Q
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What is the difference between fire-protective and fire-resistive glazing systems?
A
&#;Fire-protective&#; means the glazing defends against the spread of flames and smoke. Such materials include traditional wired glass, glass ceramics and specially tempered glass. Fire-protective glazing typically is suitable where building codes allow &#;opening protective&#; assemblies. While such glazing is available with fire ratings ranging from 20 to 180 minutes, it is subject to area and size limitations under the applicable building code and/or authority having jurisdiction.
&#;Fire-resistive&#; glass provides the same defense against flames and smoke as fire-protective glazing, and adds further protection by blocking the transfer of radiant and conductive heat. Fire-resistive glass products generally are multi-laminates incorporating several layers of glass with fire-resistive interlayers. They are typically suitable where building codes require an assembly designated &#;fire resistant&#; to enclose a space. Examples include wall applications requiring a 60-minute or greater fire rating that must meet temperature-rise criteria, such as stairwells, exit access corridors, or other fire barriers dividing interior construction. In these instances, the IBC requires the temperature rise on the non-fire side of the glass not to exceed 250 degrees Fahrenheit above the ambient temperature at the end of the fire test (generally 60 or 120 minutes). Such glass must also pass the hose stream test.
Q
What are my primary options in fire-rated glass?
A
The glass product most often associated with fire rating is polished wired glass. It has provided fire protection for more than 100 years. In North America, wired glass is typically rated for 45 minutes in lite sizes up to 9 square feet (1,296 square inches). Wired glass with a fire rating greater than 45 minutes is restricted to 100-square inch lites in doors with temperature-rise criteria. The biggest advantage of wired glass is its low cost. However, because wired glass has low impact resistance, since the International Building Code (IBC) has prohibited its use in hazardous locations in all facility types. See www.iccsafe.org for information on the IBC.
A second type of fire-rated glazing is glass ceramic. Once installed, this wireless product looks similar to ordinary window glass, which provides great design flexibility. Glass ceramic, such as the FireLite® family, provide fire ratings from 20 minutes to 3 hours, and come in sizes up to 24 square feet per lite. Like wired glass, glass ceramics are able to withstand the thermal shock of water from sprinklers or fire hoses. Where impact safety is required, they are available with up to Category II (CPSC 16CFR ) impact-safety ratings. This is the highest standard impact-safety rating available, indicating that the glass can safely withstand an impact similar to that of a full-grown, fast moving adult.
Glass ceramic is also available in insulated glass units (IGUs). The IGUs are made of two layers of glass with an air space in between. They can incorporate many types of float glass, including clear, tinted, Low-E and mirrored glass. Depending on which components are used, they can provide fire protection and comply with energy codes. IGUs are sometimes used for interior applications where sound reduction is desired.
Another category of fire-rated glass is fire-rated glass wall panels. These units are special, multi-layer assemblies that block the transfer of radiant and conductive heat. They are tested to the same fire-resistance standards as solid walls, and are not restricted to 25 percent of the wall, as may be the case with fire-protective glazing. This flexibility makes products like Pilkington Pyrostop® suitable for use in floor-to-ceiling and wall-to-wall designs, or in full lite glass doors. These large expanses of glass have obtained fire ratings up to 2 hours. They are typically used where architects desire (or building codes require) the blockage of heat transfer through the glass. Designers can thus provide clear, fire-rated glass walls that allow visibility, light and security. Like glass ceramic, fire-rated glass wall panels are available with up to Category II (CPSC 16CFR ) impact-safety ratings.
A final category of fire-rated glazing is specially tempered glass. Products such as Fireglass®20 only carry 20-minute ratings. It is important to note such products cannot withstand thermal shock, and are therefore unable to pass the &#;hose stream test&#; required for ratings greater than 20 minutes. As a result, applications for these products are generally limited to use in 20-minute fire doors.
Q
Why is the "fire hose stream" (thermal shock) test important?
A
The fire hose stream test shows how hot glass and surrounding frame assemblies will react when hit by water from a fire hose or sprinkler. Most glass is unable to withstand the thermal shock of fire and water. If nearby sprinklers activate during a fire, the heated glass may shatter and vacate the frame, thus allowing the spread of flames and smoke.
NFPA 257 states, "The hose stream test provides a method for evaluating the integrity of constructions and assemblies and for eliminating inadequate materials or constructions. The cooling, impact, and erosion effects of the hose stream provide tests of the integrity of the specimen being evaluated.&#; The hose stream test is required in the United States for glass with fire ratings greater than 20 minutes. In Canada, all fire-rated glass must pass the test.
