Not To Be Used As A Life Safety Device

Reprinted from Spring 2003 Insights

Not to be used as a life safety device – This phrase appears on a number of Kele catalog pages as a warning to customers that the particular product (usually a gas detector) is not to be relied upon to safeguard humans or animals from a particular hazard.  It may, though, be part of a control system that helps prevent the formation of a hazardous environment under normal conditions when the product is properly applied and maintained.  Even then, such devices, in most cases, cannot protect persons or animals that are intimate with the hazard.

Occasionally, a specification may insist upon a device that is listed or certified as a life safety device.  However, there is no such listing or certification available, except by manufacturers of certain firefighting equipment and personal portable gas monitors.  The only nationally recognized code that employs the term “life safety” is the Life Safety Code, and products are not specified in this code.  Its official title is actually “NFPA 101, Safety to Life from Fires in Buildings.”  There is no agency that certifies compliance with this code.

Some common products, which may be required to be applied as life safety devices, are classified into individual functional groups.  Fire alarm equipment, including smoke detectors, is such a group.  Boiler flame safety equipment is another group.  UL, Factory Mutual, CSA, and other certifying agencies will list and label approved equipment for use in these specific categories.  They are not listed and labeled as “life safety devices” but rather as “Fire Protection Signaling Equipment” or “Gas Flame Safety Equipment.”  Again, it must be emphasized that there is no such certification as “Life Safety” unless it is being applied by a manufacturer to firefighting equipment or personal portable protective equipment.  The Life Safety Code does not specify products; therefore, no product can claim compliance with it.

Even a gas sensor that is listed and labeled as a sensor by UL or another agency should never be used as a last line of defense for life.  Considering the following logic will give a better appreciation for the underlying reasons why:

Time

Gas sensors have inherent response times that are significant.  Even the fastest sensors cannot claim better than 30 seconds, and many refrigerant leak sensors may exceed 15 minutes in response to the presence of a gas.  If a multi-zone sensor is considered, response time for any one zone can exceed even the 15-minute level.  Basically, if a person or animal is in the machine room when a catastrophic release of refrigerant occurs, no sensor will save he/she or it from injury or death.

Power 

Gas sensors are powered by electricity, and there is rarely, if ever, a backup power system applied to them.  If the electricity goes out, the sensor cannot function.

Life Scan

The active portions of gas sensors have limited lives.  If not properly maintained, the sensor cannot function.

Location

Even if a gas sensor has an uninterruptible power supply and is properly maintained, it can only measure gas concentration at its specific location.  At the source of the leak, the concentration of the offending gas will be much higher.  A person or animal nearer to the source of the contaminant may be exposed to a lethal dose; however, the level may never rise to the personal exposure limit at the sensor location if the prevailing airflow is toward the source.

Absolute Protection

Absolute protection from inhalation hazards can only be provided by a source of known-to-be-clean air.  Self-contained breathing apparatus (SCBA) and remote air delivered by compressors and tubing are options that can actually safeguard a life from a toxic gas.  In areas where such exposure is likely, other codes require that such clean air sources be available.

To detect combustible or toxic mixtures at very low levels in order to protect lives in confined spaces or other potentially hazardous atmospheres, personal battery-operated monitors that perform well are available.  These can also overcome some of the shortfalls of fixed gas monitoring, as long as they are regularly tested and maintained.

In conclusion, fixed gas detectors may be used as part of a control system that is designed to prevent a hazardous situation from occurring.  These sensors may be used to signal a control system to begin ventilating a potential hazard or one that has already occurred.  However, none of these sensors can be relied upon absolutely to prevent health effects due to inhalation or combustion in a space that is already occupied.  Their reaction time, level of maintenance, source of power, and proximity to the individuals in question cannot be guaranteed.

Hazardous Atmospheres: Intrinsic Safety

Reprinted from Summer 1999 Insights

In the last edition of 20/20 Insights we covered the use of explosion proof construction to prevent a source of ignition from coming in contact with a room full of fuel and air. Strong enclosures with threaded or flanged covers can confine explosive forces within themselves and cool the escaping gases enough to prevent ignition of the surrounding atmosphere.Wouldn’t it be even better to prevent any explosion at all?

