Why This Matters on Your Plant Floor
A circuit breaker that is too small will trip under normal load. One that is too large will not protect equipment or personnel when a fault occurs. In high-density industrial zones like South Gujarat, Daman and the Mumbai belt, transformer fault levels are high and conditions are harsh. A breaker undersized for fault current can fail catastrophically, leaving switchgear exposed and the plant without power.
Choosing the wrong breaker type leads to downtime, failed electrical inspections, and safety violations. A distribution system with poor coordination between main and feeder breakers will trip at the wrong point during a fault, blacking out healthy parts of the plant. Worse, maintenance crews face the risk of live equipment if the main breaker fails to clear a fault.
The cost of a wrong choice is not the breaker itself. It is lost production, emergency call-outs, re-certification delays and potential injury. Proper selection takes time upfront but delivers reliability and compliance for years.
ACBs Versus MCCBs: Where Each Fits
Air Circuit Breakers (ACBs) are designed for main incoming switchboards and very high-current applications. They handle currents up to 6300A and have breaking capacities suited to high fault levels in main distribution systems. MCCBs (Moulded Case Circuit Breakers) are used for sub-distribution feeders and branch circuits, typically up to 1600A.
The industrial main switchboard typically uses an ACB as the main incoming breaker, backed by MCCBs on the feeder circuits. This architecture gives you adequate breaking capacity at the highest fault current, allows selective coordination, and keeps the main device serviceable. If you need very high current or very high breaking capacity, ACB is the choice. For medium currents and good breaking capacity, MCCB is cost-effective and reliable.
Load Current: The Starting Point
The first step is to know your load current. The circuit breaker's rated current (In, measured in amperes) must be higher than the expected load in the circuit. Do not confuse this with breaking capacity, which is a different figure.
Next, apply the 80% rule to continuous loads. A continuous load is one that runs for 3 hours or more without stopping. The breaker size must be at least 125% of the continuous load current. In practice, this means the continuous load must not exceed 80% of the breaker rating. For example, a circuit with a 60 A continuous load needs a breaker rated at 75 A or higher. Non-continuous loads may use 100% of breaker rating.
To size a breaker for your installation, list all loads by circuit, identify which are continuous, apply the 125% factor, and select the next standard breaker size. Use our breaker sizing calculator to confirm your selection against the load profile and site fault level.
Breaking Capacity: Matching the Fault Level
Breaking capacity is the maximum short-circuit current the breaker can safely clear. This must be higher than the prospective short-circuit current (PSC) at the installation point. If the PSC exceeds the breaker's breaking capacity, the breaker cannot clear the fault and may rupture.
Two breaking capacity figures appear on a breaker datasheet.
- Icu is the ultimate breaking capacity: the absolute maximum fault the breaker can handle. After clearing an Icu fault, the breaker may not be reusable.
- Ics is the service breaking capacity: the maximum fault the breaker can clear and still operate again without replacement.
For critical applications where downtime is unacceptable, specify Ics equal to 100% of Icu. For standard applications, Ics of 50% or 75% Icu is often acceptable. The difference affects cost and availability after a major fault.
MCCBs are available with breaking capacities from 16 kA to 150 kA depending on frame size. Large-frame MCCBs extend to 1600A current rating with breaking capacities up to 100–150 kA. Above 1600A or higher fault levels, ACBs are the choice, extending to 6300A with breaking capacities suited to main distribution fault levels.
To select the right breaking capacity, work with your licensed electrical contractor to calculate the prospective short-circuit current at your installation point. Account for transformer impedance, cable length and resistance, and the fault level of your supply authority. Then choose a breaker with Icu at or above that PSC.
Trip Elements and Load Type
Once you have sized for current and breaking capacity, you must set the thermal and magnetic trip elements to protect the specific load.
Thermal-magnetic trip units use a bimetallic element for overload protection and a magnetic coil for short-circuit protection. The thermal element has an adjustable threshold, typically from 0.7 to 1.0 times rated current. The magnetic element responds to short-circuit currents and has a fixed or adjustable instantaneous setting.
