if fitted correctly. A screw collar helps hold plug and socket together. In the single-phase standard AS/NZS 3112 used in residential environments, plugs of lower current rating may be inserted into single phase socket outlets of higher current rating but not vice versa. While this feature is somewhat reflected with multiphase sockets, it is further limited to only plugs within the same group. For example, there is one group that consists of 32 A, 40 A and 50 A connectors, and another group of 50 A, 63 A and 80 A connectors. The 32 A plug can be connected to any of the 32 A, 40 A or 50 A sockets in the first group, however it cannot be connected to any of the 50 A, 63 A or 80 A sockets in the second group. The standard notes that there are two different, incompatible 50 A connectors that are not interchangeable as they are physically different sizes. Compatibility is grouped like this, such that larger sockets in one group can take the same plug or any smaller plugs from that group only: Each of the three phase plugs and sockets are available as 4-pin (without neutral) or 5-pin (with neutral). A plug without a neutral pin can be inserted into a socket carrying neutral, but the reverse is not possible as the neutral pin blocks insertion into a socket that does not have a hole for the neutral conductor. From this it can be seen that, for example, installing a 32 A, 250 V socket will only be compatible with a 32 A plug and it will not be possible to insert 10 A or 20 A plugs. Within each size group, the plugs and sockets are keyed with certain "lugs" protruding on the outside of the plugs. Higher current plugs have more lugs to prevent them from being inserted into a lower current socket outlet, while still allowing them to be inserted into a socket outlet of the same size group rated for a higher current. For example, a 32 A 4-pin plug without neutral can plug into a 50 A 5-pin socket with neutral available. However a 10 A 5-pin plug cannot fit a 32 A 5-pin socket, as the plugs are in different groups and not only are their diameters different, but the position of the conductors also varies by a few degrees (for example the