67
Fig. 7.19 shows connection of high-speed fuses
for protection against thyristor breakdown that would
otherwise result in short-circuit flow from the AC sup-
ply side.
4. MCCBs for Lamp Mercury-Lamp Circuits
The ballasts (stabilizers) used in this type of lamp
cover a variety of types and characteristics. For 200V
applications (typical), choke-coil ballasts are used. For
100V applications a leakage-transformer ballast is
employed. Normal ballasts come in low power-factor
versions and high power-factor versions, with correc-
tion capacitors. More sophisticated types include the
constant-power (or constant-output) type, which main-
tains constant lamp current both in starting and nor-
mal running, and flickerless types, which minimize the
flicker attendant on the supply frequency.
In selecting an MCCB where normal (high or low
PF) ballasts are to be used, the determining factor is
the starting current, which is about 170% of the stable
running current. In the cases of constant-power or
flickerless types, the determining factor is the normal
running current, which is higher than the starting cur-
rent. For MCCB selection, the latter types can be re-
garded as lighting and heating general loads, as pre-
viously discussed.
For selection of MCCBs for regular ballasts, the
170% starting current is assumed to endure for a
maximum of 5 minutes. MCCBs of 100A or less frame
size have a tripping value very close to rating for over-
loads of duration of this order, so that the MCCB rat-
ing should be the nearest standard value above 170%
of the stable running current. MCCBs of above 100A
frame size can handle a current of around 120% of
the rating for 5 minutes without tripping; thus the near-
est standard MCCB rating above
1.2
1.7
= 1.4 times the
stable-running current of the lamp load is the suitable
protector.
As an example, consider MCCB selection for 10
units of 100W, 100V, 50Hz general-purpose high
power-factor mercury lamps. The stable-running cur-
rent per lamp is 1.35A. Thus:
1.35 x 10 x 1.7 = 23A, and the selection becomes
NF30-SP, 30A rated.
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