The Himel HDM3 MCCB Explained: How It Works and How to Size It

The Himel HDM3 MCCB Explained: How It Works and How to Size It

4 July 2026 · 9 min read · Trans-Digi Global

The Himel HDM3 is a moulded case circuit breaker (MCCB): a switching and protection device that carries normal load current and opens automatically on overload or short circuit. To size it, you choose three things in order: a rated current (In) at or just above the circuit’s design current, a breaking capacity (Icu) at least equal to the prospective fault current at the board, and the right pole and trip-unit configuration for the load. This guide walks through each step using the figures in Himel’s HDM3 catalogue.

In this article

• What an MCCB does, and where the HDM3 fits
• MCCB or MCB: which do I need?
• HDM3 frame sizes and breaking capacities
• Icu and Ics: the two breaking-capacity figures
• How the HDM3 trips: the numbers behind the curve
• How to size an HDM3: a step-by-step method
• Common sizing mistakes to avoid
• Why traceable sourcing matters for MCCBs
• Frequently asked questions
• Get the right HDM3, quickly
• Sources

Trans-Digi Global is an authorised Himel partner in Singapore and supplies the HDM2 and HDM3 MCCB range from local stock, with the datasheets and as-built documentation that panel submissions need.

What an MCCB does, and where the HDM3 fits

An MCCB protects the cable and equipment downstream of it against two different faults:

• Overload: a current moderately above the rating that persists, for example from an overloaded feeder or a stalled motor. The HDM3 handles this with a bimetal (thermal) element that has an inverse-time characteristic, so the larger the overload, the faster it trips.
• Short circuit: a very high fault current that must be cleared almost instantly. The HDM3 uses a magnetic element for this, which trips instantaneously.

Himel describes the HDM3 as its 3 Series range of MCCBs with a fixed thermal-magnetic trip unit, providing line protection and motor protection up to 1,600 A. Two related families sit alongside it: the HDM3E with an electronic trip unit (LSI protection), and the HDM3L earth-leakage MCCB up to 630 A (Himel HDM3 catalogue).

Because the HDM3 trip unit is fixed, you select the breaker rating to suit the circuit rather than dialling a setting on site. That matters when you compare quotations: two breakers with the same frame size but different In values are not interchangeable.

Compared with a miniature circuit breaker (MCB) such as the Himel HDB9, an MCCB covers higher currents and higher fault levels. MCBs are tested to IEC 60898-1, which covers devices up to 125 A and 25 kA short-circuit capacity, while MCCBs are tested to IEC 60947-2, the standard for industrial circuit-breakers up to 1,000 V AC.

MCCB or MCB: which do I need?

The short answer: use an MCB for final circuits, up to 125 A at most, where the prospective fault current at the board is within the MCB’s breaking capacity. Use an MCCB when the current or the fault level is higher, or when the circuit needs a motor-protection or adjustable trip unit. Whichever you choose, the same two rules apply: the design current must not exceed the rating, which must not exceed the cable rating (Ib ≤ In ≤ Iz), and the breaking capacity must be at least the prospective fault current.

Two examples from our guides show the split. A 10 A lighting circuit at a board with a fault level below 10 kA is a job for an MCB such as the HDB9H, covered in how to choose the right HDB9H MCB. The 140 A feeder with a 30 kA fault level in the worked example below needs an MCCB, here an HDM3-160. For fault-level estimates, selectivity between an MCCB and the MCBs below it, and a board-by-board selection matrix, see MCCB vs MCB: which does your panel board need?

HDM3 frame sizes and breaking capacities

The table below summarises the HDM3 technical parameters for the 50 °C-calibrated products, taken from the Himel HDM3/3L/3E catalogue. Icu and Ics are quoted at 400/415 V AC.

Other catalogue figures worth knowing: rated insulation voltage (Ui) is 800 V, rated impulse withstand voltage (Uimp) is 8 kV, the breakers are utilisation category A, and Himel states that the HDM3 meets the isolation requirements of IEC 60947-2. Standard rated currents in the 100 A frame are 10, 16, 20, 25, 32, 40, 50, 63, 80 and 100 A. Rated mechanical life is 20,000 operations for the 100 to 250 A frames, 10,000 for the 400 A frame and 5,000 for the 800 A frame; electrical life at 400/415 V is 8,000, 7,500 and 2,500 operations respectively.

