevent fires by detecting a non-working (i.e., non-intended/non-useful) electrical arc and disconnecting the power before the arc starts a fire  Dell Latitude E5400 Battery .

An AFCI should, but may not always, distinguish between a working arc that may occur in the brushes of a vacuum cleaner, on operation of a light switch, on insertion / removal of a plug into an electrical receptacle, or during the operation of other household devices and a non-working arc that can occur  Dell Latitude E4200 Battery

— for example a lamp cord that has a broken conductor in the cord from overuse. Arc faults in a home are one of the leading causes for household fires.

Annually, over 40,000 fires are attributed to home electrical wiring. These fires result in over 350 deaths and over 1,400 injuries each year  Dell Inspiron 300M Battery .

Conventional circuit breakers only respond to overloads and short circuits; so they do not protect against arcing conditions that produce erratic current. An AFCI is selective so that normal arcs do not cause it to trip. The AFCI circuitry continuously monitors the current through the AFCI  Dell Vostro A840 Battery .

AFCIs use unique current sensing circuitry to discriminate between normal and unwanted arcing conditions. Once an unwanted arcing condition is detected, the control circuitry in the AFCI trips the internal contacts, thus de-energizing the circuit and reducing the potential for a fire to occur Dell Studio 1737 battery .

An AFCI should not trip during normal arcing conditions, which can occur when a switch is opened or a plug is pulled from a receptacle.

AFCIs resemble a GFCI/RCD (Ground-Fault Circuit Interrupt/Residual-Current Device) in that they both have a test button although it is important to distinguish between the two  Dell Inspiron E1505 battery .

GFCIs and RCDs are designed to protect against electrical shock while AFCIs are primarily designed to protect against arcing and/or fire.

Electrical code requirements      Dell RM791 battery

Starting with the 1999 version of the National Electrical Code (NEC 70), also called NFPA, in the United States (US) and the 2002 version of the Canadian Electrical Code in Canada (CSA Standard C22.1) the national codes require AFCIs in all circuits that feed outlets in bedrooms of dwelling units Dell XPS M1530 battery .

This requirement is typically accomplished by using a kind of circuit-breaker (defined by UL 1699) in the breaker panel that provides combined arc-fault and overcurrent protection. Not all US. jurisdictions have adopted the AFCI requirements of the NEC as writtenDell XPS M2010 battery .

The AFCI is intended to prevent fire from arcs. AFCI circuit breakers are designed to meet one of two standards as specified by UL 1699: "branch" type or "combination" type (note: the Canadian Electrical Code uses different terminology but similar technical requirements)  Acer Aspire One battery .

A branch type AFCI trips on 75 amperes of arcing current from the line wire to either the neutral or ground wire. A combination type adds series arcing detection to branch type performance. Combination type AFCIs trip on 5 amperes of series arcing.

The advanced electronics inside an AFCI breaker detect sudden bursts of electric current in milliseconds Toshiba Satellite P10 Battery ;

long before a standard circuit breaker or fuse would trip. A "combination AFCI breaker" will provide protection against Parallel arcing (line to neutral), Series arcing (a loose, broken, or otherwise high resistance segment in a single line), Ground arcing (from line, or neutral, to ground)  SONY VGN-FZ210CE Battery ,

Overload protection (for resistance loads such as heaters; inductive loads such as motors may require additional overload protection) and from Short circuit protection.

In 2002 the NEC removed the word "receptacle" leaving "outlets"; in effect adding lights within dwelling bedrooms to the requirement  Dell Precision M70 Battery .

The 2005 code made it more clear that all outlets must be protected despite discussion in the code-making panel about excluding bedroom smoke detectors from the requirement. "Outlets" is defined in "Article 100 Definitions" of the NEC as "A point on the wiring system where current is taken to supply utilization equipment" and this includes receptacles, light fixtures and smoke alarms, amongst other things Toshiba Satellite L305 Battery .

As of January 2008 only "combination type" AFCIs will meet the NEC requirement. The 2008 NEC requires the installation of combination-type AFCIs in all 15 and 20 ampere residential circuits with the exception of laundries, kitchens, bathrooms, garages and unfinished basements  Toshiba Satellite T4900 Battery .

