A 250A chassis is an example of a type of busbar system within a distribution board where MCBs and RCBOs are installed. It serves as an electricity distributor taking in electrical current through one point from a single power source and dividing it into several points for output. The “250A” figure has no bearing with the current capacity of each MCB/RCBO installed on the chassis; rather, it is an indication of the maximum amount of current that the whole chassis can handle during its operation.
It appears as a metal frame or rail module system in the distribution board where MCBs and RCBOs are clipped or installed.
Physically, you will see a series of MCBs arranged orderly. Underneath this line or behind them is the busbar system made up of either copper or tinned copper which functions as a “power spine” for all the MCBs. It comes as a complete modular set of either 12 way, 24 way, 36 way, or 48 way.
One such example is that of a 250A mounting pan in the chassis (e.g., Schneider Acti9 MSC 18). This component is found inside the distribution board where a three-phase load is distributed to several MCBs and RCBOs. The term “250A” denotes the maximum current capacity of the entire busbar and chassis system.
Electrical Ratings and Performance
The above described chassis is designed for 415V AC three phase supply (50Hz) and has an insulation value of 500V. This makes it safe for normal usage under any low voltage distribution application. The chassis also has a short circuit breaking capacity of 25kA (0.1sec), meaning it can tolerate very high fault currents for a very short period until the necessary protection kicks in. The chassis has a surge rating of 6kV.
In addition to that, the chassis holds both MCBs and RCBOs, all linked through the busbar system. Each unit is tasked with protecting individual circuits like lighting, power sockets, and other circuits for the particular installations.
Miniature Circuit Breaker (MCB)
An MCB is an appliance which automatically cuts off the electricity supply in the event of either an overload or short circuit in order to protect cables and equipment from overheating.
An MCB is usually a small rectangular device installed on a DIN rail within a distribution board. The device has an on/off switch lever at the front and is rated in terms of current ratings like 10A, 16A, 20A, 32A and so forth. These are switches within a switchboard that you can switch off when there is a trip or fault in the circuit.
MCB can be used in the subcircuit of an installation. They can also be used in the end-use circuit, such as lights, general purpose outlet, air conditioner, water heater, motor, and other permanent fixtures, based on the electrical installation and capacity of the breaker.
Low voltage distribution board MCB ratings are usually between 6A and 125A, but in cases where a system has a 250A frame, the rating will be between 6A and 63A, but some can go as high as 80A or 100A.
Supply Voltage and Design Considerations
The power supply system in Australia usually is 230V single-phase (nominal), and 400/415V three-phase. Therefore, while performing the MCB sizing calculation, you will use 230V single phase, and 415V for three phases.
First, find the load current Ib by applying the power formula (single-phase: I = P ÷ 230V). After that, choose an appropriate MCB with a value equal to or exceeding the calculated current (In ≥ Ib). Besides, it is essential to examine the conductor current-carrying capacity to make sure that the current flowing through the MCB will not exceed the maximum current that can be carried by the conductor (In ≤ Iz).
Example:
In a single-phase circuit at 230V, with a 4,600W heater, the current will be:
I = P ÷ V = 4,600 ÷ 230 = 20A
Therefore, the design current will be 20A and the choice of an MCB will be 20A (B Type or C Type depending on the nature of the load). However, it is important that you ascertain that the cable is capable of carrying 20A safely.
It should be noted that in the real world of design, one does not base his/her calculations on actual calculated currents. There are other factors such as the nature of the load i.e. whether it is a continuous or an intermittent type of load that you have to consider. Therefore, if for instance the heater operates continuously, a 125% rule is usually applied to the design value which brings it to 25A. It means that you can now choose to go for a 25A or a 32A MCB.
The bottom line is simple if the load is 20A, then the cable will also have to be capable of carrying 20A without overheating. Should the current carried by the cable be more than its capacity, there might be chances of the MCB failing to disconnect the circuit and as such the cable will overheat and pose danger.
RCBO (Residual Current Breaker with Overcurrent Protection)
The Residual Current Circuit Breaker with Overcurrent Protection (RCBO) is a device with the dual functions of offering overload and short-circuit protection (MCB) along with earth leakage protection. The use of RCBO is made when you need protection against electric shock or fire dangers from earth leakage of electricity.
MCB provides protection from overload and short-circuit only. On the other hand, RCBO is used to protect against all three factors: overload, short-circuiting, and earth leakage.
Like MCBs, RCBOs look similar too, with the difference of one test button (often either blue or orange color) in front of them. This is used to test the earth leakage protection in case of need. Also, RCBO, like MCB, is mounted on the DIN rail, and it also has a switch lever, just like an MCB.
Rating of 400V, 50Hz relates to the standard operating voltage. Rating of 6 kA indicates the short-circuit breaking capacity and shows the amount of fault current that may safely be interrupted by the device. The international standard regulating RCBOs is IEC 61009-1.
Earth leakage fault is caused by the flow of current between the live conductor and the earth instead of between the live and neutral conductors. It is caused by either the damage of the insulator, loose connections, or any contact between live conductors and metal enclosures. Such faults pose risks of both shocks and fires. Hence, use of RCBOs/RCDs.
RCBOs cannot be used to protect against earth leakage faults when absent. Therefore, MCBs must be used, which do not offer earth leakage fault protection. In this case, earth leakage fault protection is normally offered by an RCD (either main or group protection), implying that the number of circuits individually protected is lower than in an RCBO arrangement.
Configuration of a 250A Chassis (Ways System)
The number of MCBs and RCBOs is not fixed in a 250A chassis. This will depend upon the pole capacity of the chassis, such as 12-way, 24-way, 36-way, or 48-way, and the configuration of the switchboard. This could consist entirely of MCBs, or RCBOs, or a combination of MCBs and RCBOs that would fit into the configuration and layout.
For instance:
- • 12-Way – 8 MCBs + 4 RCBOs
- • 24-Way – 16 MCBs + 8 RCBOs
- • 36-Way – 24 RCBOs + 12 MCBs
- • 48-Way – 30-40 RCBOs + MCBs
In a 48-Way, RCBOs are generally used for critical services, such as wet circuits or for dedicated machinery circuits, and MCBs are used for general lighting and power circuits with RCDs upstream of these devices.
Meaning of “Ways” and “Slots”
Ways (12 way, 24 way, 36 way, 48 way) denote the number of exit points provided on the chassis to mount MCB or RCBOs. That means it is the number of breaker positions provided in the distribution system with the help of busbar arrangement.
The external look of all the different types of chassis is similar in design, that is, there will be a mounting frame which looks like a rectangle and is placed inside the switchboard with slots made to install the MCB or RCBO.
There is no difference other than the size and number of slots. The 12-way chassis is small in size and provides less number of breaker slots, while 24-way, 36-way, and 48-way have long slots for installing the breakers.
“Ways” denote the total number of positions in the chassis, whereas “slots” or mounting positions are used to denote the actual physical position at which the MCB/RCBO gets installed. In other words, “ways” are nothing but breaker positions. Hence, in a 24-way chassis, there are 24 breaker positions.
The various configurations in which 250A chassis may be provided by different companies include 24 way, 36 way, 48 way, among others.