Intrinsically safe systems are designed to do just that. Remember that even if the ideally explosive mixture of fuel and air exists, an ignition source of sufficient energy and duration is required to light it up. Intrinsically safe design limits the available release of energy in the hazardous area to a level well below the minimum ignition energy of the worst-case gas mixture. It also maintains this limiting function in the event of two simultaneous worst-case faults. Since this type of protection is very fail-safe and requires no enclosure maintenance, most engineers (and property insurance carriers) consider it to be even safer than explosion proof construction. Surprisingly, it is usually less expensive to boot!

How does it work? First of all, let’s consider a room filled with hydrogen and air in exactly the best proportion for ignition (about 30 percent hydrogen by volume – see Figure 1). This mixture takes an instantaneous release of about 20 mJ in order to start burning, or else its temperature must be raised above the autoignition temperature of hydrogen (932 deg F or 500 deg C). For electrical devices we wish to place in this room, the amount of stored energy and the rate at which it can be released under worst-case conditions must be kept below these critical levels, and the surface temperature (see Table 2 for ratings) must be kept below the autoignition temperature. If these conditions are met (to the satisfaction of UL, FM, CSA, BASEEFA, etc… ), then the device may be listed and labeled as intrinsically safe. This means that, with a properly applied safety barrier, cabling and ground, it cannot start a fire even under the worst-case conditions. There is one subset of such intrinsically safe devices that can be used without a listing or label (although a barrier, proper cabling and ground are still required). These are designated “simple apparatus” and are a group of things that obviously neither produce nor store any energy (RTD’s, thermistors, switches, and a few others). They may be used as if they did have a label, as long as the operating limitations shown in Table 1 are strictly adhered to.

So, if we have an RTD and an intrinsically safe temperature transmitter in a hazardous location, can we wire them up to our controller and power supply in the safe area and turn them on? Not yet! Three more steps are needed first. While the devices in the hazardous area cannot ignite the gas mixture on their own, the controller and power supply in the safe area may each be capable of transmitting enough energy through the wires into the hazardous area to do the job anyway! An intrinsic safety barrier will prevent this, and is required for every intrinsically safe device (except listed self-contained battery powered units). Barriers range from simple to sophisticated, but they all use fail-safe components to limit the voltage and current that can be passed through them even in the event of worst-case wiring errors. For example, suppose someone in the safe area accidentally hooks 240 VAC up to our temperature transmitter signal lines, and one of the lines is frayed enough at the transmitter to cause an arc at that voltage. Without an intrinsic safety barrier, there may not be enough evidence left to figure out what happened. However, a barrier rated for the atmosphere, cabling, and its field device will save the day even under these circumstances. Figure 2 illustrates a simple zener diode barrier circuit. A high voltage at the safe side terminals will cause the zener diode to draw a high current and blow the input fuse. The series resistors limit the current to the hazardous side. Barriers are also rated for how much capacitance and inductance are allowed on the hazardous side.

This leads us into the cabling between the hazardous area device and its barrier. If the cable is very long and has a high capacitance, it could possibly store enough energy to cause an ignition in the event of a wiring fault. It must be checked against the rating of the barrier. If the field device (the transmitter in our example) has a high capacitance, the combined capacitance of the cable and device must be less than the barrier rating. Cable and device inductance are treated in the same manner.

The final factor to consider is grounding. What good is all this built-in electronic safety if a nearby lightning strike raises the ground potential a couple of thousand volts above the potential of the cable shield in our hazardous area? The resulting arc from the shield to ground can be every bit as effective as a butane lighter in touching off an explosion, and we needn’t have bothered with using intrinsically safe products. An intrinsic safety ground, bonded to the earth ground, is the last essential link that makes the system work. Normally provided at the barrier location, it keeps the cable shields at or near the same potential as the earth, even as that value moves around during storms.

Once it is all together, as shown in Figure 3, the benefits of an intrinsically safe system are many. There is no need to power systems down and”safe” the atmosphere for maintenance. Calibrate, adjust, even swap out bad parts with the system turned on and the atmosphere at its worst – a properly designed system cannot release enough energy to do any damage. 

 

Hazardous Atmospheres: Explosion Proof

Reprint from Spring 1999 Insights

In the last edition of 20/20 Insights we discussed the elements that must be present in order to produce an explosion. The three legs of the “fire triangle” (fuel, oxygen, and an ignition source) are required to support combustion. In addition, the volume ratio of fuel to air must be within the fuel’s explosive limits, and the ignition source must release sufficient energy to ignite the mixture. Removing any of these three elements will eliminate the explosion hazard.