For distribution circuits, instantaneous trip settings are typically fixed at 10In (ten times rated current). For motor circuits, the setting must be above the motor starting current, usually 6 to 8 times rated current, to avoid nuisance tripping when the motor starts.
Electronic trip units offer more flexibility. They are adjustable for long-time (overload) protection from 0.4 to 1.0 times rated current, and for short-time (fault) protection from 1.5 to 10 times rated current in increments of 0.5In or 1In. This allows fine-tuning to match the exact starting transient of your load and to coordinate with upstream and downstream breakers.
Always verify that the trip setting is appropriate for the load before commissioning. A setting too low causes false trips and production loss. A setting too high leaves equipment unprotected.
Worked Example: Sizing a Feeder Breaker
Suppose your plant has a feeder to a machine area with a continuous load of 480 A at 415 V. The prospective fault level at that point is 25 kA. You need a feeder breaker.
Step 1: Apply the 80% rule. Continuous load 480 A requires a breaker rated at least 480 ÷ 0.8 = 600 A. The nearest standard MCCB size is 630 A.
Step 2: Verify breaking capacity. Your PSC is 25 kA. A large-frame MCCB rated 630 A with a breaking capacity of 50 kA or higher (commonly 100 or 150 kA) is suitable.
Step 3: Set the thermal element. For a non-motor load, the thermal threshold can be set close to 1.0 times rated current, roughly 630 A. An overload lasting several minutes will trip the breaker.
Step 4: Set the magnetic element. For this feeder, a short-time setting of 2.5 to 5 times rated current (say 1.6 to 3.2 kA) ensures selective coordination with downstream MCCBs on individual circuits, which might trip at 10In.
Result: A 630 A MCCB with 100 kA breaking capacity, thermal setting 1.0In and magnetic setting 3In provides overload and fault protection, clears faults safely, and coordinates with branch breakers.
These figures are indicative, based on the assumptions stated. Confirm the selection against the manufacturer's current datasheet and site conditions with a licensed electrical engineer. If your load profile differs, fault level is higher, or you have motor circuits, settings and breaker type may change.
Coordination: Protecting the Distribution Tree
A factory distribution system is a tree: the main ACB at the root, MCCBs on each primary feeder branch, and more MCCBs on sub-feeders and circuits. When a fault occurs deep in the tree, only the breaker closest to the fault should trip, isolating that branch while the rest of the plant stays live.
Selective coordination is achieved by setting breaker trip curves and thresholds so that a fault at level N trips breaker N before breaker N-1 upstream. This requires that each upstream breaker has a higher current threshold or time delay than the one below it.
Coordination is not guaranteed by trip curves alone. Work with your contractor to perform a coordination (or discrimination) study, plotting the trip curves of main and feeder breakers against the same fault-current axis. If the curves overlap, the system will not coordinate and you risk blacking out the entire plant on a single branch fault.
Most importantly, selective coordination requires breaking capacity at every level to be adequate. If an upstream breaker cannot clear the fault, it will not stay in service long enough for a downstream breaker to trip first.
Standards and Compliance
Circuit breaker selection and coordination in India must comply with relevant standards. Refer to the current edition of IS 13947 (Specification for low-voltage switchgear and controlgear), IS 3043 (Code of practice for earthing), and IEC 60947 (Low-voltage switchgear and controlgear) and confirm with your licensed electrical contractor. These standards cover rated current, breaking capacity, trip characteristics and coordination requirements. They do not mandate specific breaker models or settings but establish the framework for safe selection.
What We Stock
Legrand DPX3 MCCBs are stocked by Vimal Electric Corporation for distribution and branch protection across the industrial belt. They are available in a range of current ratings and breaking capacities to match your load and fault level. Legrand products are supplied as complete units with manufacturer warranty and datasheet. Service and technical support go to the brand's authorised service centre.
To check availability of a specific breaker size and breaking capacity, or to discuss your distribution system requirements, send a load list or single-line diagram to our enquiry page. Include the prospective fault level at your point of supply, the continuous and peak loads by feeder, and details of any motor or special circuits. Our team will help you select the right breaker combination and can refer you to a licensed contractor for coordination studies and site commissioning.