Four good reasons for not using mcbs in motor protection
Industrial motors are the backbone of industry and commerce, and protective devices ensure their safe and reliable operation. Most machine builders and industrial engineers treat motor protection with care due to the downtime costs if they fail. Yet there are pressures on contractors and machine builders for smaller and lower cost solutions.
According to Schneider Electric, approximately 80% of motors worldwide are less than 15kW. This leads to the question “why do we need a special motor protection circuit breaker (or MPCB)? Isn’t a normal miniature circuit breaker enough?”
There are distinct trip curves for MCBs, and their tripping characteristics are for cable protection, not motor loads. For example, a type C MCB protects general electrical distribution circuits from short circuits and overloads. On the other hand, a type D MCB is for protecting inductive circuits, that can include motors. But, there is more to understand about the standards.
1. Standards matter when it comes to motor protection circuit breakers
When comparing standards for MCBs and MPCBs, it is clear there are some significant differences:
MPCBs conform to IEC 60947-4, which is the standard for motor protection. Yet, since they too are circuit breakers, they also conform to IEC 60947-2.
MCBs conform to IEC 60898-1 (residential applications), though many also conform to IEC 60947-2. For clarification, IEC 60898 devices are not meant for industrial environments. For example, it defines ambient temperature as 30 ⁰C, which is far too low for a typical factory setting. They have no compensation for ambient air temperatures like MPCBs. They can cause nuisance tripping, with all the associated costs of downtime.
2. Optimal short circuit performance
Consider the performance of the devices under a short-circuit. First, a type C MCB trips at 5-10 times the rated current, whereas MPCBs trip at about 12 times the rated current.
With an induction motor, we expect to see an initial surge of current 8-10 times the rated current by design. The MPCB design handles the higher surge currents and avoids nuisance tripping. New high-efficiency IE3 and IE4 have higher starting currents, making it worse.
Oversizing MCBa to allow for the surge current will avoid nuisance tripping but may need larger conductors. A D type MCB, on the other hand, will provide for short-circuit protection. However, no MCB provides a coordinated motor starter protection, so does not provide a safe solution.
Also, the short-circuit breaking capacity of most MCBs is under 10 kA, and too low for industrial environments. Compare this with MPCBs that can offer up to 100 kA breaking capacity to ensure the motor and starter remain safe.
3. Overload performance
Motor overloads are the most common faults seen and account for over half the total motor failures worldwide. It is here that the MPCB excels over the MCB. The table below shows the tripping behaviour of an MPCB under overload conditions, as defined by trip classes in IEC 60947-4.
Now consider a similar table but for MCBs. The difference is clear.
Repeated overloads over 50% can damage the motor. MCBs do not trip fast enough to protect the motor windings. Also, if you look at the second table, the issue becomes even more clear. The C type MCB treats this as a short circuit and the D type curve trips within one minute, whereas the MPCB is much more sensitive and trips within 10s. As discussed earlier, oversizing a C-curve MCB also risks not protecting the motor under overloads.
4. Sensitivity to phase failure
There is one more problem with using an MCB for motor protection – it is not sensitive to phase failures. A phase failure in a motor is a genuine issue that causes a surge in current in the other phases, again leading to overheating and damage to the winding. The MPCB will detect this as an overload in the other phases and trip as per the table above: the MCB will not.
In summary, an MCB cannot provide full protection to a motor and using one is unsafe. Apart from this, an MPCB is easier to select as the motor ratings are on the product itself. It is also possible to adjust it to the particular application’s reference current. The operating life of an MPCB is also much longer than that of an MCB.
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