Perhaps the most common and familiar way to eliminate ignition sources from a hazardous location is through the use of explosion proof construction. An enclosure that is rated explosion proof (NEMA 7, 8, 9, or 10) for a particular hazard group (explained below) is strong enough to withstand the pressure of a worst-case explosion inside itself. Additionally, it is designed to vent the resulting hot gases in such a way that they are cooled below the ignition temperature of a worst-case explosive mixture outside the box.

To contain the pressures of an explosion, these enclosures are made from heavy cast steel or cast aluminum. To cool escaping gases, flanged enclosures have extra-wide flanges that are ground to a smooth finish and tight tolerance – thus yielding a very thin, very long path to the outside as shown in the illustration. Enclosures with threaded covers (and threaded connections to either type of enclosure) produce the same effect by virtue of the long, narrow path through the threads. As the hot gases from an internal explosion pass through these long, narrow channels, they give up heat to the metal and their pressure is reduced. These two effects team up to lower the temperature of the gases to a safe level before they can come in contact with the atmosphere surrounding the enclosure.

With flanged enclosures, it is very important to torque the cover bolts evenly and as close as possible to the recommended value. Also, the flange surfaces must not be scratched or marred in any way. Improper torque or damaged surfaces can allow hot gases to escape and ignite an explosive mixture outside the enclosure. Threaded connections or covers must engage at least five full threads to maintain the integrity of the system.

One additional step is needed to control the spread of hot gases – conduits entering the enclosure must be sealed within the code-required distance (usually 18”) of the box to prevent the buildup of pressure within the raceway system or the leakage of combustion products into the room. If a jacketed cable passes through a conduit seal, the jacket should be removed within the seal so that the sealing compound can completely surround each insulated conductor. An alternative is to seal the cable at the end of the jacket as shown in the illustration.

So where can these types of enclosures be used? As usual in our industry, there are no easy answers! Each explosion proof enclosure will be listed or labeled for use in a particular environment as defined in the National Electrical Code (NEC) or IEC Standards. In turn, the hazardous area itself must be classified according to the same standards. The enclosure must have a listing that meets or exceeds the classification of the area in which it is to be used. In the NEC, considerations are Class, Division, and Group as shown in the table below. IEC standards use different code letters from the NEC but generally follow the same logic.

For gases (the majority of our industry’s hazards), explosion proof enclosures are readily available for Class I, Division 1, Groups C and D. Enclosures rated for Group B (hydrogen) can also be found, but generally only in small sizes since it is so easily ignitable and has high explosive energy. Almost nothing is offered for Group A (acetylene) environments because of its easy ignition and tremendous explosive energy.

Using explosion proof enclosures removes the ignition leg from the “fire triangle” in a potentially hazardous location. Although it is costly and requires care to maintain the system’s integrity, it is an effective method for working with electricity in combustible atmospheres. In a future 20/20 Insights we will discuss intrinsically safe systems, another way to remove potential sources of ignition from hazardous locations.

Hazardous Atmospheres: Introduction

Reprinted from Winter 1998/1999 Insights

Automation dealers are continuing to gain business that was once reserved only for specialty and industrial contractors. It’s a trend that is accelerating very rapidly, and the fastest growth is in the areas of hazardous locations and the monitoring of toxic and combustible gases. Kele is committed to providing the products and technical support needed to assist our customers in these important areas. By way of introduction, this article covers the basics of a hazardous atmosphere and the equipment we use to monitor combustible gases. Future editions of 20/20 Insights will contain information relating to principles and application of intrinsically safe systems, explosion proof systems, and other available means of dealing with electrical equipment in hazardous atmospheres.

The widely known “Fire Triangle” illustration shows the three components required to support combustion. All three must be present, and the methods we use to prevent explosions are designed to eliminate one of the three legs of the triangle. What the triangle doesn’t show, though, is that fuel and oxygen must be mixed in the proper proportion in order to burn. If the fuel is methane (CH4, the major component of natural gas), the concentration in air must be between 5 percent and 15 percent or else the mixture will not ignite. Those of us old enough to have worked with finicky carburetors on gasoline engines are familiar with this principle. If the mixture was too lean (not enough fuel) or too rich (too much fuel), the engine would not start. The same applies to ignition of any combustible gas in air.