Q
Why is fire-rated glazing required to have a permanent label?
A
Because fire-rated glazing is available with a wide variety of performance characteristics, specifying an appropriate one for a given application is critical for life and property safety. To help ensure the proper use of glass for various fire-rated applications, a multi-faceted product labeling system was implemented in the IBC and further simplified in the IBC [see Table 716.3 (Marking fire-rated glazing assemblies)].
The fire-rated glass marking system includes a range of information, including the product name, basic characteristics (e.g., tempered, laminated, etc.), compliance with impact safety requirements, and listing information for the applicable independent testing agency, such as Underwriters Laboratories. See our IBC Glass Label Guide to get started.
Q
I recently received product information for a fire-rated glass that indicated several limitations on use. Should this be of concern?
A
Yes. Architects and designers should always be wary of product "listings" that carry what appear to be unusual limitations. For example, one fire-rated glazing material on the market indicates a fire rating of "60 minutes", but then goes on to say, "This product does not meet the hose stream requirements of the test standards". Further, "This product protects from fire from one direction only. The identified face MUST be installed facing the direction of expected fire attack." Such limitations should raise red flags, and prove how important it is to thoroughly read manufacturers&#; literature. This clearly indicates how a laboratory "listed" product may not be exactly what you thought it might be. For more information, see this Alert.
Q
Is fire-rated glass really necessary if I use sprinklers? Can't I just use tempered or heat strengthened glass with a water "deluge" system?
A
Some manufacturers had been submitting engineering reports to Authorities Having Jurisdiction (AHJ) in which fire-resistance ratings were obtained using fire suppression systems (i.e., deluge sprinklers to cool the glass during the fire test). However, the IBC now clarifies that fire ratings must be established based solely on a material&#;s own performance. According to section 703.4, &#;...the fire-resistance rating of a building element, component or assembly shall be established without the use of automatic sprinklers or any other fire suppression system...&#;
While sprinklers do much to save lives, they are no substitute for the use of passive fire-rated glazing materials. If sprinklers do not activate due to faulty manufacturing, loss of water pressure, or other reasons, fire-rated glass will perform its critical function of compartmentation&#;with or without water from the sprinklers.
Q
Generally speaking, the "wireless" fire-rated glazing materials are more expensive than polished wired glass. How can I persuade my building owner that those products are worth the extra cost?
A
We generally see architects and designers using the higher price products for improved aesthetics and/or higher safety. The manufacturing processes for high performance wireless products are complicated, and frequently make use of specialty materials. While costs are coming down as production volumes increase, we suspect they will never reach the levels of wired glass. Interestingly, we often find the wireless products are in line with the architectural construction costs - they are just more expensive than traditional wired glass. In addition, the amount of fire-rated glazing used in most projects is rather small comparatively. Increasingly, we see architects and designers willing to use the newer products for aesthetics reasons, such as opening up entire glass walls that have high fire ratings. Characteristics such as higher fire ratings, larger glass sizes, increased clarity, higher impact ratings, meeting energy codes, etching and beveling, etc., also contribute to their increased use.
Q
What are some of the latest developments that could enhance building designs?
A
New uses for fire-rated glass and frames have gone hand-in-hand with aesthetic improvements. For years, design professionals were limited to traditional hollow metal steel frames. While functional, these bulky, wrap-around frames forced many to compromise on appearance in order to provide life safety. Manufacturing advances have led to thinner, fire-rated frames that can be custom painted or powder coated to match virtually any color scheme. There are even hardwood, aluminum and stainless steel fire-rated framing options.
Manufacturing innovations have also enabled more sophisticated fire-rated glass and framing assemblies. Advanced systems include fire-rated glass floor systems and fire-rated applications that have the appearance of a structural silicone glazed systems, such as the Fireframes SG Curtainwall&#; Series.
Q
Can other rated frames be used with TGP fire-rated glass?
A
TGP offers a range of fire-rated frames that can be used in conjunction with FireLite® glass ceramic. Compatible frames include the Fireframes® Designer Series, Fireframes® Aluminum Series, Fireframes® Hardwood Series and Fireframes® Curtainwall Series. FireLite can also be used with hollow metal steel frames. However, since fire-rated framing and glass work in tandem to provide compartmentation, the frames and glass must carry the same fire rating and classification (fire protective or fire resistive) in accordance with the IBC.
Q
Who is responsible if the wrong glass or framing is installed?
