Combustion Gas Analysis
Combustion Gas Analysis Based on the ASTM D5424 Test for Electrical Cables
W.F. Powers, P.E.
Southwire Company
Carrollton, Georgia
In the wire and cable industry, tests involving burning wire and cable have been used for a long time. Recent issues of the effects of burning have been the cause of much debate within the wire and cable industry. Central issues involve the measurement of Smoke, Corrosivity and Toxicity. Smoke has been addressed in the recent ASTM D5424-97. The UL 1685 version contains smoke limits. If those smoke limits are not exceeded the cable may obtain an “LS” (for limited smoke) rating. The issues of corrosion and toxicity remain unresolved. Small scale tests involving burning wire and cable materials, exposing the smoke to corrosion targets, conductivity targets, white mice, etc. have been proposed and debated. There is a wealth of information on complicated tests and hard to understand data. There are few conclusions.
In the meantime, customers of wire and cable, with specific end uses, need data on which to base decisions. Easily understood data on combustion products is required. Customers could then make conclusions about how these products of combustion effect their specific environment where wire is installed.
The purpose of this paper is to demonstrate industry standard flame tests can be adapted to measure data that is both easily understood and applicable to evaluating the potential hazards involved with burning wire and cable.
The end user can then use this easily understood data and apply it to a range of environments. The test is easily understood. The data can be used to compare to existing studies to determine relevance and meaning in specific uses.
The tendency of late has been to work on small scale tests with the hope of developing a large scale test later. The approach of this paper is the opposite. A large scale test is modified by using standard technology from other industries.
Practical Application
The specific wire studied is a CSPE jacketed/EPR insulated cable design of “Network Cable” used by Union Electric of St. Louis. Similar designs are used by many utilities as a distribution cable in the cities’ underground distribution network. Union Electric has had a recent incident with cable fires in their network system burning as a result of transformer malfunctions. Thick black smoke accompanied by choking fumes prevented the ready access of the fire. This prevented firefighters from gaining access to the fire and prevented the repair of the substation in a timely manner. Fumes and smoke contributed to “non thermal” damage of the substation.
Samples of the cable presently used are a 500 kcmil copper conductor CSPE /EPR construction vs. a Non Halogen XLPO/EPR were compared.
Basis of the Flame Test and Combustion Gas Test
The Underwriters Laboratory Standard 1685 is a procedure used to perform flame test and generate smoke data sufficient to give a smoke rating. The flame test are the vertical ladder test that gives a cable the rating it must have in order to be installed in cable trays. The criteria for the vertical ladder flame test used in the US and CSA is included in the procedure. The vertical ladder test that are similar are CSA FT4, IEEE 383, IEEE 1202, and UL 1581. All these test involves a vertical ladder with cables mounted with certain spacing, and a 70,00 BTU burner in a specified chamber. The burner is applied for twenty minutes and the question is how far up the ladder will the flame damage the cable.
The smoke test in UL 1685 involves the measurement of the opacity at a point in the ductwork the is far enough down to insure good mixing. It is at this point that combustion gases can be measured. Standard technology used in different industries can be of help.
In environmental safety technology, it is normal to test various environments for certain gases that may cause health hazards. There are devices to measure the concentration of many different gases. These devices are used for many different reasons. Common reasons will be the evaluation of work place environments and measurement for compliance with EPA guidelines.
These measured concentrations can be compared to the concentrations of gases published by the National Institute for Occupational Safety and Health (NIOSH), so that measurements can be compared to a standard number.
NIOSH standards have two numbers IDLH and TWA:
The IDLH is a concentration that is immediately dangerous to life and health. A person should be able to escape exposure to this concentration without permanent injury or loss of life. As a safety margin, IDLH values might occur for a 30 minute exposure. However every effort should be made to leave the environment immediately.
The Time Weighted Average (TWA) is a concentration for an exposure for up to 10 hours during a 40 hour week.
