UL1666 CMR Vertical Shaft Cable Tray Fire Test Chamber for Flame Retardant Cable Certification The SL-1666 CMR cable test chamber reproduces the burning condition of multiple cables vertically installed in a ventilated shaft and evaluates their flame retardant performance in accordance with UL 1666. Passing results are rated as CMR grade. Multiple specimens are laid on a vertical shaft and a burner with a heat release of 154.5 kW is applied for 30 minutes. The pass criterion: