Frequently Asked Questions
- What is RCBO?
- What is the Difference between RCBO, RCD and RCCB?
- What are the ways to eliminate RCBO tripping?
- How dose RCBO work?
- How does MCB work?
- What does MCB B C D Curve mean?
- What accessories can be fitted?
- What is a Type A RCCB/RCBO?
- What is the selection principle of RCBO?
- Which is the difference between direct contacts and indirect contacts?
- What are the precautions for installing the RCBO?
- What are the precautions for the wiring of the RCBO?
- What does the kA rating of a circuit breaker imply?
- RCD Nuisance tripping or Unwanted tripping ?
- What effect does ambient temperature have on the operation?
What is RCBO?
The Residual current circuit breaker with over-current protection (RCBO), is actually a kind of circuit breaker with leakage protection function. The RCBO has the protection function of leakage, electric shock, overload and short circuit, which can prevent the occurrence of electric shock accidents and avoid fire accidents caused by electric leakage. , Has obvious effect. RCBOs are installed in our common household distribution boxes to ensure people’s personal safety.
RCBO is a low-voltage safety protection electrical appliance, which is an effective protection for direct and indirect contact electricity in the low-voltage power grid. The protection action current is determined by the maximum load current of the line in normal operation. The RCBO reflects the residual current of the system. During normal operation, the system The residual current is almost zero. In the event of leakage and electric shock, the circuit generates residual current. This current is not enough for MCBs and Fuses to operate, while leakage protectors will operate reliably.
The common size of RCBO’s width is 18mm, 36mm (the same size as 2P RCCB RCD) or larger (the leakage module is separated from the MCB). RCBO can directly protect the load with a circuit loop, which has both leakage protection, short circuit and overload protection. So, the system used for the terminal switch can be more flexible, and compact.
What is the Difference between RCBO, RCD and RCCB?
The residual current circuit breaker (RCCB) with another appellation of Residual current device ( RCD) achieves the following protections:
- protection for users against electric shocks by direct contacts (<30mA),
- protection for users against electric shocks by indirect contacts (300 mA),
- protection of the installations against fire risks (300 mA).
Usually, The RCCB/RCD should be associated with MCBs for power distribution system.
But the RCBO achieve the above mentioned protections (with different settings) in addition to protection against short circuits and cable overload.
What are the ways to eliminate RCBO tripping?
- Dividing line elimination method
If the RCBO trips, you can disconnect the branch circuit of the grid first, and only carry out the power transmission test on the main line. If there is no problem with the main line test, then the branch and terminal lines are tested and eliminated in turn to find the fault point.
- Intuitive inspection method
Carry out a careful inspection of the protector and the protected line equipment, such as the corners, branches, crossovers and other complex and prone fault points of the line to find out the fault points.
- Numerical comparison method
You can also use an instrument to test the line and compare the measured value with the previous value to find the fault point.
- Trial delivery method
Finally, check the fault of the RCBO itself. It is recommended to cut off the main circuit breaker, remove the load side wiring of the tripped RCBO, then turn on the RCBO and test the test button. If the RCBO still does not work, it means that the RCBO itself has a problem and needs to be repaired or replaced. It cannot be put into operation. If there is no problem with the RCBO, you need to find the switchboard and wiring. Check whether the insulation of each electrical circuit and instrument is good, etc., and check one by one until the fault point is found. If it is really unclear, please ask professionals to come to repair it.
How dose RCBO work?
RCBO=MCB+RCD, so its working principle is actually RCCB, RCD combine with MCB.
The working Principle of RCCB RCD:
- When electrical equipment with leakage current, there are two abnormal phenomena:A fault occurs the current balance of line and neutral did not matches (imbalance occurs, as the fault current finds another earthing path of current). The second is that the non-charged metal shell has a voltage to the ground (under normal conditions, the metal shell and the earth are at zero potential).
- The basic operating principle lies in the Transformer shown in the diagram containing three coils. There are two coils say Primary (containing line current) and Secondary (containing neutral current) which produces equal and opposite fluxes if both currents are equal. The RCDobtains the abnormal signal through the current transformer detection, and transfers it through the intermediate mechanism to make the actuator operate, and the power supply is disconnected through the switch device. The structure of a current transformer is similar to that of a transformer. It consists of two coils that are insulated from each other and wound on the same core. When the primary coil has residual current, the secondary coil will induce current.
