Science Fair Project Encyclopedia
A distribution board (known in the United States as a circuit breaker panel or just breaker panel) is a mounting enclosure for multiple electrical circuit breakers. These are generally placed in two rows. Small single-phase boxes, with the breakers in just one row, are known as consumer units. Distribution boards are typically found in central locations inside buildings and often serve as the point at which electricity is distributed within a building. (For this reason, circuit breakers usually also function as switches to manually deactivate electrical circuits within a building when wiring is being serviced.) American breaker panels commonly have many live parts exposed with the lid off. British distribution boards by contrast generally have live parts enclosed to IP20, even with the lid off. This makes testing and live working much safer, especially given the voltage (415 volts) present in such boards.
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, as shown below:
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.
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. Ideally the number of rows is a multiple of 3, so there are the same number of breakers on each leg of the three-phase supply, but this is not always the case; some three-phase panels have 40 or 50 breaker slots, rather than 42 and 48.
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 208V 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.
Inside a North American panel
Inside a UK distribution board
Down the left side of the phase busbars are two two-pole RCDOs and two single-pole breakers, one unused. Down the right side of the busbars are a single-pole breaker, a two-pole RCDO 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.
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 Cutler Hammer panels and breakers of the type shown in the above and below picture illustrations are interchangeable. Therefore, these two types of breaker panels have gained widespread acceptance as a "standard". The two panels shown (one Siemens, and the other Cutler Hammer) seem to fit GOULD Type QP, ITE type QT, Cutler Hammer Type BR, and Siemens breakers.
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.
Larger buildings or facilities with high 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.
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.
Sometimes it is desired to have a portable breaker panel, e.g. 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, and are sometimes referred to as "Dief boxes" because John Diefenbaker tripped over one when they began to go into widespread use. The american one pictured 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 20A circuit. The 15A circuits each go to a triplex-box. The 20A circuit goes to an L21-20 receptacle, and one leg of it goes to a 20A duplex receptacle shown at the upper left. The neon nightlights on the upper right triplex box are to show the phase sequence.
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