AC Breaker Pakistan: Curve B C D Selection
AC Breaker Pakistan 2026 — Choosing Curve B, C, or D for Home, Office & Industrial Loads
Updated: May 2026 · CNC Electric Pakistan
AC Breaker Curve — Pakistan Quick Answer (2026)
Pick the AC MCB trip curve by the inrush behaviour of your load: Curve B (3-5×In) for lighting and electronics; Curve C (5-10×In) for sockets, ACs, fridges, fans, the everyday Pakistani home default; Curve D (10-20×In) for motors, pumps, welding sets, transformers. Wrong curve = nuisance tripping (Curve B on a motor) or undetected fault (Curve D on a sensitive load). CNC AC breakers in Pakistan: 1P from Rs.245, 2P from Rs.450, 4P 63 A from Rs.1,350.
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What an AC Breaker Actually Does
An AC breaker (formally an AC MCB — Miniature Circuit Breaker) sits on your DIN rail and does two protection jobs simultaneously:
- Thermal trip on sustained overload — the bimetallic strip inside bends as it heats. A 20 A breaker on a 25 A load trips in about 60-200 seconds. A breaker on a 60 A load trips in 1-5 seconds.
- Magnetic trip on short-circuit — a coil senses fault current and snaps the contacts open in <10 ms when current exceeds the curve threshold.
The trip curve (B / C / D) defines the magnetic threshold — how high above rated current the breaker tolerates before snapping open instantaneously. This is what makes the difference between a breaker that nuisance-trips every time your fridge compressor starts, and one that doesn't.
Curve B vs C vs D — IEC 60898 Explained
| Curve | Magnetic trip range | Tripping behaviour | Use for |
|---|---|---|---|
| B | 3× to 5× rated current | Most sensitive — fastest to trip | Pure resistive: incandescent lighting, heaters, electronics, computer / IT rooms |
| C | 5× to 10× rated current | Balanced — Pakistani household default | Mixed loads: sockets, fridge, AC, washing machine, microwave, fan, LED lights |
| D | 10× to 20× rated current | Tolerates heavy inrush — slowest to trip | Motors, pumps, transformers, welding sets, X-ray, high inrush HVAC |
The breaker is marked with the curve letter on the front (e.g. "C16" = Curve C, 16 A rated). Always match curve to load behaviour — the rated amperage alone is not enough.
Why Wrong Curve = Real Problems
The classic Pakistani complaint: "My fridge breaker keeps tripping"
Typical scenario: 1.5 HP fridge or 1 ton inverter AC on a 16 A Curve B MCB. Steady running current ~5 A, well within the breaker's 16 A rating. But the compressor inrush — 6 to 8× running current for 100 ms — peaks at 30-40 A. The B curve trips at 3-5× = 48-80 A → mostly OK, but a marginal breaker or a tired compressor pushes it over. Result: random morning trips, no fault, just inrush.
Fix: replace Curve B with Curve C. Same 16 A rating, same physical size, same DIN slot. C trips at 5-10× = 80-160 A, comfortably above the inrush spike. Problem disappears.
The industrial complaint: "Pump trips on startup, runs fine after"
10 HP 3-phase pump on a 32 A Curve C MCB. Running current 16 A. Starting inrush: 7× = 112 A for 200 ms. C curve trips at 5-10× = 160-320 A → marginal. Some starts succeed, some don't. Result: hot mornings the breaker trips, cool nights it doesn't (cable resistance changes with temperature).
Fix: Curve D. Trips at 10-20× = 320-640 A, well above the 112 A inrush. The breaker still protects on real faults (a phase-to-ground short produces 1000-5000 A — far above 640 A). Pump starts every time.
The IT/server complaint: "Server crashes during local thunderstorms"
Servers on a 16 A Curve D MCB — chosen "for headroom". Lightning surge induces a 200 A spike for 5 ms. D curve doesn't trip until 160 A sustained. Spike passes through, takes out the power supplies.
