What is RCCB and Why it is needed ?

Residual Current Circuit Breaker (RCCB)

 Much Needed Introduction....

A Residual Current Circuit Breakers is another different class of Circuit Breakers. A Residual Current Circuit Breaker (RCCB) is essentially a current sensing device used to protect a low voltage circuit in case of a fault. It contains a switch device that switches off whenever a fault occurs in the connected circuit.

Why needed  RCCB..?

Residual Current Circuit Breakers are aimed at protecting an individual from the risks of electrical shocks, electrocution and fires that are caused due to faulty wiring or earth faults.

RCCB is particularly useful in situations where there is a sudden earth fault occurring in the circuit.

e.g. A person accidentally comes in contact with an open live wire in the circuit.

In such situation, in absence of an RCCB in the circuit, an earth fault may occur and the person is at the risk of receiving an electrical shock.

However, if the same circuit is protected with RCCB, it will trip the circuit in fraction of a second thus preventing the person from receiving an electrical shock. Therefore, it is a good and safe practice to install RCCB in your electrical circuit.

Variants of RCCBs....

2 Pole RCCB: It is used in case of a single phase supply that involves only a live and neutral wire. It is as displayed in image below. It contains two ends where the live and neutral wires are connected. A Rotary switch is used to switch the RCCB back to ON or OFF positions. A test button helps to periodically test the RCCB functionality.

4 Pole RCCB: It is used in cases of a three phase supply connection involving three phase wires and a neutral. It is as displayed in image below. It consists of two ends where the three phases and neutral wire is connected. Besides this it is similar in construction and operation as 2 Pole RCCB.

RCCBs come in different ratings like: 30mA, 100mA, 300mA

How does it Protect?

As explained above, RCCB is meant for protection from earth faults and associated risk to human life such as electrical shocks.

The underlying fundamental principle behind operation of RCCB is that in ideal situations the current flowing in to the circuit through live (hot) wire should be same as the returning current from the neutral.

In case of an earth fault, the current finds a passage to earth through accidental means (such as accidental contact with an open wire etc.). As a result the returning current from neutral is reduced. This differential in the current is also known as “Residual Current”.

RCCB is designed such way that it continuously senses and compares for difference (residual current value) in current values between the live and neutral wires. Any small change in the current value on account of such event would trigger the RCCB to trip off the circuit.

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INTRODUCTION OF DIFFERENT CURVES IN MCBs

Peoples are confused at some point while buying MCBs for protraction in there house/ office/industries etc.

What is meant by B, C, D, K and Z curves in MCBs?

MCB is a device designed to protect a circuit from short circuits and over currents. Trip curves of MCB's (B, C, D, K and Z curves) tell us about the trip current rating of Miniature Circuit breakers. Trip current rating is the minimum at which the MCB will trip instantaneously. It is required that the trip current must persist for 0.1s.

An MCB with trip curve class B means that the MCB trips at as soon as the current rises above 3 to 5 times its rated current In.   Similarly, MCB with trip curve class C means that the MCB trips at as soon as the current rises above 5 to 10 times its rated current In and so on..

In some applications, frequent current peaks occur for a very short period (100ms to 2s). For such applications class K type fuses shall be used. Class K type fuses are used in circuits with semiconductor devices. 

TRIP CURVE CLASS B:         Above 3 to 5 times rated current. Suitable for cable protection.

TRIP CURVE CLASS C:        Above 5 to 10 times the rated current. Suitable Domestic and residential                                                              applications and electromagnetic starting loads with medium starting                                                                   currents

TRIP CURVE CLASS D:   Above 10(excluding 10) to 20 times the rated current. Suitable for inductive                                                       and motor loads with high starting currents.

TRIP CURVE CLASS K:    Above 8 to 12 times the rated current. Suitable for inductive and motor                                                               loads with high inrush currents.

TRIP CURVE CLASS Z:    Above 2 to 3 times the rated current. These type of MCBs are highly                                                                     sensitive to short circuit and are used for protection of highly sensitive                                                                 devices such as   semiconductor devices.

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An Over view about MCCB (Moulded Case Circuit Breakers )

Molded Case Circuit Breakers

          Molded Case Circuit Breakers are electromechanical devices which protect a circuit from  Over-current and Short Circuit. It provides Short Circuit Protection and also Over-current and for circuits ranging from 63 Amps up to 3000 Amps.

             Their primary functions are to provide a means to manually and automatically open a circuit under overload or short circuit conditions. The over-current, in an electrical circuit, may result from short circuit, overload or faulty design.

