Overload setting, instantaneous trip and frame selection, featuring the CJMM3 series moulded case circuit breaker from C&J Electrical
An MCCB can be the right frame size, on the right feeder, with the right breaking capacity, and still be set wrong. The frame decides how much current the breaker can carry. The trip setting decides when it gives up and opens the circuit. Those are two separate decisions, and only the first one is settled by the frame table.
This article looks at what the trip setting controls, where a fixed setting is enough, where an adjustable one earns its place, and how the CJMM3 series is specified.
What the trip setting does
A moulded case circuit breaker carries two settings in one device. The overload element is the slow one: it allows a modest overcurrent to run for a while, because that is what starting a motor or switching on a group of loads looks like. The instantaneous element is the fast one: above a much higher current it opens immediately, because that is a short circuit and nothing is gained by waiting.
Set the overload element too low and the breaker trips on normal starting current. Set it too high and it stops protecting the cable. The instantaneous element has the same problem in a shorter time frame: too low and it trips on inrush, too high and it lets a fault run.
Fixed or adjustable?
On a fixed device the two settings are built in. The overload threshold follows the rated current and the instantaneous threshold sits at a multiple of it, so the user selects the rating and nothing else. On the CJMM3 series a fixed device is set for distribution protection at 8/10 In or for motor starting protection at 12 In, depending on the version ordered.
That is usually enough where the load is known and does not change. It becomes awkward where one breaker rating has to suit several feeders, where the load is expected to grow, or where the protection has to be fine tuned on site rather than on the drawing board. That is the case the adjustable version is built for.
What you can adjust
The adjustable version of the CJMM3 keeps the same frame and the same breaking capacity, and adds two dials. The overload long-delay tripping setting can be moved between 0.7 and 1.0 In, or between 0.8 and 1.0 In, depending on the option ordered. The short-circuit instantaneous tripping setting can be moved between 5 In and 10 In.
Both settings are optional, and the adjustment range is quoted against In, so the same dial covers a wider span of loads than the rated current alone would suggest. The catalogue also states how the short-circuit protection is verified: at 0.8 times the setting the device does not operate within 0.2 s, and at 1.2 times the setting it does. That is the window in which the setting is shown to hold.
- ⚡Wider load range per rating. One frame can be dialled to suit several feeders instead of one.
- ⚡Field adjustment. The setting can be moved on site when the load turns out different from the design.
- ⚡Margin for growth. A feeder that is expected to grow can be set low now and raised later.
- ⚡Verification stated. The catalogue gives the 0.2 s non-operation and operation points rather than only a range.
Matching setting to load
The starting point is the load current, not the breaker rating. For a distribution feeder the design current of the cable has to sit below the setting with enough margin for the breaker not to trip on normal peaks, and above the load current with enough margin to protect the cable. For a motor feeder the calculation runs the other way: the setting has to sit above the starting current, which is several times the full load current, and the instantaneous element has to be set clear of the inrush that comes with it.
Frame and breaking capacity
The CJMM3 series covers five frame grades, 125, 160, 250, 400 and 630 A, with rated currents listed for each. Breaking capacity is offered at two levels, L for standard and H for high, so the same frame can be ordered to suit a higher or lower prospective fault current.
The CJMM3 series
The CJMM3 series is a moulded case circuit breaker manufactured by Zhejiang C&J Electrical Holding Co., Ltd. It is declared to IEC 60947-1 and IEC 60947-2, comes in 2-pole, 3-pole and 4-pole versions, and covers the frame grades above with a rated operational voltage of 400/415 V or 690 V AC. Both single magnetic and thermal magnetic releases are available, and the series lists an isolation function on every frame.
Key technical data of the CJMM3 series:
| Type | Specifications |
|---|---|
| Standard | IEC 60947-1 / IEC 60947-2 |
| No. of poles | 3P and 4P |
| Rated current In (A) | 125: 16-125 · 160: 50-160 · 250: 125-250 · 400/630: 225-630 |
| Rated operating voltage Ue | 400/415 V, 690 V AC · 50/60 Hz |
| Breaking capacity level | L (standard) / H (high) |
| Protection type | Distribution (8/10 In) · Motor (12 In) |
Note: Confirm the final selection and the trip settings against the load, the cable and the applicable installation requirements.
Applications
- Distribution feeders. Frame and breaking capacity selected from the fault level at the point of installation.
- Motor starters. The 12 In motor starting characteristic is available on the fixed version.
- Feeder groups with mixed loads. The adjustable version allows one rating to be dialled to suit more than one feeder.
- Panels built for export. 690 V capability on the larger frames covers systems above 400/415 V.
- OEM and replacement work. Frames from 125 A to 630 A with a choice of L or H breaking capacity.
About C&J Electrical
Zhejiang C&J Electrical Holding Co., Ltd. manufactures low-voltage electrical products, mainly circuit breakers and related protection devices. The company supports OEM brand customization and can ship products as SKD kits for local assembly.
For more information on the CJMM3 series or to request a quotation, please contact us.
Post time: Sep-29-2026