Abstract: A transport refrigeration unit is provided with a control box 16 arrangement in which a hard-wired electrically passive assembly of receptacles 23, 24 and electrical terminal strips 25-27, and interconnecting wires 28-30 at the rear wall 18 of the box is arranged to be electrically connected to the interface module 33 through the plugs 39, 40 being received in the passive assembly receptacles, the interface module having both high voltage and low control voltage elements 41-48, the interface module also having a forwardly facing low voltage receptacle 49 which receives the front module 33 plugs 60 at the rear of the front module, the front module having only low control voltage elements 61-63 and a door 73 for the box provided with control switches 75 connected through wiring 76 and plug 78 to receptacle 79 so that all of the electrical elements of the front module and the door are of low control voltage.