Type D is the highest-inrush curve among the common B/C/D MCB choices. It is useful for transformer magnetizing current, high-inertia direct-on-line motors, welders, X-ray equipment, UPS rectifier/capacitor inputs and similar loads. The same tolerance that prevents nuisance tripping also makes fault-loop verification more demanding.

Type D trip band and PL10 examples

The magnetic band is a screening calculation. The manufacturer's time-current curve, installation standard and test tolerances govern the actual behavior. The thermal element still protects against sustained overload and must coordinate with the conductor and load.

When Type D is justified

• Transformer magnetizing inrush is documented above the practical C-curve tolerance.

• A direct-on-line motor has a high starting multiple and long acceleration time, and the motor-protection design supports a D curve.

• A UPS, power supply or capacitor bank draws a short charging pulse that repeatedly trips a correctly rated Type C MCB.

• Welding or medical imaging equipment has a manufacturer recommendation for a D-curve device.

• The minimum fault current at the end of the circuit is high enough to operate the D curve within required disconnection times.

Worked comparison: C16 vs D16

Suppose a transformer-fed load draws 9 A continuously and produces a 125 A energization pulse. A C16 begins its magnetic band around 80 A, so nuisance trips are plausible. A D16 begins around 160 A and is more likely to ride through 125 A. However, the D16 choice is valid only if the cable is protected and the minimum short-circuit current is sufficient. If the end-of-line fault current is only 140 A, it is below the lower edge of the D16 instantaneous region; the design must be checked against thermal-trip time and required disconnection time rather than assumed safe.

Complete Type D selection process

1. Measure or obtain the load inrush magnitude and duration; do not select from load category alone.

2. Confirm continuous load current and conductor ampacity after all derating factors.

3. Compare the start profile with manufacturer time-current curves for the candidate C and D models.

4. Calculate prospective maximum short-circuit current to verify breaking capacity.

5. Calculate minimum end-of-circuit fault current or loop impedance to verify disconnection time with the higher D threshold.

6. Check upstream/downstream selectivity and the load manufacturer's required protective device.

7. Select the exact pole model: for example PL10-D16, PL10-D16/2, PL10-D16/3 or PL10-D16/4.

8. Review alternatives such as soft starters, pre-charge circuits, inrush limiters, motor-protection breakers or a correctly sized transformer fuse if Type D does not satisfy protection constraints.

When not to move to Type D

• The nuisance trip is caused by overload, loose terminals, a failing motor or insulation fault rather than normal inrush.

• The circuit has high loop impedance or a long cable run, leaving insufficient magnetic fault current.

• The equipment specification requires a different protective device or maximum rating.

• The available short-circuit current exceeds the MCB breaking capacity.

• A soft starter, VFD, pre-charge resistor or sequenced energization solves the inrush at system level more appropriately.

Frequently Asked Questions

Can PL10-D16 replace PL10-C16?

It is physically related within the PL10 family, but electrically it is a different protection curve. Replace only after inrush, loop impedance, disconnection time, cable protection and coordination checks.

Which Type D model is used for a three-phase 32 A load?

PL10-D32/3 is the three-pole model pattern. The rating and curve must still be validated against load current, start current and fault conditions.

Is Type D safer for motors?

No. It is less sensitive in the instantaneous region. It is appropriate only when high starting current requires it and fault protection remains adequate.

Can a D curve stop all nuisance trips?

No. It will not correct sustained overload, equipment faults, undervoltage, poor connections or an undersized circuit. Diagnose the trip cause first.

Bottom line

Type D is a specialist high-inrush solution. Translate the decision into an exact model such as PL10-D16/3 only after measured inrush, cable protection, breaking capacity and minimum fault current have all been verified.

For a Type C vs Type D check, provide ASBEAM with the load nameplate, start method, inrush current/duration, cable length/size, supply voltage, poles and available fault current.

Technical references

• ASBEAM - Eaton PL10 B/C/D model range

ASBEAM - Eaton PL10-C32 comparison product