Fix: Curve B + Class 2 SPD upstream. The SPD diverts the surge; if it gets through, the fast-trip B curve catches it. Use Curve C if you want a single breaker without SPD.
Pakistan Application Matrix — Pick Your Curve
| Pakistani load | Typical running A | Inrush multiplier | Curve | Rated A |
|---|---|---|---|---|
| LED lighting circuit (8-12 lamps) | 1-2 A | 10× for 100 µs | B | 6 A or 10 A |
| Ceiling fan + room lights | 2-4 A | 3-5× | C | 10 A |
| General socket circuit | 5-13 A | depends on plug | C | 16 A or 20 A |
| Fridge / freezer dedicated | 3-6 A | 6-8× | C | 10 A or 16 A |
| 1 ton inverter AC | 5-7 A | 3-5× (soft-start) | C | 16 A |
| 1.5 ton conventional AC | 8-10 A | 6-8× | C or D | 20 A |
| 2 ton conventional AC | 12-15 A | 6-8× | D | 25 A |
| Electric geyser 2 kW | 9 A | 1× (resistive) | B or C | 16 A |
| Water pump 1.5 HP single-phase | 7 A | 7-9× | D | 16 A |
| Borewell pump 5 HP 3-phase | 8 A | 7-9× | D | 16 A or 20 A |
| Welding inverter | 15-30 A | 3× during strike | D | 32 A or 40 A |
| Shop LED signage + fans | 5-8 A | 10× | C | 16 A |
| Office IT server + UPS | 10-20 A | 3-5× | B (with SPD) | 20 A or 32 A |
CNC AC Breaker Range — Models & Prices (Pakistan 2026)
YCB7H Series — Workhorse AC MCB
The most popular CNC AC MCB. IEC 60898 compliant, 6 kA breaking capacity, available in B / C / D curves and 1P / 2P / 3P / 4P. DIN-rail snap mount, 18 mm width per pole.
- 1 Pole 6-63 A (B / C / D) — Rs.245 to Rs.380. Standard for branch circuits.
- 2 Pole 6-63 A (B / C / D) — Rs.450 to Rs.900. For wet-area circuits (geyser, bathroom socket) and 2P safety isolation.
- 3 Pole 6-63 A (C / D) — Rs.900 to Rs.1,300. 3-phase branch circuits and motor protection.
- 4 Pole 6-63 A (C / D) — Rs.1,350 to Rs.1,800. 3-phase + neutral, main isolation duty.
YCB6H Series — Higher Breaking Capacity (10 kA)
For installations downstream of larger transformers or where prospective fault current exceeds 6 kA. Same physical size as YCB7H. Use where IEC 60898 10 kA is required (commercial main DBs).
- 1P / 2P / 3P / 4P, 6-63 A, B / C / D curves — Rs.420 to Rs.2,400 depending on configuration.
Pairing with RCCB / RCBO
AC MCBs only handle overload and short-circuit. They do NOT protect against electric shock from earth leakage. Pair with an RCCB (30 mA) on the main feed or RCBOs (combined) on final circuits in wet areas. Pakistani electrical code requires earth-leakage protection on bathroom, kitchen, outdoor, and water heater circuits. See our RCCB selection guide.
Pakistani Home DB — Sample Build with Curves
Typical 14 kW single-phase Pakistani home DB box, designed by curve and amperage:
- WAPDA service drop → meter
- Main MCB 2P 63 A Curve D (Rs.900) — isolation, fault protection
- Voltage protector YC7VA 63 A (Rs.2,800) — over/under voltage protection
- SPD YCS6-C 40 kA (Rs.700) — surge protection
- 4P 63 A 30 mA RCCB (Rs.3,200) — earth-leakage on the whole house
- Branch circuits:
- 2× 1P 10 A Curve B — bedroom/lounge lighting (Rs.245 each)
- 4× 1P 16 A Curve C — general sockets (Rs.245 each)
- 1× 2P 16 A Curve B — bathroom geyser (Rs.450)
- 2× 1P 20 A Curve C — kitchen heavy sockets (Rs.290 each)
- 2× 1P 25 A Curve D — AC dedicated circuits (Rs.330 each)
- 1× 1P 16 A Curve D — water pump (Rs.290)
- 1× 1P 32 A Curve D — workshop / heavy tool circuit (Rs.380)
Total panel cost: approximately Rs.13,000-15,000 in breakers + Rs.6,700 in safety devices = Rs.20,000-22,000. Compared to a Rs.400,000 home equipment value at risk, that's 5% of equipment value for full protection.