Icu and Ics: the two breaking-capacity figures

Breaking capacity is the largest fault current a breaker can interrupt safely. It is quoted in kiloamps (kA) at a stated voltage. IEC 60947-2 defines two values, summarised clearly in this technical note on LV breaker fault ratings:

• Icu, rated ultimate short-circuit breaking capacity: the test sequence does not require the breaker to carry its rated current afterwards. Think of it as the maximum fault the device can clear.
• Ics, rated service short-circuit breaking capacity: the test sequence does require the breaker to carry its rated current afterwards. It is stated as a percentage of Icu, with the standard proposing 25%, 50%, 75% or 100%.

The selection rule is simple: the Icu of the breaker must be equal to or greater than the prospective short-circuit current at the point where it is installed. If the fault level at a sub-main board is 30 kA, an HDM3-160 in the S class (Icu 35 kA) meets that rule, while the HDM3-100 S class (Icu 25 kA) does not. Where you want the breaker to remain in service after clearing a fault at that level, compare the Ics figure as well.

In Singapore, the prospective fault level is part of the installation design under SS 638, the Code of Practice for Electrical Installations, so it should come from the project’s electrical design, not from a rule of thumb. If you share the fault level from your single-line diagram or board schedule, we will match the breaking class to it.

How the HDM3 trips: the numbers behind the curve

Overload (thermal) protection

The HDM3 catalogue gives the conventional test points for the thermal element: at 1.05 × In the breaker must not trip within 1 hour (In ≤ 63 A) or 2 hours (In > 63 A), starting cold; at 1.30 × In it must trip within the same time, starting immediately after the first test. In practice, a correctly rated HDM3 rides through normal load swings but will not let a cable run warm indefinitely.

Short-circuit (magnetic) protection

For distribution (line) protection, the instantaneous magnetic setting Ii is fixed at 10 × In. For the motor-protection version, which has a magnetic-only trip unit, Ii is fixed at 12 × In, giving more headroom for a motor’s starting current. The catalogue test confirms no trip at 80% of Ii and a trip at 120% of Ii. Himel notes there is no motor-protection version in the 800 A frame.

When a magnetic-only MCCB protects a motor, overload protection comes from a separate device, usually a thermal overload relay with a contactor. How the breaker, contactor and relay behave together under fault is called coordination, defined as Type 1 or Type 2 in IEC 60947-4-1.

How to size an HDM3: a step-by-step method

1. Find the design current (Ib) of the circuit from the load schedule.
2. Pick In so that Ib ≤ In, and check that In does not exceed the current-carrying capacity of the cable it protects. With a fixed trip unit, choose the nearest standard rating rather than oversizing.
3. Pick the frame that contains that In: 100, 160, 250, 400 or 800 A.
4. Pick the breaking class (S, F or N) so that Icu is at least the prospective fault current at the board. Check Ics if service continuity after a fault matters.
5. Choose the trip-unit type: thermal-magnetic for distribution circuits (Ii = 10 × In), or magnetic-only for motor circuits used with an overload relay (Ii = 12 × In).
6. Choose poles: 3P for three-phase loads, 4P where the neutral must be switched. The catalogue lists three 4P neutral arrangements: N connected directly without contacts, N switched but unprotected, or N switched and protected.
7. Add accessories the panel logic needs: shunt trip, undervoltage release, auxiliary contacts or a motor operator.
8. Check discrimination (selectivity) with the upstream and downstream devices, so a fault on one outgoing circuit trips only its own breaker.

Worked example

A three-phase feeder to a sub-board has a design current of 140 A, and the designer’s calculated fault level at that board is 30 kA. Step 2 gives In = 160 A. Step 3 gives the HDM3-160 frame. On Icu, the S class (35 kA) already meets the 30 kA fault level. If the specification also requires Ics at or above the fault level, the S class (Ics 21 kA) falls short and the F class (Ics 30 kA) meets it. The cable must, of course, be rated for at least 160 A.

