Specifications

NAIROBI OPERATIONAL HUB
UNOPS Building, UN Lane, Off UN Avenue Tel: [phone]
P.O Box 783,Village Market, 00621 E-mail: [e-mail]
Nairobi, Kenya Website: www.unops.org
UNOPS is ISO 9001 certified
clearly indicates the three positions ‘ON’, ‘OFF’ and ‘TRIP’ thus
indicating clearly abnormal conditions of the circuit. In order to ensure
suitability for isolation complying with IEC-947-2, the operating
mechanism shall be designed such that the toggle or handle can only be in
OFF position if the power contacts are all actually separated. The molded
case circuit breakers shall be able to receive a locking device in the
“isolated” position and there shall be a “push to trip” button in front to test
operation and the opening of the poles. The circuit breaker rating, the
‘push to trip’ button, outgoing circuit identification and the contact
position indication must be clearly visible and accessible from the front,
through the front panel or the door of the switchboard. Single pole breaker
with handle tie or bar equivalent construction are not acceptable for a
multi-pole breaker. Molded case circuit breakers shall be the fixed type.
Plug in type breaker connections are not acceptable.
C. Breakers shall have the rating and rated service breaking capacity (Ics) as
per IEC 947-2 as indicated in the drawings. The breakers shall be of
current limiting type. For short circuits, the maximum thermal stress I²t
shall be limited to 10
6
A²s for ratings up to 250A and 5 x 10
6
A² s for
ratings above up to 630A.
D. Circuit breakers shall have inverse time tripping characteristic with
automatic release secured through action of a combination of thermal-
magnetic or electronic trip units which shall trip free of the handle and
operate in response to an overload or a short circuit.
D. It shall be possible to equip the moulded case circuit breaker with a motor
mechanism if needed and closing of mechanism shall take place in less
than 80 ms. The operating mechanism shall be of the stored energy type
only. The addition of motor mechanism or a rotary handle shall in no way
affect circuit breaker characteristics and shall not block device settings.
E. The MCCB’s shall be designed for adding auxiliary contacts such as shunt
or undervoltage releases after installation at site. The auxiliaries shall be
separated from power circuits. It shall be possible to install auxiliary
switches for fault/status indication in already energized MCCB without
the need to trip the MCCB.
G. It shall be possible to assemble earth fault protection of MCCB’s by
adding a residual current device directly to the circuit breaker case and it
shall operate without an auxiliary power supply. The add on RCD’s shall