6 adjustable tripping settings of a circuit breaker you must understand

7/26/2019 6 Adjustable Tripping Settings of a Circuit Breaker You MUST Understand

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6 Adjustable Tripping Settings of a Circuit Breaker You MUST

Understand

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6 Adjustable Tripping Settingsof a Circuit BreakerYou MUST Know

Basic tripping settingsof CB

There are (at least) six basic adjustable tripping settings(functions)you must understand in order to fully

understand how circuit breakeractually works. All these adjustable functions actually shape the time-current curveof

circuit breakerand allows proper tripping according to the network parameters and also the proper coordination

between upstream and downstream devices.

Note that modern circuit breakers (MCCB, ACB) mostly have an electronic tripping unitwhich is much more

advanced comparing to these explained here, but the basics are the same, very same.

So, lets start with explanation!

1. Continuous Amps (Ir)

2. Long-Time Delay

3. Short-Time Pickup

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4. Short-Time Delay

5. Instantaneous Pickup

6. Ground Fault Pickup

Circuit breakeradjustable tripping functions

The below time-current curvereflects one possible setup for a 1200 ampere circuit breakerwith a nominal

(maximum continuous ampere) rating of 1000 amps. This time-current curvewill be the basis for

discussing adjustable tripping settingsof the circuit breakers.

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Time-current curveof a circuit breaker

1. Continuous Amps (Ir)

Continuous Amps (Ir) varies the level of current the circuit breaker will carry without tripping. Ir is a percentage

of the circuit breakers nominal rating (In). Continuous amps can be adjusted from 20 to 100 percentof the circuit

breakers nominal rating.

For example, a 1000 amp breaker can be changed from 1000 amps to 800 amps by adjusting the

breaker continuous amps setting to 80%.

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Continuous current [Amps]

Go back to CB tripping settings

2. Long-Time Delay

Long-time delaycauses the breaker to wait a certain amount of time to allow temporary inrush currents, such as

those encountered when starting a motor, to pass without tripping.

The adjustment is from 2.2 to 27 seconds at six times the continuous amps (Ir) setting.

As shown below, the long-time delayeffects the position of an I2T slope. This means that lower levels of current will

allow the breaker to remain online for longer periods of time.

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Long time delay tripping settings


3. Short-Time Pickup

Short-time pickupis used for selective tripping.The short-time pickupfunction determines the amount of current

the breaker will carry for a short period of time, allowing downstream protective devices to clear short-circuits withouttripping the upstream device.

Short-time pickupis adjustable from 1.5 to 10 times the trip unit ampere setting (Ir) .

For example, a 1000 ampere frame can be adjusted to trip anywhere from 1500 to 10,000 amps. The switch also has

an OFF position to eliminate short-time pickupand short-time delay.

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Short-time pickupused for selective tripping


4. Short-Time Delay

Short-time delay, used in conjunction with short-time pickup, controls the time involved in postponing a short-time

pickuptrip.

There are two modes: fixed time, or I 2T ramp.Fixed time is adjustable from .05 to .5 seconds.The I2T ramp

mode is adjustable from .18 seconds to .45 seconds, providing a short inverse time ramp.

This allows better coordination with downstream thermal-magnetic circuit breakers and fuses. A fixed instantaneous

trip point of 10,000 amps trips the breaker automatically and overrides any pre-programmed instructions.

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Short-time delay, used in conjunction with short-time pickup


5. Instantaneous Pickup

Instantaneous pickupis used to trip the circuit breakerwith no intentional delayat any current between 2 and 40

times the breakers continuous ampere setting (Ir).

In this example instantaneous pickuphas been set to 10 times the continuous amp setting , or 10,000 amps (10 x

1000) with a continuous amp setting of 1000 amps. In this case a higher setting would still trip at 10,000 amps due to

a fixed instantaneous override of 10,000 amps which automatically trips the breaker regardless of the instantaneous

pickupsetting.

If the continuous amp setting had been 300 amps, setting the instantaneous pickupat 10 would make

the instantaneous setting equal to 3000 amps, well below the fixed instantaneous override.

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Instantaneous pickup Used to trip the circuit breakerwith no intentional delay at any current between 2 and40 times the breakers continuous ampere setting (Ir)


6. Ground Fault Pickup

Ground fault pickupcontrols the amount of ground fault currentthat will cause the breaker to interrupt the circuit. The

adjustment can be set from 20 to 70% of the maximum breaker rating. In compliance with NEC 230-95 (A), no trip

point setting exceeds 1200 amps.

Theground fault pickupis divided into three sections; .1s, .2s, and .4s.This feature adds a

time delay of .1, .2, or .4 seconds to the breakers trip when a ground fault occurs.

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Ground fault pickupcontrols the amount of ground fault current that will cause the breaker to interrupt thecircuit

Circuit breaker coordination

The ground fault pickuptime delay feature is useful for circuit breaker coordination. In the following scheme,

upstream breaker A has been set to .2sand downstream breakers B and C have been set to .1s.

A ground fault occurring in the circuit supplied by B will trip the B breaker without disturbing A or C .


Reference //STEP 2000 Molded Case Circuit Breakers (MCCB) by SIEMENS

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The ground fault pickuptime delay feature is useful for circuit

breakercoordination

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6 adjustable tripping settings of a circuit breaker you must understand

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