Don't get tripped up by trip curves

Trip Curves, also known as Time Current Curves are quite simply a graphical representation of the expected behaviour of a circuit protection device. These protection devices come in various forms, such as fuses, miniature circuit breakers, moulded case circuit breakers, supplementary protectors, motor protection circuit breakers, electronic fuses, overload relays and air circuit breakers.

Why?

The purpose of a circuit breaker is to protect wires and electrical equipment from damage should there be an electrical overload, ground fault or short circuit. In order to help prevent fire, injury or equipment damage, circuit protection is designed to cut off power to the circuit in the event of sudden power surges, overloaded power outlets, lightning or other events that could otherwise result in a dangerous situation for people and/or equipment.

MCB's

MCB’s are used nowadays in low voltage electrical networks rather than fuses, as MCB’s are much more sensitive to overcurrent than a fuse. MCB’s are also electrically safer than fuses and allow for quicker restoration of power supply. When a continuous overcurrent flows through an MCB, the internal bimetallic strip is heated and bends. This deflection releases a mechanical latch which causes the MCB contacts to open, turning the MCB off and therefore stopping the current flow in the circuit. During short circuit, the sudden dramatic rise of electric current causes electro-mechanical displacement of the plunger from the tripping coil or solenoid. The plunger strikes the trip lever causing the immediate release of catch mechanism, opens the contacts and breaks the circuit. It is important to note that MCBs do not protect humans against electrical shock caused by “earth leakage” – this service is provided by RCD’s and RCBO’s.

Tripping Curves

The “tripping curve” of an MCB allows for real world (and often entirely necessary) surges in power. For example in industrial environments large machines often require an initial surge of power in excess of their normal running current to overcome the inertia of large motors. The brief surge, lasting just a few seconds, is allowed by the MCB as it is safe to do so for a very short period of time. There are three principle Curve Types which allow for the surges in different electrical environments:

1. Thermal Trip Curve. This is the trip curve for the bi-metallic strip, which is designed for slower over-currents to allow for in rush as described above.

2. Magnetic Trip Curve. This is the trip curve for the coil or solenoid, designed to react quickly to large overcurrents, such as a short circuit condition.

3. The Ideal Trip Curve. This curve shows what the desired trip curve for the bi-metallic strip is. Because of the organic nature of the bi-metallic strip, and changing ambient conditions, it is difficult to precisely predict the exact tripping point.

The top of the chart shows the thermal trip curve for the bi-metallic strip. It tells us that at 1.5X the rated current the quickest the circuit breaker will trip is forty seconds (1). Forty seconds at 2X the rated current is the slowest the circuit breaker will trip (2). The bottom of the chart is for the magnetic trip of the coil/solenoid; 0.02 to 2.5 seconds at 3X the rated current is the soonest the circuit breaker will trip (3). The same duration, 0.02 to 2.5 seconds, at 5X the rated current, is the longest it will take the circuit breaker to trip (4). The area shaded in between is the Tripping Zone.

Important!

Trip curves represent the predicted behaviour of a circuit breaker in a cold state at ambient room temperature. A cold state is when the bimetallic strip is within the specified ambient operating temperature for the breaker. If the breaker has experienced a recent thermal trip, and has not cooled down to the ambient temperature, it may trip sooner. Also important to note is the difference between AC and DC MCB's - as general purpose AC type MCB's will not work on DC circuits, so special attention must be made to choose the correct protection.