When to Step Up to MCCB
AC MCBs cap out at 63 A and 6-10 kA breaking capacity. The moment you need more, graduate to an MCCB. Triggers:
- Load above 63 A — single largest load like a 5 ton central AC, lift motor, factory machinery
- Fault current above 10 kA — usually downstream of 500+ kVA transformers
- Need for adjustable trip settings — selectivity with downstream protection
See our MCCB sizing chart guide for the full step-up criteria.
Frequently Asked Questions
What is the difference between Curve B, C, and D AC breakers?
Curve B trips at 3-5× rated current (most sensitive, for lighting and electronics). Curve C trips at 5-10× (default for sockets, ACs, fridges). Curve D trips at 10-20× (for motors, pumps, transformers with heavy inrush). All three protect equally against sustained overload — the difference is in tolerance for inrush spikes.
What is the price of an AC breaker in Pakistan?
CNC AC MCBs in Pakistan 2026: 1P 6-63 A from Rs.245, 2P 6-63 A from Rs.450, 3P 6-63 A from Rs.900, 4P 6-63 A from Rs.1,350. Higher breaking capacity (10 kA YCB6H series) costs 30-50% more.
Which curve do I need for a 1.5 ton AC?
For a 1.5 ton inverter AC use a Curve C 16 A breaker. For a conventional (non-inverter) 1.5 ton AC, step up to Curve D 20 A because the conventional compressor inrush is much higher than soft-start inverters.
My breaker keeps tripping when the fridge starts. What's wrong?
Most likely you have a Curve B breaker on a fridge circuit. The fridge compressor inrush (6-8× running current) exceeds the 3-5× B-curve threshold. Replace with the same amperage Curve C breaker — same DIN slot, same physical size, problem fixed. If a Curve C still trips, check the breaker amperage matches the cable size and the compressor isn't end-of-life.
Can I use a Curve D breaker everywhere "to be safe"?
No. Curve D tolerates 10-20× rated current before tripping — a 16 A D-curve breaker only trips instantly above 160 A. On a sensitive circuit (electronics, lighting), a 100-150 A fault can pass through long enough to damage equipment. Match the curve to the load behaviour — over-curving is as bad as under-curving.
Do I need an RCCB if I have AC MCBs?
Yes. AC MCBs protect against overload and short-circuit (equipment damage). RCCBs protect against earth-leakage (electric shock to people). They serve different purposes. Pakistani electrical code requires RCCBs on bathroom, kitchen, outdoor, and water-heater circuits. See our RCCB guide.
Can I use an AC MCB on a DC solar string?
No — never. AC breakers rely on the natural AC zero-crossing to extinguish the arc when contacts open. DC has no zero-crossing, so the arc sustains and burns through the breaker — potentially starting a fire. Use a purpose-built DC breaker for solar PV and battery circuits.
Pick the right AC breaker curve and amperage
Send us your load list (lights, sockets, ACs, fridge, pump etc.) and we’ll spec the full DB layout with the right curve for each circuit.
Related Collections & Guides
- CNC AC MCBs (YCB7H, YCB6H)
- All Circuit Breakers (MCB, MCCB, RCCB)
- DC Breakers — for solar and battery
- RCCB / RCBO Earth-Leakage Protection
- Voltage Protectors
- Surge Protection Devices
- MCB vs MCCB vs ACB vs RCCB Selection Guide
- MCCB Sizing Chart Guide (when to step up)
- CNC Electric Pakistan 2026 Price List