If a motor isn't adequately protected, a short circuit will likely result in premature replacement. Then there's the 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 to start-up. This leads to a trial-and-error process to establish a breaker setting, particularly when working with motor circuit protectors (MCPs), a UL-recognized instantaneous trip breaker used exclusively on branch motor circuits. Once installed and after setting the appropriate dial on the MCP's faceplate, if the breaker doesn't trip upon start-up, it is usually left alone, possibly leaving the setting too high for proper protection. Another school of thought is to reduce the trip level until it trips, then raise it slightly, thus achieving the breaker setting. However, that setting may also not provide the best overall protection for the motor.
It's not an exact science, but neither are the complex mathematical algorithms that have been developed to get closer to an appropriate setting. Complicating the issue even further are the various types of motors available, including standard and energy-efficient models.
The nuances of sizing and setting MCPs for branch motor circuits to protect against short circuits requires you to have a firm understanding of the NEC and overcome the challenges of initial motor inrush currents. The advent of electronic MCPs, which use a microprocessor to guarantee the correct settings, can help take the guesswork out of the initial setup of these types of protective devices.
Initial current in-rush
When installing a piece of equipment or replacing branch-circuit protection for existing equipment, initial start-up can be nerve-racking, and the reason for all the trepidation is motor current inrush, or the surge in current that occurs at start-up to get through the motor's resting torque. Depending on the motor type — a standard motor or one of the new energy-efficient motors that are now on the market — and other circuit parameters, the duration of that initial current spike can be ⅛ to two full cycles before decaying to the motor's operating current (FLA). For example, the operating current for a motor may be 1A, but it may take a full 0.05 seconds for the current to decay to that level. At start-up, current inrush may top out at 6A. An MCP has to be able to “ride through” this inrush without tripping. If a trip occurs, it's back to the drawing board to alter the setting — much to the chagrin of the plant manager.
The NEC states in Table 430.52 that short-circuit protection for MCPs shall be no more than eight times the FLA. There is an exception that allows protection to be set to 13 times the FLA for standard motors (types A, B, C, and D) and up to 17 times the FLA for high-efficiency motors. However, it's also important to note that our own field experience and research has found that the average set point is a whopping 23 times higher than FLA for both standard and energy-efficient motors, which not only violates NEC requirements but also leaves the motor unprotected.
Obviously, this is not an optimal situation. If breaker protection is oversized, it may not trip during a current spike, which will result in motor burnout and early replacement. This means added costs for the plant — not only for a new motor and the labor to install it, but also for troubleshooting time to set the circuit protection for that new motor. If breaker protection is undersized, annoying nuisance tripping becomes the problem, meaning more time has to be taken for adjustments.
The solution is to locate the fine line between oversized and undersized protection on the front end of the process and set the breaker accordingly. But ascertaining that fine line is easier said than done.
Conventional MCPs are electro-mechanical trip mechanisms that work purely on magnetics, meaning they are designed to protect a motor from short-circuit events. An MCP senses a difference in current quickly, and instantaneously opens in the event of a short circuit. An MCP is designed to allow for the initial inrush current of the motor it's connected to.
An MCP typically has a single dial called a magnetic adjustment range. After determining the motor's FLA from its nameplate, the electrical worker refers to NEC Table 430.52 and selects the appropriate initial breaker trip level, then rotates the dial with a screwdriver to the selected setting. Following that is the process of trial and error described earlier — if the MCP doesn't trip upon start-up, the trip level should be reduced until it does, and vice versa if it does trip initially.
But a conventional MCP is somewhat limited, because although it is designed to “dial back” protection after initial current in-rush, there can still be a protection gap above the motor's locked rotor current. For example, a contractor may set an MCP to protect for up to 15A of in-rush current, before setting back to 8A for operating current protection. But what if the locked rotor current is only 3A, a figure that might not be known at the time the MCP is set? That creates a gap between locked rotor current (3A) and maximum operating current protection (8A). In other words, the MCP won't trip until it senses 8A of current, but a spike could reduce the operational life of a motor, requiring early replacement and related costs and hassle.
Mathematical equations have been developed to reduce this protection gap, incorporating locked rotor current along with factors such as percent loading, operating temperature, and in-rush characteristics, and thus allow a conventional MCP to better protect the motor by reducing the protection gap. Although these calculations have been used for decades, they take time and offer no guarantee that the fine line between short-circuit and overload protection is achieved.
In the prior example, perhaps the calculations reduce the protection gap by 2A, meaning the MCP won't trip until it senses 6A of current. However, the danger of burning out the motor is still present. Worst of all, the electrical worker might not even know it, and could be in for some heavy explaining later.
An alternative now entering the market is the electronic MCP, which uses an internal microprocessor to calculate the algorithms necessary to not only reduce the gap in short-circuit protection on initial current in-rush, but also to eliminate it. For example, an electronic MCP could be set to ride through the initial current in-rush of 8A, then automatically set back protection to the locked rotor current amperage of 4A, allowing the motor to run on an appropriate operating current. In the event of a short circuit, the microprocessor will order the breaker to trip instantaneously.
Achieving proper balance between overload and short-circuit protection, along with gaining NEC compliance, doesn't have to be a source of angst. New technologies are entering the market that will make the process easier, provide adequate, compliant motor circuit protection, and reduce the incidence of burned-out motors or nuisance tripping.
Bardsley is a senior product specialist for Square D/Schneider Electric in Cedar Rapids, Iowa.