Basic Information on Circuit Breaker Maintenance

Circuit Breaker Tester

Billions of circuit breakers are installed in residential and non-residential buildings, or are used for industrial applications. These automatic electrical switches make it possible to switch currents on and off during the normal operation of the grid, and to automatically switch the current off if it exceeds the permissible value for a certain period of time.

In order to allow for typical appliance-inrush-currents, the circuit breaker must remain stable during a minor short-term overload. However, if the overload persists, the circuit breaker must open the circuit after a certain period of time. During a short-circuit, a circuit breaker must interrupt the current safely in order to minimize damage.

Design and operating principles of a circuit breaker

A circuit breaker has a rather sophisticated design, consisting of two separate tripping mechanisms for overloads and short circuits. Circuit breaker tripping characteristics show a thermal trigger for overloads, and a magnetic trigger for short-circuits.

Inside of a circuit breaker, the path the current is taking across the closed contacts is visible, as are the tripping components. The higher the amplitude of the overcurrent, the faster a circuit breaker must trip.

This is achieved with a metallic element consisting of two different metals which are heated up by the current flow. The two metals are expanding at different rates while they are heating up. An overcurrent that is present long enough will eventually activate the breaking element. This mechanism is tripped when the current exceeds its rated value by a relatively low figure of 1.45 – 2 times. At the same time, the tripping time, which depends on the current value (the higher the current, the faster the heating time for the metallic element), can reach up to 2 hours.

A circuit breaker must open a circuit instantaneously when it encounters a short circuit at several times the nominal current. A solenoid and a piston are the elements used for instantaneous tripping. A short circuit creates an instant magnetic field that moves the piston and operates the breaker.

Another important point is that interrupting high fault currents can result in heavy arcing. Due to magnetic forces, the arc moves through a stack of metal plates, where it is cooled, split, and finally extinguished at the next zero crossing.

Testing circuit breakers

The quality of the materials used by the manufacturers differ widely.
Therefore, it is a good idea for users to test their circuit breakers and not simply rely on them.

During testing, the circuit breaker is disconnected from the circuit and connected to a primary injection test set, which, in essence, generates different currents in order to simulate load conditions and fault currents.

Circuit Breaker Tester UPA-16

In thermal-magnetic circuit breakers both the electromagnetic and thermal tripping elements need to be tested. The testing current is selected based on the technical parameters of the given circuit breaker provided in the respective documentation. These parameters stipulate at what current load and in what time the circuit breaker should trip.

As for the testing sequence, the short circuit tripping element is tested first. Then, the overload tripping element is tested, followed with a stability test with nominal current.
If either of the tripping elements fails to trip the circuit breaker in the set amount of time, the circuit breaker under test is considered to be out of order and must not be operated any longer.

Circuit breaker testers UPA-series by KEP

Circuit breaker testers UPA are intended for automatic testing of circuit breakers with alternating current. The devices allow measuring the values of the supplied current and timing the circuit breaker tripping time. The UPA operates on the principle of varying the power in the primary circuit of the matching power transformer and, respectively, varying the output current flowing through the circuit breaker under test.
Power adjustment can be done either through an external voltage source or through a built-in thyristor controller.

Alexei Tiatiushkin
Marketing manager
KharkovEnergoPribor Ltd.
marketing@keppowertesting.uk
http://www.kep.ua

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