Distribution board
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A distribution board (known in the United States as a (circuit) breaker panel, panelboard, or load center or for old ones, fuse box) is a mounting enclosure for multiple electrical circuit breakers. These are generally placed in two columns. Small single-phase boxes, with the breakers in just one row, are known as consumer units in Britain. Distribution boards are typically found in central locations inside buildings and often serve as the point at which electricity is distributed within a building. Circuit breakers can be used to manually de-energize electrical circuits when the downstream wiring is being serviced.
Circuit breaker panels are always dead front, that is, the operator of the circuit breakers cannot contact live electrical parts. During servicing of the distribution board itself, though, when the cover has been removed and the cables are visible, North American breaker panels commonly have some live parts exposed. British distribution boards generally have the live parts enclosed to IP20, even when the cover has been removed for servicing.
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[edit] Breaker arrangement
Breakers are usually arranged in two columns. In a US-style board, breaker positions are numbered left-to-right, along each row from top to bottom. For 120/240 volts, hot wires (that which are live) are black and red (blue is used as the third leg of three-phase power [120/208 volts]) and white for neutral. For 277/480 volts (always three-phase) the hot wires are brown, orange and yellow and grey is the neutral. Green or bare wires are used as grounds in both configurations.
| Phase | Breakers | |
|---|---|---|
| X | 1 | 2 |
| Y | 3 | 4 |
| Z | 5 | 6 |
| X | 7 | 8 |
| Y | 9 | 10 |
| Z | 11 | 12 |
These breakers cycle through two or three phases, labelled as X, Y, and Z in the above diagram. This numbering system is universal across various competing manufacturers of breaker panels.
In a UK-style board, breaker positions are numbered top to bottom in the left hand column, then top to bottom in the right column. Each number is used to label one position on each phase, as below. It remains to be seen how the new wiring colours recently introduced in the UK will affect this labelling.
| Phase | Breakers | |
|---|---|---|
| Red | R1 | R4 |
| Yellow | Y1 | Y4 |
| Blue | B1 | B4 |
| Red | R2 | R5 |
| Yellow | Y2 | Y5 |
| Blue | B2 | B5 |
| Red | R3 | R6 |
| Yellow | Y3 | Y6 |
| Blue | B3 | B6 |
In both labelling styles the reason for the alternating pattern of phases is to allow for common trip breakers to have one pole on each phase.
In North America it is common to wire large heating 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. This practice is much less common in countries that use a higher line-neutral voltage. Large motors, air conditioners, subpanels, etc., are typically three-phase (where available). Therefore a three-pole breaker is needed which takes three slots in the breaker panel.
[edit] Inside a North American panel
The picture to the right (Click to enlarge it) shows the interior of a standard residential service, North-American General Electric style breaker panel. 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 an electrical receptacle with something plugged into it.
[edit] 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 an isolator switch in the centre of the panel. The incoming neutral connects to the neutral busbar at the centre right of the board, which is in turn connected to the neutral busbar at the top left side of the board. The incoming earth wire connects to the earth busbar at the centre left side of the panel, which is in turn connected to the earth busbar at the top right of the board. The cover has been removed from the neutral bar at the right of the board; 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.
The two-pole RCDOs in the picture are not connected across two phases, but have supply-side neutral connections exiting behind the phase busbars.
It is likely that the manufacturer produces 18- and 24-position versions of this panel using the same chassis which explains why there appears to be so much unused space.
[edit] 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. 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.
It is commonly known in North America that Siemens and General Electric panels and breakers of the type shown in the above and below picture illustrations are interchangeable one-inch wide breakers. Therefore, these two types of breaker panels have gained widespread acceptance as a "standard". The two panels shown seem to fit GOULD Type QP, ITE type QT, Cutler Hammer Type BR, Square D "Homeline", Siemens, and General Electric breakers. The other two standards commonly found in North America are the Cutler-Hammer "CH" standard and the Square D "QO" standard (both 3/4 inch breakers). These systems allow the use of breakers which accommodate two individual circuit breaker functions within the width of a standard (1" or 3/4") case, but not all positions in all distribution boards may allow the use of such dual breakers.
Numerous older systems are still in use in older buildings and are still manufactured for these legacy applications, such as Zinsco and others.
[edit] Location and designation
For reasons of aesthetics and security, circuit breaker panels are often placed in out-of-the-way closets, attics, garages, 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), or where there is insufficient space for a worker to access it.
In large buildings or facilities with higher electric power demand may have multiple circuit breaker panels. 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". 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.
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 sub divide the building. They are commonly designated as either three-phase or single-phase and normal power or emergency power. 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.
Distribution boards may be surface-mounted on a wall or may be sunk in to 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 situation. 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 in to the wall.
[edit] 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. These are common in the movie industry. The American one pictured at the right has a cord with an L21-30 plug to supply power. 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 nightlights on the upper right triplex box are to show the phase sequence.