A
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Potentially, multiple parties involved in the selection, specification, approval, or installation of products could be held liable. The building owner might look to the architect, who in turn could point to the code officials and the glazing contractor. Code officials will say their approval has the disclaimer that it is "subject to errors and omissions". Glaziers are the glass experts, and architects rely on them for advice. If a glazier sees a problem, he needs to alert the architect about suitable alternatives. The excuse, "I just bid what the architect specified...&#; may not go far in a court of law.
When it comes to life safety in a building, it is important to gather all the details to avoid making costly or dangerous mistakes. After examining the application and narrowing the option for glazing and framing materials, review an individual&#;s product literature in detail. Look for any special requirements, limitations or exclusions.
Q. I&#;d like to understand the differences between available resistor types and how to select the right one for a particular application.
A. Sure, let&#;s talk first about the familiar &#;discrete&#; or axial-lead type resistors we&#;re used to working with in the lab; then we&#;ll compare cost and performance tradeoffs of the discretes and thin- or thick-film networks.
Axial Lead Types: The three most common types of axial-lead resistors we&#;ll talk about are carbon composition, or carbon film, metal film and wirewound:
- carbon composition or carbon film-type resistors are used in general-purpose circuits where initial accuracy and stability with variations of temperature aren&#;t deemed critical. Typical applications include their use as a collector or emitter load, in transistor/FET biasing networks, as a discharge path for charged capacitors, and as pull-up and/or pull-down elements in digital logic circuits.
Carbon-type resistors are assigned a series of standard values (Table 1) in a quasi-logarithmic sequence, from 1 ohm to 22 megohms, with tolerances from 2% (carbon film) to 5% up to 20% (carbon composition). Power dissipation ratings range from 1/8 watt up to 2 watts. The 1/4-watt and 1/2-watt, 5% and 10% types tend to be the most popular.
Carbon-type resistors have a poor temperature coefficient (typically 5, 000 ppm/°C); so they are not well suited for precision applications requiring little resistance change over temperature, but they are inexpensive&#;as little as 3 cents [USD 0.03] each in 1, 000 quantities.
Table 1 lists a decade (10:1 range) of standard resistance values for 2% and 5% tolerances, spaced 10% apart. The smaller subset in lightface denote the only values available with 10% or 20% tolerances; they are spaced 20% apart.
Table 1. Standard resistor values: 2%, 5% and 10%
10
16
27
43
68
11
18
30
47
75
12
20
33
51
82
13
22
36
56
91
15
24
39
64
100
Carbon-type resistors use color-coded bands to identify the resistor&#;s ohmic value and tolerance:
Table 2. Color code for carbon-type resistors
digit
color
multiple
# of zeroes
tolerance
&#;
silver
0.01
&#;2
10%
&#;
gold
0.10
&#;1
5%
0
black
1
0
&#;
1
brown
10
1
&#;
2
red
100
2
2%
3
orange
1k
3
&#;
4
yellow
10k
4
&#;
5
green
100k
5
&#;
6
blue
1m
6
&#;
7
violet
10m
7
&#;
8
gray
&#;
&#;
&#;
9
white
&#;
&#;
&#;
&#;
none
&#;
&#;
20%
- Metal film resistors are chosen for precision applications where initial accuracy, low temperature coefficient, and lower noise are required. Metal film resistors are generally composed of Nichrome, tin oxide or tantalum nitride, and are available in either a hermetically sealed or molded phenolic body. Typical applications include bridge circuits, RC oscillators and active filters. Initial accuracies range from 0.1 to 1.0 %, with temperature coefficients ranging between 10 and 100 ppm/°C. Standard values range from 10.0 ohms to 301 kohms in discrete increments of 2% (for 0.5% and 1% rated tolerances).