Limitations

AFCIs are designed to protect against fires caused by electrical arcing faults. However they provide no specific protection against "glowing" connections, excess current, high line voltages or low line voltages Toshiba PA3399U-2BRS battery .

Glowing connections occur when relatively high electric current exists in a relatively large resistance object. Heat comes from power dissipation. Power (in watts, W) equals the current (in amperes, A) squared, multiplied by the resistance (in ohms, symbol ?) Toshiba Satellite A200 Battery .

For example a 60 watt lamp operating on a 120 V circuit draws 1/2 ampere of current. An 1,800 watt space heater on a 120 V circuit draws up to 15 amperes. If a bad wiring junction in a circuit has a resistance of 1 ohm, then a 60 W lamp will cause it to dissipate 0.25 watt of power (0.5 A * 0.5 A * 1 ?)  Toshiba Satellite 1200 Battery .

In contrast an 1,800 W heater could theoretically cause the bad wiring junction to dissipate 169 watts (13 A * 13 A * 1 ?). Note that the current is less than 15 A because of the combined resistance of the heater plus the bad wiring junction. This energy, when dissipated in a small junction area, can generate temperatures above 1000 degrees Celsius and can ignite most flammable materialsToshiba Satellite M300 Battery .

Bad wiring junctions can occur in utilization equipment, cords or in-situ wiring and especially in a defective switch, socket, plug, wiring connection and even at the circuit breaker or fuse panels. Terminal screws loosened by vibration, improper tightening or other causes offer increased resistance to the current  SONY VGP-BPS13A/Q Battery ,

with consequent heating and potential thermal creep, see "Coefficient of expansion" in aluminium wire, which will cause the termination to loosen further and exacerbate the heating effect. In North America, high resistance junctions are sometimes observed at the terminations ofaluminum wire circuits SONY VGP-BPS13A/Q Battery ,

where oxidation has caused increased resistance, resulting in thermal creep. No technology located in a circuit breaker or fuse panel could detect a high-resistance wiring fault as no measurable characteristic exists that differentiates a glow fault from normal branch circuit operation SONY VGP-BPS13B/Q Battery .

Power Fault Circuit Interrupters (PFCI) located in receptacles are designed to prevent fires caused by glowing connections in premise wiring or panels. From the receptacle a PFCI can detect the voltage drop when high current exists in a high resistance junction SONY VGP-BPS13B/Q Battery .

In a properly designed and maintained circuit substantial voltage drops should never occur. Proper wire terminations inside utilization equipment, such as appliances, and cords prevent high-resistance connections that could lead to fires.

Excess current can heat entire lengths of wire  SONY VGP-BPS13/Q Battery .

Thermal circuit breakers are designed to protect against excess current through the permanent circuit wiring. However excess current through the smaller wires in equipment can exist at levels below the trip thresholds of a circuit breaker. Overload fault circuit interrupters (OFCI) are designed to protect against excess current drawn by utilization equipment SONY VGP-BPS13/Q Battery .

OFCIs must be located within receptacles. Both thermal circuit breakers and OFCIs are required to prevent fire ignition from excess current.

High line-voltage creates excess power and heat in utilization devices such as heaters, light bulbs, appliances, motors and other electronics  SONY VGP-BPS21A/B Battery .

In extreme cases, this heat can ignite fires. One common source of high line voltage occurs from a 'neutral' path opening within a multi-wire branch circuit. A three-conductor cable supplying two 120V circuits using two ungrounded conductors ('hot legs') at 240V to each other, and one grounded conductor ('neutral') at 120V to each 'hot'   SONY VGP-BPS21/S Battery ,

may deliver 240V to connected 120V-rated loads if the ungrounded conductor is opened because of a failed connection or worker error. When the grounded conductor breaks or opens, the utilization equipment voltage can almost double to over 200 V with large leg-to-leg load imbalances  SONY VGP-BPS21B Battery .

This extreme situation can result in almost four times the normal power and heat under load. During such overheating, some utilization equipment can reach self-ignition temperature in less than 10 minutes. Power fault circuit interrupters (PFCIs) are designed to prevent fires caused by excess voltage across loads SONY VGP-BPS21A Battery .

