Hydraulic Magnetic Principles

Hydraulic Magnetic Circuit Breakers operate on the magnetic force produced by the load current flowing through a series connected solenoid coil which is wound around a hermetically sealed tube containing an iron core, a spring and a dampening fluid.

Normal Overload Operation

At currents below the circuit breaker rating, the magnetic flux in the solenoid is insufficient to attract the core towards the pole piece due to the spring pressure.

Where an overload occurs, i.e currents above the circuit breaker rating, the magnetic flux in the solenoid produces sufficient pull on the core to commence its movement toward the pole piece. During this movement, the hydraulic fluid regulates the core's speed of travel, thereby creating a controlled time delay, which is inversely proportional to the magnitude of the current. This time delay is useful in that if the overload is of short duration, i.e. start up of motors etc., the core returns to its rest position once the overload disappears.

If the overload persists, the core reaches the pole piece after a time delay particular to that current and in so doing, the reluctance of the magnetic circuit drops considerably, so that the armature is attracted to the pole face with sufficient force to collapse the latch mechanism (toggle) and consequently trip the breaker.

The contacts separate, current ceases to flow, and the core returns to its rest position.

Short circuit operation

With high values of overloads or short circuit, the magnetic flux produced by the coil is sufficient to attract the armature to the pole face and trip the breaker even though the core has not moved. This is called the instantaneous trip region of the circuit breaker characteristic.

Unlike thermal circuit breakers, the hydraulic magnetic circuit breakers trip point is unaffected by ambient temperature. After tripping, the breaker may be reclosed immediately since there is no cooling down time necessary.

By the nature of the principle of operation, it is possible to obtain any variation of time/current characteristic.


  • Always holds 100% rated current independent of ambient temperature
  • Always trips at 125% or 130% of rated current depending on the product specification, independent of ambient temperature
  • Immediate resetting after trip
  • Any current rating possible
  • Large range of time delays possible