What are miniature CB and their uses?
Miniature circuit breakers are electromagnetic devices that protect an electric circuit against short circuits and over-current.
It is used in domestic and industrial distribution panels and also may be used in low-voltage switchgear as well.
I can see obviously that, almost all control circuits I have seen in my work as an electrical engineer have one or more MCB.
How Many Types of MCB Are There?
Based on MCB tripping curve there are about 6 types of MCB, Type A, B, C, D, K, and Z. Based on the number of poles, there are 4 MCB types.
Miniature circuit breakers (MCB) are widely used in almost all electrical fields, and the MCB types are based on poles number and may be based on curve type.
Based on Curve Class:
- Type B
- Type C
- Type D
- Type K
- Type Z
MCB Types And Their Uses Based On Tripping Curve
Electrical loads are not the same, loads vary from application to application. Domestic loads are not the same as industrial loads.
Home appliances have no inrush currents at their starting, while factories and industrial applications loads may need high starting inrush currents, like heavy three-phase induction motors and x-ray machines.
Using the right MCB for the load protects against sudden trips at the start because of the inrush current.
MCBs are classified according to fault-current tripping range as follows:
- Type B MCB: this MCB types trip between 3 and 5 times of full load current.
Used in residential applications. - Type C MCB: Trips between 5 and 10 times of full load current.
Mainly used in industrial type of applications where there are chances of higher short-circuit current values in the circuit - Type D MCB: Trips between 10 and 20 times of full load current.
Is used in Specialty industrial uses with loads like heavy motors, welding, electrical transformers, x-rays, etc. - Type K MCB: trips between 8 to 12 times the rated current, These are used for inductive loads which have a chance of high inrush currents.
- Type Z MCB: operates for a current value between 2 to 3 times of its rated current.
Class K and Z are highly sensitive miniature circuit breakers that operate very quickly in a short time these MCB types are used to protect sensitive devices.
Based on Breaker Poles Number
- Single pole miniature circuit breaker.
- Two poles.
- Three poles
- And four poles MCB
Another way to classify MCB types is based on the number of poles of the (Miniature circuit breaker) MCB
- Single pole MCB or one pole, these miniature circuit breaker types protect only one phase of the electrical circuit
- Two poles MCB (double poles), these miniature circuit breaker types protect one phase and also the neutral of the circuit
- Three poles MCB (triple poles), provides protection and switching for three-phase loads in an electrical circuit such as three-phase motors
- Four poles MCB (Three poles with neutral), This type provides additional neutral switching besides the three phases of protection and switching
Example
Say we have Type B, Type C, and one more type D miniature circuit breaker with a current capacity of 32 A for each of them, now if the load draws an AC current of 100 A which is about 3 times the 32 A.
In this case each type of mcb with the same load and the same rated current, But each one of them will act as its own design;
- Type b will be the first to trip after a short period, While Type C will trip also but after a longer duration than type b miniature CB.
- Type D will be the last one to act and trip with this 100 A value.
If we noticed all of the miniature CB types in the example above will trip, but the difference between them is the duration to act or to trip.
What Type of MCB Should I Use With Domestic Loads?
Generally, Type B MCBs are suitable for domestic applications and home loads.
What is The Main MCB Function?
MCB or Miniature Circuit Breakers are protection devices to switch OFF electrical circuits during any abnormal condition in the electrical circuits, such as short circuit and overload conditions.
Based on the curve of the MCB it trips during any over-current situation to protect the load.
What does the mcb rating mean?
The miniature circuit breaker current rating is the maximum current the mcb can withstand in normal operation conditions without tripping.
While the voltage rating of the miniature circuit breaker is the operating voltage of the circuit the breaker will protect. The voltage rating can be higher than the circuit voltage, but can’t be lower than it.
If the mcb rating is lower than the load current it will trip. While in case of the voltage rating is lower than the circuit voltage the breaker insulation will get damaged and a possible short circuitmay occur.
What is the meaning of 10kA in MCB?
10KA on an mcb is the short circuit withstand capacity or ultimate breaking capacity of the circuit breaker. It is the maximum short circuit current the circuit breaker can interrupt safely.
If the short circuit current is higher than this 10KA value, the mcb will get damaged and will be a bad breaker.
In the case of a short circuit, the current is in kilo amperes -KA- if the circuit breaker’s short circuit withstand capacity is lower than the circuit short circuit current the mcb, as well as, the circuit will get damaged.
So it’s important to consider the KA rating of the mcb in the design phase.
Can I use mCB as an on/off switch?
It’s not recommended to use mcb as an on/off switch. They are not switches, they are there to protect the circuit from overcurrent and short circuits.
Also, the frequent using of the miniature circuit breaker as a switch shortens its life span. Yes, you can use it to isolate the circuit in case of maintenance and troubleshooting activities.
I know, using mcbs are widely used as switches. The mcb will work safely and effectively. But its life span will be shorter than usual.
On the other hand, using mcb as a switch is not safe for nontechnical persons in many cases. This is because, in many electrical panels, some mcb terminals are not properly isolated.
What are the differences between fuse and MCB?
Fuse and MCB are both overcurrent protection devices but there are some major differences:
- The MCB is reusable and the fuse is self-destructed.
- MCB works based on current electromagnetic and thermal effects. It is an electromechanical device.
- Fuse has a metallic wire that melts if the current passing through it exceeds the fuse ratings.
- MCB has an electromagnetic mechanism that acts in case of overcurrent or short circuits.
- Replacing the fuse is not safe and needs an electrician to perform it, while anyone can re-close the MCB in case of overload tripping.
- Generally fuses cost less than mcb.
Which is better MCB or fuse?
For the above-mentioned advantages of MCB over the fuse it’s clear that using mcb is better and safer than fuses.
What causes a breaker to spark?
Two main reasons for sparking MCB:
- Loosen connection of the wires connected to the breaker. In this case, the spark will be continuous or intermittent. The wire and the MCB will be smoky.
- Heavy loads are connected to the MCB. In this case, the spark happens only at closing it on time.
If the MCB is sparking in normal condition and keeps sparking, it’s a serious situation and you should switch all loads off, then switch the breaker off and call an electrician for repair.
If it sparks only when closing it on, this means that heavy loads are connected to the MCB circuit.
Recheck loads and make sure they are suitable for the MCB rating. If you switch all loads before closing the breaker on, there will be no spark.
What causes a miniature breaker to melt?
Loosen connection of wires is the main reason for melting circuit breakers. The loosened wire creates a gap between the wire and the MCB contact parts.
This gap produces a spark. this spark will take some time to heat up the MCB body, unfortunately, the breaker is not able to detect this spark as it’s not an overcurrent.
With the continuation of this spark, the body of the breaker melts.
