Shock and Fracture
Shock and Fracture
An engineer recently received an electric shock whilst working from a ladder. The shock caused him to fall a distance of 2 metres onto the floor below. He sustained a cut to the palm of both hands. The injuries were such, that after cleaning himself up, he was able to continue his work.
The accident occurred whilst checking the operation of defrost heaters in an evaporator. The engineer had set the system to defrost, then placed the top of a 3 metre long wooden ladder against the evaporator and footed it's base against the store wall. He then climbed the ladder and removed the evaporator end-cover in order to expose the defrost heater wires for testing with his clamp ammeter. Unfortunately, some of the wires were buried in ice. Whilst holding onto the earthed cooler with one hand and removing the ice with the other, he touched a live 240volt conductor and received a shock that caused him to fall from the ladder.
The engineer was lucky not to have received more serious injuries. The shock could have been fatal, as could the fall. Therefore, please take a moment to consider the following facts and advice.
Effects of current flowing through the human body
(in milliamps ac @ 50 Hz) Effect
Less than 0.5 No sensation
0.5 to 2 Perception
2 To 10 Muscular contraction
5 To 25 Painful shock
Over 25 Severe shock with violent muscular contraction
50 to 100 Ventricular fibrillation - Heart failure - after 40 milliseconds
Over 100 Breathing failure
Typical shock currents through the body to earth from various supplies
Supply Voltage Current through the Body
110v safety transformer 28 milliamps
110v control circuit 55 milliamps
240v mains supply 120 milliamps
Although many of you may have experienced a shock from 240 volts, this has not proved fatal because you were, for example, standing on or wearing some insulating material, which will have reduced the current flow to below the fatal 50 to 100mA. However, in many engineering situations such as illustrated by the above accident, it is very easy to provide a very low resistance route (E.g. hand to hand.) for electricity to flow through the body to earth.
REMEMBER in your own interest when working with electricity:
• Do not take short cuts.
• Ensure that before covers are removed that power is removed by safe isolation and lock-off procedures.
• Test for dead with a well-maintained instrument before touching conductors.
• If you need to clear ice, only do so with the supply isolated.
• Question constantly the need to work live.
• Work within your level of knowledge, experience and training.
• When testing live circuits ensure that you are well insulated, use prescribed rubber mats to stand on, wear rubber-soled boots, do not hold earthed-metalwork with bare hands.
• Use an RCD if using 240volt portable tools.
• Be familiar with any risk assessments or method statements.
• Ensure safe access to coolers. Use a cage, MEWP or similar, AVOID LADDERS.
• Accompaniment is required when working at height with live supplies.
• Ensure that a responsible person on site knows that you are working on site and that you may require accompaniment.
• If in doubt - STOP.