the basics of circuit breaker tripping units _ eep.pdf

7/25/2019 The Basics Of Circuit Breaker Tripping Units _ EEP.pdf

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5/23/2016 The Basics Of Circuit Breaker Tripping Units | EEP

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The Basics Of Circuit Breaker Tripping Units

The Ba sics Of Circuit Breaker Tripping Units (on photo: Siemens molded case circuit breaker 'Sentron'

Series 400 Amp Frame - 400 Amp Trip)

Circuit breaker releases / Tripping units

The main objective of circuit breaker tripping units and protective functions in general is to detect faults and to selectively isolate faulted parts of the system. It

must also permit short clearance times to limit the fault power and the effect of arcing faults.

The protective function of the circuit breaker in the power distribut ion system is determined by t he s election of the appropriate release (see Figure 1). Releases can

be divided into:

Thermal-magneti c t ripping units TMTU, also called electromechanical releases and

Electronic tripping units ETU

Figure 1 Variants of circuit breaker tripping curves

Thermal-magnetic tripping units //

The thermomagnet ic trip unit consists of two parts:

The thermal trip unit Made up b y a bimetal thermal device which actuates the opening of a circuit breaker with a delay depending on the overcurrent value. This

trip unit is intended for the protection against overloads.

The magnetic trip unit Made up by a n electromagnetic device, with fixed (fixed instantaneous trip) or adjustable (adjustable instantaneous trip) threshold, which

actuates the instantaneous trip of the circuit breaker on a pre-determined overcurrent value (multiple of the In) with a constant trip time (about some tens of

milliseconds). This trip unit is intended for the protection against short circuit.

Figure 2 TM thermal-magnetic and MA magnetic tripping units

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Electronic tripping units //

The electronic trip units instead use a microprocessor to process the current signaland operate the circuit breaker opening in case of fault. In addition to this,

electronic tripping units offer more tripping criteria which are not feasible with electromechanical releases.

By digital processing of the signal, they provide the following protection functions:

1. Long time-delay trip function (ANSI code: 51, AC time overcurrent relay)

2. Short time-delay trip function (ANSI code: 51, AC time overcurrent relay)

3. Instantaneous trip function (ANSI code: 50, instantaneous overcurrent relay)

4. Ground-fault trip function (ANSI code: 51 N, AC time earth fault overcurrent relay).

Figure 3 Microprocessor tripping unit (Overloads: Long time protection (Ir) Short-circuits: Short-time protection (Isd) Short-circuits: Instantaneous protection (Ii), Additional ground fault

protection (Ig) and Neutral protection)

1. Overload protection

Designation: L (LT: long-time delay) , previously a-release.

Depending on the type of release, inverse-time-delay overload releases are also available with optional characteristic curves.

This adjustab le function simulates the effect of a bimetal conductor in a t hermal-magneti c c ircuit breaker. It reacts to overload conditions and determines how much

current the circuit breaker will carry continuously.

The nominal pickup point where a circuit breaker trip unit detects an overload is at 1.075 times the selected ampere rating . After the circuit breaker has picked up,

t will not trip until the delay determined by the long-time delay adjustment has been achieved.

2. Neutral conductor protection

Inverse-time-delay overload releases for neutral conductors are available in a 50% or 100% ratio of the overload release . The neutral must have specific

rotection if:

It is reduced in size compared to the phases

Nonlinear loads generating third order harmonics are installed

It may be necessary to cut off the neutral for functional reasons (multiple source diagram) or safety reasons (working with power off).


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3. Short-circuit protection, instantaneous

Designation: I (INST: instantaneous), previously n-release

Depending on the application, I-releases can either be used with a fixed or an adjustable release current Ii as well as with a switch-off or non-switch-off function.

The instantaneo us pickup function simulates the magnetic characterist ic of a thermal-magnet ic circuit breaker. This function trips the circuit breaker with no

ntentional time delay.

In circuit breakers with both short-time and instantaneous pickup, the instantaneous pickup will override the short-time pickup if the instantaneous pickup is set at

the same or lower setting than the short-time pickup.

4. Short-circuit protection, delayed

Designation: S (ST: short-time delay), previously z-release.

To b e us ed for a t ime adjustment of protective functions in series. Besides the standard curves and settings, there are also optional functions for special application s:

Definite-time overcurrent releases

For this standard S function, the desired delay time (tsd) is defined as of a set current value (threshold I sd) (definite time, similar to the function of definite- time

overcurrent-time protection (DMT) at the medium voltage level).

Inverse-time overcurrent releases

In this optional S function, the product of I 2t is always constant. In general, this function is used to improve the selectivity response (inverse time, similar to the

function of inverse-time overcurrent-time protection at the medium voltage level.

5. Earth-fault protection

Designation: G (GF: ground fault) , previously g-release.

Besides the standard function (definite-time) an optional function (I 2t = current-dependent delay) is also available.

Figure 4 Ground-fault Characteristic Curve

The ground-faul t function is divided into pickup and delay components (see Figure 4). The pickup portion determines at what point the circuit breaker will begin

detecting a ground fault.

The delay adjustment determines how long the circuit breaker will delay tripping after a ground-fault has been detected . It is supplied with both an It IN and an It OUT

function on the circuit breakers.

In a circuit breaker with the ground-fault function, there is a maximum unrestrained ground-fault delay provided. This delay determines the maximum amount of time

the circuit breaker will delay during a ground-fault condition when not restrained by a downstream breaker. The maximum delay is shown by a single line and stays

constant for all ground-fault delay switch settings.

6. Fault-current protection

Designation: RCD (residual current device), previously also DI (differential current interrupter).

It detects differential fault currents up to 3 A, similar to the FI function for personal protection (max. 500 mA).

Go back to CB tripping units

References //

Planning of Electric Power Distribution Technical Principles by SIEMENS

Circuit Breaker Characteristic Trip Curves and Coordination by Schneider Electric

Low Voltage Circuit Breakers complying with the Standards UL 489 and UL 1066 by ABB

Recommended EE stuff //
https://www.industry.usa.siemens.com/services/us/en/industry-services/training/self-study-courses/quick-step-courses/Documents/circuit_breakers.pdfhttp://electrical-engineering-portal.com/inspection-of-ground-fault-protection-systems


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About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (