Electrical Connection Technical Information
Electrical Connection Fires
Electrical connectors are the simplest type of component used in electrical engineering systems. Their very simplicity can make them liable to be neglected and their importance overlooked. The electrical connector connects electrical conductors and must achieve very low electrical contact resistance. This is achieved by the contact force (compression) within the connection and applied by the terminal screw. The ability for the electrical connector to maintain contact force, and therefore low contact resistance is essential and fundamental for the safe performance of the connector.
Each year there are many incidents, recorded by Fire & Rescue Services, of fires in homes and businesses caused by high contact resistance / high temperature electrical connections. Categorising these incidents has highlighted that this risk is especially common in electrical supply intake equipment (main intake fuse, electricity meter & consumer unit connections).
If a high resistance/temperature electrical connection begins to develop, it may steadily get worse without anyone becoming aware of it. High resistance electrical connections can be initiated by simple local environment changes i.e. through temperature fluctuations (including cyclic heating from electrical loading) or vibration and even if the connection cables have been moved or disturbed slightly. They can also be caused by sub-standard installation or maintenance works. A high resistance/temperature electrical connection can take weeks, months or even years to develop. Eventually, the connection may become hot enough to ignite the insulating materials around it, or eject sparks through electrical arcing to ignite combustibles in the vicinity. The ensuing fire can then spread destroying property, life and business.
Developing high resistance/temperature electrical connections cannot be easily detected, without the use of specialist equipment. It is therefore easy to understand how, once developed, they can deteriorate and cause fire.
The Stages of Electrical Connection Ignition
- Heat develops in the electrical connection.
- Electrical connection contact force reduces slightly causing a higher resistance connection.
- Heat develops in the electrical connection.
- If the temperature rises above 90°C copper contact surfaces oxidise more rapidly and the connection resistance increases generating more heat.
- If the copper creep temperature limit is achieved (135°C) the connection contact force can decrease due to changes in copper elasticity/plasticity. This again increases connection resistance and generated heat.
- Connection resistance & heat increases until eventual ignition of local insulation materials or electrical arcing ejects sparks which ignite local combustibles.
Brief Glossary of Terms
Whenever two electrical conductors are held together to enable the flow of electrical current from one to the other, there is some additional electrical resistance at the point of surface contact. The effect of this contact resistance is to cause a voltage drop and also cause heat generation at the point of contact.
Connection resistance is related to the surface contact, contact force and the elasticity of the copper. If the contact force is reduced, the connection resistance rises and so does the heat developed in the connection.
Elasticity is the ability of a material to resist a distorting influence or force and to return to its original size and shape when the force is removed. If the force is too great the material will deform plastically (permanently). The elasticity of the copper cable and connector body maintain the contact force and low contact resistance within an electrical connector.
In materials science, creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical forces and is known to be more severe when materials are subjected to heat. The rate of creep is a function of the material properties, exposure time, exposure temperature and the applied forces.
The rate at which the creep occurs depends on the rise in temperature above a material CTL (creep temperature limit). The CTL temperature for Copper is 135°C. Once the CTL has been reached the elasticity of the copper will be adversely affected.
Copper creep can be a problem within an electrical connector as it has the potential to reduce internal contact force (tightness) and therefore increase contact resistance.
IET Wiring Regulations
IET 17th Edition Wiring Regulations (BS 7671);
Chapter 13; Fundamental Principles.
134; Erection and Initial Verification of Electrical Installations
This regulation is reproduced below as it currently appears in BS7671:
‘Good workmanship by skilled or instructed persons and proper materials shall be used in the erection of the electrical installation. The installation of electrical equipment shall take account of manufacturers’ instructions’
Manufacturers’ instructions include the required torque settings for their equipment cable connections.