 The lowest concentration of a gas in air that will ignite is its lower explosive limit (LEL), and the highest concentration that will ignite is its upper explosive limit (UEL). These values are also sometimes referred to as the lower and upper flammability limits (LFL, UFL). Limits for some common fuels are shown in the table of flammability limits at right. If a system is designed to keep the fuel concentration below the LEL, the fuel leg is effectively removed from the fire triangle. Under certain conditions (in an oil field, for example), it is easier to maintain the concentration above the UEL. In this case, the oxygen leg is eliminated. In either case, combustion cannot happen.

Table of Flammability Limits

FlamTable Note: Multiply percentages by 10,000 to convert to parts per million (ppm).

It is often advisable to monitor the concentration of fuel in air, in order to take action or sound an alarm if it is moving toward an explosive level. Kele has gas monitors that are ideal for this purpose. Some sensors provide a 4-20 mA signal over the range of 0 to 100 percent of the LEL, with an alarm relay set to energize at 25 percent of the LEL (an industry standard alarm point). For example, if the gas is methane, (5 percent LEL), the sensor will output 4 milliamps at 0 percent methane and 20 milliamps at 5 percent methane. The alarm relay will energize at 1.25 percent methane. With these devices, the alarm relay or an automation system responding to the analog signal can cause electricity (source of ignition) to be shut off in an area if the gas concentration is rising toward the explosive level (LEL). If the sensor itself must remain energized, an explosion proof enclosure is available to prevent it from becoming the source of ignition itself.

The system described here is based on removing the fuel leg from the fire triangle, then having an automatic means of removing the ignition source leg if the fuel begins to return. This is the first of many ways we will discuss to work safely with electricity in hazardous locations.

Measuring Flow in Tight Spots

Often, one of the most challenging aspects of applying a flow-sensing device is the hunt. Tracking down the elusive and mysterious twenty diameters of straight, accessible pipe that the sensor manufacturer demands can be impossible at times. Let’s face it – it isn’t often that the Architect, Engineer, General Contractor, and all the subcontractors conspire to make the automation guy’s job easier, is it?

If the length of straight pipe upstream and downstream of a flow measuring device doesn’t meet the manufacturer’s published specs, then the manufacturer’s guarantee of accuracy no longer applies. But just how bad will the results be? As long as a few common criteria are met, the answer is “not as bad as you’d expect!”

To insure the best accuracy possible, the flow profile of the fluid to be measured must be as uniform as possible across the pipe or duct, and the velocity must be high enough to ensure turbulent flow. With uniform turbulent flow, almost any placement of a differential pressure or turbine device across the pipe or duct diameter will give a good representation of the average fluid velocity. As the flow profile loses uniformity (close to an elbow or tee, for example), error is introduced since the device placement might be in a region with a higher or lower velocity than the actual average. The same effect can occur if the velocity slows enough to create laminar flow. The flow profile illustration to the right indicates these effects.

Robert Benedict’s Fundamentals of Temperature, Pressure, and Flow Measurement cites several studies in the International Journal of Heat and Fluid Flow which demonstrate that about 8 upstream diameters of straight pipe after an elbow are sufficient to produce ±1 percent variance with an orifice meter. Reduction of straight pipe to only 4 diameters yields ±2 percent variance if a constant correction factor of 0.98 is applied to the orifice discharge coefficient. In either case, the downstream straight pipe need only be two to four pipe diameters in length. If the upstream problem is more obstructive than a simple elbow (a valve, perhaps, or a bullhead tee), the lengths of straight pipe needed are closer to 16 diameters upstream for ±1 percent, and 8 diameters for ±2 percent. Table 1 is derived from Benedict’s work, and is valid for any fluid in fully developed turbulent flow. In typical HVAC applications, turbulence is pretty certain. If in doubt, check to see if the flow in question has a product of velocity (V, feet per minute) and the equivalent circular pipe or duct diameter (D, in inches) that meets the following criteria:

For Air, V x D > 650
For Water, V x D > 20
For Steam, V x D > 110

In summary, even if there isn’t enough straight pipe or duct to meet the manufacturer’s requirements, it is often possible to get a “pretty good” reading anyway, and the results will be very repeatable even if they’re off by a few percent.

