Circuit breaker releases / Tripping units
In the majority of the cases, a tripped circuit breaker mostly cause by circuit. To trip when there is a problem, when they don't, the main circuit breaker in the. How about some basic circuit breaker information and limit the technical verbosity on at least one page. Please.24.145.62.13 23:30, 28 October 2009 (UTC) But an encyclopedia is not a catalog. Choosing the appropriate type of circuit breaker for a given application requires more expertise than can be reasonably put into an encyclopedia article.
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 distribution system is determined by the selection of the appropriate release (see Figure 1). Releases can be divided into:
- Thermal-magnetic tripping units – TMTU, also called electromechanical releases and
- Electronic tripping units – ETU
Thermal-magnetic tripping units //
The thermomagnetic trip unit consists of two parts:
The thermal trip unit – Made up by 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 an 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.
Electronic tripping units //
The electronic trip units instead use a microprocessor to process the current signal and 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:
- Long time-delay trip function (ANSI code: 51, AC time overcurrent relay);
- Short time-delay trip function (ANSI code: 51, AC time overcurrent relay);
- Instantaneous trip function (ANSI code: 50, instantaneous overcurrent relay);
- Ground-fault trip function (ANSI code: 51 N, AC time earth fault overcurrent relay).
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 adjustable function simulates the effect of a bimetal conductor in a thermal-magnetic circuit breaker. It reacts to overload conditions and determines how much current the circuit breaker will carry continuously.
![Circuit Circuit](/uploads/1/2/5/8/125844775/133737084.jpg)
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, it 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 protection 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).
Basic tripping settings of CB
There are (at least) six basic adjustable tripping settings (functions) you really should understand in order to fully understand how circuit breaker actually works.
All these adjustable functions actually shape the time-current curve of a circuit breaker and 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 unit which is much more advanced comparing to these explained here, but the basics are the same, very same.
So, let’s start with explanation!
The below time-current curve reflects one possible setup for a 1200 ampere circuit breaker with a nominal (maximum continuous ampere) rating of 1000 amps. This time-current curve will be the basis for discussing adjustable tripping settings of the circuit breakers.