- The working principle of the leakage protector is to install the leakage protector in the circuit, the primary coil is connected with the line of the power grid, and the secondary coil is connected with the trip unit in the leakage protector. When the electrical equipment is operating normally, the current in the line is in a balanced state, and the sum of the current vectors in the transformer is zero.The current flowing back and forth in the transformer is equal in magnitude, opposite in direction, and positive and negative cancel each other). Since there is no residual current in the primary coil, the secondary coil will not be induced, and the switching device of the leakage protector is operating in a closed state. When a leakage occurs to the equipment shell and someone touches it in time, a shunt occurs at the fault point. This leakage current passes through the human body, The earth. The work is grounded and returns to the neutral point of the transformer (without current transformer), causing the current flowing in and out of the transformer to appear unbalanced (the sum of current vectors is not zero), and the primary coil produces residual current. Therefore, the secondary coil is induced, and when the current value reaches the operating current value limited by the leakage protector, the automatic switch trips and cuts off the power supply
How does MCB work?
The miniature circuit breaker (MCB), is actually a kind of circuit breaker with overloads and short -circuits protection function, When we look on the inside of an MCB we can see how that actually works, MCB has two tripping protection modes：
For overload protection：
It is protection depend on the heated bimetal , through which the current passes (Blue area). If When the working current passes through MCB, it exceeds the MCB rated current and reaches a certain value, the bimetal heats up to a greater extent and, after a certain period of time, this causes the switching mechanism to trip.
For Short-circuit protection:
It is located in the electromagnetic coil (green area). In the event of a short-circuit, the current rises very sharply and the coil creates a magnetic field that both trips the switching mechanism and opens the contacts via a quick release mechanism. The additional quick release for opening the contacts in the event of a short-circuit helps to keep the energy of the short-circuit to a minimum, which in turn keeps the ‘stress’ the wires are subjected to, as low as possible.
In both cases of short circuit or overload, the tripping process results in an electric arc between the contacts of the MCB. This electric arc is much stronger when trying to separate the two circuits. In order to extinguish the arc, it must be directed away from the contacts, over the arc runners, then past the prechamber plate to the arc chamber (red area). In the arc chamber, the formerly powerful electric arc is split into several smaller arcs until the driving voltage is no longer sufficient and they are extinguished.
What does MCB B C D Curve mean?
There are 3 curve characteristics for magnetic operation:
Type B devices are designed to trip at fault currents of 3-5 times rated current (In).
For example a 6A device will trip at 18-30A. They are generally suitable for domestic applications, may be used in light commercial applications where switching surges are low or non-existent.
Type C devices are designed to trip at 5-10 times In (30-60A for a rated current 6A device). they be use in lighting and power circuits, most common, widely available
Type D devices are designed to trip at 10-20 times In (60-120A for a rated current 6A device). They be use in highly inductive load, motors, transformers, some discharge lighting, welder and some types of lighting.
What accessories can be fitted?
The MCB’s accessories include auxiliary contacts (on/off condition), signal contacts (MCB tripped due to fault), shunt trip (remote operation off), undervoltage (35-70% of nominal causes MCB to trip), locking device, and heat dissipation inserts.
What is a Type A RCCB/RCBO?
Type A RCCB RCBO are sensitive to both AC and pulsating DC sine waves. Recommended for welding machine protection where a DC offset may be used by the machine operator (a DC offset may saturate the differential relay of a standard Type AC device and it may not trip when required). Type AC RCCB RCBO are sensitive to AC sine waves only.
What is the selection principle of RCBO?
The number of poles of the RCBOshould be selected according to the line characteristics. 1P+N RCBOs apply to single-phase lines, like home appliances with separate circuits, single-phase outdoor socket boxes, etc, and 3P+N RCBOs apply to three-phase four-wire lines equipment, power and lighting. When selecting the rated operating current value of the RCBO, the normal leakage current value that may occur in the protected circuit and equipment should be fully considered. If necessary, the leakage current value of the protected circuit or equipment can be obtained through actual measurement
Which is the difference between direct contacts and indirect contacts?
A direct contact refers to a person coming into contact with live parts or conductors that are normally live: the main protection against direct contacts is the physical prevention of contact with live parts by means of barriers, insulation, inaccessibility, etc.
An indirect contact refers to a person coming into contact with an exposed conductive part which is not normally live, but it has become live accidentally (due to insulation failure or some other problems). The protection against indirect contacts is mainly realized by disconnection of the supply, by means of a residual current device. RCD RCBO of earth-leakage high sensitivity (l△n ≤30mA) are able to provide both protection against electric shock from direct contact and indirect contact.
What are the precautions for installing the RCBO?
- Before installation, check whether the data on the nameplate of the RCBOis consistent with the usage requirements.
- When the operating current of the RCBOis greater than 8 mA, the enclosure of the equipment protected by it must be reliably grounded.
- The power supply mode, voltage and grounding form of the system should be fully considered.
- After installing theRCBO, the original grounding protection measures of the original low-voltage circuit or equipment cannot be removed. At the same time, the neutral line of the load side of the circuit breaker shall not be shared with other circuits to avoid malfunction.
- The neutral wire and the protective grounding wire must be strictly distinguished during installation. The neutral wire of three-pole four-wireRCBOshould be connected to the circuit breaker.