Table 3. Standard values for film-type resistors
1.00
1.29
1.68
2.17
2.81
3.64
4.70
6.08
7.87
1.02
1.32
1.71
2.22
2.87
3.71
4.80
6.21
8.03
1.04
1.35
1.74
2.26
2.92
3.78
4.89
6.33
8.19
1.06
1.37
1.78
2.31
2.98
3.86
4.99
6.46
8.35
1.08
1.40
1.82
2.35
3.04
3.94
5.09
6.59
8.52
1.10
1.43
1.85
2.40
3.10
4.01
5.19
6.72
8.69
1.13
1.46
1.89
2.45
3.17
4.09
5.30
6.85
8.86
1.15
1.49
1.93
2.50
3.23
4.18
5.40
6.99
9.04
1.17
1.52
1.96
2.55
3.29
4.26
5.51
7.13
9.22
1.20
1.55
2.00
2.60
3.36
4.34
5.62
7.27
9.41
1.20
1.55
2.00
2.60
3.36
4.34
5.62
7.27
9.41
1.22
1.58
2.04
2.65
3.43
4.43
5.73
7.42
9.59
1.22
1.58
2.04
2.65
3.43
4.43
5.73
7.42
9.59
1.24
1.61
2.09
2.70
3.49
4.52
5.85
7.56
9.79
1.27
1.64
2.13
2.76
3.56
4.61
5.96
7.72
9.98
Metal film resistors use a 4 digit numbering sequence to identify the resistor value instead of the color band scheme used for carbon types:
- Wirewound precision resistors are extremely accurate and stable (0.05%, <10 ppm/°C); they are used in demanding applications, such as tuning networks and precision attenuator circuits. Typical resistance values run from 0.1 ohms to 1.2 Mohms.
High Frequency Effects: Unlike its &#;ideal&#; counterpart, a &#;real&#; resistor, like a real capacitor (Analog Dialogue 302), suffers from parasitics. (Actually, any two-terminal element may look like a resistor, capacitor, inductor, or damped resonant circuit, depending on the frequency it&#;s tested at.)
Factors such as resistor base material and the ratio of length to cross-sectional area determine the extent to which the parasitic L and C affect the constancy of a resistor&#;s effective dc resistance at high frequencies. Film type resistors generally have excellent high-frequency response; the best maintain their accuracy to about 100 MHz. Carbon types are useful to about 1 MHz. Wirewound resistors have the highest inductance, and hence the poorest frequency response. Even if they are non-inductively wound, they tend to have high capacitance and are likely to be unsuitable for use above 50 kHz.
Q. What about temperature effects? Should I always use resistors with the lowest temperature coefficients (TCRs)?
A. Not necessarily. A lot depends on the application. For the single resistor shown here, measuring current in a loop, the current produces a voltage across the resistor equal to I x R. In this application, the absolute accuracy of resistance at any temperature would be critical to the accuracy of the current measurement, so a resistor with a very low TC would be used.
A different example is the behavior of gain-setting resistors in a gain-of-100 op amp circuit, shown below. In this type of application, where gain accuracy depends on the ratio of resistances (a ratiometric configuration), resistance matching, and the tracking of the resistance temperature coefficients (TCRs), is more critical than absolute accuracy.
Here are a couple of examples that make the point.
1. Assume both resistors have an actual TC of 100 ppm/°C (i.e., 0.01%/°C). The resistance following a temperature change, ΔT, is
R = R0(1+ TC ΔT)
For a 10°C temperature rise, both RF and RI increase by 0.01%/°C x 10°C = 0.1%. Op amp gains are [to a very good approximation] 1 + RF/RI. Since both resistance values, though quite different (99:1), have increased by the same percentage, their ratio hence the gain is unchanged. Note that the gain accuracy depends just on the resistance ratio, independently of the absolute values.
2. Assume that RI has a TC of 100 ppm/°C, but RF&#;s TC is only 75 ppm/°C. For a 10°C change, RI increases by 0.1% to 1.001 times its initial value, and RF increases by 0.075% to 1. times its initial value. The new value of gain is
(1. RF)/(1.001 RI) = 0. RF/RI
For an ambient temperature change of 10°C, the amplifier circuit&#;s gain has decreased by 0.025% (equivalent to 1 LSB in a 12-bit system). Another parameter that&#;s not often understood is the self-heating effect in a resistor.
Q. What&#;s that?
A. Self-heating causes a change in resistance because of the increase in temperature when the dissipated power increases. Most manufacturers&#; data sheets will include a specification called &#;thermal resistance&#; or &#;thermal derating&#;, expressed in degrees C per watt (°C/W). For a 1/4-watt resistor of typical size, the thermal resistance is about 125°C/W. Let&#;s apply this to the example of the above op amp circuit for full-scale input:
Power dissipated by RI is
E2/R = (100 mV)2/100 ohms = 100 µW, leading to a temperature change of 100 µW x 125°C/W = 0.°C, and a negligible 1ppm resistance change (0.%).
Power dissipated by RF is
E2/R = (9.9 V)2/ ohms = 9.9 mW, leading to a temperature change of 0. W x 125°C/W = 1.24°C, and a resistance change of 0.%, which translates directly into a 0.012% gain change.