          MCCB is an alternative to a fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCCB can be easily reset after a fault and offers improved operational safety and convenience without incurring operating cost.

      Molded case circuit breakers generally have a  Thermal element for over-current and Magnetic element for short circuit release which has to operate faster.

         MCCBs are manufactured such that end user will not have access to internal workings of the over-current protection device. Generally constructed of two pieces of heavy-duty electrically insulated plastic, these two halves are riveted together to form the whole. Inside the plastic shell is a series of thermal elements and a Spring-loaded trigger.

       When the thermal element gets too warm, from an over-current situation, the spring trips, which in turn will shut off the electrical circuit.

Types of MCCBs

Larger molded case circuit breakers have adjustable range setting on the face of the device. Molded case circuit breakers can range in size from 32 Amperes up to 3000 Amperes.

Molded Case Circuit Breakers have the following Specifications

• Current Rating - Amperes
• Current Setting Range - Amperes
• Short Circuit Rating - Kilo Amperes (KA)
• Operating Characteristics - Normal / Current Limiting Type

MCCBs are now available with a variety of Releases or Operating Mechanisms these are given below

• Thermal Magnetic Release
• Electronic Release
• Microprocessor Release
• Thermal Magnetic Release MCCB

               Thermal-magnetic circuit breakers use bimetals and electromagnetic assemblies to provide over-current protection. Their characteristic inverse time tripping under overload conditions is ideally suited for many applications varying from residential to heavy industrial loads. For higher level (short circuit) over currents, instantaneous trip characteristics allow molded case circuit breakers to interrupt with no intentional delay.

               The adjustable overload protection is from 70% to 100% of the nominal current and short circuit setting from 5 to 10 times of the rated current is possible.

              The minor disadvantage of the release is that operating characteristics of the breaker may vary depending on the ambient temperature.

Electronic Release MCCB

           Electronic or Static Release Molded Case circuit breakers use power electronic circuitry to provide over-current protection. The Continuous adjustable overload protection from 60% to 100% of the nominal current and short circuit setting from 2 to 10 times of the rated current is possible.

         The advantage of the release is that operating characteristics of the breaker is independent of the ambient temperature.

                This wide flexibility takes care of future increases in load capacity of an installation and ensures better planning at an optimum cost.

Microprocessor release MCCB

             Microprocessor release Molded Case circuit breakers use microprocessors to provide over-current protection. The Microprocessor release works on monitoring of current True R.M.S value. It is simulated and calculated from peak values, which installed microprocessor, can detect.

                There is high Flexibility through multiple adjustments of protection settings, High repeat accuracy and High reliability.

               Time delays can be provided for Short Circuit Release better discrimination and co-ordination using LCD display. System Diagnosis is possible as it stores the Trip history within the internal memory. Trip current indication is also available for understanding of type of fault and set-up programming at site.

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Applications and forms of differential relays   

      Differential relays take a variety of forms, depending on the equipment they protect. The definition of such a relay is “one that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined amount. It will be seen later that almost any type of relay, when connected in a certain way, can be made to operate as a differential relay. In other words, it is not so much the relay construction as the way the relay is connected in a circuit that makes it a differential relay.

ABB Residual Current Breaker with Overload Protection RCBo

   Most differential-relay applications are of the “current-differential” type. The simplest example of such an arrangement is shown in Fig. 14. The dashed portion of the circuit of Fig. 14 represents the system element that is protected by the differential relay. This system element might be a length of circuit, a winding of a generator, a portion of a bus, etc.

   A current transformer (CT) is shown in each connection to the system element. The secondaries of the CTÕs are interconnected, and the coil of an overcurrent relay is connected across the CT secondary circuit.

This relay could be any of the a-c types that we have considered.

Fig. 14. A simple differential-relay application

Now, suppose that current flows through the primary circuit either to a load or to a short circuit located at X. The conditions will be as in Fig. 15. If the two current transformers have the same ratio, and are properly connected, their secondary currents will merely circulate between the two CT’s as shown by the arrows, and no current will flow through the differential relay.

Fig. 15. Conditions for an external load or fault

But, should a short circuit develop anywhere between the two CT’s, the conditions of Fig. 16 will then exist. If current flows to the short circuit from both sides as shown, the sum of the CT secondary currents will flow through the differential relay. It is not necessary that short-circuit current flow to the fault from both sides to cause secondary current to flow through the differential relay. A flow on one side only, or even some current flowing out of one side while a larger current enters the other side, will cause a differential current.