Pipes
Table 1: FlowChart

Table 1: Effects of Upstream Obstructions on Flow Profiles. Add the “Resulting Variance” to the manufacturer’s stated accuracy to get an idea of expected behavior of a flow transmitter when the published straight pipe (or duct) length exceeds those in the table. While we cannot guarantee these figures for every application, they are valid for most HVAC/R flow ranges, and may even be improved upon with the use of straightening vanes or honeycomb flow straighteners.

Lighting Controls Can Brighten Business Picture

Reprinted from January 1992 Insights

Forty to sixty percent of the average commercial utility bill is for lighting.  That bit of gloomy news for building owners is good news for companies that sell and install devices designed to control lighting and lighting costs.  Thanks to the availability of a variety of lighting control options, contractors can take advantage of this potentially lucrative business with a minimum of effort.  Convincing a customer to decide on a new or retrofit control installation is made easier because of attractive paybacks that can be projected.  These payback projections can be enhanced when tied to utility company rebates and investment programs that encourage these installations.  As a result, the customer saves money, relationships are reinforced, and energy waste is curbed as much as possible.

Lighting control methods can be relatively simple to extremely sophisticated – with price tags that escalate with complexity.  Among the more common of these methods in use today are: on-off control, occupancy sensors, and light level control.

On-Off Control

The simplest of all lighting controls is the use of mechanical or electronic time clocksTheir function is to turn lights on and off at pre-set times of the day. Each time clock can control one or more lighting circuits.  The down side to time clock control is lack of flexibility and override capability.

Flexibility can be achieved by interfacing with lighting panels that have either their own time-of-day capability or are controlled from a building automation system. The lighting panel allows multiple lighting circuits to be controlled from one switch or control point on an automation system.  These panels also allow flexibility in the assignment of those circuits to various time schedules.

Most lighting panels or automation systems allow for some sort of override function for after hours use and a “flash system” to notify tenants of an impending “lights out” condition.  Another way of accomplishing the override is with a SENTRY switch that replaces the normal light switch to provide manual override of the time-of-day function in individual zones and which can be “swept” off from the lighting panel or automation system.  A sweep function will reset the lights to an “off” condition periodically during unoccupied hours.  These types of systems may also incorporate a remote dial-in function.

The “time-of-day” capability can be especially effective in retail stores and supermarkets.  The store could automatically be illuminated at 100% during store hours, reduced to 60% during stocking, then further reduced to 40% illumination for janitorial duties.  As a side note, these time-of-day system scan be tied to a security system (like an override) and all the lights can be turned on if the building is burglarized.

Occupancy Sensors

Occupancy sensors provide another method of lighting control.  These devices will automatically turn lights off after a pre-determined amount of time if there is no one in a given area. There are a number of different types of occupancy sensors on the market today.  The most common of these are the infrared and ultrasonic occupancy sensors.  The infrared occupancy sensor detects body heat and the ultrasonic type senses a breakup of the ultrasonic signal due to motion. Some occupancy sensors have a wall switch replacement type of detector for easier installation.  In all cases, care must be taken in selecting the location, sensitivity and length of time for which these sensors are set, based on the type of sensor and where it is to be used.

Light Level Control

Lighting effectiveness is being built into modern office buildings and shopping malls as more windows and skylights are being used than ever before.  To take advantage of this, some lighting control systems allow for a footcandle setpoint and then control the level of lighting required based on a photocell input that senses actual footcandles in a given space.  A number of dimming and ballast control systems are available to achieve this type control.  Two types of sensors, photo resistive or photo diode, are available as inputs.  This method allows the natural lighting in an area to be utilized to its fullest.

Another advantage to this type of control is that various lighting levels can be set based on need.  Hallways and open areas would require less light than stores or office areas.

Parking lot and store sign lights can also be controlled. Typically the store lights would come on earlier than the parking lot lights for their advertising effect. The reverse would happen at closing. The store sign lights would go out first to let people know the store is closed.  The parking lot lights could remain on long enough to allow store employees to close the store or until the following dawn for security reasons.

Future Trends in Lighting

Higher efficiency lights and new types of reflectors that put more light into the space are the latest in attempts to reduce lighting costs.  As control manufacturers study and design new control systems, improved dimming and ballast controls, astronomical time of day control and new strategies in demand control using lighting will be introduced and enhanced.  This new lighting technology is continually creating a wide array of innovative products that offer increased savings for customers and, at the same time, provide contractors potentially greater profits.

Between now and the turn of the century, lighting controls could prove to be one of the brightest spots in your business plan.