Voltage-trip circuit breakers detect excess line voltages but are unable to detect sub-circuit open neutral conditions.

Low line voltage can cause electro-mechanical relays to repeatedly turn off (relay opens) and on (relay closes again) SONY VGP-BPS21 Battery .

If current is flowing through the load contacts it will cause arcing across the contacts when they open. The arcing can oxidize, pit and melt the contacts. This process can increase the contact resistance, superheat the relay and lead to fires. Power fault circuit interrupters are designed to prevent fires from low voltage across loads  Sony VGP-BPS21A/B Battery .

A distribution board (or panelboard) is a component of an electricity supply system which divides an electrical power feed into subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit, in a common enclosure  Sony VGP-BPS21/S Battery .

Normally, a main switch, and in recent boards, one or more Residual-current devices (RCD) or Residual Current Breakers with Overcurrent protection (RCBO), will also be incorporated.

Other names     Sony VGP-BPS21A Battery

Distribution boards are also referred to as a:

North American breaker panels

The circuit breakers are generally placed in two columns. Circuit breaker panelboards are always dead front, that is, the operator of the circuit breakers cannot contact live electrical parts  Sony Vaio PCG-5G3L Battery .

During servicing of the distribution board itself, though, when the cover has been removed and the cables are visible, North American breaker panelboards commonly have some live parts exposed.

Breaker arrangement

Breakers are usually arranged in two columns  Sony Vaio PCG-5J1L Battery .

In a US-style board, breaker positions are numbered left-to-right, along each row from top to bottom. This numbering system is universal across various competing manufacturers of breaker panels.

Each row is fed from a different phase (A, B, and C below), to allow 2- or 3-pole common-trip breakers to have one pole on each phase  Sony Vaio PCG-5K2L Battery .

In North America, it is common to wire large permanently installed equipment line-to-line. This takes two slots in the panel (two-pole) and gives a voltage of 240V if the supply system is split phase and 208 V if the supply system is three phase.

The picture to the right shows the interior of a standard residential service, North American General Electric style breaker panelSony Vaio PCG-5J2L Battery .

The three power lines can be seen coming in at the top (One going to the neutral busbar to the left with all the white wires, the other two attached to the main breaker). Below it are the two rows of circuit breakers with the circuit's hot wire leading off. A line can be seen directly exiting the box and running to a NEMA 5-15 electrical receptacle with a power cord plugged into it Sony Vaio PCG-5K1L Battery .

UK boards

In the UK, domestic and small commercial or public installations usually have single-phase supplies at 230V (nominal standard). The main distribution boards in these installations are called consumer units (CUs), though they may be known as fuse boxes; older consumer units used fuses until the advent of mini-circuit breakers (MCBs) Sony Vaio PCG-6S2L Battery .

A consumer unit normally has a single horizontal row of fuses or MCBs, though some older units grouped four fuses in a square arrangement. For two-rate supplies (standard/off-peak), a second CU may be added (stacked). Multiple CUs are also found in larger premises Sony Vaio PCG-6S3L Battery .

Larger commercial, public, and industrial installations generally use three-phase supplies, with distribution boards which have twin vertical rows of breakers. Larger installations will often use subsidiary distribution boards.

In both cases, modern boards handling supplies up to around 100 A (CUs) or 200 A (distribution boards) use circuit breakers and RCDs on DIN rail mountings  Sony Vaio PCG-6V1L Battery .

The main distribution board in an installation will also normally provide a main switch (known as an incomer) which switches the phase and neutral lines for the whole supply. (n.b., an incomer may be referred to, or sold as, an isolator, but this is problematic, as it will not necessarily be used as an isolator in the strict sense.)  Sony Vaio PCG-6W1L Battery .

For each phase, power is fed along a busbar. In split-phase panels, separate busbars are fed directly from the incomer, which allows RCDs to be used to protect groups of circuits. Alternatively RCBOs may be used to provide both overcurrent and residual-current protection to single circuits Sony Vaio PCG-6W2L Battery .

Other devices, such as transformers (e.g., for bell circuits) and contactors (relays; e.g., for large motor or heating loads) may also be used.

