Protecting high-efficiency motor circuits.

Some types of overcurrent protective devices may not prove suitable for all high-efficiency motor applications.We received a series of questions related to instantaneous-trip circuit breakers (ITCB). The questioner noted that the high inrush characteristics of these motors has been covered in EC&M previously, and he was concerned about nuisance tripping, particularly in the case of instantaneous-trip

Some types of overcurrent protective devices may not prove suitable for all high-efficiency motor applications.

We received a series of questions related to instantaneous-trip circuit breakers (ITCB). The questioner noted that the high inrush characteristics of these motors has been covered in EC&M previously, and he was concerned about nuisance tripping, particularly in the case of instantaneous-trip circuit breakers. His application involved a listed combination controller that had coordinated overload, short-circuit and ground-fault protection for the circuit conductors. He wanted answers to the following questions:

1. Could the ITCB in question be used if its possible trip settings (as it came from the manufacturer) can be increased above the Code maximum of 1300% of the full-load motor current? He noted that several manufacturer's literature mention these higher settings.

2. Would the NEC allow higher settings (above 1300%) if there were an acceptable engineering evaluation of the particular application?

3. How were the maximum percentage allowances in the NEC determined, and does the 1300% parameter take premium efficiency motors into account?

4. If nuisance tripping at 1300% in fact occurs on one of these motors, how can the installation proceed or be corrected within the limits of the Code?

The EC&M panel's response

Motor circuits are unique in that they don't have overcurrent protection in the usual sense. These circuits usually divorce the protection against overloads from the protection against short circuits and ground faults. Overloads are long-time currents that are high enough to eventually cause damage but that don't need to be cleared immediately. Short circuits and ground faults must be cleared immediately, however, since these faults release enormous amounts of energy, easily enough to start a fire. Since overload protection and the short-circuit and ground-fault protection are provided by separate protective devices, there are different requirements for each.

The questions only relate to the short-circuit and ground-fault aspects of protection. Regardless of which device is chosen for this purpose, you still need to select the overload protection in accordance with Part C of Art. 430. Since those rules aren't in question here, we will focus on the basic principles of selecting appropriate short-circuit and ground-fault protective devices, as covered in Part D of the article.

The short-circuit and ground-fault protection must protect both the motor itself and the conductors and other elements of the motor circuit from the effects of these high-level faults. Therefore, the maximum safe ratings of these devices is related to the full-load current rating of the motor. Since there are definite rules that relate the allowable ampacity of the motor circuit conductors to the motor current, basing the protection on the motor also protects the conductors.

The first step is to realize that, unlike running overload protection that is customized to the actual motor, in these cases the motor currents listed in Tables 430-147 to 430-150 must form the basis for any calculations. That rule occurs in Sec. 430-6(a), which reads in part as follows:

(a) General Motor Applications. Other than as specified for torque motors in (b) below and for ac adjustable voltage motors in (c) below, where the current rating of a motor is used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, etc., the values given in Tables 430-147, 430-148, 430-149, and 430-150, including notes, shall be used instead of the actual current rating marked on the motor nameplate. Separate motor overload protection shall be based on the motor nameplate current rating.

The next step is to multiply the motor current by the multipliers in Table 430-152. This result gives the maximum allowable settings. For an ITCB, this allows a typical maximum result equal to 700% of the motor current. In answer to the third question, these percentages are based motor performance characteristics, and they have stood the test of many years of enforcement. ITCBs, however, have some unique rules. The first is that they must only be used in listed combination controllers. The questioner is clearly aware of this requirement, which is in the first sentence of Sec. 450-52(a), 2nd paragraph:

An instantaneous trip circuit breaker shall be used only if adjustable, and if part of a listed combination controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor, and if it will operate at not more than 1300 percent of full-load motor current.

The reader may well wonder where the 700% indicated previously comes in, since the rule allows 1300%. The 1300% is unintended, and resulted from a panel error in the 1993 NEC. During the proposal period, the panel accepted a proposal to go to 1300% by right, bypassing any consideration of 700%. During the comment period, however, the panel decided to retain the prior 700% outer limit, and then modify the exception that follows the paragraph quoted above to allow higher settings if the motor were unable to start, or if an engineering evaluation showed a higher setting would be needed. The question on engineering evaluations is undoubtedly related to this change, which added the last two sentences to the exception:

Exception: Where the setting specified in Table 430-152 is not sufficient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current. Trip settings above 700 percent shall be permitted where the need has been demonstrated by engineering evaluation. In such cases, it shall not be necessary to first apply an instantaneous trip circuit breaker at 700 percent.

The intent was simply make sure that no one was compelled to actually purchase a lower-rated ITCB, watch it fail to start the motor, and only then be able to use the proper size. Unfortunately, when the panel modified the exception, it forgot to go back to the main rule and remove the 1300% figure. This error will be corrected in the next Code. It certainly was never intended to allow an engineering evaluation to supersede the 1300% upper limit.

Protecting high-efficiency motors

The reason for the concern about routine uses of the 1300% factors has to do with high-efficiency motors, such as those described in the question. These motors achieve their efficiency by reducing resistive losses in the motor windings, which simply appear as waste heat during operation. If a motor has very low resistance, however, that means that at standstill the motor will draw a very high current. These currents may well be appreciably higher than even the 1300% parameter, and that is what motivates these questions.

We think that the parameters in the NEC must be observed. The fact that the Code recognizes these devices doesn't mean that if they don't work within Code limits, then those limits may be exceeded. The purpose of the Code is safety, not to protect the market share of any particular type of device. The fact that the product literature mentions higher settings doesn't mean those settings can be used. Furthermore, if those settings are simply expressed (or calculated) in amperes, the higher settings may be entirely appropriate for larger motors.

If the ITCB won't start the motor, then the type of short-circuit and ground-fault protective device needs to be reevaluated. The appropriate way to protect these motor circuits is to use a different type of device that won't nuisance trip, while staying within code rules.

An example

Suppose a 30 hp 460V motor were at issue. According to Table 430-150, this must be figured at 40A for calculation purposes. Suppose that even at the 1300% level (520A) this motor won't start reliably. The solution would be to use an inverse-time circuit breaker or a time delay fuse. Note that although Table 430-152 generally allows 250% or 175% respectively, Sec. 43052(a) Ex. 2 allows 400% or 225% respectively as the upper limit (cannot be rounded up) if the motor just won't start. In this case, that would be 150A or 90A. Taking a closer look at the time-delay fuse, by industry standards it must be able to hold five times its rating for at least 10 sec. In this case it would be able to hold 11 times the full-load current for a full 10 sec. The same fuse will hold eight times its rating (which would be 18 times the full load current) for 2.3 sec. We think that one of these other devices will surely be able to accelerate the motor as required.

TAGS: Design
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