In order to study telecommunications cable combustion characteristics, a 100m long, ventilated full-scale tunnel was constructed to facilitate measurement of the temperatures inside the tunnel and those of its wall as well the velocities of two-layer countercurrents. Experimental results showed that the temperature distribution along the length of the tunnel was linear on logarithmic temperature. More than 80% of the dissipated calories obtained from the results were seen to be absorbed into the tunnel wall, and over 10% of the dissipated calories escaped as a gas flow out of the tunnel.It was further found that a great volume of ethylene gas, which was generated from polyethylene cable sheath by heat decomposition, was included in the flow of the fire products. This gas did not burn because air-inflow from the tunnel fan was observed to be limited. Measured temperatures inside the tunnel indicated that polyethylene ignition point temperatures were never exceeded beyond about 40m downstream of the burned end of the cables. A 40m long fire-proof cable compartment design was evaluated and found to be effective in stopping the fire spread along an unenclosed upstream segment of cable bundle.