Three-phase Installations -- Pump Control Panels
A pump control panel has four major components which provide four basic functions in a three-phase system. First, the disconnect is the means of disconnecting the pump and its control equipment from incoming power. Second, the contactor switches the pump on and off as directed by a control device (pressure switch, float switch, timer, etc.) Third, fuses or circuit breaker are the primary protection for short circuit electrical faults, and fourth, overload devices protect equipment from being damaged by overload faults. Figure 1 shows a full voltage start pump control panel. Figure 2 is a schematic for the same panel.
The EnclosureThe components providing the four functions listed above are housed in a protective enclosure. One of the primary functions of a pump control panel is to protect you, your customer, and the general citizenry from being injured or killed by the high voltage inside the panel.
I had first-hand experience with the danger of high voltage when I was in high school. I was hooking up my firs welder, a Sears 90-amp buzz box, to the 240-volt drier circuit in my Dad's basement. Being a brainless, macho kind of kid, and not having any respect for electricity, I didn't bother turning off the circuit before attempting the hookup. I figured it would be about like changing a light switch, which I had done several times without turning off the power, by wearing tennis shoes for insulation from the ground, working with one wire at a time, and being careful not to touch any other wires or the metal box. I had been shocked a couple of times, which shocked me more than it hurt.
The bottom line is I somehow got one hand on one leg of the 240 volts and the other hand on the other leg, and was literally thrown across the basement by the force of the electricity. The veins in my arms felt like they were on fire. My arms ached for days. I was lucky to be alive. Had it been 480 volts, I wouldn't be. From that day on, I have had great respect for high voltage electricity.
The safety features of a pump control panel that provide shock protection are the metal or fiberglass enclosure, the lockable door, and the interlock function of the disconnect switch which keeps the door from being opened when the panel is turned on.
Beyond safety, the enclosure protects the electrical components from the panel's environment, dust, dirt, and weather. A NEMA 3R enclosure rating is standard in most pump control panels. Figure 3 is a chart we printed several months ago on the various types of environmental and safety protection offered by the most common NEMA ratings offered in pump control panels.
Some other things to look for in the enclosure are:
- The paint finish should be suitable for the environment in which it will serve.
- It should have mounting brackets, hubs, etc, and hardware necessary to mount the panel and bring power in from the bottom or from the top.
Good-quality latches and door gaskets are important to provide the necessary seal over the life of the panel.
The DisconnectThis component turns the electricity to the pump on and off, much like how the main disconnect (just below your electric meter) turns electricity on and off to your house. Often the disconnect incorporates short circuit protection, fuses or a motor circuit protector.
It should be lockable in the off position so the service person can work on the pump equipment without fear of someone unexpectedly turning on the power. The disconnect handle itself should be interlocked, so the panel door can only be opened with the disconnect switch in the off position.
Short circuit protection is provided by fuses or a motor circuit protector (MCP or circuit breaker as it is often called). Each varies greatly in its ability to respond to short circuit faults. For partial shorts, just larger than an overload, MCPs are faster to respond than fuses, but for a dead short, fuses are faster. The severity of a dead short will vary depending on how close the short is to the transformer, but it could be in the range of 10,000 to 30,000 amps. In this case, a MCP would respond in about one cycle, or 16 milliseconds. A Class R fuse would respond in about 1/2 cycle, or 6 milliseconds, and a Class J fuse in about 1/4 cycle, or about 4 milliseconds.
The J Class fuses are so fast the fuse manufacturers offer a replacement guarantee for components downline from the fuses. If any component is damaged by a short during life of the panel, they will replace the damaged component at no charge. Of course, there is some fine print, but the guarantee says something about their confidence in the J Class fuses.
Overload Faults vs. Short Circuit FaultsThere is sometimes confusion on difference in protection needed for overloads vs. short circuits. Why don't fuses protect for overloads, and conversely, why don't overload devices protect against shorts? Let's review a couple of concepts we talked about in previous articles. First, a motor will draw close to the nameplate current under normal operating conditions. But during the short time when a motor is starting and accelerating to speed, it can draw up to six times the nameplate current. The acceleration time varies depending on motor size, from a fraction of a second for a small motor to several seconds for a large motor.
Secondly, wiring in the system from the pole to pump is sized to carry the amount of current drawn by the pump during normal operation, plus a small safety margin. Any excessive current draw will cause wiring to heat up, increasing its resistance, which causes it to heat up more. Even the in-rush current at start-up causes some heat build-up. The function of fuses and motor current protectors is to provide a weak link in the circuit that will fail before the wiring heats up to the point of melting the insulation and causing any damage.
So, to provide good short circuit protection, the fuses or MCP must be sized small enough to trip before any damage occurs, and large enough to not trip during start up. Or, they must have a time delay built into them to allow the motor to fully accelerate before tripping. Fuses used in pump control panels are dual element, time delay type, so they can be sized at around 125 percent of the nameplate amperage.
MCPs used in the pumping industry are usually magnetic only, instantaneous type, so they must be sized at greater than the 600 percent in-rush value. They are usually set up at 700 percent of nameplate current, but if they trip on start up, the code allows for up to 1300 percent of the full load current. Most MCPs are adjustable, and obviously, the lower the setting, the better the protection.
It is true if a system does not have separate overload protection, and the overload fault is larger than the rating of the fuses or the MCP setting, they will open up, and provide overload protection. However, in a three-phase system, an overload can occur in any of the three legs of the three-phase power, which is why the National Electric Code requires a separate overload device in each leg in a three-phase system.
Overload devices are designed to trip relatively quickly for large overloads and more slowly for small overloads. They are rated by how quickly they respond to a 600 percent overload in terms of seconds. For instance, a class 10 overload will respond in 10 seconds to a 600 percent overload. In other words, if you have a motor that operates at 100 amps and have the overload protection properly set up, it will trip in 10 seconds if the load reaches 600 amps. This gives ample time for the motor to accelerate.
Next month we will finish this section on pump motor control panels by talking a little more about overloads and covering the contactor. We will also touch on reduced-voltage start panels, which are used when the power distribution system cannot handle the in-rush current of a full-voltage start and some accessories often added to a pump control panel.