Centre tapped to earth

X- 110-volt centre tapped to earth supply X- residual current device (ROD) in 240-volt supply X- means to discharge static electricity charges... 

V Portable hand lamps Secondary winding centre tapped to earth (25 volt) No No Yearly... 

V Portable and hand-held tools, extension leads, site lighting, moveable wiring systems and associated switchgear Secondary winding centre tapped to earth (55 volt) Weekly Monthly Before first use on site and then 3 monthly... 

This type of system is becoming less common as a result of the increasing use of reduced voltage (llOV centre-tapped-to-earth) and even battery-operated Class II hand tools in construction sites and other harsh environments. It does, however, have a role in ensuring the integrity of earthing systems where there are high levels of protective conductor current (see BS 7671, section 607). 

The first of these measures is the most widely used, and its requirements are generally understood by the electrically qualified, except perhaps for the bonding of some extraneous metalwork such as metal framed windows where it is often a matter of risk assessment and judgement see section 7.2. The relation between disconnection times and degree of risk is explained, including the 5 s llOV centre tapped to earth (CTE) system used on construction sites. 

The construction industry has an unenviable accident record and, until the 1990s, had more electrical fatalities than any other industry. The accident rate dropped during the 1990s, largely as a result of the emphasis placed on the use of 110 V centre-tapped-to-earth supplies to hand tools and the use of safer distribution systems. However, serious accidents and fatalities still occur, with contact with overhead power lines being an all too frequent occurrence. 

All types Less than 110 V with secondary winding centre tapped to earth = 55V Not required Not required Not generally required except yearly for hand lamps... 

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