IEEE 690:2018 pdf free download – IEEE Standard for Design and Installationof Cable Systems for Class 1E Circuitsin Nuclear Power Generating Stations
5. Conductor sizing
This clause establishes cable conductor sizing requirements for various types of cable installations for Class1E circuits. Power cables shall be sized in accordance with National Electrical CodeR(NEC*) (NFPA 70*),IEEE Std 835, or NEMA WC 51/ICEA P-54-440.The ampacity tables provided in these documents are veryspecific to the cable installation method. It is important to understand the bases for the ampacities providedin their tables so that the appropriate selection is made based on the cable installation (e.g.,random fill, non-spaced versus single-layer for open top trays).Additional guidance for cable ampacity determination relatingto maintained spacing and conduit grouping factors can be found in IEEE Std S-135-1.
Cables shall be sized to carry load current with the following special considerations:
a)Unless specific site ambient conditions or the safety analysis of the plant supports other ambient temperatures, the minimum ambient temperatures used in calculating cable ampacities shall be 40 °cfor exposed installations. For buried installations, earth ambient will vary depending on geographicallocation (typically between 20 °C to 30 °C).Ampacity tables in IEEE Std 835 are based on 25 °Cearth ambient. IEEE Std S-135-1 provides additional discussion. The ratings for buried cables aresignificantly affected by the earth thermal resistivity and soil thermal stability. Knowledge of theseparameters is important in rating of a buried cable system (IEEE Std 835).Conductor size shall beselected to carry required normal, emergency overload, and short-circuit current without exceedingrated temperature of the insulation for the respective condition，and considering appropriateenvironmental and operational design parameters. Temperature extremes under design basis eventsshall be addressed by the qualification of the cable in accordance with IEEE Std 383.
b)Cable ampacity shall consider different conditions such as (direct buried, sun exposure, exposed conduit, cable penetration fire stops,covered cable trays,wireways, proximity to hot steam lines,raceway fill, ambient temperature, soil thermal resistivity, conductor temperature, application of firewrap/barriers/coatings and conduit groupings) and the conductor size shall be selected or de-rated forthe most limiting condition.
c)Selection of conductor size shall also consider voltage regulation requirements, shield circulating currents, and mechanical strength in addition to cable current carrying requirements.
11.2Fixed fire-extinguishing system requirements
The following are requirements for the selection and application of fixed fire-extinguishing requirements:
a)A fire hazard analysis(see NFPA 804 and NFPA805)shall be conducted to determine if fixed automatic fire-extinguishing systems are necessary for areas of high cable concentration and spaces below raisedfloors or above false ceilings containing exposed cables.
b) When the analysis in a) determines that fixed extinguishing systems are required, automatic water-spray systems designed in accordance with NFPA 13, or NFPA 15 shall be used. If the activation offixed automatic water-spray discharge could cause undesirable consequences to sensitive equipmentthat would negate single failure criteria, such equipment shall be protected from the spray and sealedagainst potential water damage due to water traveling along the cable system.If the equipment cannotbe protected, an extinguishing system utilizing another extinguishing agent, such as carbon dioxide(NFPA 12) or a clean agent (NFPA 2001), shall be provided in accordance with the appropriate NFPAstandard. These alternative gaseous suppression systems can also be used for spaces below raisedfloors.
c)System operational testing shall be in accordance with NFPA standards.
d)Fixed fire-extinguishing systems, whether manual or automatic, shall alert control-room operators of system operation or of any abnormal condition. Fire-extinguishing systems shall be electricallysupervised to comply with the requirements of NFPA 72, and Class 1 circuits as defined in NFPA 70,Article 725.
e) In areas where forced ventilation would circulate smoke or a gaseous extinguishing agent, or both,to other areas,mechanical ventilation systems shall be shut down prior to system actuation, and firedampers shall be closed by mechanical or electrical release devices prior to fire-protection systemdischarge.