T 1.6 Burning cables

When a cable catches fire, either by spontaneous ignition or exposure to flames, there can be a variety of consequences:

• When the wire insulation is destroyed, short circuits and arcing can occur, which then triggers the corresponding protection devices (circuit breaker or fuse) and disrupts the supply of power.
• Individual wires or the entire cable can break and disconnect. Especially critical is when only the ground conductor (PE) is broken while the live wires (L and N) are still in operation. The protective measures will have no effect in this case. An immediate threat is when the PEN conductor fails in a TNC network. The housings of Protection Class 1 devices would suddenly be live in this case. This obviously results in a shock hazard.

• Aggressive gases can form. Such gases can be corrosive and adversely affect the IT and communications equipment. They can also be toxic and lead to personal injury (e.g. poisoning). Corrosive gases can also attack structural components of the building in the concrete walls and floors, causing problems with the statics of the building when repairing the damage done by the burning cables.

• Cables which do not have fire-resistant or self-extinguishing insulation material may help to spread the fire. Even fire sealing cannot prevent this completely and merely delays the spreading of the fire.
• In tightly packed cable trays, smouldering fires can ignite and remain undetected for a long time, causing the fire to spread long before the fire breaks out openly. The conductivity of a cable decreases as it gets warmer, which in turn increases the loop resistance. This can generate additional heat that then promotes the continuation of the critical processes.

Burning cables often only cause a slight increase in the temperature while the fire is forming. This then creates a risk of generating significant amounts of "cold" smoke before the smoke detectors mounted on the ceiling are triggered.

Example:

• In an administrative building, the existing electrical cables were not replaced for cost reasons and were instead overloaded, in spite of knowing better. The necessary modifications were not made as the staff was expecting to relocate soon to a new administrative building.
• The overloaded lines heated up, and because they were packed so tightly together, heat accumulated and then caused a smouldering fire. The fire was only discovered after the lines failed due to the intense heat. It took several days to restore the workstations affected by the fire to proper operating condition again.