New British distribution boards generally have the live parts enclosed to IP20, even when the cover has been removed for servicingSony Vaio PCG-6W3L Battery .

Modern CU

A typical new domestic CU used as a main panel may have from 6 to 24 ways for devices (some of which may occupy two ways), and will be split into two or more sections (e.g. a non-RCD section for alarms etc., an RCD-protected section for socket outlets, and an RCD-protected section for lighting and other built-in appliances)  Sony Vaio PCG-7111L Battery .

Secondary CUs used for outbuildings usually have 1 to 4 ways plus an RCD.

Older CUs

Recent (pre-17th edition wiring regulations) CUs would not usually have RCD protected sections for anything other than socket outlets, though some older CUs used RCD Incomers Sony Vaio PCG-7112L Battery .

Before the 1990s RCDs (and split busbars) were not standard in CUs.

Fuse Boxes usually use cartridge or rewirable fuses with no other protective device, and basic 4-ways boxes are very common. Some older boxes are made of brown-black bakelite, sometimes with a wooden base Sony Vaio PCG-7113L Battery .

Although their design is historic, these were standard equipment for new installs as recently as the 1980s, so they are very common. Fuseholders in these boxes may not provide protection from accidental contact with live terminals Sony Vaio PCG-7133L Battery .

The popular 4-way fusebox usually has two lighting and two socket circuits, with heavy or sustained loads such as immersion heater and oven on a socket circuit. This arrangement is not recommended practice today, but it is common in existing installations Sony Vaio PCG-7Z2L Battery .

Larger boxes with more ways will have separate fuses for large loads such as immersion heater, oven and shower.

Circuit breaker retrofits

Some of these fuseboxes have had their fuse carriers replaced with plug-in miniature circuit breakers  Sony Vaio PCG-8Y1L Battery .

These retrofit MCBs are typically rated at 3kA breaking capacity, but many homes or properties have prospective short circuit currents as high as 6kA. Fault currents of over 3kA are thus interrupted by the incomer fuse, should they ever occur, and the MCB would not survive  Sony Vaio PCG-8Y2L Battery .

Historic fuseboxes

A small number of pre-1950 fuseboxes are still in service. These should be treated with caution because exposed live parts are common on these boxes. The installations they supply will not meet modern standards for electrical safety Sony Vaio PCG-8Z1L Battery .

Another characteristic of very old installations is that there may be two fuses for each circuit; one on the live and one on the neutral. In rare instances, old ring circuits may be encountered with no less than 4 15 A fuses per ring, one on each of L and N, and this duplicated for each of the 2 feeds for the ring  Sony Vaio PCG-8Z2L Battery .

Inside a UK distribution board

This picture shows the interior of a typical 12-position UK distribution panel. The three incoming phase wires connect to the busbars via a main switch in the centre of the panel  SONY VAIO VGN-FW31J Battery .

On each side of the panel are two busbars, for neutral and earth. The incoming neutral connects to the lower busbar on the right side of the panel, which is in turn connected to the neutral busbar at the top left. The incoming earth wire connects to the lower busbar on the left side of the panel, which is in turn connected to the earth busbar at the top right  SONY VAIO VGN-FW31E Battery .

The cover has been removed from the lower-right neutral bar; the neutral bar on the left side has its cover in place.

Down the left side of the phase busbars are two two-pole RCBOs and two single-pole breakers, one unused. Down the right side of the busbars are a single-pole breaker, a two-pole RCBO and a three-pole breaker  SONY VAIO VGN-FW31B Battery .

The two-pole RCBOs in the picture are not connected across two phases, but have supply-side neutral connections exiting behind the phase busbars.

The illustrated panel includes a great deal of unused space; it is likely that the manufacturer produces 18- and 24-position versions of this panel using the same chassis.

In a UK-style board, breaker positions are numbered top to bottom in the left hand column, then top to bottom in the right columnSONY VAIO VGN-FW31M Battery .

Each number is used to label one position on each phase, as below, and can be seen faintly in the photograph to the right. It remains to be seen how the new wiring colours recently introduced in the UK will affect this labelling  SONY VAIO VGN-FW31Z Battery .

