Code of Practice for the Electricity (Wiring) Regulations - 2009 Edition

(c) The letters L and N should be used to indicate the terminals
corresponding respectively to the line socket contact (L) and the neutral
socket contact (N) and letter E or symbol for the earthing socket
contact (E). The letters and symbols should be as close as practicable to
the corresponding terminals.
(10)Temperature rise
(a) The permitted temperature rise of the accessible external surface of the
socket outlet after the test of subparagraph (b) should be 50°C.
(b) The following procedures of the test on the temperature rise of the
accessible external surface of the socket outlet are acceptable:
(i) Temperature rise of socket outlets should be measured by fine wire
thermocouples.
(ii) Surface mounted socket outlets should be mounted as in use with
their accompanying mounting block or backplate fixed to a vertical
plywood board 25 mm thick and having a flat surface extending at
least 150 mm in each direction beyond the extremity of the socket
outlet.
(iii) Flush mounted socket outlets should be mounted on socket outlet
boxes with an internal depth of 35 mm. The flush mounted socket
outlet box is placed in a block of wood, so that the front edges of
the box are between 2.5 mm and 5 mm below the front surface of
the block. The size of the block should be such that there is a
minimum of 25 mm of wood surrounding the box on all four sides
and the back.
(iv) Socket outlets under test should be subjected to its rated loading
connected via a plug for a minimum continuous period of 4 hours or
longer until stability is reached with a maximum duration of 8 hours,
stability being taken as less than 1°C rise within 1 hour.
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