Coastal areas in Vietnam and Bangladesh are hard on disconnect switches. Salt spray, long rainy seasons, and typhoons mean you can't just follow inland installation practices. Many of these lines run along rural dirt roads with limited maintenance access, so whatever goes up on the pole has to last for years without much attention. Below are installation notes for the most common models.
GW9-12/630 Pole-Mounted Disconnect Switch This is the most common type on the high-voltage side of distribution transformers, usually mounted on the same pole with drop-out fuses and surge arresters. Before installation, check the base hole spacing against the crossarm holes. If they don't line up, drill new holes in the crossarm - don't force it. Make sure you get the incoming and outgoing terminals right: the line side goes toward the fuse, the load side toward the transformer. Reversing them makes operation and maintenance a headache later.
Fasten the base with hot-dip galvanized bolts, flat washers, and spring washers. After tightening, dab a bit of anti-loosening paint on the nut. The contact is a blade type. When closed, the blade must fully enter the stationary contact, not just catch half of it. Pull the blade by hand - it shouldn't be loose, but also shouldn't be so tight you can barely move it. Contact pressure comes from the spring clips on both sides of the stationary contact. If the clips are deformed or weak, take them off and fix or replace them before you put the switch on the pole.
Leave enough clearance between the disconnect switch and the arrester leads. The arrester usually sits below the switch. The leads should be as short as possible, but they can't interfere with the operating rod or crossarm braces. Don't bend the leads too sharply, or wind vibration will eventually break strands. In coastal areas, use copper-aluminum transition clamps and apply conductive grease to the contact surfaces to prevent galvanic corrosion.
GW4-40.5/1250 Outdoor Disconnect Switch This one is used on 35 kV lines or simple substations. The base rotates horizontally, so all three bases must be on the same level. The elevation error of the embedded parts should not exceed 5 mm, otherwise the three phases won't close at the same time. After fixing the bases, close the switch by hand a couple of times and check whether the three moving contacts enter the stationary contacts together. Adjust the synchronism with the adjusting nuts on the operating linkage - don't pry the contacts.
The contacts are copper with silver plating. Don't touch them with bare hands during installation; sweat accelerates oxidation. If you find black oxidation on the contact surface when unpacking, lightly sand it off with fine sandpaper and wipe with alcohol. After closing, the contact marks should be even. Check with a 0.05 mm feeler gauge; it shouldn't go in more than a quarter of the contact face width.
The mechanism box mounts on the side of the pole. The operating handle height depends on local maintenance habits, usually 1.2 to 1.5 meters above ground. The door seal must be intact; after closing the door, no water should get through the gap. Don't block the drain hole at the bottom of the box, or water will collect during the rainy season and rust the internal transmission parts. If an earthing switch is included, test the mechanical interlock between the main switch and the earthing switch repeatedly. When the main switch is closed, the earthing switch cannot close; when the earthing switch is closed, the main switch cannot close. Getting this interlock wrong is a serious mistake.
GW5-40.5/1250 Outdoor V-Type Disconnect Switch This type is often used in substation bays. It has a V-shaped double-post structure, so the relative positions of the two post insulator bases must be accurate. Check the center distance of the bases against the drawings; if it's off, the switch won't close properly. When lifting the post insulators, use soft slings. Don't let steel wire rope bite directly into the porcelain; chips in the glaze will cause problems sooner or later.
After adjusting the closed position, connect the drive rod to the operating mechanism. For motor-operated mechanisms, cut the control power first and crank manually to the middle position before testing with the motor, to prevent the motor from stalling and burning the coil. Set the limit switches properly so the switch stops accurately at both fully closed and fully open positions; it shouldn't rely on hitting the stops. After operation, the auxiliary switch signals must match the actual position; remote control point-to-point checks are done here.
In coastal substations, this device needs extra protection against salt spray. If the galvanizing on the base or linkage is scratched during transport, touch it up with zinc-rich paint on site. If the scratched area is large, have it re-galvanized. Use molybdenum disulfide grease on the bearing parts; ordinary grease fails quickly in a salt-spray environment. Wipe the insulator surfaces with a dry cloth before energization, so salt deposits and dust don't form a conductive layer in humid weather.
General Installation Notes No matter which model, grounding must not be neglected. The switch base must be connected to the grounding grid with copper braid or galvanized flat steel. Clean the contact surfaces to bare metal, tighten the bolts, and then measure the grounding resistance. In coastal areas, grounding resistance can be low because of high soil salinity, but that doesn't mean you can skip the measurement. Sometimes the ground electrode rusts through and the resistance shoots up just as easily.
After installation, operate the switch manually five times and electrically five times. Record the operating time and synchronism. When installed together with surge arresters and drop-out fuses, do an overall check of the lead clearances. Leave enough margin for wind swing. In typhoon-prone areas, add vibration dampers or support insulators to the leads to prevent long-term swinging from wearing through the conductor.
Finally, put together the installation records and adjustment data. Note down the three-phase synchronism deviation, contact resistance, and grounding resistance values. Give a copy to the owner. Many projects in Vietnam and Bangladesh are in remote places; complete documentation will save the maintenance crew a lot of trouble later. If the installation of these disconnect switch models is done carefully, they won't need much attention during operation.