Handbook of Electrical Installation Practice - BeKnowledge

104 Handbook of Electrical Installation Practice
Transformer assembly (TA/1/3)
The MDAs distribute the mains voltage direct to the larger electrical loads or to the
TAs which are located at load centres wherever possible. They operate from a 400
V three-phase supply or a 230V single-phase supply and step down the voltage to
110V. The standard design of a TA comprises a double-wound transformer housed
in a weatherproof enclosure and fitted with socket-outlets and protective devices.
The star point of the secondary side of the three-phase unit is earthed and the singlephase
unit secondary is centre-tapped to earth, reducing the potential to earth to
63.5V and 55V respectively. The widespread use of transformers providing power
at 110V centre or star-point earthed has resulted in a considerable reduction in
injuries due to electric shock since the early 1970s.
There are two main types of TAs: 1kVA to 25kVA continuously rated, and 0.5
kVA to 3kVA power tool rated. It is usual to associate power transformers with
final distribution on a construction complex, and the power tool rated portable
transformers with smaller projects, such as service industries and house building.
Figure 5.4 shows the types of TAs.
The same basic parameters apply to each type of transformer: 110V centretapped
or star-point earthed. Secondary distribution is provided by 110V socket
outlets to BS EN 60309 (BS 4343). The quantity of socket-outlets available on a
TA can vary slightly, depending on company standards. Table 5.2 identifies the
Fig. 5.4 Typical transformer assemblies.