Circuit Breaker Tripping Units.


Draw-Out Circuit Breaker


A trip unit is the part of a circuit breaker that automatically opens the contacts of the circuit breaker in the event of a short circuit, ground fault, or thermal overload.

There are two types of trip units; the thermal-magnetic trip unit, and the electronic trip unit.

A thermal-magnetic trip unit has moving parts. It consists of a bimetallic thermal device that opens the circuit breaker with a delay depending on the magnitude of overcurrent. It also consists of a system where magnetic forces proportional to the current flowing through the circuit breaker are used to actuate the breaker. Simply put, in the event of a short circuit, high current flows and a strong magnetic field is created. This magnetic field is used to actuate the trip mechanism.

 An electronic trip unit is a programmable device that measures and times the current flowing through a circuit breaker and initiates a trip signal when appropriate.

There are 6 common current protection adjustments (adjustable trip settings) usually found on the front panel of a trip unit.  These settings are usually adjusted for proper co-ordination of cascaded circuit breakers.

It is important to note that not all types of circuit breakers come with an adjustable trip unit but most draw-out type circuit breakers racked directly into the bus bar of low-voltage switchgears come with a trip unit.

Without any more delay let us highlight the adjustable trip settings on a Trip Unit and what they mean.
Trip Unit - Static Trip II 
1)     Continuous Amps (Ir) – This setting is used to change the level of current the circuit breaker would carry without tripping. Ideally, it is used to derate the breaker as it is usually a percentage of In where In is the normal rating of the breaker.

For example, a circuit breaker is rated at 800A(In) if the continuous amps dial (Ir) is tuned to 80 this means that the breaker has now been de-rated to 80% of In. The breaker is now rated at 640A and all other overcurrent settings would be dependent on this one.
The continuous Amps (Ir) setting is also known as the Long Time Pickup or Long Time band

2)       Long Time Delay – Turning this dial allows the circuit breaker to carry overcurrent excess of the Long Time pickup setting for a period of time before the trip unit begins to initiate a trip action. This is typically set to allow for inrush currents without initiating a trip. A good example would be those encountered while starting a motor.

It is important to note that the delay is inversely proportional to the square of the magnitude of the overcurrent condition. This means that higher overcurrent conditions have shorter delays and, conversely, lower overcurrent conditions have longer delays.
The long time delay allows the circuit breaker to carry overcurrent at six times the Long Time Pickup (Ir) setting.

3)       Short-Time Pickup – tuning this dial sets the amount/ the magnitude of high current the breaker is allowed to carry for a short period of time. This time is to allow downstream protective devices to clear short-circuits without tripping the upstream circuit breaker.
The short-time pickup setting is a multiple of the long-time pickup setting. This means that for our 800A breaker example if we set the continuous rating (Ir) at 640A and short-time pickup at 3, the short-time pickup current level would actually be 640 times 3, not 800 times 3.

4)       Short-Time Delay – This setting is used in conjunction with short-time pickup to postpone tripping for some seconds when a current in excess of the short-time pickup current level has been detected.

5)       Instantaneous Pickup – This setting is used to set the level of high current that the trip unit will trip the circuit breaker with no intentional delay. It is normally used in response to severe overcurrent conditions.  It could be set at any current between 2 and 40 times of the long-time pickup setting (Ir). The trip unit use above the maximum setting available is 12x the long-time pickup.

6)       Ground Fault Pickup – this setting selects the level of ground-fault current that will flow before the trip unit trips the circuit breaker. The ground fault pickup has a ground fault delay function to allow for proper coordination between downstream and upstream devices.

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