Fan Engineering: Spark Resistance Ratings

Every once in a while we will see a specification for "explosion proof" fans. While this may be a desirable characteristic, "explosion proof" is not a specifiable option, and usually is included because of confusion with electrical component (i.e. motors, disconnects) specifications.



Instead, fans are generally classified by "spark resistance". AMCA has created a standard that defines three different levels of spark resistance, classes A, B and C.

These classes, listed in decreasing order of assurance, are generally concerned with the prevention of sparks caused by the rubbing together of spark-producing metallic components (generally ferrous materials). These classes only address spark risks due to an explosive airstream, and do not address explosive conditions outside the fan. A summary of the different levels of protection is found in this helpful engineering paper from Twin City Fans.

Type C: The fan is designed so that if the impeller or shaft comes loose and shifts during operation, two ferrous parts will not come into contact.

Type B: In general, this requires a nonferrous impeller and a nonferrous rubbing ring around the shaft hole. Also, extra locking systems are required to prevent the fan impeller, shaft, and bearings from shifting.

Type A: This requires a nonferrous airstream. Also, the extra locking systems are required as in Type B.

As with any engineering decision, the correct level of spark resistance to specify depends strongly upon the particulars of the project: The gasses or substances expected, the concentration of these contaminants, the location of the air-moving device, etc.

Smarter Starters

As anyone who has specified or supplied them on projects knows, motor starters can be a big headache. And despite the growth in the use of VFD's in today's energy conscious designs, motor starters still are used on about 60-70% of the motors provided in the industry. Typical problems can range from the very basic to the more involved. Some of the problems our customers report are:

• Incorrect overload protection provided or heaters incorrectly sized
• Incorrect control signal available on the site
• Not enough BAS points to provide needed damper or valve interlocks

For the most part, starters are old technology. Cerus Industrial re-examined starters and found that a lot of benefit can be gained from re-working these components to take advantage of today's technology.

Cerus BAS Starter

Protecting The Motor

Besides starting the motor, the main function of a motor starter is to provide a measure of protection to the motor in case of an electrical malfunction. Typically, this protection is provided by means of an overload protection in the starter that trips power in case the motor pulls more amps than it is safely rated for. Traditionally, this has been provided by means of small heater elements that are rated for the motor FLA. These are interchangeable and different motor operating conditions will require different heaters.

In practice, these heaters can be problematic--Essentially, they must be sized correctly for the operating amps of the motor, and this may or may not be known at the time they are purchased. Many things can change a motor duty point from the design to the installation: product substitution, air or water balancing, changes to the design, etc. And since these heaters have very small operating ranges, even a small change in any of these criteria can result in a heater change. In fact, replacing starter heaters are one of the most commonly reported change orders.


Adjustable thermal overloads have been developed to allow a greater amp range for a starter overload protection, but the biggest ranges available are provided via solid state overloads. Solid state overloads typically provide about a 5:1 amperage adjustment (from 1A to 5A, or 4.4A to 22A in two typical sizes). This essentially allows a single overload device to protect any motor that would reasonably be started by a given contactor, meaning that starters equipped with solid state overloads will always have the right overload protection for the motor they are starting. And solid state overloads add phase loss protection, giving your equipment even better security.

Cerus has standardized on solid state overloads for their HVAC starter line, to greatly simplify the process of selecting the correct starter and to avoid last-minute trips to the electrical supply house to get the right overload protection.


Communications

Almost all available starters use 120v relays to communicate with building controls. However, almost all available building operating systems use 24v power for their signals. This means that in order to provide a control signal to the starter from the BAS, interposing relays are required to convert the 24v signal to a 120v signal. These relays are usually field wired, and therefore add complexity and field labor costs to the installation of motor starters.

Cerus has greatly simplified the interface between the BAS and their starters.


All of their standard starters have a terminal strip that can accept a 120vac, 24vac or 24vdc signal directly. The terminal is self-calibrating, and will adjust to whatever signal is provided automatically, greatly simplifying field wiring. In addition, this terminal strip has a terminal that will accept a dry contact, if that is the preferred method of control.

But even greater communication flexibility is provided in Cerus' new line of BACnet enabled starters. Integrating BACnet allows direct communication with the BAS system via a single network connection. The BAS system can provide start/stop signals, interlocked damper control, receive proof of flow via current sensing, monitor the runtime and HOA position remotely, detect phase failure or phase loss, and enact fireman's overrides or emergency shutdowns. These starters provide unequaled ease of use and troubleshooting capabilities and require the absolute minimum in field labor to accomplish this.

BACnet points matrix


Simplified Interlocks

Many fan systems have control dampers that are intended to close when the fan is not operating. These dampers can serve to isolate a single fan in a multiple fan array, or may serve to prevent undesired airflow when the fan is not operating. However, these sorts of systems present a problem--The damper must be opened before the fan is turned on in order to prevent overpressurization of the ductwork. This is typically is handled by the BAS sending a signal to open the damper, having and endswitch on the damper send a signal back to the BAS to indicate that it is open, and then having the BAS send a signal to start the fan. This requires three control points on a BAS controller. Often, these control points are limited, and may or may not be available without a significant cost impact. Cerus has again simplified things by providing for a direct interlock between the fan motor and damper. In this system, the BAS sends a signal to start the fan, then the starter sends a signal to the damper and waits for a confirming end-switch signal before it will initiate the fan. This eliminates two control points from the BAS and greatly simplifies this critical interlock.



Additionally, the control terminal strip for the Cerus BAS starter has dedicated contacts for emergency shutdown and fireman's override.

And More to Come

Cerus is quickly adding functionality to their BAS starter line and upcoming features will include revenue-grade power metering and automatically calibrating motor overload protection.

Smarter starters use today's technology to reduce installation costs, minimize field problems and simplify troubleshooting.

Cerus BAS motor starter specification
Cerus BACnet motor starter specification