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There can be unpleasant repercussions for many if a circuit breaker in improperly sized or set for abranch motor circuit

If a motor isn't adequately protected, a short circuit will likely result in premature replacement. Then there'sthe problem of nuisance tripping upon initial start-up because the magnetic setting of the breaker is too low.Both of these situations can cause any electrical worker to sweat, as the facility manager hovers over him,impatiently waiting for his production line to fire up so he can make quota.

Traditionally, an electrician has had to rely on the manufacturer's setting, which is inevitably changed prior tostart-up. This leads to a trial-and-error process to establish a breaker setting, particularly when working withmotor circuit protectors !"#s$, a %&-recognied instantaneous trip breaker used e(clusively on branchmotor circuits. )nce installed and after setting the appropriate dial on the !"#'s faceplate, if the breakerdoesn't trip upon start-up, it is usually left alone, possibly leaving the setting too high for proper protection.

*nother school of thought is to reduce the trip level until it trips, then raise it slightly, thus achieving thebreaker setting. +owever, that setting may also not provide the best overall protection for the motor.

Tracking that breaker setting can be a little nerve-racking.

It's not an e(act science, but neither are the comple( mathematical algorithms that have been developed toget closer to an appropriate setting. "omplicating the issue even further are the various types of motors

available, including standard and energy-efficient models.

The nuances of siing and setting !"#s for branch motor circuits to protect against short circuits requiresyou to have a firm understanding of the " and overcome the challenges of initial motor inrush currents.The advent of electronic !"#s, which use a microprocessor to guarantee the correct settings, can help takethe guesswork out of the initial setup of these types of protective devices.

Initial current in-rush

hen installing a piece of equipment or replacing branch-circuit protection for e(isting equipment, initialstart-up can be nerve-racking, and the reason for all the trepidation is motor current inrush, or the surge incurrent that occurs at start-up to get through the motor's resting torque. /epending on the motor type 0 astandard motor or one of the new energy-efficient motors that are now on the market 0 and other circuitparameters, the duration of that initial current spike can be 1 to two full cycles before decaying to themotor's operating current 2&*$. 2or e(ample, the operating current for a motor may be 3*, but it may take afull 4.45 seconds for the current to decay to that level. *t start-up, current inrush may top out at 6*. *n !"#

has to be able to 7ride through8 this inrush without tripping. If a trip occurs, it's back to the drawing board toalter the setting 0 much to the chagrin of the plant manager.

The " states in Table 9:4.5; that short-circuit protection for !"#s shall be no more than eight times the2&*. There is an e(ception that allows protection to be set to 3: times the 2&* for standard motors types *,B, ", and /$ and up to 3< times the 2&* for high-efficiency motors. +owever, it's also important to note thatour own field e(perience and research has found that the average set point is a whopping ;: times higherthan 2&* for both standard and energy-efficient motors, which not only violates " requirements but alsoleaves the motor unprotected.

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)bviously, this is not an optimal situation. If breaker protection is oversied, it may not trip during a currentspike, which will result in motor burnout and early replacement. This means added costs for the plant 0 notonly for a new motor and the labor to install it, but also for troubleshooting time to set the circuit protectionfor that new motor. If breaker protection is undersied, annoying nuisance tripping becomes the problem,meaning more time has to be taken for ad=ustments.

The solution is to locate the fine line between oversied and undersied protection on the front end of theprocess and set the breaker accordingly. But ascertaining that fine line is easier said than done.

MCPs defined

"onventional !"#s are electro-mechanical trip mechanisms that work purely on magnetics, meaning theyare designed to protect a motor from short-circuit events. *n !"# senses a difference in current quickly,and instantaneously opens in the event of a short circuit. *n !"# is designed to allow for the initial inrushcurrent of the motor it's connected to.

*n !"# typically has a single dial called a magnetic ad=ustment range. *fter determining the motor's 2&*from its nameplate, the electrical worker refers to " Table 9:4.5; and selects the appropriate initialbreaker trip level, then rotates the dial with a screwdriver to the selected setting. 2ollowing that is theprocess of trial and error described earlier 0 if the !"# doesn't trip upon start-up, the trip level should bereduced until it does, and vice versa if it does trip initially.

But a conventional !"# is somewhat limited, because although it is designed to 7dial back8 protection after

initial current in-rush, there can still be a protection gap above the motor's locked rotor current. 2or e(ample,a contractor may set an !"# to protect for up to 35* of in-rush current, before setting back to >* foroperating current protection. But what if the locked rotor current is only :*, a figure that might not be knownat the time the !"# is set? That creates a gap between locked rotor current :*$ and ma(imum operatingcurrent protection >*$. In other words, the !"# won't trip until it senses >* of current, but a spike couldreduce the operational life of a motor, requiring early replacement and related costs and hassle.

!athematical equations have been developed to reduce this protection gap, incorporating locked rotorcurrent along with factors such as percent loading, operating temperature, and in-rush characteristics, andthus allow a conventional !"# to better protect the motor by reducing the protection gap. *lthough thesecalculations have been used for decades, they take time and offer no guarantee that the fine line betweenshort-circuit and overload protection is achieved.

In the prior e(ample, perhaps the calculations reduce the protection gap by ;*, meaning the !"# won't tripuntil it senses 6* of current. +owever, the danger of burning out the motor is still present. orst of all, the

electrical worker might not even know it, and could be in for some heavy e(plaining later.

*n alternative now entering the market is the electronic !"#, which uses an internal microprocessor tocalculate the algorithms necessary to not only reduce the gap in short-circuit protection on initial current in-rush, but also to eliminate it. 2or e(ample, an electronic !"# could be set to ride through the initial currentin-rush of >*, then automatically set back protection to the locked rotor current amperage of 9*, allowing themotor to run on an appropriate operating current. In the event of a short circuit, the microprocessor will orderthe breaker to trip instantaneously.

Achieving balance

*chieving proper balance between overload and short-circuit protection, along with gaining "compliance, doesn't have to be a source of angst. ew technologies are entering the market that will makethe process easier, provide adequate, compliant motor circuit protection, and reduce the incidence ofburned-out motors or nuisance tripping.