Emergency Lighting

if the test is then repeated at the other end of the cable and the mean position of X is taken as the point of fault.

AB L L X X / [ ( )]/

Hence

AB L XX / ( )/ AX LB BX AX BX LB XA B LB ( ) X 2 2 2 2

2LB A B)

Variable resistors A and B are usually incorporated in a single bridge instrument, and the resistances are adjusted for a zero reading.

EMERGENCY LIGHTING

There is not a lot to go wrong with these systems other than the cable faults and internal failure of the unit. Cable faults are located in the way previously described; however, do not conduct insulation resistance tests with the unit still connected, or the internal electronic circuitry may be damaged.

Individual units have LEDs that indicate a healthy state of charge. If this is out, 1. Check the mains supply. If it is all right, then: 2. Check the output of the charging unit at the battery terminals. If this is all right, check the battery separately. If the LED is lit but the emergency light does not function when the mains supply is removed, check the lamps. If these are functioning, the circuitry is probably faulty.