Lampholders and ceiling roses

In public buildings the luminaires are fixed as part of the electrical installation . In housing , the choice of the lampshade or luminaire is usually left to the owner or tenant and is made once the dwelling is occupied . plain lampholders are , therefore , provided which will accept ordinary 100w and 150 w tungsten bulbs , and which usually have a ring or skirt to which a normal lampshade or similar liminaire can be attached . The top of the lampholder screws down to grip the flexible cable cord on which it is suspended from the ceiling . Typical lampholers are shown in Figure 1.13.
  The flexible cord on which the lampholder is suspended performs two functions . It carries the electric current to the lamp , and it supports the weight of the holders , lamp and shade . Its physical strength , therefore , just as important as its current carrying capacity and it has to be selected with this in mind . At the ceiling itself , the wiring in ( or on ) the ceiling must be connected to the flexible cord . The connection is made by means of a ceiling rose , which is illustrated in Figure 1.14 . it consists of a circular plastic housing with a terminal block inside and a bushed opening on the underside where the flexible cord to the lampholder can come out of the rose . In installations which have the main wiring inside ceiling , this wiring enters the rose through the back or top of the rose ; when the main wiring runs exposed on the surface of the ceiling ,it enters the rose through a cut-out in the inside of the rose .
  Ceiling roses are made with three line terminals in addition to an earth terminal . The reason for the third terminal is used , it temains live even when the light atttached to the ceiling rose is switched off  . It must , therefore , be shrouded so that it cannot be touched by accident if ever the flexible cord is being replaced ; complete circuit isolation for this task is strongly recommended . Ceiling roses are available which incorporate a plug and without disruption to other parts of the same circuit .