Switch Disconnector / Fused Switch

What sets a switch disconnector apart

A switch disconnector does two things: it makes and breaks load current manually, and it provides a visible or positively indicated isolating gap so that work can be done safely downstream. A fused switch disconnector combines that function with fuse links, so the fuses give the short-circuit protection and the switch gives the isolation. The difference from a circuit breaker matters on board: a breaker detects an overload or fault and trips automatically, whereas a switch disconnector only operates when someone moves the handle. Many a switchboard therefore uses fused switches for motor feeders and distribution outgoers where fuses are the chosen protection, and breakers where automatic and resettable tripping is wanted.

Main designs

• Switch disconnectors without fuses - plain isolating and load-breaking switches, often used as incomers or section isolators
• Fused switch disconnectors - the switch carries fuse links in the moving part or in fixed fuse holders, commonly NH fuse links in large sizes
• Fuse switch disconnectors for busbar mounting - the unit sits directly on a busbar system, saving space in distribution boards
• Rotary and handle-operated types - front handles with positions for on, off and sometimes earth, with padlock provisions for lock-out
• Three-pole and four-pole versions, with the fourth pole switching the neutral

Main components

• Switching mechanism with spring-assisted contacts so that make and break speed does not depend on how fast the operator moves the handle
• Main contacts and arc chambers rated for the load they must break
• Fuse holders or fuse carriers, with the fuse size and type fixed by the housing
• Position indication that follows the contacts, not just the handle position
• Padlock facility and cover interlocks, so the enclosure cannot be opened while the switch is on

Selection and sizing

• Rated operational current and voltage for the circuit, with derating when the switch is mounted in a warm or poorly ventilated enclosure
• Utilisation category under IEC 60947-3: the category tells whether the switch may be used for resistive loads or for switching motors
• Rated short-time withstand current, and for fused units the conditional short-circuit current with the fuse fitted
• Fuse size, characteristic and rated current, chosen to protect the cable and coordinate with the upstream device
• Pole number, mounting type and terminal arrangement, so the unit fits the board layout and the cable cross-section
• Degree of protection if the switch is mounted in its own enclosure, see IEC 60529

Regulations and class

The product standard is IEC 60947-3 for switches, disconnectors, switch disconnectors and fuse combination units, while fuse links follow IEC 60269. Electrical installations on ships fall under the IEC 60092 series and the class rules, and switchgear in essential-service circuits is normally accepted on a type approval from the classification society. The approval is specific to a product range and rating, so a switch with the same housing but a different rating needs its own paperwork.

Typical faults

• Overheating at terminals or fuse contacts from loose connections or oxidised surfaces, often visible as discoloured plastic
• Worn or pitted main contacts after many load operations or after switching a fault
• Broken spring mechanism or damaged handle shaft, so the contacts do not follow the handle
• Single-phasing of a motor feeder after one of three fuses blows, leaving the motor running on two phases
• Wrong fuse fitted at a previous service, with the wrong rating or breaking capacity
• Damaged interlock or padlock hardware, which defeats the lock-out arrangement

What to look for in a supplier

• Type-approval certificate that covers the exact range, rating and fuse size ordered
• Utilisation category stated clearly, not just the current rating
• Spare fuse carriers, contacts and handle kits for the same series
• Terminal and busbar adapter options that match the existing board
• Drawings and mounting data, so a replacement can be fitted without modifying the panel

After a fuse blows on a three-phase feeder, replace all three fuses, not just the one that failed. Mixed aged and new fuses will not share a fault current as the protection study assumed.