Handbook of Electrical Installation Practice - BeKnowledge

Switchgear 371
The other recent development in arc extinction technique uses the thermal energy
of the arc to create an increase in gas pressure. Typically, the contacts part in a
chamber of which the only opening is blocked by the moving contact at the early
part of the opening travel. Thus the pressure inside the chamber rises until the tip
of the moving contact passes through the opening and the gas flow is released. A
strong deionising action is created along the arc path, causing interruption at the
next current zero.An arcing contact of small diameter is better for this type of interruption,
so the normal current carrying is via a set of parallel main contacts.
This type of circuit-breaker is usually limited to voltages up to about 15kV and
breaking currents up to 20kA.
In addition to the virtual elimination of fire risk in the new technologies of
vacuum and SF 6 circuit-breakers, these devices are sealed-for-life equipments, where
it is not expected to make repairs or to replace the contact system during a normal
service life. This makes the effectiveness of the sealing almost the most important
aspect of performance. The vacuum interrupter is usually sealed by a brazing or
welding technique, the most difficult part being the seal between the metal parts
and the insulating enclosure. Many years of experience show that this is very
reliable.
One of the original ‘advantages’ claimed for the SF6 circuit-breaker was the
facility that a pressurised device gave for the checking of its contents for leakage.
Today, the emphasis on a sealed-for-life enclosure leads to efforts to omit pressure
measuring devices so as to reduce possible leakage sites.
Switches
In many distribution systems there exist situations where a simpler and cheaper
switching device than the circuit-breaker would be adequate for control purposes.
The switch does everything that the circuit-breaker can do except interrupt fault
currents. The breaking capacity is limiting to rated load current at a power factor
of 0.7.
Until recent years switches used oil or air as the insulating medium. In the
case of the oil switch, the oil also served as the interrupting medium, but with
air insulated switches some help was required for the interruption of the load
currents. Various techniques were used from air ‘puffers’ to gas producing plastics
materials.
Now, this field has also been changed through the introduction of SF6 gas and
vacuum. On the whole, the vacuum interrupter is too expensive for use when the
objective is a cheap device, and at present there are very few vacuum switches on
the market. On the other hand SF6 lends itself to very simple arc control devices
when limited to load currents, and a large number of manufacturers have introduced
this type of equipment.
In application, the protection of the circuit controlled by the switch relies on some
other fault interrupting device, usually either an upstream circuit-breaker or a highvoltage
fuse. It is common to incorporate high-voltage fuses with the load-break
switch to give a protective equipment particularly suitable for connecting distribution
transformers to the network. Many thousands of such fuse-switches are
installed and working very well in this application. In the UK these were all oil