Circuit Breakers- Discrimination /Selectivity and Its Techniques

 A circuit breaker is a switching device which can be operated manually and automatically for control and protection of electrical power circuit.

The primary purpose of a circuit breaker is to protect the system from the damage in series connected equipment & to minimize the area of damage and to minimum duration of power loss. However the first consideration in the discrimination is the correct circuit sizing breaker of main and branch breaker.
Discrimination is the coordination between the operating characteristics of circuit breakers placed in series. When the fault occurs in system only the circuit breaker placed immediately upstream of the fault will trip.

Discrimination as per IEC 60947-2 can be defined as follows:

1. The total discrimination: This is a type of current based discrimination where in there are two circuit breakers in series, the circuit breaker on the load side effects the protection without causing the upstream circuit breaker to operate/trip
2. Partial Discrimination: This is a type of current based discrimination where in there are two circuit breakers in series, the circuit breaker on the load side effects the protection up to a defined level of overcurrent, without causing the upstream circuit breaker to operate/trip.

Principles of discriminative tripping (selectivity)

Discrimination may be total or partial, and based on the principles of current levels, or time-delays, or a combination of both. A more recent development is based on the logic techniques Discrimination is achieved by automatic protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately Upstream of the fault, while all other protective devices remain unaffected.

• Protection against overload: discrimination based on current levels
  This method is realized by setting successive tripping thresholds at stepped levels, from downstream relays (lower settings) towards the source (higher settings). Discrimination is total or partial, depending on particular conditions, as noted above.
• Protection against low level short-circuit currents: discrimination based on stepped time delays
  This method is implemented by adjusting the time-delayed tripping units, such that downstream circuit breakers have the shortest operating times, with progressively longer delays towards the source.

Current-level discrimination – Principle

Current-level discrimination is achieved with stepped current-level settings of the instantaneous magnetic-trip elements. Current-level discrimination is achieved with circuit breakers, preferably current limiting, and stepped current-level settings of the instantaneous magnetic-trip elements.
The downstream circuit-breaker is not a current-limiter device
Total discrimination in this situation is practically impossible because
Isc A ≈ Isc B, so that both circuit-breakers will generally trip simultaneously. In this case discrimination is partial, and limited to the Im of the upstream circuit-breaker.
The downstream circuit-breaker is a current-limiting device. Improvement in discriminative tripping can be obtained by using a current limiter for circuit-breaker B. For a short-circuit downstream of B, the limited level of peak current IB would operate the (suitably adjusted) magnetic trip unit of B, but would be insufficient to cause circuit-breaker A to trip.

Time-based discrimination- Principle

This technique requires:
The introduction of time-delays into the tripping mechanisms of circuit breakers
Circuit Breakers with adequate thermal and mechanical withstand capabilities at the high current levels and time delays considered Two circuit-breakers A and B in series (i.e. carrying the same current) are discriminative if the current-breaking period of downstream circuit-breaker B is less than the non-tripping time of circuit-breaker A
C&S offers total discrimination up to fault level of 50kA. For total discrimination, the customers can select Winmaster2 Air Circuit Breakers (ACBs) & Winbreak2 Molded Case Circuit Breakers (MCCBs). Customers can refer selection of up- stream and down-steam breaker details (kA, Rating & AF) from the selection tables provided by C&S Electric.

RCCB – Residual Current Circuit Breaker

 

Table of Content

1. Principle behind RCCB.
2. Benefits Of RCCB.
3. Sensitivity Of RCCB.
4. Limitations Of RCCB.
5. Classification Of RCCB.

While electricity has become an indispensable component of our lives, the fact is, it comes with its own hazards to human life and property. Electrocution and fire being the two major risks associated with electricity, one cannot afford to be negligent when it comes to insulating equipment.
A Residual Current Circuit Breaker (RCCB) is an important safety measure when it comes to protection of electrical circuits. It is a current sensing device, which can automatically measure and disconnect the circuit whenever a fault occurs in the connected circuit or the current exceeds the rated sensitivity.

Aimed at protecting an individual from the risk of electric shocks as well as electrocution and fires, RCCB is particularly helpful in instances of sudden earth fault. The presence of RCCB ensures that in such cases, the circuit will trip immediately and the person is thus protected from an electric shock.

Principle behind RCCB

RCCB works on the principle of Kirchhoff’s law, which states that the incoming current must be equal to the outgoing current in a circuit. RCCB thus compares the difference in current values between live and neutral wires. Ideally, the current flowing to the circuit from the live wire should be the same as that flowing through the neutral wire. In case of a fault, the current from the neutral wire is reduced, the differential between the two known as Residual Current. On spotting a Residual Current, the RCCB is triggered to trip off the circuit.

A test circuit included with the Residual Current device ensures that the reliability of RCCB is tested. When the test button is pushed, the current starts to flow through the test circuit. As it creates an imbalance on the neutral coil of the device, the RCCB trips and supply is disconnected thereby checking RCCB’s reliability.

Benefits of RCCB

• Provides protection against earth fault as well as any leakage current
• Automatically disconnects the circuit when the rated sensitivity is exceeded
• Offers possibility of dual termination both for cable and busbar connections
• Offers protection against voltage fluctuation as it includes a filtering device that guards against transient voltage levels.

 

 

Sensitivity of RCCB

A human being is able to sustain an electric shock to the extent of 30 mA. While upto 10 mA may just evoke a prickling sensation, 10 mA onwards may lead to muscular contraction, further leading to a respiratory paralysis at around 30mA. RCCBs are therefore designed to look for small changes in residual current. In cases where protection from fire is sought, RCCBs are also used to track higher changes in residual current of up to 300mA.

Limitations of RCCB

While RCCB has many advantages, it has some limitations as well:

• RCCB does not guarantee to operate if none standard waveforms are generated by loads. It’s mainly because RCCB is designed to operate on normal supply waveforms.
• There might be some unwanted tripping of RCCB. It’s mainly because whenever there are sudden changes in electrical load, there can be small current flow to earth especially in the old appliances.
• RCCB does not protect from current overload. It has been designed to protect only when the live current and neutral current are different. However, a current overload cannot be detected.
• RCCB does not protect against line-neutral shocks. It’s mainly because current in them is balanced. The current gets balanced as both terminals are held together.
• RCCB does not protect from the overheating that strike if conductors are not properly screwed into terminals.

 

Classification of RCCB

RCCB are of two types; the 2 Pole RCCB and 4 Pole RCCB.

2 Pole RCCB:

This is used in case of a single-phase supply connection that has only a live and a neutral wire.

4 Pole RCCB:

This is used in case of a three-phase supply connection.

Rating from 10 Amp ….100 Amp
Sensitivity 30,100,300 m Amp

RCCB is therefore extremely important in providing real time protection for circuits. In industries and high voltage commercial set ups especially, its importance cannot be undermined as there is always risk of shocks and accidental deaths on account of it. At C&S Electric, we offer Wintrip RCCB, a state-of-the-art product fit for industrial, residential and commercial application. C&S RCCBs conforms to IEC 61008 – 1 and is used for both control and isolation of electrical circuits. Being a respected global brand for power management products for over 50 years, with C&S Electric you can rest assured knowing that you are in safe hands.