Industrial power plug - Definition and Overview

Industrial power plug is used here to refer to those power connectors that, unlike ordinary mains power plugs, are not used for the average domestic appliance but rather for more heavy-duty applications such as arc welders, cranes, cinemas, large cooking ranges, etc.

See Domestic AC power plugs & sockets for further information.

Some plugs are borderline cases. For instance, Italy has large, 16-ampere, three-prong plugs competing with the more normal 5-ampere ones, but modern sockets in homes take both types. The US goes so far as to use two phases for larger domestic appliances: these can be either split phase (from a centre-tapped transformer) or two phases from a three-phase system. The situation in the United Kingdom is slightly more clear-cut, with BS 1363 plugs (3 to 13 amperes, depending on the fuse inside) almost universal in homes. The 5- and 15-ampere BS 546 plugs are occasionally seen in older homes, but this is getting rarer, and these plugs have remained in use in a few niche markets like theatre lighting. The IEC.309 range of single and three-phase plugs are used mainly for industrial purposes, although most people in the UK will have seen the smaller varieties around swimming pools and caravan sites.

Unlike domestic plugs, industrial connectors often use three-phase rather than single-phase electrical power. Such polyphase systems have several advantages: first, they provide a better ratio of cable size to maximum voltage present; second, they allow a higher voltage present on trunk lines than is finally fed into single-phase appliances (though split phase can also achieve this); and third, they allow large motors and generators to operate far more efficiently.

Almost all three-phase power plugs have an earth (also known as "ground") connection, but many do not have a neutral, because large appliances such as circular saws, air conditioners, etc. tend to be delta connected. Such plugs have only four prongs (earth, and X, Y, Z phases). An example of a socket with neutral is the L21-30 (30 amp) and the L21-20 (20 amp) both of which have five pins (earth, neutral, and X, Y, Z phases).

Countries have different standards and regulations (or lack of them) for industrial power plugs, but there are also international standards and norms. The colour-coding of wires, etc. may or may not be the same as for normal mains plugs.

Europe

In Europe the the most common range of heavy commercial and industrial plugs are made to IEC 309 and BS EN 60309-2 (formerly BS 4343) (the two refer to the same plug types though BS EN specifications tend to be stricter). These are often referred to in the UK as CEE industrial or simply CEE plugs.

Voltage is represented by a color code (in three-phase plugs the stated voltage is the phase-phase voltage, not the phase-neutral voltage). The different voltages have the key (or keys in the case of ELV plugs) in different places relative to the earth pin so it is impossible to mate e.g. a blue plug with a yellow socket. It is also not possible to mate different pin configurations or current ratings so for example a 16A 3P+E 400V plug will not mate with a 16A 3P+N+E 400V socket and a 16A 2P+E 230V plug will not mate with a 32A 2P+E 230V socket.

The voltage ranges are:

25V: purple

50V: white

100V-130V:yellow

200V-250V:blue

346-460V:red

500-750V:black

high frequency >50V: green

Plugs are available in 2P+E (single phase) 3P+E (3 phase no neutral) and 3P+N+E (three phase with neutral).

Yellow 2P+E blue 2P+E yellow 3P+E red 3P+E and red 3P+N+E are by far the most common arrangements.

Current ratings available are 16A, 32A 63A, and 125A.

A comprehensive summary of this information can be seen here (http://www.mkelectric.co.uk/PDF/select/296.pdf) (PDF).

North America

Type L21

The type L21-30-P and L21-20-P are 30 Amp and 20 Amp plugs for 208Y/120V connections. These plugs connect to type L21-30-R and type L21-20-R receptacles, respectively. The middle prong is ground, and the prong with a right angle on the tab (nearest the "L2130P" indicia on the plug pictured) is neutral

NEMA 5-20, 5-30, 5-50, 6-15, 6-20, 6-30, and 6-50

These are variants on the 5-15 plug. The 20 A plug has the neutral pin rotated 90 degrees, and the receptacle has a T-shaped neutral hole, to accept both 5-15 and 5-20 plugs. The 5-30 uses an L-shaped neutral, and both it and the 5-50 are over-sized, but these configurations are uncommon, as "twist-lock" plugs are generally used for high-current applications.

The 6-15, 6-20, 6-30, and 6-50 are 250 V variants, with the 6-15 and 6-20 being grounded versions of the 2-15 and 2-20; again, the higher-current versions are rare, with "twist-lock" plugs and direct wiring being far more common; generally 6-series non-locking plugs are used for such appliances as electric clothes dryers and ranges, and the occasional home arc welder. There are other configurations, both twist-lock and otherwise, for higher voltages and currents, and for more conductors.

NEMA 14-30 and 14-50

These offer two hots, a neutral and a ground, allowing for both 120V and 240V (or a bit lower if the supply system is three phase rather than split phase) appliances. They differ in rating and shape of the neutral pin. The 14-30 has a rating of 30A and an L-shaped neutral pin. The 14-50 has a rating of 50A and a straight neutral pin. Other sizes also exist with different shapes of neutral pin which are not pictured here.

NEMA 10-20, 10-30 and 10-50

These serve a similar purpose to the plugs above but lack the ground connection. They are seen on older installations that were put in before grounding became a requirement.

Duplexing and triplexing

Triplex outlet. The top three are used to run a phase sequence indicator comprising three neon night lights. The bottom three run the desired triplex load.

Most receptacles in the USA and Canada are duplex receptacles. The top and bottom sockets can also be separated, if desired, and, for example, supplied by separate breakers with a common neutral. This is typically done in kitchens where a high load will likely be placed on both sockets. In this case, a common trip two-pole breaker is often used.

The concept of duplexing can be generalized to triplexing, so that three duplex receptacles can be supplied by a common neutral, from a three-phase supply. Typically, a three-pole common trip 15 A breaker is used to supply such a socket. This enables three single phase loads to be supplied in a phase-sequenced manner. An example of such a load is a light fixture having three bulbs. For flicker-free operation, three bulbs are each fitted with a separate plug, and driven 120° out of phase with one another, from a triplex receptacle. The top receptacles shown in the figure, are fitted with neon night lights to indicate phase sequence, for triplex loads where proper phase sequence is desired. This can, however, create problems in environments that have large quantities of dimmed circuits, such as theatres. Harmonic 'noise' created by dimming equipment can effectively overload a combined neutral and result in a fire. Combined neutrals can also cause inconsistent response from individual circuits in this situation. Although not the case in older facilities, in all new installations of large-scale theatrical dimming equipment, manufacturers require individual neutrals to maintain warranty status of the equipment.

This fact should also be taken into account in wiring office buildings, as a large number of computer power supplies can also overload a combined neutral.

External links

• Household wiring (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html#c2) (Georgia State University physics department)
• NEMA socket configurations (http://www.et-sales.com/Receptacles.htm): diagrams from a vendor of plugs and receptacles
• Diagrams of locking North American receptacles (http://www.stayonline.com/reference/nema_twistlock.asp)
• Diagrams of non-locking North American receptacles (http://www.stayonline.com/reference/nema_straight_blade.asp)