The latest regulation requires all plastic enclosures to be changed to metal and here is why?
Regulation 421.1.201 was introduced because of reports from fire investigators of a recent trend of increasing numbers of fires involving consumer units having a moulded thermoplastic enclosure. See Figures 1, 2 and 3.
The cause of the fires investigated was almost invariably found to be resistance heating as a result of poor electrical connections due to poor workmanship or lack of maintenance. Examples of such poor workmanship are thought to include:
failure to tighten neutral conductor connections to the same torque as line conductor connections due to a misconception that neutral conductors carry less current
inadvertently taking cable insulation inside the terminal at a connection, resulting in the securing screw or the clamp of the terminal not making proper contact with the conductor
failure to check factory-installed connections for tightness where required to do so by the manufacturer, and
tightening connections to an incorrect torque and/or with inappropriate tools.