7.16.4 - Reduced voltage
In situations where the power
requirements are high, extra-low voltage systems would need
to deliver very high currents. If an increased voltage can
still lead to a safe system, the current required will be
reduced. We are therefore considering a voltage
higher than extra-low but lower than low voltage.
The highest voltage permitted for such
systems is 110 V between conductors for both single and
three-phase systems, with voltages to earth of 55 V for
single-phase and 65 V for three-phase supplies {Fig 7.26}.
The reduced voltage supply can be taken
from a double wound isolating transformer, or from a suitable
motor generator set, provided it has the unusual feature
of a centre-tapped winding. The system must be insulated
and protected against direct contact as for a low voltage
installation. Earth-fault loop impedance must allow automatic
disconnection in a maximum of 5 s (see {Table 7.9}), or
an RCD with an operating current of no more than 30
mA must be provided. The result of multiplying earth-fault
loop impedance by the RCD operating current (A) must not
exceed 50 (V). Plugs, sockets and cable couplers in the
reduced voltage system must all be provided with protective
conductor contacts (earth pins) and must not be interchangeable
with those of any other system in use at the same location.
Fig 7.26 - Reduced voltage
Systems
Table 7.9 - Maximum
earth-fault loop impedance for 5 s
----------------- disconnection
time in reduced voltage systems |
| Device rating (A) |
MCB type 1 |
MCB type 2 |
MCB types C & 3 |
BS 88 fuse |
| - |
55 V |
63.5 V |
55 V |
63.5 V |
55 V |
63.5 V |
55 V |
63.5 V |
| 6 |
2.30 |
2.65 |
1.32 |
1.52 |
0.92 |
1.07 |
3.20 |
3.70 |
| 10 |
1.38 |
1.59 |
0.79 |
0.91 |
0.55 |
0.64 |
1.77 |
2.05 |
| 16 |
0.86 |
0.99 |
0.49 |
0.57 |
0.34 |
0.40 |
1.00 |
1.15 |
| 20 |
0.69 |
0.80 |
0.40 |
0.46 |
0.28 |
0.32 |
0.69 |
0.80 |
| 25 |
0.55 |
0.64 |
0.32 |
0.36 |
0.22 |
0.26 |
0.55 |
0.63 |
| 32 |
0.43 |
0.50 |
0.25 |
0.28 |
0.17 |
0.20 |
0.44 |
0.51 |
| 50 |
0.28 |
0.32 |
0.16 |
0.18 |
0.11 |
0.13 |
0.25 |
0.29 |
Note : 63.5 V
is the voltage to earth (to the star point) of a 110
V
---------- three-phase
supply. |
|
