The breaker and assembly shall be suitable for continuous outdoor operation in tropical areas at altitudes of up to 2200m above sea level, humidities of up to 90%, average ambient temperature of +30ºC with a minimum of -1ºC and a maximum of +40ºC and saline conditions along the coast. The assembly shall be exposed to direct sunlight.
LV CIRCUIT BREAKER.
The circuit breaker shall be mounted inside an enclosure conforming to IP-54.The enclosure will form an integral part of the circuit breaker assembly.
The circuit breaker shall be 3phase 4 wire molded case type with air as the arc quenching medium.
The circuit breaker shall be capable of being operated (RESET or ARM /ON/OFF) from the ground level by means of an operator’s rod (link sticks). Facilities to insert the operator’s rod shall be provided on the housing.Mechanical indications for ON (red) and OFF(green) shall be connected to the housing so as to be seen from the ground level.
The main contacts shall be separate from the arc-breaking contacts.
The main contacts shall be made of electrolytic copper and shall be silver faced.
The arcing contacts shall be non-welding and conductive.
The terminals shall be screw type and capable of accommodating the rated cable size.
The circuit breaker shall have three main elements viz., (a) thermal overload protection, (b) short circuit current sensing (c) latching and tripping.
The circuit breaker shall have a label indicating the size of the applicable transformer.
Thermal overload protection using transformer thermal replica
Each phase of the breaker shall be fitted with a thermal overload protection.
The thermal overload sensing device shall be coordinated thermally with the transformer rating to follow as close as possible the variations of winding temperature due to load fluctuations and ambient temperatures.
The manufacturer shall provide the formula for the oil-immersed transformer thermal model used and time-current curves in the protection for each size of the transformer.
The following information could be used
> The top oil temperature rise for the transformers is 55°C
>The type of oil used is mineral oil (for determination of oil time constant )
A signal light (on each phase), controlled by the thermal overload relay shall come on when the loading reaches 75% indicating that the transformer is almost fully-loaded and up-rating or load balancing exercise shall have to be scheduled without causing an unplanned service interruptions.
The thermal overload relay shall trip out the breaker when the overload reaches 90% of the transformer full rating.
The signal lights shall be positioned and wired on the housing so as to be visible from the ground level.
Short circuit sensing device
A short circuit sensing element shall provide instantaneous tripping of the circuit breaker under high fault current conditions.
The short-circuit element shall have a mechanically reset indication/flag (marked S/C trip) when operated.
The indication shall be connected on the housing so as to be seen from ground level.
Mechanical.Tripping and latching
The circuit breaker shall be opened via the protection devices (thermal and shortcircuit) and/or manually via the operator’s rod (link stick).
The circuit breaker shall be trip free.
Closing of the breaker after opening shall be achieved by resetting or arming the breaker first.
HOUSING OF CIRCUIT BREAKER
The circuit breaker housing shall be outdoor type conforming to IP-54.
The housing shall be made of any of the following:
1 hot dip galvanized steel finished in epoxy
2 or fiberglass
3 or fiber glass/steel combination
INSTRUCTIONS, MARKING AND PACKING.
Instructions for storage, handling and installation of the breaker and housing shall be submitted in the English Language.
All breakers and housing shall be clearly and indelibly marked by the manufacturer to indicate the following:
a) the manufacturer’s identification mark and reference/catalogue number
b) the breaker ratings
c) The circuit breaker shall have a label indicating the size of the applicable transformer.
d) The circuit breaker housing shall have a label indicating the size of the applicable transformers.
e) The manufacturer’s identification mark and a part number on all components forming part of an accessory. This part number shall be referenced in the bill of materials. Components that are physically impossible to mark shall be individually packed and the packaging shall be marked.
f) The expiry date (visible on the packaging of all components or consumables that are subjected to a shelf life limitation). These components or consumables shall be individually packed.