Circuit integrity
From Wikipedia, the free encyclopedia
Circuit integrity refers to the operability of electrical circuits during a fire. It is a form of fire-resistance rating. Circuit integrity is achieved via passive fire protection means, which are subject to stringent bounding.
Contents |
[edit] Fireproofing of Cables
"Fireproofing" cables or cable trays or electrical conduit can mean two different things. Cable coating is generally considered a "fire retardant", which lowers the spread of flame and generation of smoke along the combustible cable jacketing. Some cable coating systems are able to achieve a measure of circuit integrity, but this must be demonstrated and quantified through product certification listings. True circuit integrity measures typically mean an enclosure, which is meant to keep cables operational during a fire. Alternatively, one may use cables that achieve fire-resistance ratings on their own, such as Mineral-insulated copper-clad cable, or MI cable. Mica insulated cables have also demonstrated a measure of circuit integrity for small cables.
[edit] Testing and Certification
In Canada, testing is run in accordance with ULC-S101, as required by the local building code. Unfortunately, S101 is ill equipped to deal realistically with circuit integrity, particularly for enclosures. For circuit integrity cables, one simply uses a full scale wall panel test, loops the cables through the fire, energises the cables and quantifies the current carrying capacity of the cables during the fire.
There are two ways of achieving circuit integrity. One may either choose mineral insulated or otherwise fire-resistant (tested for that purpose) cables, or one may use an enclosure that was tested for that purpose. This is where "grandfathered" systems still find acceptance in certain parts in North America. A prime example of this is Canada, where the code indicates that 2" of concrete coverage over or around electrical circuits is sufficient to obtain an unquantified duration of circuit integrity. No testing documentation exists to qualify this measure, according to the Institute for Research in Construction, a part of the National Research Council of Canada. 2" of concrete, regardless of the conductor configuration, percentage fill, etc. is of course a judgment call.
[edit] The Mechanical Ducting Precedent
The other grandfathered approach of course is drywall shaftwall systems. Drywall shaftwalls were tested as a flat wall, no corners, no turns. This approach has pretty much been negated for use around ductwork (i.e. pressurisation and grease ducting, which are required to have a fire-resistance rating) since the adoption of the more suitable ISO6944 test regime by ULC as well as Underwriters Laboratories, whereby a duct is suspended from a full scale floor slab and the enclosure is built around the duct (or an inherently fire resistant duct is similarly tested without an enclosure, since it already contains a layer of insulation), for a more realistic 3D configuration and exposure. Drywall shaftwall systems were entirely grandfathered for this application and ceased to be legally representative of due diligence the instant a properly and purposely tested system with bona fide listings became available. The same thing applies to circuit integrity enclosures.
For the mechanical ductwork, a Canadian entrepreneur got ISO6944 passed by the ULC Standards Council and then performed testing. This made all grandfathered systems legally indefencible.
This has yet to occur in Canada for circuit integrity, but it has long been standard construction work in Europe and also in the US, through work done by UL and other laboratories. Since UL is accredited by the Standards Council of Canada [1] in Canada and its listings are considered public record up all over North America including Canada, one is ill advised to use grandfathered systems for circuit integrity anywhere.
Importantly, drywall shaftwall systems have only been qualified as straight walls in panel furnaces, not 3D enclosures with corners.
[edit] Current Test Methods
Germany has standardised this sort of testing via DIN4102 Part 12, dated January 1991, Fire behaviour of building materials and elements, Fire resistance of electrical cable systems, Requirements and testing. Part 12 encompasses both enclosures for cabling and bus ducts, as well as inherently fire-resistive cables, such as mineral insulated cables. Enclosures for ductwork as well as wiring are a regular part of passive fire protection there. It is also not nearly as expensive as North American qualified approaches. Typically, lightweight mineral boards are used, such as calcium silicate and bonded vermiculite.
The North American state of the art is UL1724 Standard for Tests of Thermal Barrier Systems for Electrical System Components as well as its cousin, UL2196 Standard for Tests of Fire Resistive Cables. UL1724 had its origin with USNRC Generic Letter 86-10 Supplement 1, issued by the Nuclear Regulatory Commission. "Supplement 1" was to address lessons learned from the widely publicised Thermo-lag 330-1 scandal, which resulted on Congressional hearings and a large amount of remedial work.
Supplement 1 is a particularly difficult and expensive test to pass. No testing is done in anything less than a full scale fire test, running easily into 6 figure costs per burn multiplied by all the applications one desires to test. In order to pass, one must test the smallest as well as the largest application (12" and 36" cable tray, 1/2" and 6" conduit). Accordingly, the approved materials are costly, as manufacturers must get a return on the large test investment.
In concept, it is simple to devise systems that will pass the test. As far back as the 1970's, it was apparent that when one uses enough high temperature qualified insulation, such as ceramic fibre, one is assured of a rating. However, this comes at the price of significant ampacity derating. Also, the concept that more fireproofing is better, was defeated by industry tests of Thermo-lag 330-1 (which is not a fibrous insulation). No matter what was done to this material (used for fireproofing purposes over electrical circuits in full scale fire testing) by various nuclear power plant owners (USNRC licensees) who sponsored extensive testing, where more of the old Thermo-lag was applied onto the older substrate, no satisfactory results were achieved. In order for licensees to come into compliance, otgher methods, replacements, overlays and MI Cable were used to fix the problem. Also, since the fore-runner of this testing was the USNRC, and the commercial version of it (UL1724) has undergone various revisions, the UL systems listed in the UL Building Materials directory are not necessarily qualified to the latest USNRC compliant or the latest UL version. But that does not mean that the older listings are simply discarded or that the manufacturers performed all new tests. Therefore, users must closely review the versions of the tests deemed acceptable in an end-user facility.
[edit] What Circuit Integrity Methods Are Used Where
Ordinarily, small runs of cables are indicidually run with cables that are fire-resistance rated on their own. Larger bundles and trays full of wiring may be less expensive to clad or wrap on the outside. The concrete cover method is most often used in Canadian construction, as the code and common practice permit this, despite the absence of testing data that gives the required "carte blanche" for all cables and indefinite ratings.
[edit] Cladding and Wrapping Considerations
The added weight of the wrap systems must be included in static and seismic calculations. Fireproofing of the hanging system must also be considered. Regular maintenance must be considered, as cladding and wraps are not load-bearing and can be damaged during normal building or facility operations. Ampacity-derating may be mitigated by the use of purpose-designed intumescent or mechanically/electronically activated "windows" that permit heat venting. Like everything else in passive fire protection, all such methods are subject to stringent bounding.
[edit] Circuit Integrity Cable Considerations
Termination points and junction boxes, in other words: the entire circuit, must be completely protected. Oftentimes, termination points are left out, providing a weak link. Therefore, some enclosures are needed to be used in conjuction with MI cables. One may run MI cable into a box in an electrical room. However, just because that room may be a "service room" and may be subject to compartmentalization (fire protection), this does not mean one no longer requires a rated box or wrap around the electrical outlet box or junction box where the wiring is terminated because that box may be disabled as a result of a fire within the room. The probability of electrical fires are a strong motivating factor for compartmetalisation to begin with. The cable may thus be operable but the circuit as a whole may be defeated because the junction box would not have been protected. Such omissions are not entirely uncommon in the field.
