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Overload Protection and Sensing

Every component making up an electrical installation is designed for use at a particular rated current. When it is used within these limits 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 limit beyond which the insulation begins to deteriorate and the service life will be shortened. 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. It is possible to detect an overload condition by monitoring the current flowing into an item of equipment and the time for which it flows.

Thermal sensing

Thermal sensing is the oldest technology that 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 sensing

Hydraulic-magnetic sensing eliminates the inconvenience of early tripping of thermally operated circuit breakers at elevated ambient temperatures. Hydraulic-magnetic circuit breakers also offer the advantages of more accurate calibration of trippingcurves, together with the ease of achieving a variety of tripping curves to suit specific application requirements including fractional ampere ratings.

Hydraulic-magnetic circuit beakers 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 nonmagnetic cylinder. The design has both a time delay operation (overload trip) and an instantaneous operation in the case of a short circuit.

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.

Solid state electronic sensing

Solid State electronic sensing is often combined with microprocessor controllers, and is generally restricted to larger frame circuit breakers mainly due to cost considerations.


While the purpose of the fuses and circuit breakers is the same, there are fundamental differences, which are important to understand when making selections for protection of equipment and people.

  • Fuses must be replaced after it has interrupted an overcurrent event.
  • Has a lower initial cost and no maintenance
  • Only open on overcurrent events
  • No capability for optional protective features

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