A cord must be strong enough to withstand the physical abuse anticipated for the equipment. The common mechanical concerns are flexing and pull. Excessive flexing of any cord will cause the copper strands of each conductor to weaken and break. If the cord is used after the failure of the copper strands, arcing will occur internally and eventually the arcing will explode through the covering. This catastrophic event can startle the user, cause burns and fires. If your cord must be flexed frequently then choose only copper conductors made with 34 AWG or 36 AWG strands. Stranding information is part of the cord description found in the catalog. All cords marked SVT must have 34 or 36 AWG strands. Cords used for vacuum cleaners are marked SVT, but may be found on other appliances and electronic equipment as well. Cords marked SJT may have larger 30 AWG strands and therefore not suitable for high flex-life applications. SJT cords have thicker insulation and can withstand greater abuse for use in power tools and similar applications. Check the UL standard for the end-product equipment to see if a cord marked SJT is required. Equipment or appliances that are likely to be moved around a lot with stress on the cord requires a round cord. They have a minimum of 150 pounds pull before any stress is applied to the copper strands. This is for extra safety that prevents damage to the conductors that may lead to shortages and fires. Flat, or parallel, cords are not intended for use where high pull stress or frequent flexing is likely. The equipment may be light duty and permit the use of a parallel cord, such as SPT-2. Parallel cord types are usually applicable for small appliances and electronic equipment that are stationary during use. Round cords are normally used when the appliance is often moved with the cord connected. Round cords provide more flexibility and flex life for durable appliances and equipment that moves frequently. In North America round cords must withstand a straight pull force of at least 150 pounds. The National Electric Code allows some pull force on a cord for pendant use. The construction of each cord includes strength members and perhaps fillers to meet this requirement.