Manufacturer differences

Most of the time, the panel and the breakers inserted into it must both be from the same company. Each company has one or more "systems", or kinds of breaker panels, that only accept breakers of that type  SONY VAIO VGN-FW38M Battery .

In Europe this is still the case, despite the adoption of a standard DIN rail for mounting and a standard cut-out shape, as the positions of the busbar connections are not standardised.

Certain panels use seemingly interchangeable 1-inch-wide (25 mm) breakers  SONY VAIO VGN-AW41JF Battery .

However, a given manufacturer will often specify exactly what devices are permitted to be installed in their equipment. These assemblies have been tested and approved for use by a recognized authority. Replacing or adding equipment which "just happens to fit" can result in unexpected or even dangerous conditions  SONY VAIO VGN-AW41JF/H Battery .

Such installations should not be done without first consulting knowledgeable sources, including manufacturers.

Location and designation

For reasons of aesthetics and security, circuit breaker panels are often placed in out-of-the-way closets, attics, garages SONY VAIO VGN-AW41MF Battery ,

or basements, but sometimes they are also featured as part of the aesthetic elements of a building (as an art installation, for example) or where they can be easily accessed. However, current US building codes prohibit installing a panel in a bathroom (or similar room), in closets intended for clothing, or where there is insufficient space for a worker to access it SONY VAIO VGN-AW41MF/H Battery .

Specific situations, such as an installation outdoors, in a hazardous environment, or in other out-of-the-ordinary locations may require specialized equipment and more stringent installation practices.

Large buildings or facilities with higher electric power demand may have multiple circuit breaker panels SONY VAIO VGN-FW30B Battery .

In this case, the panels are often indicated by letters of the alphabet. One case is The Decon Gallery, a modern building in downtown Toronto, which has 11 breaker panels designated A, B, C, D, and so on. A backstage outlet is therefore labeled C27  SONY VAIO VGN-FW50B Battery .

In many such buildings, each outlet is on its own circuit breaker, and the outlets are labelled in the above specified manner to facilitate easy location of which breaker to shut off for servicing, rewiring, or the like  SONY VAIO VGN-FW51B/W Battery .

In even larger buildings, such as schools, hospitals and sports/entertainment venues it is not uncommon to have scores of panels, specially designated for each building depending on how the architects and electrical engineers subdivide the building. They are commonly designated as either three-phase or single-phase and normal power or emergency power SONY VAIO VGN-FW51MF Battery .

In these set-ups they may also be designated for their use, such as distribution panels for supplying other panels, lighting panels for lights, power panels for equipment and receptacles and special uses for whatever type of building they are used in. It is also not uncommon for these panels to be located throughout the building in electric closets serving a section of the building SONY VAIO VGN-FW51MF/H Battery .

In a theatre a specialty panel called a dimmer rack is used to feed stage lighting instruments. A US style dimmer rack has a 208Y/120 volt 3-phase feed. Instead of just circuit breakers, the rack has a solid state electronic dimmer with its own circuit breaker for each stage circuit. This is known as a dimmer-per-circuit arrangement  SONY VAIO VGN-FW51ZF Battery .

The dimmers are equally divided across the three incoming phases. In a 96 dimmer rack, there are 32 dimmers on phase A, 32 dimmers on phase B, and 32 on phase C to spread out the lighting load as equally as possible. In addition to the power feed from the supply transformer in the building  SONY VAIO VGN-FW51ZF/H Battery ,

a control cable from the lighting desk carries information to the dimmers in a control protocol such as DMX-512. The information includes commands on levels, fade times, and which dimmers come up and go out during the lighting changes of the show (light cues)  SONY VAIO VGN-FW52JB Battery .

Distribution boards may be surface-mounted on a wall or may be sunk into the wall. The former arrangement allows for easier alteration or addition to wiring at a later date, but the latter arrangement may look neater, particularly in a residential situationSONY VAIO VGN-FW54FB Battery .

The other problem with recessing a distribution board into a wall is that if the wall is solid a lot of brick or block may need to be removed - for this reason recessed boards are generally only fitted on new-build projects when the required space can be built into the wall  SONY VAIO VGN-FW70DB Battery .

