Screw connections of mains cables are very common – they are used in almost every piece of electrical equipment, but fires caused by them are rare. RFA has recently completed investigations into a number of small fires in such equipment, and found that you cannot treat mains connections casually.

A number of small fires in mains powered equipment were investigated. The fires were centred on a mains connection into the equipment carrying up to 20 Amps. The mains cables were connected into pcb-mounted, screw-tightened terminal-blocks, although the full current was not necessarily passing into the pcb. Two completely different failure modes were seen, according to the detail of the design of the connector.

There is very little doubt that the initial cause of the problem was poor connection of the cable into the block. This was probably caused by inadequate tightening of the clamp screw, but in one case un-stripped wire insulation was inserted part of the way into the clamp. Both of these faults would have resulted in a reduced area of contact between the wire and the connector metal, and thus a higher current density than was designed. Neither of these situations caused any problems for many months, but the defect was set to generate enough warmth in the connection to gradually and progressively impair the already inadequate connection. When the problem became so acute as to cause thermal run-away, it actually appeared at two different points.

• In one connector design, the metal path between the screw connection and the solder tag to the pcb was relatively long and slender. There was sufficient metal to carry the current, but the thermal path was not generous. The heat was largely retained at the screw connection producing progressive oxidation and eventual melting of the copper wire.
• In another connector design, the metal path between the screw connection and the solder tags to the pcb was much shorter and more robust. This provided a greater thermal conduction path, and the heat from the screw connection was carried down to the solder joints made to the pcb. These joints eventually melted, thus becoming very much higher in electrical resistance, and caused a small fire centred at that point.

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