The IDLH and TWA for several combustion gases expected in burning wire and cable are shown in Table 1.
Table 1
|
|
IDLH |
TWA |
|
|
|
|
|
Carbon Monoxide |
220 ppm |
35 ppm |
|
30 ppm |
3 ppm |
|
Hydrogen Chloride |
50 ppm |
5 ppm |
|
Hydrogen Fluoride |
30 ppm |
3 ppm |
|
Hydrogen Cyanide |
50 ppm |
4.8 ppm |
Network Cables
Standard cable for network applications are constructed of an insulation, usually EPR and a jacket usually CSPE (Chlorosulfonated Polyethylene). This cable has a high flame resistance and is perceived to be a very rugged construction. The flame retardant system in the jacket is based on the polymers’ chlorine. The chlorine forms Hydrogen Chloride, a clear gas that shields the fire from oxygen needed to sustain the flame. This “suffocation” method of flame retardancy is accompanied by smoke and incomplete combustion. In addition, the Hydrogen Chloride gas when dissolved in water results in Hydrochloric Acid. This contributes to corrosion and noxious effects.
As a contrast, a “Non Halogen” network cable can be made with polymers to pass the same flame retardant test as required by most network cables specification using polymers that are loaded with fillers containing molecular water. The water releases upon exposure to flame and is closely analogous to pouring water onto the fire. The combustion that takes place takes place completely and so there is very little smoke. Also there is no halogen to make acids.
The Test Results
In the test conducted to UL 1685, the two different cables had similar performance in the flame portion of the test. This flame test is the only performance test required in most network cable specifications. The performance of the cable are shown in Graph 1.
Graph 1
The smoke performance is different between the two cables. Both cable constructions pass the smoke criteria. The size of the copper conductor is a positive influence on the degree of burning and smoke on this test. For the same materials and constructions smaller sizes will do more poorly on the smoke and flame performance.
Performance of the smoke is shown in Graphs 2 and 3 and is compared to performance required to obtain a U.L. “LS” (Limited Smoke and soon to change to “ST1”) rating.
Graph 2
Graph 3
Also the concentration of two different combustion gases were measured, one was Carbon Monoxide (Graph 4), and Hydrogen Chloride (Graph 5). Carbon Monoxide is said by many people to be the gas to cause most fatalities in a fire. When there is a facility in which only wire is burning, Hydrogen Chloride would appear to be of more concern than Carbon Monoxide.
Graph 4
Graph 5
The following conclusions can easily be made from the graphic data:
1. Hydrogen Chloride can be a bigger problem than carbon monoxide. During the burn the concentrations of Hydrogen Chloride for the EPR/CSPE Network cable are above the IDLH. In the case of Carbon Monoxide this is not the case.
2. The Non halogen smokes much less.
3. The Non halogen generates no combustion gases in excess of the IDLH
Network cables are installed in conditions where concentrations are likely at or above the concentrations found in the UL 1685 test. Air flow could expect to be near the same or worse. There are many like environments where wire is installed like Network power cables. There are also environments with less cables and more open spaces where the advantages of the Non Halogen cable would not be as pronounced and concentration as measured in this test much lower.
Using a standard flame test method for the source of the flame and giving PPM is an easily understood and applied method for accessing the relative dangerous level of combustion gases. This makes the decision making process in many critical applications much easier and more understandable by the end user.
References
“Pocket Guide to Chemical Hazards”
U.S. Department of Health and Human Services
June 1994
Table One
Gases possible found in burning wire and cable
“Pocket Guide to Chemical Hazards”
“Pocket Guide to Chemical Hazards”
U.S. Department of Health and Human Services
June 1994
UL Standard 1685 “Vertical-Tray Fire-Propagation and Smoke-Release Test for Electrical and Optical-Fiber Cables”
ASTM D5424-97 “Standard Test Method for Smoke Obscuration of Insulating Materials Contained in Electrical or Optical Fiber Cables When Burning in a Vertical Cable Tray Configuration1”