In other words, the differential-relay current will be proportional to the vector difference between the currents entering and leaving the protected circuit; and, if the differential current exceeds the relay’s pickup value, the relay will operate.

Fig. 16. Conditions for an internal fault

It is a simple step to extend the principle to a system element having several connections. Consider Fig. 17, for example, in which three connections are involved

Fig. 17 A three-terminal current-differential application

The principle can still be applied where a power transformer is involved, but, in this case, the ratios and connections of the CTÕs on opposite sides of the power transformer must be such as to compensate for the magnitude and phase-angle change between the power transformer currents on either side. This subject will be treated in detail when we consider the subject of power-transformer protection.It is only necessary, as before, that all the CT’s have the same ratio, and that they be connected so that the relay receives no current when the total current leaving the circuit element is equal vectorially to the total current entering the circuit element. A most extensively used form of differential relay is the “percentage-differential” type. This is essentially the same as the overcurrent type of current-balance relay that was described earlier, but it is connected in a differential circuit, as shown in Fig. 18.

Fig. 18. A percentage-differential relay in a two-terminal circuit

The differential current required to operate this relay is a variable quantity, owing to the effect of the restraining coil. The differential current in the operating coil is proportional to I1 – I2, and the equivalent current in the restraining coil is proportional to (I1 + I2)/2, since the operating coil is connected to the midpoint of the restraining coil; in other words, if we let N be the number of turns on the restraining coil, the total ampere-turns are I1N/2 + I2N/2, which is the same as if (I1 + I2)/2 were to flow through the whole coil.

The operating characteristic of such a relay is shown in Fig. 19. Thus, except for the slight effect of the control spring at low currents, the ratio of the differential operating current to the average restraining current is a fixed percentage, which explains the name of this relay. The term “through” current is often used to designate I2, which is the portion of the total current that flows through the circuit from one end to the other, and the operating characteristics may be plotted using I2 instead of (I1 + I2)/2, to conform with the ASA definition for a percentage differential relay.

The advantage of this relay is that it is less likely to operate incorrectly than a differentially connected overcurrent relay when a short circuit occurs external to the protected zone.

Fig. 19. Operating characteristic of a percentage-differential relay

Current transformers of the types normally used do not transform their primary currents so accurately under transient conditions as for a short time after a short circuit occurs.

This is particularly true when the shortcircuit current is offset. Under such conditions, supposedly identical current transformers may not have identical secondary currents, owing to slight differences in magnetic properties or to their having different amounts of residual magnetism, and the difference current may be greater, the larger the magnitude of short-circuit current. Even if the short-circuit current to an external fault is not offset, the CT secondary currents may differ owing to differences in the CT types or loadings, particularly in power-transformer protection. Since the percentage-differential relay has a rising pickup characteristic as the magnitude of through current increases, the relay is restrained against operating improperly.

Figure 20 shows the comparison of a simple overcurrent relay with a percentage-differential relay under such conditions. An overcurrent relay having the same minimum pickup as a percentage-differential relay would operate undesirably when the differential current barely exceeded the value X, whereas there would be no tendency for the percentage-differential relay to operate.

Fig. 20. Illustrating the value of the percentage-differential characteristic

Percentage-differential relays can be applied to system elements having more than two terminals, as in the three-terminal application of Fig. 21. Each of the three restraining coils of Fig. 21 has the same number of turns, and each coil produces restraining torque independently of the others, and their torques are added arithmetically.

The percent-slope characteristic for such a relay will vary with the distribution of currents between the three restraining coils.

Fig. 21. Three-terminal application of a percentage-differential relay

   Percentage-differential relays are usually instantaneous or high speed. Time delay is not required for selectivity because the percentage-differential characteristic and other supplementary features to be described later make these relays virtually immune to the effects of transients when the relays are properly applied. The adjustments provided with some percentage-differential relay will be described in connection with their application.

   Several other types of differential-relay arrangements could be mentioned. One of these uses a directional relay. Another has additional restraint obtained from harmonics and the d-c component of the differential current. Another type uses an overvoltage relay instead of an overcurrent relay in the differential circuit. Special current transformers may be used having little or no iron in their magnetic circuit to avoid errors in transformation caused by the d-c component of offset current waves. All these types are extensions of the fundamental principles that have been described, and they will be treated later in connection with their specific applications.

    There has been great activity in the development of the differential relay because this form of relay is inherently the most selective of all the conventional types. However, each kind of system element presents special problems that have thus far made it impossible to devise a differential-relaying equipment having universal application.

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