Mobile operation

Sometimes it is desired to have a portable breaker panel, for example, for special events. In this case, a breaker panel is mounted to a board, together with various sockets. The American one pictured at the right has a cord with an L21-30 plug to supply power SONY VAIO VGN-FW71DB/W Battery .

Power leaves the board through four three-phase circuits: three 15 ampere circuits; and one 20 A circuit. The 15 A circuits each go to a triplex-box. The 20 A circuit goes to an L21-20 receptacle, and one leg of it goes to a 20 A duplex receptacle shown at the upper left. The neon night-lights on the upper right triplex box are to show the phase presence SONY VAIO VGN-FW72JGB Battery .

The use of a load center in this type of configuration is dangerous and violates UL and NEC rules for their use. When power distribution is required on movie sets, concert stages and theatrical venues it should be provided via products Listed "for portable power distribution SONY VAIO VGN-FW73JGB Battery ."

In March 2005, the

'European Hydrogen and Fuel Cell Technology Platform' adopted a research agenda for accelerating the development and market introduction of fuel cell and hydrogen technologies within the European Community. This agenda called for funding by the EC and organisations from the public- and private sectors SONY VAIO VGN-FW74FB Battery .

On 19 December 2006, the agenda of the Technology Platform were adopted by the Council Decision 2006/975/EC within the EC's Seventh Framework Programme. The prospect of further financing from the European Investment Bank (in particular through its Risk-Sharing Finance Facility) had been established in an earlier decision (2006/971/EC) SONY VAIO VGN-FW81HS Battery .

In March 2007 the European Council concluded that the Union's member states had an interest in taking a lead position in renewable energy programs. On 10 October 2007, the European Commission adopted two proposals to further the development and marketing of clean and safe hydrogen vehicles   SONY VAIO VGN-FW81NS Battery.

One proposal was to simplify the regulatory procedures for hydrogen-powered vehicles. The other proposal was the establishment up of the 'Fuel Cells and Hydrogen Joint Technology Initiative' as called for by the Hydrogen and Fuel Cell Technology Platform  SONY VAIO VGN-FW81S Battery .

The Commission's second proposal was duly considered by the European Parliament and the Council of Ministers, and on 30 May 2008 the Council passed regulation number 521/2008 setting up the "Fuel Cells and Hydrogen Joint Undertaking" that will run until 31 December 2017  SONY VAIO VGN-FW82DS Battery.

Article 2 of the regulation stipulated a contribution to the implementation of the Seventh Framework Programme, in particular to its energy-, nanotechnologies-, environment-, and transport-specific programmes. The "Joint Technology Initiative on Fuel Cells and Hydrogen"  SONY VAIO VGN-FW82JS Battery .

would accordingly receive appropriations in the general budget of the European Union allocated to those programmes. Article 5 established a maximum community contribution cap of € 470 million. The objectives of the fuel cell initiative were to be pursued by pooling resources from the public and private sectors   SONY VAIO VGN-FW82XS Battery

The 'Fuel Cells and Hydrogen Joint Technology Initiative' was launched on 14 October 2008  during the General Assembly of Fuel Cells and Hydrogen Stakeholders  SONY VAIO VGN-FW83DS Battery .

A press release from the European Hydrogen and Fuel Cell Technology Platform reiterates an estimate "that the activities of the JTI will reduce time to market for hydrogen and fuel cell technologies by between 2 and 5 years  SONY VAIO VGN-FW83JS Battery ."

Membership and structure

The public-private joint initiative operates under the auspices of the DG Research of the European Commission, representing the European Communities, and industry.  SONY VAIO VGN-FW83XS Battery

Governance of the 'Fuel Cells and Hydrogen Joint Technology Initiative' (FCH JTI) lies with the 'Fuel Cells and Hydrogen Joint Undertaking'. Members of that body are the European Community and the 'JTI Industry Grouping' SONY VAIO VGN-FW90HS Battery

with the latter being "a not-for-profit organisation which brings the sector's industrial key players and which is open to any private legal entity sharing the objectives of the FCH JTI."

As of December 2008, the chairman of the governing board is Gijs van Breda Vriesman of Shell Hydrogen SONY VAIO VGN-FW90NS Battery .

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