Hazard Knowledge

The Energy Networks Association has produced Safety Information for the Fire Service. This contains information on emergency situations, including rescues, involving electricity. 

A person may be entrapped by being: 

• In contact with the ground, within 5m of a source of high-voltage electricity 
• In a vehicle that is in contact with or surrounded by high-voltage-electricity; the vehicle’s tyres may insulate it from the ground 
• On an insulated structure, such as a raised wooden platform, that is surrounded by high-voltage electricity 

When a source of high-voltage electricity is in contact with the ground, there is a risk of the ground itself becoming live and conducting electricity to people and objects in the vicinity. 

Electrocution may also occur if a person attempts to move from a position of safety, as, any contact with the insulated or protected position and ground at the same time will offer the electricity a path to the ground. 

The highest voltage is on the ground closest to the point of contact. It gradually reduces, in concentric rings away from the point of contact. This is known as potential gradient or voltage gradient. 

When walking towards or away from the point of contact, each foot can have a difference in electrical potential, resulting in one foot touching one voltage while the other is touching another voltage. The two voltages will try to equalise by flowing up one leg, through the body and down the other leg, which may be indicated by a tingling sensation. 

If the ground has become live, a similar situation can occur when standing with a charged hose. The voltage differential will be between where personnel are standing and where the hose is in contact with the ground.