How Do Hydraulic Magnetic Circuit Breakers work?

Hydraulic magnetic circuit breakers combine overload and short circuit protection features into a single mechanism. The operation is completely independent of temperature allowing them to be used in environments subjected to extreme temperatures, without fear of failure.


The main advantage of using a hydraulic magnetic circuit breaker is that its operation is independent of temperature and thus, can be used in environments where extreme or volatile temperatures are present. Hydraulic magnetic circuit breakers also allow for a large range of delay times in which the overload condition will trip the breaker.

General Operation:

The hydraulic magnetic circuit breaker combines both the overload and fault protection into a single mechanism rather than using separate thermal and magnetic mechanisms as with the thermal magnetic circuit breaker. The circuit breaker contains a single solenoid coil which is combined with a spring-loaded actuator that is housed inside a cylinder filled with a dampening fluid.  Figure 1 below shows the inner workings of a hydraulic magnetic circuit breaker.

Figure 1: Hydraulic Magnetic Circuit Breaker [1]

A magnetic field is produced by the current flowing through the breaker. The magnetic field strength varies depending on the amount of current flowing through the solenoid. At normal operating currents the magnetic field will not be strong enough to cause the switch trip [2].

Normal Overload Operation:

When an overload condition arises, i.e. the current drawn through the breaker is greater than the rated current, the magnetic flux within the solenoid produces a magnetic field that is strong enough to move the core to the pole position. The hydraulic fluid within the cylinder dampens this movement thereby creating a time delay in the breaking of the current. This time delay is important provided it is short enough to not damage the devices connected to the breaker. This time delay allows for a moment of inrush current that many devices, such as motors and lights, require on start-up without tripping the breaker.  However, if the overload condition persists the core piece will reach the pole position which significantly drops the reluctance of the magnetic circuit causing the armature to be attracted to the pole face with sufficient force to collapse the latch mechanism which in turn trips the breaker [3].

[Photo Taken at Switchboard Manufacturers Johannesburg]

Short Circuit Operation:

When a short occurs in the system, a very large amount of current is drawn through the solenoid which creates a strong magnetic field. The strong magnetic field attracts the armature to the pole face even without the core having moved. This is known as the instantaneous trip region of the circuit breaker. Unlike thermal circuit breakers, the trip point is unaffected by the ambient temperature and thus the hydraulic magnetic circuit breaker can immediately be reset without requiring a cool down period [3].


In conclusion, hydraulic magnetic circuit breakers use a single solenoid coil which is combined with a spring-loaded actuator housed inside a cylinder that is filled with a dampening fluid to achieve the overload and short circuit fault protection. These circuit breakers make use of varying strength magnetic fields to break the circuit when a fault occurs. They are especially suited for environments where the temperatures vary as their operation is independent of temperature.


[1] (2017). Energy Efficiency | CBI-electric (Circuit Breaker Industries). [online] Available at: [Accessed 14 Dec. 2017].

[2] Chan, L. (2017). Hydraulic Magnetic Circuit Breaker Theory. [online] Available at: [Accessed 14 Dec. 2017].

[3] (2017). Hydraulic Magnetic Principles | CBI-electric (Circuit Breaker Industries). [online] Available at: [Accessed 14 Dec. 2017].

Author: Brendon Swanepoel

2nd Year Electrical Engineering Student, University of the Witwatersrand

Brendon is completing Switchboard Group’s 6 week Learnership and Training program offered to students looking to further their practical skills.

Switchboard Manufacturers, Empowering South Africa’s youth.