Thermocouple Effects: Wirewound precision resistors have another problem. The junction of the resistance wire and the resistor lead forms a thermocouple which has a thermoelectric EMF of 42 µV/°C for the standard &#;Alloy 180&#;/Nichrome junction of an ordinary wirewound resistor. If a resistor is chosen with the [more expensive] copper/nichrome junction, the value is 2.5 µV/°C. (&#;Alloy 180&#; is the standard component lead alloy of 77% copper and 23% nickel.)
Such thermocouple effects are unimportant in ac applications, and they cancel out when both ends of the resistor are at the same temperature; however if one end is warmer than the other, either because of the power being dissipated in the resistor, or its location with respect to heat sources, the net thermoelectric EMF will introduce an erroneous dc voltage into the circuit. With an ordinary wirewound resistor, a temperature differential of only 4°C will introduce a dc error of 168 µV which is greater than 1 LSB in a 10V/16-bit system!
This problem can be fixed by mounting wirewound resistors so as to insure that temperature differentials are minimized. This may be done by keeping both leads of equal length, to equalize thermal conduction through them, by insuring that any airflow (whether forced or natural convection) is normal to the resistor body, and by taking care that both ends of the resistor are at the same thermal distance (i.e., receive equal heat flow) from any heat source on the PC board.
Q. What are the differences between &#;thin-film&#; and &#;thick-film&#; networks, and what are the advantages/disadvantages of using a resistor network over discrete parts?
A. Besides the obvious advantage of taking up considerably less real estate, resistor networks&#;whether as a separate entity, or part of a monolithic IC&#;offer the advantages of high accuracy via laser trimming, tight TC matching, and good temperature tracking. Typical applications for discrete networks are in precision attenuators and gain setting stages. Thin film networks are also used in the design of monolithic (IC) and hybrid instrumentation amplifiers, and in CMOS D/A and A/D converters that employ an R2R Ladder network topology.
Thick film resistors are the lowest-cost type&#;they have fair matching (<0.1%), but poor TC performance (<100 ppm/°C) and tracking (<10 ppm/°C).They are produced by screening or electroplating the resistive element onto a substrate material, such as glass or ceramic.
Thin film networks are moderately priced and offer good matching (0.01%), plus good TC (<100 ppm/°C) and tracking (<10 ppm/°C). All are laser trimmable. Thin-film networks are manufactured using vapor deposition.
Table 4 compares the advantages/disadvantages of a thick film and several types of thin-film resistor networks. Table 5 compares substrate materials.
Table 4. Resistor Networks
Type
Advantages
Disadvantages
Thick film
Low cost
Fair matching (0.1%)
High power
Poor TC (>100 ppm/°C)
Laser-trimmable
Poor tracking TC
Readily available
(10 ppm/°C)
Thin film on glass
Good matching (<0.01%)
Delicate
Good TC (<100 ppm/°C)
Often large geometry
Good tracking TC (2 ppm/°C)
Low power
Moderate cost
Laser-trimmable
Low capacitance
Thin film on ceramic
Good matching (<0.01%)
Often large geometry
Good TC (<100 ppm/°C)
Good tracking TC (2 ppm/°C)
Moderate cost
Laser-trimmable
Low capacitance
Suitable for hybrid IC substrate
Thin film on silicon
Good matching (<0.01%)
Good TC (<100 ppm/°C)
Good tracking TC (2 ppm/°C)
Moderate cost
Laser-trimmable
Low capacitance
Suitable for hybrid IC substrate
Table 5. Substrate Materials
Substrate
Advantages
Disadvantages
Glass
Low capacitance
Delicate
Low power
Large geometry
Ceramic
Low capacitance
Large geometry
Suitable for hybrid IC substrate
Silicon
Suitable for monolithic
Low power
construction
Capacitance to substrate
Sapphire
Low capacitance
Low power
Higher cost
In the example of the IC instrumentation amplifier shown below, tight matching between resistors R1-R1', R2-R2', R3-R3' insures high common-mode rejection (as much as 120 dB, dc to 60 Hz). While it is possible to achieve higher common-mode rejection using discrete op amps and resistors, the arduous task of matching the resistor elements is undesirable in a production environment.
Matching, rather than absolute accuracy, is also important in R2R ladder networks (including the feedback resistor) of the type used in CMOS D/A converters. To achieve n-bit performance, the resistors have to be matched to within 1/2n, which is easily achieved through laser trimming. Absolute accuracy error, however, can be as much as ±20%. Shown here is a typical R-2R ladder network used in a CMOS digital analog converter.
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