Each component of an electrical installation is designed for use at a particular rated current. When the component is used within the designed current limit it will have a specific service life expectancy, which is the length of time for which the insulation will remain operationally safe.
When a component is overloaded, the temperature in the insulation exceeds the design limit, the insulation begins to deteriorate and the service life will be reduced. Quantitatively, this deterioration depends on both the temperature rise and time for which the insulation is exposed to the overload. Precautions should be taken to avoid, or at least reduce to a minimum, overloading of electrical components. An overload condition can be detected by monitoring the current flowing into an item of equipment and the time for which it flows.
The method of overload sensing incorporated into circuit breakers is usually achieved through one of three different technologies:
* Solid-state electronic sensing.
* Thermal-magnetic sensing.
* Hydraulic-magnetic sensing.
Hydraulic-magnetic mechanism as current increases
Solid state electronic sensing
This technology is often combined with microprocessor controllers and is generally restricted to larger frame circuit breakers due to cost considerations.
Hydraulic-magnetic mechanism showing trip bar activated
Thermal sensing
This is the oldest technology and has been used since the first appearance of miniature and moulded case circuit breakers. Thermal sensing components such as bimetals, are supported by instantaneously operated magnetic trips for short circuit protection.
Hydraulic-magnetic mechanism remains latched after trip
Hydraulic-magnetic sensing
This technology is widely used in South Africa and eliminates the inconvenience of early tripping of thermally operated circuit breakers at elevated ambient temperatures. Hydraulic-magnetic circuit breakers have the advantage of more accurate calibration of tripping curves and make possible a variety of tripping curves to suit application-specific requirements including fractional ampere ratings.
Hydraulic-magnetic circuit breakers operate on the principle of the opposing forces of a spring and a viscous fluid controlling the magnetic attraction on a ferrous piston inside a non-magnetic cylinder. The design has both a time delay operation (overload trip) and an instantaneous operation in the case of a short circuit.
Hydraulic-magnetic mechanism at low current
When an overcurrent occurs, the magnetic force produced in the coil overcomes the core spring and the core moves towards the pole piece. The closer the core gets to the pole piece, the more magnetised the pole piece becomes. This attracts the armature, which in turn actuates the trip bar. The viscosity of the fluid and the characteristics of the spring govern the time delay. If the overcurrent is excessive, the magnetic field is such that the armature is immediately attracted to the pole piece without the influence of the core.
Four-digit multifunction instrument Vepac Electronics
Electrical Power & Protection
The OM 403 model series comprises four-digit panel programmable instruments designed for maximum usefulness and user comfort, while maintaining their fair price.
Read more...New function/arbitrary waveform generator Vepac Electronics
Electrical Power & Protection
Siglent has released the SDG1000X Plus series Function/Arbitrary Waveform Generator, providing engineers with a more flexible and efficient signal generation solution.
Read more...World’s fastest 14-bit arbitrary waveform generator Vepac Electronics
Electrical Power & Protection
The ARB Rider AWG-7000 is the world’s fastest 14-bit arbitrary waveform generator, with a 20 GS/s real time update rate and 14-bit vertical resolution. Thanks to Simple Rider software, it offers best-in-class performances and an easy to use interface.
Read more...Steam on demand
Electrical Power & Protection
Rental boilers are increasingly popular, offering the convenience of on-demand power and heat without the commitment of a permanent installation. They are an ideal solution while a customer awaits repairs, or only needs a boiler for a particular period, or does not have the capital to invest upfront.
Read more...High-voltage backup power solutions
Electrical Power & Protection
Businesses are facing increasing pressure to rethink their approach to power. As a result, there’s been a significant uptick in interest in high-voltage installations, either as standalone battery energy storage systems, or as solar installations feeding into battery storage.
Read more...Rotary friction welding – a first for Tutuka power station
Electrical Power & Protection
Steinmüller Africa has successfully used rotary friction welding in a project with engineering consultant, eNtsa. This innovative technique was applied to fabricating four headers used for high-pressure water heater boiler units installed at Tutuka power station in Mpumalanga.
Read more...Why it is vital to size soft starters carefully WEG Africa
Electrical Power & Protection
The application of soft starters has become a well-established strategy to improve the uptime and lifespan for motors and mechanical equipment driven by motors, but they need to be chosen carefully.
Read more...Changeover switch between two power supplies Schneider Electric South Africa
Electrical Power & Protection
Schneider Electric South Africa has unveiled the Resi9 Changeover Switch product range which provides the perfect solution to switch manually between two power supplies or circuits.
Read more...Partial discharge monitoring solution for MV switchgear Schneider Electric South Africa
Electrical Power & Protection
Schneider Electric South Africa has unveiled its PowerLogic PD100 Partial Discharge Monitoring Solution, designed to be installed on MV switchgear to detect anomalies early, to prevent unplanned downtime.
Read more...Boiler update at food processing plant transforms production process
Electrical Power & Protection
The replacement of 78 year-old boilers at a Western Cape food processing plant by Associated Energy Services has not only proven to be a worthwhile investment for the client, but has transformed its entire production process.
printer friendly version