- After the installation is completed, the test button should be operated to check whether the RCBOcan operate reliably. Under normal circumstances, it should be tested more than three times, and it can work normally.
What are the precautions for the wiring of the RCBO?
- For single-phase lighting circuits, three-phase four-wire distribution lines or equipment that use a working neutral line, the neutral line must pass through a zero-sequence current transformer.
- Wiring should be done in accordance with the power supply and load marks on the leakage circuit breaker, and the two should not be reversed, Unless there is a special indication that the RCBO can be use as reversed. (Some RCBO can be reversed, like TOBN1 TOBD5).
- In lines where single-phase and three-phase loads are mixed under three-phase four-wire system or three-phase five-wire system, the three-phase load should be balanced as much as possible.
What does the kA rating of a circuit breaker imply?
The kA marked on the circuit breaker represents the breaking capacity of the current carried by the circuit breaker, and the circuit breaker contains two key specification as below:
Service breaking capacity (Ics): The largest current the circuit breaker can interrupt without suffering permanent damage.
Ultimate breaking capacity (Icu): The maximum current can be interrupted by circuit breaker, although it will suffer permanent damage if the value exceeds Ics. If a fault current exceeds Icu, the circuit breaker can not interrupt it and the fault must be cleared by the main breaker, which has a higher breaking capacity by design.
For example, if a circuit breaker has an Ics of 4500 Amperes and an Icu of 6000 Amperes:
Any fault below 4.5kA will be cleared with no problem.
A fault between 4.5kA and 6kA will cause permanent damage when cleared.
Any current exceeding 6 kA can’t be cleared by this breaker.
Selection of breaking capacity greatly depends on the application. For example, the fault currents that can be expected in a small residential installation are of much lower magnitude than those found in the main switchboard of a large industrial facility.
All Our circuit breakers have been subjected to short circuit testing at their marked rating and are capable of successfully interrupting the fault current without undue damage to the circuit breaker. The circuit breaker should not be installed into an area where the prospective fault level is higher than the rating of the circuit breaker. Commercial installations and installations close to distribution transformers will have relatively higher fault levels. Consult your energy distributor for the fault level at a given installation.
RCD Nuisance tripping or Unwanted tripping ?
It is very tempting to describe RCD tripping due to intermittent electrical fault as ‘Nuisance Tripping’. However, ‘Nuisance Tripping’ probably best describes an RCD that trips for no electrically based reason what so ever.
Intermittent tripping that typically occurs after a new installation, maintenance or wiring modification would suggest that the RCD is performing the very function for which it was designed / installed for (i.e. fault detection & protection). This intermittent or ‘Unwanted Tripping’ can actually highlight potential problems within the installation, turning the simple exercise of fitting of an RCD, into a huge fault finding exercise. This is not a relished thought for any sparky!
Typically ‘Unwanted Tripping’ on RCDs can stem from misplaced or combined Neutrals. At times, Neutrals intended for protection by the RCD are incorrectly wired to the ‘pre-RCD’ Neutral bar. Other times, current is accidentally shared between the ‘pre-RCD’ Neutral bar and ‘post-RCD’ Neutral bar (e.g. via a common bond that should not exist in the first place). Another important consideration is the effect of Standing Leakage Current & how it relates to ‘Unwanted Tripping’ .
Standing Leakage Current is inherently present in all electrical appliances due to the RFI filters and suppressors inside switch mode power supplies on modern appliances such as LCD TVs, Hi-fi systems, PCs and Laptops. This also occurs in leaky cable appliances with pre-existing poor insulation resistance, or an insulation breakdown developed over time.
Typically ‘Unwanted Tripping’ is blamed on the RCD being overly sensitive. More often than not, it is the Standing Leakage Current that is the problem. The steady state sum of Standing Leakage Current in the circuit must be significantly less than the RCD tripping threshold. If this is very close to the RCD tripping threshold, then even the slightest transient disturbance will cause the RCD to trip.
Generally, RCDs may trip at any value in excess of 50% of the rated residual current (e.g. 15mA on 30mA RCD). Further care should be taken on installations that are susceptible to high transient disturbances or where particularly leaky appliances may be connected. The recommended Standing Leakage Current steady state threshold is less than 33% of the rated residual current (i.e. 10mA on 30mA RCD).
As an example, for a 30mA RCD to stay under the threshold and avoid ‘Unwanted Tripping’, it is recommended that a maximum of four computers (desktops/towers) are connected to a single RCD circuit at any one time. The number of computers may need to be further reduced if they have particularly high Standing Leakage Current or where the installation is particularly susceptible to transient disturbances.
What effect does ambient temperature have on the operation?
The circuit breaker have thermal/magnetic characteristics that are affected by ambient temperature. So the different circuit breaks with different ambient temperature requirement.
Please refer to the circuit breaker technical Information when installing.