Common sizing mistakes to avoid

• Matching frame size instead of In. A 250 A frame fitted with a 160 A rating protects very differently from a 250 A rating.
• Reading Ics as Icu, or the reverse. Tender documents sometimes specify one while quotations show the other. Compare like with like.
• Ignoring the calibration temperature. Himel publishes separate selection tables for 40 °C and 50 °C products. Make sure the datasheet you approve matches the reference you order.
• Using a distribution breaker on a motor feeder without checking that the 10 × In magnetic setting will ride through the motor’s starting current.
• Leaving accessories off the order. Shunt trips and auxiliary contacts are easier to specify up front than to retrofit during testing and commissioning.

Why traceable sourcing matters for MCCBs

An MCCB’s breaking capacity is only as good as the test certification behind it. Himel lists KEMA, TÜV and CE certification across the HDM3 frames in its catalogue. Buying through an authorised channel means the device you install is the device those certificates describe. As our partners page sets out, everything we supply is traceable to its source, with the documentation to support tender and as-built submissions.

Our team supports panel builders, M&E contractors and consultants with product selection and sizing, the same approach behind our published project references, from HDB estates to Seletar Mall.

Frequently asked questions

What is the difference between the Himel HDM2 and HDM3?

Both are Himel MCCB ranges and both are on our HDM series product page. Specifications differ between the two, so confirm rated current, breaking capacity, poles and trip unit against the datasheet for the exact reference.

Can I adjust the trip settings on an HDM3?

No. The standard HDM3 has a fixed thermal-magnetic trip unit, so overload and short-circuit thresholds are set by the rated current you choose. Himel’s HDM3E family uses an electronic trip unit with adjustable settings.

What breaking capacity do I need?

At least the prospective short-circuit current at the point of installation. This figure comes from the electrical design of the installation. Send it to us with the board schedule and we will match the breaking class.

Which standard does the HDM3 comply with?

Himel’s catalogue refers to IEC 60947-2, the international standard for low-voltage circuit-breakers, which builds on the general rules in IEC 60947-1.

When should I use a 4-pole MCCB?

Where the neutral must be switched or isolated together with the phases. The HDM3 offers 4P versions with an unswitched neutral, a switched but unprotected neutral, or a switched and protected neutral.

Do you stock the HDM3 in Singapore?

Yes. The HDM series is held in local Singapore stock. Send us the rating, poles and fault level and we will confirm the model and availability the same business day.

Get the right HDM3, quickly

Tell us the design current, number of poles and fault level, or simply send the board schedule, and we will confirm the HDM3 frame, breaking class and accessories. Browse the Himel range or the full product catalogue, or contact our team.

Related reading: How to Choose the Right Himel HDB9H MCB for Your Project · Why the Himel HDR3S Thermal Overload Relay Is Essential for Motor Safety · Why Contactor Selection Matters for Reliable Motor Control

Sources

• Himel: HDM3/3L/3E Series MCCB catalogue (selection guide, technical parameters, trip characteristics)
• Himel: Panel Builder brochure (MCB and MCCB panoramas, HDB9 and HDB3w ratings)
• IEC 60947-2:2024, Low-voltage switchgear and controlgear, Part 2: Circuit-breakers
• IEC 60947-1:2020, Low-voltage switchgear and controlgear, Part 1: General rules
• IEC 60898-1:2015/AMD1:2019, Circuit-breakers for household and similar installations
• IEC 60947-4-1:2023, Contactors and motor-starters
• Singapore Standards: SS 638:2018+C1:2020+A1:2022, Code of Practice for Electrical Installations
• Singapore Standards eShop: IEC 60947-2:2024 listing
• myElectrical: Understanding LV circuit breaker fault ratings (Icu, Ics, Icw, Icm)
• Trans-Digi Global: Himel MCCB HDM series (HDM2/HDM3) product page